Colour temperature in photography

colour-temperature-in-photography

colour-temperature-in-photography

 Color basic technique for the photographer has a relationship with the ligh some technique of color temperature and unrelated to the light.

PHOTOGRAPHY BASIC LIGHT?

Lighting Tips For Photography

Phrases like ‘red hot’ and ‘white hot’ are in everyday use. In the first Industrial Revolution of the

late 18th and early 19th centuries, it became important to judge accurately the temperature of

industrial processes such as smelting and glass-making. Traditionally, this was done by

observing the colour of the furnace. William Thompson, the 19th-century physicist and later

Lord Kelvin, formalised these observations and the unit of colour temperature, kelvin (not

degrees kelvin), is named after him.

If a dull block of iron is heated to white-hot, it passes through the full range of colours from dull

red through yellow. The colour temperature scale relates directly to this idea of the colour

change seen when heating an object. Kelvin took the actual temperature and added the value

of absolute zero to create his scale. The block of iron analogy runs out at ‘white-hot’, as in the

air this is where the iron would begin to burn (oxidize). Exclude the air by putting the block in a

vacuum; keep pumping in more energy and the colour will change from white to blue (we have

just invented the light bulb).

 

colour temperature measure of ‘whiteness’ of light, measured in kelvin

The sun is a source of radiant energy. Many of our light sources are incandescent (glowing)

sources, usually created by heating a metal filament in a vacuum. In colour photography, we

need to take into account the quality (temperature) of the illuminating light. Time of day has a

big effect on the colour of light.To be able to photograph in a range of lighting conditions, yet

keep white appearing as white and not blue-white or yellow-white, we need to be able to

either filter the light source or use an appropriate film stock for that light. Digital cameras

too can be adjusted to give neutral whites across a range of light sources with varying

Colour temperature of common light sources

The sun is a source of radiant energy. Many of our light sources are incandescent (glowing)

sources, usually created by heating a metal filament in a vacuum. In colour photography, we

need to take into account the quality (temperature) of the illuminating light. Time of day has a

big effect on the colour of light.To be able to photograph in a range of lighting conditions, yet

keep white appearing as white and not blue-white or yellow-white, we need to be able to

either filter the light source or use an appropriate film stock for that light. Digital cameras

too can be adjusted to give neutral whites across a range of light sources with varying

colour temperatures.

Working out which filter will convert the colour of one light source to another is done using

a nomograph. Simply draw a line connecting the colour temperature of the original light

source with the converted source. The necessary mired shift to achieve the change can be

read off from the centre scale. The filters that produce the necessary shift are also shown.

This nomograph shows only photographic filters for use on the camera; you will find other

charts that additionally show the coloured gels that can be used on the light sources

themselves, a practice more common in the film and TV industries.

Colour balance in photography

The human eye and brain will adjust our perception of white whatever the colour quality of the

illuminating light source. Digital cameras make something like this adjustment when switched

into automatic white balance (AWB). With colour film, if adjustment is needed to avoid a colour

cast – and it usually will need at least fine-tuning – then a filter has to be used.

Colour films are balanced to give neutral results with very specific colour temperatures.

Daylight film (see pages 58-9 for a description of photographic daylight) is balanced for a

colour temperature of 5500K. Tungsten film is balanced for a temperature of 3200K. (Older

technical books refer to two types of tungsten film – Type A balanced for 3400K and Type B

at 3200K.)

By the correct use of conversion filters, daylight film can be balanced for use with artificial

lights or artificial light film (tungsten) balanced for daylight use. These are the extreme cases;

more likely is the subtle adjustment necessary of film to a light source that is close to, but not

precisely at, the film’s specified colour temperature.

Colour conversion filters let the photographer match the colour temperature of the lighting to

the film in use. They can also be used to modify the colour balance. In Europe, the filters are

described as KB (bluish) filters, which increase colour temperature, or KR (reddish) filters,

which reduce it.

There is an older system of describing filters called the Wratten numbering scheme. This was

named after Frederick Wratten who sold his coloured filter company to Eastman Kodak at the

beginning of the 20th century. Wratten numbers can seem confusing but are still used in both

the USA and the UK today.

The Wratten colour conversion filters in the deep blue 80 series and deep orange 85 series are

used for making major shifts in colour temperature. The Wratten 81 series filters (yellow-amber)

lower the colour temperature and 82 series filters (light blue) raise the colour temperature over

a range of several hundred degrees. They are referred to as colour balancing or light

balancing filters. These raise or lower colour temperature in much smaller steps than the

colour conversion filters.

colour balance truthfulness of colours across the spectrum

colour balancing (light balancing) filters amber and blue filters used when making colour temperature

changes, sometimes referred to as warming or cooling filters respectively or light balancing filters

colour cast unwanted, overall colour change in an image

colour conversion filters deep blue and orange filters used to achieve significant shifts in colour temperature, and to correct white balance when using film and lighting of a different target colour temperature filter factor filters cut out certain wavelengths of light, reducing the total amount of light that reaches the film or sensor – for a correct exposure an allowance must be made. A filter that cuts out half the light will have a factor of x2 and one stop must be added to the exposure indicated by an incident light meter reading. On the whole TTL meters in cameras are not affected…

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