Troubleshooting, Designing, & Installing Digital & Analog Closed Circuit TV Systems

1

Light and Optics

This chapter discusses the general aspects of light and the response of the human eye towards light with different wavelengths and frequencies. Sources of light and the comparison between eye and camera are given. Reflection, refraction, the measurement of lens speed and losses as per the F and T numbers and depth of field are explained. Lens formats, angle of view of lenses, C and CS mounts and back focusing of lenses are also discussed. Learning objectives: After studying this chapter you will have a basic understanding of:

• The response of human eye towards light • The different sources of light • Eye persistence in motion pictures • The difference between eye and camera • Optical elements • The basics of lenses and lens types • Back-focus adjustment and ND Filters

1.1 Introduction In general, objects are classified into two types. They are:

• Luminous bodies • Non- Luminous bodies

Luminous bodies are those that can generate light. For example, the sun, stars, comets and other such bodies are luminous bodies. On the other hand, objects that cannot produce light by themselves, but reflect the light that falls on them, are called non-luminous bodies. For example, the Moon, planets, humans, bricks, plastic, metals etc are non-luminous bodies. The Sun, Stars and glow-worms are natural sources of light. Apart from these natural sources of light, there are some man-made, artificial sources of light such as a wax-candle, oil-lamp, torch, electric light bulb and Light Emitting Diode (LED) etc. The intensity of light at any place is measured in terms of lumens present on unit area. This unit is called foot candle (Lux is the modern unit). One foot candle is equal to 10.76 lux.

2 Troubleshooting, Designing and Installing Digital and Analog Closed Circuit TV Systems 2

1.2 Human eye and light Light is an electromagnetic radiation which is measured in wavelength or frequency, the same as radio signals. Wavelength is usually measured in nanometres (nm) while frequency is measured in hertz (Hz). Higher the frequency, the shorter is the wavelength. Out of the different frequencies, visible light falls in the range of 400nm to 700nm as shown in the Figure 1.1. Here, 400nm corresponds to violet and 700nm corresponds to red. There are various colours between the violet and red wavelengths, blue, blue-green, green, yellow-green, yellow, orange. All these colours produce different effects on the human eye retina.

Figure 1.1 Spectrum distribution of light, with associated wavelengths Ref: http://www.sciencesway.com/vb/t6526

The eye shows its greatest reaction to green light. Green light produces the highest output on the retina where as red and violet light produces the least output. The eye’s spectral sensitivity is higher at green light, not at red or violet, because the largest amount of the sun’s energy that penetrates the earth’s atmosphere is in the range of 555nm. This is represented graphically in Figure 1.2.

Figure 1.2 Response of eye at different wavelengths Ref: http://www.bunkerofdoom.com/laser/response/index.html

Light and Optics

3

1.2.1 Additive and subtractive mixing Red, green and blue are referred to as primary colours. To produce any secondary colour we have to mix two or three primary colours in different proportions, resulting in magenta, yellow, etc. The mixing of primary colours to produce new colours is called additive mixing and the reproducing of primary colours from the secondary colours is called subtractive mixing. This is illustrated in Figure 1.3.

Figure 1.3 Principle of additive mixing and subtractive mixing Television screens use additive mixing to produce the colour images we see on the screen. Printing, on the other hand, uses the secondary colours to produce a colour image on the page or photo quality paper. Hence the use of yellow, magenta and cyan toners in colour printers.

1.2.2 The human eye The Retina is a photosensitive area of the eye which is composed of millions of cells called cones and rods. Cones are sensitive to medium and bright intensity light and can also sense colour. Rod cells are sensitive to lower light levels and cannot distinguish between colours. We use the rod cells to see at night which makes it harder to distinguish colours. Cones cease to react once the level of light falling on them drops below 1 lux. The rods are 10,000 times more sensitive than the cones. There are approximately 10 million cones and over 100 million rod cells in each eye.

The cross section of a human eye is shown in Figure 1.4.

Figure 1.4 Cross section of the human eye Ref: http://academia.hixie.ch/bath/eye/home.html


Focusing of the eye is necessary to see objects at different distances. If focusing of the eye is affected then the eye may be considered defective. Major defects found in the eye due to lack of correct focusing are:

• Hyperopia (farsightedness) • Myopia (nearsightedness)

There are many other defects in the eye, although the above defects are the most common. These defects can be corrected by using different lenses. A convex lens is used to correct hyperopia (see Figure 1.5) and a concave lens is used to correct myopia (see Figure 1.6).

Figure: 1.5 Correcting hyperopia through glass

Figure: 1.6 Correcting myopia through glass Ref: http://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision/Optics/RefractiveErrors.html

In the same way that different types of lenses are used to assist focussing in the human eye, different types of lenses are used in CCTV to focus objects at different distances.

1.3 Sources of light For video surveillance in outdoor areas, artificial lighting by different light sources, such as tungsten, tungsten- halogen, metallic arc, mercury, sodium, xenon, IR lamp, LED IR arrays etc are required. These are selected according to the requirement, safety consideration and the quality of the video picture. The colour temperature of different light sources is shown in Figure 1.7.

Light and Optics

5

Figure 1.7 Colour Temperature of different light sources Artificial lighting is the process of illuminating a scene with a high level of evenly distributed light so that each and every object, place or person available at that point is clearly visible.

Figure 1.8 Spectral output range for common sources of artificial light Ref: http://www.olympusmicro.com/primer/lightandcolor/lightsourcesintro.html

The temperature at which an imaginary perfect black body is heated and consequently produces light is called colour temperature. Max Plank, a German physicist and the founder of quantum theory, explains the relation between the peak wavelengths radiated and the temperature to which the body is heated as: λm=2896/T Where:

• λ is wavelength in microns • T is temperature in degrees Kelvin.

There are five different standard sources of “standard white light”. They are referred to as Illuminants A, B, C, D6500, and E.


Illuminant A: is a warm light with a color temperature of 2845°K. A tungsten lamp at 2800°K produces this shade of white light. Illuminant B: is close to sunlight at noon. Filtering the light from Illuminant ‘A’ gives radiation of Illuminant B i.e. white light. The correlation colour temperature of B is 4880oK. Illuminant C: is the same as diffused light from an overcast sky. When Illuminant A is filtered Illuminant C is obtained. The correlation colour temperature of C is 6770oK. Illuminant D6500: is the standard used for color TV. An average daylight colour temperature is represented by source D6500. It is a mix of direct and diffused skylight and is not obtained by filtering any other source. The correlation colour temperature is 6500oK Illuminant E: is referred to as “equal energy white”. It is a hypothetical white and is equivalent to a light source comprising all wavelengths of visible equal light. Source E has uniformly distributed radiation which looks like a flat horizontal line. It is used for calculation only.

1.3.1 Light units The science that deals with light is called photometry and the units used are called photometric units. Important terms associated with light and their units are discussed below.

Luminous intensity (I)

Luminous intensity is the illuminating power of a primary light source, radiated in all directions. The associated unit of measurement is the candela (cd). 1 candela is equal to approximately the amount of light energy generated by a standard candle. 1 cd is 1/60 the light emitted from 1cm2 of a black body heated to a temperature of molten platinum 2042°K.

Luminous flux (F)

Luminous Flux is the measure of the flow of light in one second and is measured in lumens (lm). One lumen is the quantity of luminous flux that falls on a unit area surface at a unit distance from a light source of 1 candela in one second. Luminous flux = luminous intensity (lumens)/ 4П

Illumination (E)

Illumination of a surface is the amount of luminous flux falling on a surface and is measured in lumens per unit area. One lux = 1 lumen per square meter (lm/m2)

Figure 1.9 Light units and their meanings Ref: http://www.handprint.com/HP/WCL/color3.html

Light and Optics

7

When luminous flux of one lumen falls on an area of 1m2 then it is called meter candela or lux (lx). Different light units and their meanings are shown in figure 1.9.

The Inverse Square Law of Illumination: The illumination of a surface decreases as the square of the distance of the illuminated surface from the light source. At 1 meter from a 1 candela source the illumination of the surface is 1 lux (1 lumen per m2). At 2 meter the illumination is 1/22 = 0.25 lux. In basic terms this can be shown as: E = I / d2 lux Where E = Illumination (lux) I = Luminous intensity of source (cd)

d = the distance from the light source (m) Different levels of indoor and outdoor illuminations are shown in Figure 1.10 and Figure 1.11 respectively.

Figure 1.10 Typical levels of indoor illumination Ref: http://a-kingnet.com/index.php?route=product/category&path=39_40

Figure 1.11 Typical levels of illumination at outdoor Ref: http://a-kingnet.com/index.php?route=product/category&path=39_40

The vergence is inversely proportional to the distance. This can be explained by considering the following example. Consider a source which emits light or radiation onto a surface at different points. If the points are at differing distances from the source as shown in Figure 1.12 the following results:

• Distance A from the source: At this point or on the surface the rays are highly divergent and wavefronts are strongly curved


• Distance B from the source: Here the rays are less divergent and wavefronts are less curved

• Infinity distance form the source: Rays are parallel in nature and the wavefronts are a plane.

Figure 1.12 The amount of divergence of the light is inversely proportional to the distance from the source of light at distance ‘A’ has more vergence than that at B Ref: http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v1/v1c030.html

Luminance (L)

Luminance is the measure of light emitted from a surface. This light may be in the form of radiated light or reflected light. Luminance can be measured in two ways, using either candela or lumens per square area. 1 nit = 1 candela per sq m (cd/m2) 1 stilb (sb) = 1 candela per sq cm (cd/cm2) 1 apostilb = 1 lumen per sq m (lm/m2)

1 lambert = 1 lumen per sq cm (lm/cm2)

Table 1.1 shows the light levels under day time and night time conditions.

Table 1.1 Light levels under daytime and night time conditions

Illumination

Conditions Foot-Candles (FtCd)

(lux)

Comments

Direct sunlight 10,000 107,500 Full daylight 1000 10,750 Overcast day 100 1,075 Very dark day 10 107.5

Twilight 1 10.75 Deep twilight 0.1 1.075

Daylight range

Full moon 0.01 0.1075 Quarter moon 0.001 0.01075

Starlight 0.0001 0.001075 Overcast light 0.00001 0.0001075

Low light level range

Light and Optics

9

International Standards for Units (SI units)

SI units are the standardized units used for the measurement of quantity or quality of components which are accepted world wide. Table 1.2 shows the international standards used in the measurement of light and related factors.

Table 1.2 SI units

Unit Symbol Measure

Kelvin [K] temperature

Candela [cd] luminous intensity

The derived SI unit

The table below shows the derived SI units.

Table 1.3 Derived SI unit

Quantity Unit Symbol / Definition

Velocity Meter/second m/s

Acceleration Meters per second per second 2s

m

Frequency Hertz Hz =1/s

Density Kilograms per cubic meter 3m

kg

Energy Work Joule J=N·m Illumination Lux lx = lm/m2

Luminous flux Lumen lm = cd·steradian

Luminance Nit nt = cd/m2

1.4 Eye persistence – motion pictures Eye persistence is the most important “eye defect” used in cinematography and television. Persistence depends on the intensity of light and brightness to see the picture in motion. Pictures have to change quickly so as to avoid flicker and to give a feeling of motion when logically consecutive pictures are played. When these pictures move faster than the persistency of the eye, the motion of pictures looks like a moving picture. The eye responds differently to different wavelengths as shown in Figure 1.13.


Figure 1.13 Response of eye towards different wavelength Ref: http://videocodecs.blogspot.com/

A video camera records images with a speed of 25/30 pictures per second. For a low intensity projector, 24 frames per second, is required.

Television uses the same principle as that of the human eye. It projects pictures by a method called ‘interlaced scanning’ as shown in Figure 1.14. The pictures are created by scanning and the pictures are projected by lines. As the number of lines increase, the clarity increases. These lines are projected from left to right and top to bottom.

Three major television systems exist. They are: • PAL (Phase Alternating Line) • NTSC (National Television System Committee) • SECAM (Sequential Couleur Avec Memoire or Sequential Colour with Memory).

Although the picture projection and scanning is the same, the lines and the frames, or images, per second changes and the method of colour coding is different.

• PAL : 625 Scanning Lines / 50 Interlaced Pictures Per Second • NTSC : 525 Scanning Lines / 60 Interlaced Pictures Per Second • SECAM: 625 Scanning Pictures (Used To Be 819) / 50 Interlaced Pictures Per

Second.

Figure 1.14 Two interlaced fields build up a complete TV picture frame Ref: http://www.hardwarezone.com/features/view/123917/page:2

Light and Optics

11

1.5 Comparison between eye and camera The similarities between eye and camera are illustrated in Figure 1.15.

Figure 1.15 Eye and camera similarities Both eye and camera are similar in nature. In the same way that the retina captures pictures, the camera sensor captures the picture, scans it and produces images. The camera lens can focus on an object that is close or far away, according to the settings made. Where as a human eye can focus on the near or far away object automatically according to the requirement. In the human eye there is a spot referred to as the blind spot where nothing can be seen. When we consider both the eyes at the same time we cannot have the same view from both the left and right eye because of the presence of the blind spot. Thus, what we see is the combinational image of both the left and right eye. This differs from a camera where there is no blind spot. To transmit the information from the camera to a monitor we have to connect cables, where as in the case of the eye, nerves transfer the information to the brain. The information stored in a camera will not be deleted unless and until it is deleted manually, but this is not the case with the human eye. The pupil that is present in the eye can change its size rapidly according to the requirement, where as in case of a camera it does not change rapidly.

1.6 Optical elements

1.6.1 Lenses as optical elements The basic lenses used in optics are concave and convex lens. Concave lenses are used when the objects are to be shown smaller when compared to the actual size of the object. They have a negative focal length with a virtual focus. On the other hand, convex lenses are used when the objects are to be shown bigger compared to the actual size of the object. They have a positive focal length with a real focus.


The important properties of lenses are: • Optical plane: The plane passing through the centre of the lens • Optical axis: The axis perpendicular to the centre of the optical plane • Focus: The point where the rays falling parallel to the optical axis converge • Focal length: The distance between the optical plane and the focus (in meters) • Diopter: An inverse value of the focal length, where the focal length is stated in

meters.

Figure 1.16 shows the properties of a lens.

Figure 1.16 Properties of a lens There are different types of lenses with varying lens sizes and materials. Some of the different types are as follows:

• Plano–Convex (A lens which is plane on one side and convex on the other side) • Plano-Concave (A lens which is plane on one side and concave on the other side) • Bi-convex, also referred to as a Converging lens (A lens which is convex on both

sides) • Bi-concave, also referred to as a Diverging lens (A lens which is concave on both

sides) • Convex-Concave (A lens which is convex on one side and concave on the other side).

Figure 1.17 shows the different types of lenses.

Figure 1.17 Types of lenses Different focal lengths can be achieved by the combined use of different types of lenses with different focal lengths. Lenses can be joined by applying transparent glue. To obtain a particular focal length, design engineers use mathematical calculations. The assembly of a lens is shown in Figure 1.18.

Light and Optics

13

Figure 1.18 Lens assembly 1- Lens Clip; 2- Lens 1; 3- Lens spacer; 4- Lens 2; 5- Lens 3; 6- Lens holder The factors that determine lens quality are lens design, element manufacture, mechanical composition and electronics (refers to auto iris and motorized lenses). Following are the parameters considered under each factor.

Lens design: • Number of elements • Relative position • Aberration correction in the design stage.

Lens element manufacture:

• Glass type • Technology and type of glass manufacturing (heating, cooling, cleanness) • Precision of grinding and polishing (very important) • Anti reflection coatings of the glass (micrometer layers for minimizing losses).

Lens mechanical composition:

• The lens’ positional fixing and stability (shock, temperature) • The lens’ moving mechanics (especially zooming, focusing, iris leaves) • Internal light reflections (matte black absorption) • Gears used for motorized lenses (plastic, metal, precision).

Electronics (refers to auto iris and motorized lenses):

• Auto iris electronics quality • Electric consumption • Zoom and focus control circuitry.

1.6.2 Reflection and Refraction Light travels from one medium to another. When light travelling in a straight line strikes a plane mirror it bounces back at an angle, this is referred to as reflected light. This phenomenon is called reflection and Figure 1.19 shows this in detail.


Figure 1.19 Light Reflection from a plane mirror Ref:http://www.cssforum.com.pk/css-compulsory-subjects/everyday-science/everyday-science-notes/17029-glossory-physics-2.html When light travels from one medium to another (for example, consider a glass slab or water as one medium and air as another medium), it bends slightly at the boundaries of the medium, rather than reflecting back. This process is referred to as refraction and Figure 1.20 shows this in detail.

Figure 1.20 Light refraction through glass and water Ref: http://micro.magnet.fsu.edu/optics/lightandcolor/refraction.html Due to refraction, light loses its speed. The wavelength of the light changes as it penetrates from one medium to another. When a white light passes through a prism, its wavelength changes and appears as a spectrum of seven colours referred to as rainbow colours. Figure 1.21 shows the details of light spectrum.

Figure 1.21 White light refraction through a prism Ref: http://www.123rf.com/photo_7114848_light-dispersion-illustration-hi-res-3d-rendering.html

Light and Optics

15

Images captured by the eye or the camera are possible because light penetrates through the eye or the camera lens and leaves an image on them.

1.7 Basis of lenses

1.7.1 Contrast Transfer Function (CTF) and Modulation Transfer Function (MTF) Contrast Transfer Function (CTF) is used to evaluate and compare the performances of different microscopes. Definition: CTF is the function which modulates the amplitudes and phases of the electron diffraction pattern formed in the back focal plane of the objective lens. A chart consisting of white and black strips is used to measure the resolution of the lens. The white and black stripes are usually referred to as lines per millimetre. CTF is a characteristic which shows the response of the lens to various densities of the lines per millimetre. In TV lenses, the characteristic of a lens is similar to a continuous variation because the optical signal is converted in to an electrical signal. This characteristic is known as Modulation Transfer Function (MTF).

In practice, it is easy to produce black and white strips than producing continuous variation between the black and white strips. CTF is much easier to measure than MTF.

CTF is analogous to Modulation Transfer Function (MTF). In electrical engineering, MTF is used for modulation in the output signal to the signal frequency. CTF graphs the percentage contrast as a function of spatial frequency and thereby characterizes the information transmission capability of optical systems. The intensity recorded at zero spatial frequency in the CTF is a quantification of the average brightness of the image.

CTF is a performance measure of an imaging system and can be determined for the functional components of the imaging system. Performance of a system can be determined as a product of the CTF curves for each component. Modifications performed in magnification and concomitant adjustments of pixel geometry will result in improvement of CTF.

Figure 1.22 CTF and MTF Ref: http://www.microscopyu.com/articles/optics/mtfintro.html


CTF is given by the following equation:

( ) ⎥⎦

⎤⎢⎣⎡ ΠΔ+Π

−= 24352

sin kfkCkT λλ

Where: Cs = The quality of the objective lens defined by spherical aberration coefficient l = Wave-length defined by accelerating voltage Δf = The defocus value k = Spatial frequency

Figure 1.23 CTF plot for an imaginary 200 keV plot microscope (a) When envelope functions are not applied (b) When envelope functions are applied

MTF is similar to CTF except that MTF uses sine wave spatial frequencies and CTF uses square wave. MTF is an important aid to objective evaluation of the image forming capability of an optical system. Due to the errors in the optics, such as manufacturing, assembly and alignment errors, the overall performance of the system decreases. As a result the dark and light shades, or the image in the image plane, will not be the same as that of the original patterns. At zero spatial frequency, MTF is normalized to unity. If spatial frequency is low the value of MTF is close to 1 and decreases as the spatial frequency increases and reaches zero. This is the limit of resolution and the cut-off frequency for a given optical system.

Spatial frequency

Spatial frequency is the characteristic of any structure that is periodic in space. It refers to the number of pairs of bars imaged within a given distance of a retina.

Spatial frequency is never constant and it varies up and down and from point to point. Modulation ‘M’ for a given spatial frequency ‘ν’ is given as:

I I) I - (I

M(v)minmax

minmax

+=

Where: I max = maximum intensities I min = minimum intensities

Light and Optics

17

MTF is given as:

(v) M(v) M

(v) MTFobject

image=

Figure 1.24 MTF curves for two different lenses

1.7.2 Camera Lens Lens selection is based on a number of lens characteristics and specifications. The science of optical lenses is a very large subject and will not be covered in any great depth in this manual. However a few lens terms need to be defined. Note a very simplistic approach has been used in the following cases.

Focal Length (f): the distance between the optical centre of the lens and the principle focus. See Figure 1.25.

Effective Aperture (N'): the diameter of the opening within the lens system which controls the amount of light passed by the lens to the camera pickup device. See Figure 1.25.

Relative Aperture (N): also referred to as the f/stop number. It is obtained by dividing the FOCAL LENGTH of the lens by the EFFECTIVE APERTURE. The effective aperture is shown, for example, as f/5.6. The following list shows the standard f/stops - f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22.

Iris: usually an almost circular, variable diameter opening within the lens system, which when reduced in diameter, reduces the amount of light transmitted by the lens to the pickup device. As the diameter of the iris is reduced the Effective Aperture is reduced, the brightness of the image reduces and the Depth of Field is increased. See Figure 1.25.

Field of View (FOV or w): also referred to as Angle of View. This is the vertical and horizontal angles which the lens covers and is related to the Focal Length of the lens. A short focal length results in a large angle of view. As the focal length increases the angle reduces. See Figure 1.26.

Depth of Field (T): the distance over which a subject may extend when the lens is focussed as sharply as possible on one part of it, without the image becoming noticeably un-sharp. See Figure 1.27.

All these characteristics interact with one another and figure 1.25 shows the basic lens parameters. In reality the lens of the camera would be a number of lens elements, but this diagram will aid in explaining the characteristics of the lenses that must be considered.


Figure 1.25 Basic lens parameters

Figure 1.26 Field of view

Figure 1.27 Depth of Field Figure 1.26 shows how the field of view is reduced as the focal length is increased. This results in a close up or telephoto type of lens and the short focal length results in a wide angle lens.

Light and Optics

19

A normal basic camera lens has a focal length of about 50mm which gives a field of view equal to 47ø, while 200mm gives 12ø (telephoto) and 20mm gives 94ø (wide angle). The image size is set by the size of the pickup device being used in the camera and is usually slightly smaller than the maximum scanned area. The focal plane is the point at which the pickup device is placed in the optical system. Thus the image is focused at this point to give an image of height 'h'. Depth of Field is an important consideration, as already stated, this is the area over which an image will remain in focus. Figure 1.27 shows four lenses. In all of these diagrams the image size is the same and the bottom two (b, d) are exactly the same, while the top two (a, c) have one of the lens parameters changed, resulting in a reduction of the Depth of Field. In order to aid in the comparisons, the object distance has remained the same. The object height has also remained the same, except in Figure 1.27 (a), which is smaller, as discussed below. The two lenses in Figure 1.27(a) and 1.27(b) are shown with the same aperture setting, only the focal length has been adjusted. The lens in Figure 1.27(a) shows the focal length increasing, resulting in a smaller angle of view and a reduced Depth of Field. Another point to note is that a smaller object will result in an image of the same size, as the one below. This is due to the focal length of the lens also setting the angle of view. Short focal lengths give a wider angle of view, while a long focal length results in a narrow angle of view. Thus, for a wide angle lens (short focal length) focusing becomes easier and more of the image will be in focus. The aperture of a fixed focal length lens also controls the Depth of Field, as shown in Figures 1.27 (c) and 1.27(d). The larger the aperture, which is the same as using a larger lens, the smaller the Depth of Field. The smallest aperture gives a very large Depth of Field, in fact a pin-hole size aperture results in an infinite Depth of Field. Although not shown in Figure 1.27, the distance from the lens that the object is focussed will also result in changing Depth of Field. The dotted lines passing through the centre of the lens shows the field of view. If these lines were extrapolated beyond the points shown and the object moved (keeping the height of the object equal to the distance between the dotted lines), the solid ray lines would move, changing the area known as the Depth of Field. Moving the object closer to the lens means the ray lines would point up and down at a very steep angle, not unlike Figure 1.27(c). While moving the object toward infinity, the ray lines begin to spread, to give an infinite Depth of Field. The height of the object must also increase to give the same size image on the pickup device. Inversely if the object has remained the same, the image is going to be significantly smaller. Note, also, that objects very close to the lens will not be in focus. This can be shown by looking at any of the lenses in Figure 1.27. The distance where the object remains in focus, from the object to the lens, is smaller than that behind the object. The use of an automatic focus system will thus affect the Depth of Field. However, this could be an advantage, particularly when reversing. In this case, the image will remain in constant focus and as the object gets so close that a collision between the object and the vehicle may occur (when the object is very close to the lens, the Depth of Field reduces), the detail will be of more use to the driver than that of an out of focus image. Adjusting the aperture has already been shown to affect the Depth of Field, but a changing aperture also reduces the amount of light being transmitted by the lens to the pickup device. The iris is used to perform this function. Another means of adjusting the amount of light transmitted is to use a Neutral Density (ND) Filter. Thus the iris can be opened further and the same amount of light can be transmitted to the pickup device, while at the same time the Depth of Field remains unchanged. Putting all this information together gives some guidelines which need to be followed when selecting a lens. The first consideration is how wide an angle (Field of View) is required to give the best coverage. Once this is decided, the speed of the lens (maximum Effective Aperture in relation to the Focal Length i.e. Relative Aperture) should be considered. Thus a lens of f/1 is faster than a lens of f/2.8.


The term speed of a lens relates to the exposure time in the photography field. The faster lens allows more light to be transmitted and would be a requirement for night driving. However during the day the iris would have to close right down. The iris and ND filters would work together to provide the best Depth of Field and image illumination of the pickup device to provide noise free pictures.

Depth of Field is the biggest problem, since for the system to work correctly, as much of the displayed image must be in focus as possible. Object focus would then be set to provide the best Depth of Field. Therefore, Depth of Field is the total area under the field in focus. A smaller depth of field has a small area in focus and vice versa.

A larger depth of field occurs in the following cases: • The type of lens used changes the depth of field (if a wide angle lens is used, depth of

field is larger when compared to that of a telephoto lens) • When the F-stop setting is high • When high resolution cameras are used.

Figure 1.28 Depth of field with different F- stops Ref: http://audster.wordpress.com/2010/05/05/back-to-basics-part-1-the-aperture/

Figure 1.29 Near and Far focus Limits of Depth of field Ref: http://www.learnslr.com/slr-beginner-guide/depth-of-field-explained

The selection of the lens will thus be dependent on the selection of the camera and its related sensitivity.

Light and Optics

21

1.7.3 Focal (F) and Transmission (T) numbers

F-number

F-number is the ratio of the focal length of a lens to the effective aperture diameter. It is a measure of lens speed and is a dimensionless quantity. Lens light gathering ability is an important factor to be considered in an indoor environment. On a camera the F number is adjusted in discrete steps called f-stops. F numbers are usually written as F/x where F is the focal length and x is the aperture.

The F-number of a lens can be measured by setting up a light source behind the F-number of a lens, thereby setting the lens to full aperture. A parallel beam of light is seen to emerge through the lens, whose diameter can be measured. These, when divided into focal length, are called f/aperture. More light will pass through the lens when the F-number is less. Decreasing the F-number by a factor of 1.414 (i.e. √2) doubles the exposure and increasing it with the same factor will decrease the exposure to half of its value. When the F-number is doubled, it increases light passing power of a lens by a factor of 4 (i.e. 22) and vice versa. For camera lens usage F-numbers are invariably rounded off to whole numbers or one decimal point (e.g. f/4 or f/5.6). Hyper focal distances and depth of field calculations always use the f/aperture of a lens, not the T stop.

T-stop

T-stop is the amount of light that passes through a lens after all the losses, such as absorption, internal reflections and scattering. T-stops are the means used to measure the efficiency and consistency of a lens; therefore the exposure settings for any situation are related to the T-stop. T-stops are used for the comparison of lenses, by illuminating a piece of translucent material in the focal plane of each lens. The intensity of light projected through the lens is measured along the optical axis with an incident light meter. If one lens is required to be set, at say f/5.6 for a given reading on the photometer, then any other lens which is set similarly should give the same reading. T-stops are always smaller (higher in number) than f/apertures, i.e. a lens set at f/5.6 may equal T6. The differential may also be expressed as a percentage and will remain consistent throughout the aperture range of the lens. Example: T= f/no of aperture/root of normalized transmission, i.e. if f=1.8 and the transmission =81% then T=1.8/ROOT 0.81 =T=2

1.8 Types of lenses

1.8.1 Manual, automatic and motorized iris lenses The Iris is a mechanism used to vary the light falling on the imaging device. It determines the light falling on the sensor.

Manual iris lenses

Definition: A lens with a manual adjustment to set the iris opening is known as manual iris lens. These lenses are used for fixed lighting applications where cameras are readily accessible and the light levels are constant. Manual iris lens cameras are not suitable for external applications as lighting levels are constantly changing in a scene and the wrong level of light will enter the camera as the lens cannot prevent it. Automatic electronic shutters in the camera will prevent the wrong level of light to some extent but they are sensitive to very small changes in light levels.


Automatic iris lenses

The drawback of the manual iris lens can be overcome be using an automatic iris lens. An automatic iris lens is best suited for cameras used for outdoor scenes as they can cope with the variation of light. Two types of automatic iris lenses exist; these are discussed below. Types of automatic iris lenses Automatic iris lenses are categorized into two types as follows:

• Video driven automatic iris lenses • DC driven automatic iris lenses.

In DC Automatic Iris lenses, input is taken from the camera that resembles a DC Signal, there by representing a particular F-stop. Video Driven Automatic Iris lenses depend upon the video signal that is originated from the camera which is determined by comparing the amplitude of a reference voltage to the video signal that is coming from the camera, in order to check whether the iris is opened or closed. The camera determines the amplitude of the video signal by calculating the mean of the most recent captured image. When the video signal amplitude is more than the reference voltage, the iris will close until the signal amplitude is reduced. The lens will open when the video signal amplitude of the iris is below the reference signal.

Motorized iris lenses

These lenses offer remote control of the iris. Here, control action is set by the operator according to the light conditions. The iris is adjusted by a small electric motor controlled by signals generated from the remote operators control system. With the development of Charged Coupled Device (CCD) cameras this type of zoom lens is becoming increasingly popular.

1.8.2 Spherical and Aspherical lenses

Spherical Lenses

Spherical lenses are those which have a sphere shaped surface i.e. they have a spherical curvature. These lenses do not bring parallel light rays to a single focus. The focus depends on the distance of the light ray from the centre of the lens. If the distance of the ray from the centre of the lens is far, then the point of convergence is nearer to the lens and vice-versa. This phenomenon is called ‘focus shift’ and is illustrated in Figure 1.30.

Figure 1.30 Focus shift of a spherical lens Ref: http://www.physicsinsights.org/simple_optics_spherical_lenses-1.html

Some of the limitations of spherical lenses are: • High definition of the large aperture lenses, compensating spherical aberrations • Correction of distorted images with wide angle lenses • Compact and high quality zoom lenses.

These problems can be minimized by using aspherical lenses.

Light and Optics

23

Aspherical lenses

These lenses have a non-spherical surface to converge all light rays to a single focal point and incorporate some optical characteristics. Aspherical lenses are smaller and lighter in weight than the lenses which employ properties of spherical elements. They are cheaper to manufacturer and improve optical performance. The surface of an aspherical lens does not conform to the shape of a sphere, as shown in Figure 1.31. The moulding techniques and hybrid elements of the glass/polymer structure of the aspherical lens allows economical production. Aspherical lenses are commonly used for the aberration correction level needed for illumination systems. Aspherical lenses have an advantage of shorter focal length and larger aperture than compared to spherical lenses of equal diameter and spherical aberration correction.

Aspherical lenses are attached to video camera lenses to reduce focal length and act as a wide angle converter. An aspherical auto iris lens is shown in Figure 1.32.

Figure 1.31 Aspherical lens http://www.arri-rental.com/camera/lenses/35mm-prime-lenses/zeiss-master-prime-lenses/zoom-in.html

Figure 1.32 Aspherical auto iris lens Ref: http://www.directindustry.com/prod/geutebruck/camera-objective-lenses-28815-210256.html

Spherical aberration

High-angle rays are brought to a premature focus by lens aberrations so that the point object appears blurred in the image plane. This is illustrated in Figure 1.33.

Figure 1.33 Spherical aberration

1.8.3 Zoom lenses Zoom lenses have the advantage of varying focal lengths. The focal length can be adjusted by turning a ring on the lens, also referred to as twist zoom. In some older versions of this type of lens,


the zooming action was performed by pushing and pulling the lens. The latest zoom lenses give a lot of flexibility as they can be adjusted by turning the zoom ring.

Figure 1.34 Image of a lily pod taken from the middle of the pond with a zoom lens

Figure 1.35 Zoom Lens

1.8.4 Fixed focal length lenses Lenses with constant focal length are referred to as fixed focal length lenses or prime lenses. Zooming is not possible with this type of lens and the camera would need to be moved closer to the object if zooming is required. The advantage of using this type of lens is that they are lighter in weight and smaller when compared to zoom lenses. Their construction does not need multiple pieces of glass (known as elements).

Figure 1.36 Fixed focal length lens

1.9 Lenses as applied in CCTV

1.9.1 Image and lens formats in CCTV A camera images the angular extent of a given scene and this is referred to as the ‘angle of vision’. This vision is equal in all directions when an object is viewed through a lens and is conical in shape. The image projected by the lens is spherical in shape and the camera sensor is rectangular in shape, irrespective of the imaging circle. For example: Consider a 1.27 cm portion on a 1.693cm chip. A lens of a bigger format will project an image circle much larger than the actual chip size. If a bigger lens is used on a smaller chip, there will be no corners cut off or any other deformation.

Light and Optics

25

When there is a reduction in the imaging pickup, a reduction of relative resolution is observed as the area used is very small. Apart from this, the CCD blocks the excessive light which is around the chip that in turn may be reflected inside the lens. The images will be affected if there are surfaces that are not neutralized sufficiently with a black metal finish.

1.9.2 Angle of view The angle of view of a camera is the angular extent of a given screen. It can be measured horizontally, vertically or diagonally, but the horizontal view is used as a reference. The angle of view differs with the focal length of the lens. The following rules should be observed when analysing the angle of view:

• As the focal length decreases, the angle of view becomes wider and vice versa • With the same lens, as the size of the CCD chip decreases, the angle of view becomes

narrower • The vertical angle of view can be determined if the horizontal angle of view is

known.

A viewfinder calculator (see Figure 1.37) and an optical viewfinder (see Figure 1.38) are the two instruments used for finding the angle of view. Most camera and lens manufacturers provide useful on-line tools to calculate the angle of view. Software CCTV Installation tools can also be used. A viewfinder calculator is round or ruler shaped. Different parameters such as the CCD chip size, camera and object distance and object width are used to determine the focal length. Optical viewfinders are used by photographers to gain focus on an image.

Figure 1.37 Various lens calculators Ref:http://sliderulemuseum.com/HSRC/38621.jpg


Figure 1.38 Optical view finder

1.9.3 C-mount, CS-mount and back focus There are two standards for distance between the back-flange of the lens and the CCD image plane. They are:

• C-mount • CS-mount.

C-mount

C-mount is denoted as 17.5mm. This standard has been used since the early days of the tube cameras. It consists of a metal ring with 1.00/32mm thread and the front surface area is 17.5mm away from the image plane.

CS-mount

A mount used for smaller cameras and lens design is CS-mount and it is denoted as 12.5mm. In this, the same thread is used as in the case of the C-mount, but is approximately 5mm closer to the image plane. This preserves compatibility with the old C-mount format. It allows cheaper and smaller lenses to be manufactured to suit smaller CCD chip sizes.

C and CS mount lens are shown in Figure 1.39.

Figure 1.39 C and CS mount lens Ref: http://www.icpdas.com/products/Vision/mavis/vision_glossary_a~m.htm

1.10 Back-focus Adjustment When a lens’ back-flange is adjusted in relation to the CCD image plane, it is referred to as back-focusing. This concept is very useful when zoom lenses are used in CCTV. The output for the zoom lens, when focused at different distance objects, is shown in Figure 1.40. The optic-CCD distance in zoom lenses has to be precise in order to achieve good focus throughout the zoom range. Back-focusing adjustment is performed with a low F-Stop.

Back focus is the distance from the last glass surface of a lens to the focused image on the sensor.

Light and Optics

27

When the camera is zoomed in and focused clearly, but the zoom out makes the images go out of focus, then it is termed as a back focus problem. This is a problem associated only with zoom lenses. The back focus adjustment is performed at the back of the lens barrel, near the camera body. Because the back focus lens is designed to stay in one place, a little lever has to be loosened in order to move this ring. Once the adjustments are final, the level should be carefully twisted back in tightly so the back focus isn’t accidentally disturbed.

Figure 1.40 Zoom lens focusing at different distance objects Ref:http://thecareyadventures.com/blog/2011/aperture-31-days-to-better-photography/

Methods that assist with opening the iris to the maximum extent are as follows: • Adjust the back-focus at low light levels in the workshop • Adjust the back-focus in the late afternoon • Adjust the back-focus at daytime by using external ND filters • If a camera with CCD iris is used then the optical iris can be opened fully, even

during daylight, as the CCD iris will compensate for excessive light.

1.11 Neutral Density filter (ND filter) A neutral density filter is a light filter which is used to reduce the intensity of light passing through the lens. It filters the light spectrum evenly without affecting the colour and contrast. They are generally gray in colour. The effect of the filter depends on the depth of the gray colour. This filter is also referred to as gray filter or ND filter.

Figure 1.41 ND4 and ND8 filters

If the shutter speed is kept constant and an ND filter is added, aperture must be increased to maintain the same exposure.


Figure 1.42 Relation between aperture and the shutter speed

An ND filter can be used with all types of film and video cameras, but is especially valuable when working with high-speed films or long-exposure motion applications.

Different ND filter manufacturers may use different ways to indicate the amount of light an ND filter can reduce or attenuate. There are two typical systems as shown in Table 1.4.

Table 1.4

Indication of the amount of light an ND filter can reduce

Density Reduction of F-stops

0.1 31

0.2

32

0.3 1 0.4

311

0.5 321

0.6 2 0.7

312

0.8 322

0.9 3 1.0

313

2.0 326

3.0 10 4.0

3113

For example, ND filters marked as 0.3, 0.6 and 0.9 reduce the light by one, two and three stops. ND’s marked as 2×, 4× and 8× indicate reducing 1 (i.e. 2=21), 2 (i.e., 4=22), and 3 (i.e., 8=23) stops. ND2, ND4 and ND8 mean they reduce the light by one, two and three stops, respectively.

Light and Optics

29

When a slower shutter speed is used, moving objects may appear blurred creating a sense of motion, see Figure 1.43.

Figure 1.43 Image when slower shutter speed is used with both ND4 and ND8 filters (Ref: http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-ND.html)

ND filters are used to open up the aperture while maintaining constant shutter speed. See Figure 1.44.

Figure 1.44 Image with larger aperture (a) With no ND filter (b) With ND4 Filter (c) With ND8 filter (Ref: http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-ND.html)

Note: Remember that a larger aperture results in a smaller Depth of Field. The images in Figure 1.44 were all taken with a shutter speed of 1/30 second. The left image used F10.7 and no filter, the statue and background are all in focus. Using a ND4 filter reduces the aperture to F5.4, the background is now blurred and the statue is isolated from the background. Using the ND8 filter reduces the aperture to F3.9, now the statue is well isolated from the background and shows a sense of distance.

ND filters can be stacked together and the effect is to multiply the attenuation, although doing so may produce underexposed images.

ND filters can be fitted to the lens externally by screwing them to the front of the lens. See Figure 1.41. ND filters can also be found built into the lens body and can be variable density discs or rotating wheel with different ND filters fitted to the disc.

1.12 Summary This chapter summarizes the following:

• Objects are classified as luminous and non-luminous bodies. Luminous bodies are those which generate light, for example, the sun, stars, etc. Non-luminous bodies are those which do not produce light, but reflect the light which falls on them. Light is electromagnetic radiation.


• Visible light falls between the range of 400nm and 700nm in the electro magnetic spectrum, where 400nm is violet light and 700nm is red. The eye shows the greatest reaction towards green colour, as and when green colour falls on it. The largest amount of the sun’s energy that penetrates through the earth’s atmosphere is in the range of 555nm.

• Red, green and blue are primary colours and secondary colours can be formed by

using the additive process. • A human eye is made up of millions of cells called cones and rods. Cones are

sensitive towards medium and high intensity light where as the rods are sensitive towards low light levels.

• Luminous intensity is the illuminating power of a primary light source, radiated in all

directions. The associated unit of measurement is candela (cd). • Luminous Flux is the luminous intensity, but in a certain solid angle. The unit of

measurement for luminous intensity is 4П (pi) radians and is measured in lumens (lm).

• Illumination of a surface is the amount of luminous flux on a unit area.

• Luminous is defined as the brightness of a surface either of a primary or secondary

light source. The unit of measurement for surface intensity is Ft-Cd (Foot-Candle). • There are three colours in a Cathode Ray Tube (CRT), red, blue and green. • The relationship between the peak wavelengths radiated, and the temperature to

which a body is heated, is given as λm=2896/T. Where, λ is wavelength, T is temperature.

• Persistence depends on the intensity of light and brightness to be able to see a picture

in motion. Pictures have to change quickly as to avoid flicker, when logical consecutive pictures are played.

• There are three basic television systems. These are PAL (Phase Alternating Line),

NTSC (National Television System Committee), and SECAM (Sequential Couleur Avec Memoire or Sequential Colour with Memory). • PAL : 625 Scanning Lines / 50 Interlaced Pictures Per Second • NTSC : 525 Scanning Lines /60 Interlaced Pictures Per Second • SECAM: 625 Scanning Pictures (Used To Be 819) /50 Interlaced Pictures Per

Second • Both the eye and camera are similar in process but different in function.

• When light travelling in a straight line strikes a plane mirror it bounces back at an

angle, this is referred to as reflected light. This phenomenon is referred to as reflection.

• Penetration of light through different mediums is called refraction • The speed and wavelength of light changes due to refraction • The basic type of lenses used in optics are concave and convex lenses

Light and Optics

31

• The factors that determine lens quality are lens design, element manufacture, mechanical composition and electronics (refers to automatic iris and motorized lenses).

• CTF is used to evaluate and compare the performances of different microscopes. • CTF is the function which modulates the amplitudes and phases of the electron

diffraction pattern formed in the back focal plane of the objective lens. • MTF is similar to CTF, except that MTF uses sine wave spatial frequencies and CTF

uses square wave. • Spatial frequency: The characteristic of any structure that is periodic in space. It

refers to the number of pairs of bars imaged within a given distance of a retina. • F-number is the ratio of the focal length of a lens to the effective aperture diameter. • T-stop is the amount of light that passes through a lens after all the losses like

absorption, internal reflections and scattering. • Depth of field is the total area under the field in focus. Smaller depth of field has a

small area in focus and vice versa. • An iris is a mechanism used to vary the light falling on the imaging device. It

determines the light falling on the sensor. • A lens with a manual adjustment to set the iris opening is known as Manual Iris lens. • An automatic iris lens is best suited for cameras used for outdoor scenes. • Automatic iris lenses are categorized into two types as follows:

• Video driven automatic iris lenses • DC driven automatic iris lenses

• Motorized lenses offer remote control of the iris. • Spherical lenses are those which have a sphere shaped surface. Focus shift occurs in

spherical lenses. • Aspherical lenses have a non-spherical surface to converge all the light rays to a

single focal point. • Spherical aberration: High-angle rays are brought to a premature focus by lens

aberrations so that the point object appears blurred in the image plane. • Zoom lenses have the advantage of variable focal lengths.

• Lenses with constant focal length are referred to as fixed focal length lenses or prime

lenses. • A camera images the angular extent of a given screen and this is referred to as the

angle of vision. • The angle of view of a camera is the angular extent of a given screen.


• A viewfinder calculator and optical viewfinder are the two instruments used for finding the angle of view.

• There are two standards for distance between the back-flange of the lens and the CCD

image plane. They are: • C-mount • CS-mount

• Back focus is the distance from the last glass surface of a lens to the focused image

on the sensor. • When the camera is zoomed in and focused clearly, but the zoom out makes the

images fall out of focus, then this is referred to as a back focus problem.

• ND Filters can be used to reduce the amount of light entering the lens.

• Some modern lenses have internal ND filter discs.

Engineering

Troubleshooting, Designing, & Installing Digital & Analog Closed Circuit TV Systems