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

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Troubleshooting, Designing and Installing Digital and Analogue Closed Circuit TV

Systems


Topics Introduction Human eye and Light Sources of light Eye persistence – motion pictures Comparison between eye and camera Optical elements Basics of a lenses Types of lenses Lenses as applied in CCTV Back-focus Adjustment Neutral Density filter (ND filter)


Introduction

Objects are classified into two types Luminous bodies

• These are the ones which can generate light

• Example: The sun, stars, comets and other such bodies

Non-luminous bodies

• Objects that cannot produce light by themselves, but reflect the light that falls on them, are called non-luminous bodies

• Example: the Moon, planets, humans, bricks, plastic, metals etc


Human Eye and Light

Spectrum distribution of light, with associated wavelengths


Human Eye and Light

Response of eye at different wavelengths


Human Eye and Light

Additive and Subtractive Mixing

Principle of additive mixing and subtractive mixing


Human Eye and Light

Cross section of the eye

Human eye


Human Eye and Light

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 as defective

Major defects found in the eye due to lack of correct focusing are Hypermetropia (farsightedness) Myopia (nearsightedness)


Sources of Light

Colour Temperature of different light sources


Sources of Light

Artificial lighting is 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

Spectral output range for common sources of artificial light


Sources of Light

Typical levels of illumination in indoor Typical levels of illumination at outdoor


Sources of LightConditions Illumination Comments

Foot-Candles (FtCd)

(lux)

Direct sunlight 10,000 107,500 Daylight range

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

Full moon 0.01 0.1075 Low light level range

Quarter moon 0.001 0.01075

Starlight 0.0001 0.001075

Overcast light 0.00001 0.0001075

Light levels under daytime and night time conditions


Comparison Between Eye and Camera

Eye and camera similarities


Optical Elements

Lens as optical elements The basic lenses used in optics are concave and convex

lens Important properties of lenses

• Optical plane

• Optical axis

• Focus

• Focal length

• Diopter

Properties of lens


Optical Elements

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 called as rainbow colours

White light refraction through prism


Basics of Lens

Contrast Transfer Function (CTF) and Modulation Transfer Function (MTF) 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

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).


Basic of Lens

CTF is given by the following equation

MTF is similar to CTF except that MTF uses sine wave spatial frequencies and CTF uses square wave

243

2sin kfkCkT S

Where Cs - The quality of objective lens defined by spherical aberration coefficientl - Wave-length defined by accelerating voltageΔf - The defocus valuek - Spatial frequency


Basics of Lens

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


Basics of Lens

Spatial frequency• It is the characteristic of any structure that is periodic in space

• Spatial frequency is never constant, it varies up and down from point to point

• Modulation ‘M’ for a given spatial frequency ‘ν’ is given as

(v) M

(v) M (v) MTF

object

image

MTF is given as

I I

) I - (I M(v)

minmax

minmax

Where

I max – maximum intensitiesI min – minimum intensities


Basic of Lens

Depth of field with different F-stops


Basic of Lens

Near and Far focus Limits of Depth of field


Lens as Applied in CCTV

Image and Lens Formats in CCTV A camera images the angular extent of a given screen and

is referred as ‘Angle of vision’

This vision is equal in all directions when an object is viewed through a lens and is in conical shape

The image projected by the lens is spherical in shape and the camera sensor is rectangular in shape, irrespective of the imaging circle.


Back-focus Adjustment When a lens’ back-flange is adjusted in relation to the

CCD image plane, it is referred to as back-focusing

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

Zoom lens focusing at different distance objects


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 is also referred to as gray filter or ND filter

ND4 and ND8 filters


ND filter

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

Image when slower shutter speed is used with both ND4 and ND8 filters


ND filter

ND filters are used to open up the aperture while maintaining constant shutter speed

Image with larger aperture(a) With no ND filter(b) With ND4 Filter(c) With ND8 filter

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