IMAGES AND INFORMATION

This section of the module deals with understanding how information is delivered by visual means.

The term 'television' now has a wider meaning than it originally had.

Today, the internet itself has become a prime means of delivering visual and audio media information.

Television- "seeing from afar" hopes to present an image of a distant scene as accurately as possible in terms of:

-- Geometry-- Brightness and contrast -- Colour conditions

Light as Electro Magnetic Radiation

wavelength symbol = λ (lambda)wavelength measured in nm (nano-meters)

1 nanometre = 1nm = 10-9 m)finer measurement: Angstrom unit = 10-10 m

Range of full spectrum (visible and invisible)

• Radio waves• Infra red• Ultra violet• x-ray• Cosmic rays

wavelengths (λ) from 3000m to 3 x 10-17m

Analysis of Visible Colour Spectrum

400 - 700 nm approximatelySpectral (ROYGBIV) colours Richard Of York Gave Battle In Vain

Green is predominant colour in all natural lightorder :Blue (400nm), Green, Red (700nm)

Light is the visible part of the electromagnetic spectrum

Colour Temperature

A light source will have a given spectral power distribution (SPD) – the energy emitted at each wavelength of light, in watts. Objects generate a SPD according to how hot they are.

• absolute temperature (Kelvin)• light colour represented by degreesK• referred to as ‘warm’ or ‘cool’ colours

Industry Standard Colour Temperatures

This is black body radiation, light sources can be matched against these

• 2800 degrees K- Tungsten filament light bulb• 6500 degrees K - Standard Daylight White

This can be done even if the light source is not a black body, e.g. a fluorescent tube, LED.

Colour temperature is used to calibrate the white balance for lighting and exposure control on cameras

Colour temperature through the day

More than one standard colour of white!!

‘cooler’ colours actually represented by higher degreesK!

• 5-6000 degreesK (midday)• 3-4000 degreesK (late afternoon/evening)

Visual Perception

It is necessary to understand the fundamentals of human vision from which the design criteria for television originate

how much of what you see is:

• received• interpreted• invented or ‘completed’ by your brain

Visual Perception

Think about...how much (or how little) visual information you need to make sense of what you see?

Human Vision• field of view• stereoscopic view• visual acuity• colour vision• sensitivity

The Human Eyehttp://webvision.med.utah.edu/anatomy.htm

retina: • the light sensitive screen at the back of the eye.

• translates light into illumination, which is passed by nerve fibres to the brain

optic nerve: • collective term for the the bunch of nerve fibres which

connect directly with the brain

iris:• adjusts light entering the eye like a camera shutter.

lens:• convex lens which focuses the image on the retina

rods & cones

cones (colour - sensitive down to 1 Lux)

• 3 types - each has different response curve• responsible for normal colour vision• 6-7 million in each eye

rods (monochrome only NOT colour)• 10,000 times more sensitive than cones • responsible for brightness and contrast

information• 100-120 million in each eye

blind spot: no rods or cones (no cells in centre at connection with optic nerve)

fovea: • area of maximum visual resolution • 34,000 cones (each with own nerve fibre)• NOT very sensitive to light

• consequently...the eye is more sensitive when looking slightly to the side of an object in low light conditions (i.e. when light is focused to the side of the fovea onto more sensitive part of the retina)

• try this looking at a star lit sky - see the difference!

What we see• Cones respond to colour• Rods respond to brightness

What we see is a highly detailed monochrome image with added colouring. The overall IMPRESSION is a sharp colour image

Wavelength response of the eye

• Although the eye can detect a wide range of colours, its ability to respond varies across the range.

• At the centre of wavelength of around 550 nm, sensitivity is greatest, falling off either side.

• The peak of the response moves towards shorter wavelengths as the light level falls. This is due to a change in the colour of the illumination.

• Moonlight and starlight are more blue than sunlight.

Visual SensitivitySpectral sensitivity functions derive from photoreceptor sensitivities.

Sensitivity of an individual From Wald G: Science 145:1007-1016, 1964,

http://www.unmc.edu/Physiology/Mann/mann7.html

day & night vision non-uniform response

Laws of light

• In general terms, white light contains an equal mix of all wave-lengths from 380 nm to 780 nm.

• Most of the field of view gives out reflected light, and the property of colour is due to selective absorption of certain wavelengths.

• A white object is so defined because it absorbs very little at any wavelength.

Measures of light

Luminous Intensity (Candela)

• one Candela (cd) = one candle power (c.p.)

Luminous Flux (Lumens)An expression of the electromagnetic radiation output of lighthttp://en.wikipedia.org/wiki/Lumen_%28unit%29

• one Lumen (lm) is the quantity of luminous flux which falls upon a surface

• in scientific terms...the quantity of luminous flux per second which would pass through one square metre of the surface of a perfectly transparent sphere with a radius of 1 metre, with a source of 1 candela at the centre

Illuminance (E)

Measure of the concentration of luminous flux falling on a surface

Illuminance expressed as Lux• one Lux = one lumen per square metre (lm/m2 )• non-metric term used to be ‘foot-candle’ • foot candle now rarely used

http://science.howstuffworks.com/light2.htm

Luminance • measure of light emitted from a surface

• either emitted or reflected/incidental light

• therefore expressed either as cd/m2 or lm/m2

Luminance is used in the video industry to characterise the brightness of displays. • A typical computer display emits between 50 and 300

cd/m2. • The sun has luminance of about 1.6×109 cd/m2 at noon

http://en.wikipedia.org/wiki/Luminance

BrightnessAnother parameter is the brightness of the perceived colour.

• If light of a certain wavelength is added to light of the same wavelength, then the total brightness is perceived as the sum of the two (Grassmann's law).

There is incoherent addition of the energies in the light.

Contrast

• Contrast is a phenomenon, which compares brightness or colours.

• The acceptability of a given contrast depends on light level.

• At low illumination, a range of 10:1 is acceptable, but at high levels, e.g. in full sun, a range of 1000:1 is needed.

• Domestic television usually achieves between 30-100:1

Colour mixing

It has been known for some time that the subjective effect of a particular colour can be simulated by light of other wavelengths.

As an example, a red light and a green one, in varying proportions, can produce a wide range from red through to orange, yellow, and finally green.

However, a green at 540nm is different from that at 560nm. Consequently, any colour mixing must take account of wavelength.

The definition of white

One of the interesting properties of the human eye is its ability to see white.

White is scientifically defined as the equality of all wavelengths.

In reality, and as far as the human observer is concerned, several colours are acceptable as white.

For example, a sheet of "white" paper appears white in direct sunlight, under fluorescent light, or under incandescent bulbs, yet each environment is different in terms of the wavelengths present.

It is for this reason that the lighting for television must be strictly controlled in terms of the spectrum.

Standardisation is to D6500• Red, at 650 nm • Green, at 530 nm • Blue, at 460 nm

• D6500 is the white usually used in colour TV and corresponds to a mixture of direct sunlight and diffuse "skylight".

• It can be generated using pixels on a colour TV screen.

• In a TV studio, tungsten lighting is almost always used, which is deficient in the short wavelengths (blue).

• The colour balance of the received picture would be erroneous, but for the electronic processing used which corrects for “white balance”

Colour starAdditive and Subtractive Colour Theory• Primary Colours - additive• Secondary - subtractive

http://www.colour.org.uk/additive.jpg

Subtractive colour mixture: overlapping cyan, magenta and yellow filters

Saturation

• Another aspect is the definition of the depth of a colour. This is known as the saturation.

• The hue of a colour is the apparent colour of the dominant wavelength, eg blue, red, yellow, etc.

• Saturation can describe a whole range of colours, from white, through pastel shades, right through to a full deepness. (A pale pink is a desaturated red.)

• Desaturated colours contain a proportion of white, as well as the predominant colour.

Saturation =

maximum value – minimum value--------------------------------- X 100% maximum value

eg. if Red = 0.5, Green = 0.5, Blue = 0.1

Saturation = 0.5 (Red or green) - 0.1 ---------------------- x 100% = 80% 0.5

The colour would be described as a 50% amplitude, 80% saturated, yellow, (Red = Green), ie the hue is "yellow".