The Science of Photography - Colour Reproduction
Forming the Image | Controlling the Image | Colour Reproduction
Chemical Image Processing | Digital Image Processing
Subject Lighting | Halftone Reproduction  | Mechanical Printing

Colour Reproduction

The Source of Colour

Light is the source of colour, this was first demonstrated by Isaac Newton in 1666 at the age of 23. Newton passed a ray of sunlight through a glass prism to produce a rainbow effect. He then passed these rays through a second prism to reproduce a single ray of white light. Newton thus proved that colour is in the light and that white light is a mixture of all the colours in the visible spectrum.

When light strikes a surface the surface will absorb some colours and reflect others. It is the reflected colours we see. 
 
Additive and Subtractive Methods of Colour Reproduction Back to the top

In 1801 Thomas Young suggested that the human eye contains three types of colour sensitive receptors. One sensitive to blue light, one to green and one to red. Varying signals from these receptors produce, in our brain, any visible colour.

In the middle of the 19th century Clerk Maxwell demonstrated the formation of colours by mixing red, green and blue light. The process of adding red, green and blue light to produce colour is called the additive system. 
 
The image on the right demonstrates the additive method, where separate coloured lights combine to form other colours. 

When green red and blue light overlap the combination is white light. 

Please click to see larger view

Another method, called the subtractive process, was first developed independently by two talented Frenchmen, Charles Cros and Louis Ducos du Hauron in the 1860ís.

The subtractive process uses three filters to remove unwanted colours from white light. There are three layers, the yellow layer absorbs blue light, the magenta layer absorbs green light and the cyan layer absorbs red light.
 
Please click to see larger view The subtractive process works by taking colour away from white light by using yellow, cyan and magenta filters. 

Where two of the filters overlap they pass red, green or blue light, depending upon the pair of filters. Where all three overlap all wavelengths are blocked.

The subtractive process maybe seen in the following diagram, if you move the cursor over the links different filters will be placed over the background.
 
No Filter
Cyan filter blocks red
Magenta filter blocks green
Yellow filter blocks blue
Please click to see Image

 
Recording Colours

The first commercial photographic processes used the additive method. The Lumiere brothers patented the Autochrome process in 1904. They recorded colour by randomly covering the film plate with millions of starch grains individually dyed red, green or blue. Each grain acted as a filter and the image is broken into tiny dots of primary colours. The plate is then processed into a positive transparency. The eye has limited resolving power and is tricked into seeing a full colour image.

Such additive processes were inefficient in terms of light passing power and were difficult to print on paper or copy to another screen material, however Autochrome plates continued in production until 1932. The additive process is now used in digital cameras with filtered CCD (Charged Coupled Device) systems. 

For film emulsions the subtractive process eventually superseded the additive method. However although "colour couplers", the mechanism for producing the required coloured dyes during film development, were invented in 1912  it took until the 1930ís before a practical solution to the complex technical problems were overcome. 

On April 15th 1935 Eastman Kodak placed on sale the colour film called Kodachrome, largely the product of two scientists Leopold Godowsky and Leopold Mannes. Kodachrome produced a positive colour transparency and used non incorporated colour couplers. In 1936 Agfa were the first company to produce film material with colour couplers anchored within the emulsion layers.

The first negative colour film, Agfacolor, appeared in Germany in 1936. Towards the end of World War II American troops seized the Agfa plant and "liberated" the technical details of Agfacolor and the formulas were soon distributed to film manufacturers around the world. Agfacolor thus became the basis for a variety of colour processes

Now all modern colour film uses the subtractive process. Multilayer colour material is basically comprised of three permanently superimposed emulsion layers of different colour sensitivity. Usually the top one is sensitive to blue light, followed by green and red sensitive layers.

Improvements are constantly being made and although the principles remain the same many films now contain multiple emulsion layers of different colour sensitivity and silver halide crystal structure. The aim being to improve both film speed and colour reproduction.
 
White Light and Colour Temperature  Back to the top

We recognise a mixture of visible light at different wavelengths as being "white" light. Although the distribution of energy at each wavelength could vary considerably from source to source our brains still perceive it as "white" light.

Move your cursor over the links below to see changes in the composition of white light depending upon the source, the chart shows wavelength against relative power.

Please click to see Image
Blue Sky
Tungsten Filament Lamp
Flash Tube
Noon Sunlight
Fluorescent Tube

Photographic processes, both digital and chemical, record subtle differences in colour. For photographic purposes a common method of gauging the balance of colours of an artificial light source is by "Colour Temperature". As an incandescent light heats up, not only does the temperature vary but so does the colour. 

Clear Blue Sky
Flash / Daylight
Tungsten Bulb
Wax Candle
Please click to see Image

The colour temperature chart above indicates the approximate colour temperature of various "white" light sources in degrees Kelvin.
 
More Information  Back to the top

For more information about colour reproduction please refer to the following sections :


Next in the Science of Photography : Chemical Image Processing

Forming the Image | Controlling the Image | Colour Reproduction
Chemical Image Processing | Digital Image Processing
Subject Lighting | Halftone Reproduction  | Mechanical Printing

 
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