Photography of shiny objects
Colour of light
('colour temperature') - Obviously the colour of objects needs to be correctly reproduced. Colour rendering depends on a number of factors, such as:
- The colour of the light used for lighting the object.
- How well film or the light-sensitive component of a digital camera can reproduce a colour (in digital cameras the white balance needs to be propertly set).
- How old is the film, and was it properly developed.
- How well can the colour and contrast be reproduced in a print;
- when using a printer: what are the printer's specifications, what those of the pigments and the paper used;
- in the case of a computer screen: how well can the screen reproduce the colours and brightness.
- What colour and brightness corrections have been carried out.
Let us consider the colour of light. As we all know from looking at the rainbow white light is made up of a large spectrum of colours. At sunrise and sunset the outside light is reddish, with the sun high in a clear sky it is bluish but white when the sun lights the earth through a layer of clouds. The light from normal (tungsten) light bulbs is yellowish or orange tinged, the light emitted by halogen light sources is closer to white, photographic flash lights emit white light. However, if you use a flash light in a space with a coloured wall, the light reflected from the wall will not be white and will influece the perception of all objects illuminated with the reflected light. Similarly in e.g. a church with stained glass windows, the filtered light will create a colour cast that affects all objects in its interior. Usually we are not conscious of the more subtle variations of the colour of light as the brain adapts to it, so that after a short while a colour cast is filtered out and goes unnoticed. The material that we use to take a photograph (film, CCD or CMOS) does not adapt and mercilessly records any colour cast .
Colour of light - An iron rod when heated starts to emit red light, then it turns more yellow, subsequently white and at an even higher temperature blueish. One can correlate the colour of the light emitted to the thermodynamic temperate of the object emitting the light; temperature is expressed in Kelvin (K) (or in mired, see below). The light emitted at a certain temperature is thus called the temperature colour. Interestingly, in everyday language red is called a warm colour, and blue a cold colour.
The figure on the right shows the colours of the rainbow (red - orange - yellow - green - blue - indigo - violet), and white in the centre. Red, green and blue are primary colours which when added yield white; the are therefore called additive colours. White is obtained by adding diagonally opposed colours (e.g. blue + yellow); diagonally opposed colours are therefore called complementary. Thus white is to be found at the centre of the spectral map. The black line is Planck's curve, used to indicate colour temperature. The table below relates the colour of environmental light to light sources:
|sunrise or sunset||2000 K||reddish-yellow|
|tungsten light 100 watt||2800 K||orange-yellowish|
|halogen light 500 watt||3200 K||yellowish|
|halogen film photoflood 500 watt||3400 K||yellowish|
|one hour after sunrise||3500 K||yellowish|
|flash bulbs||4950 K||white with minimal yellow cast|
|daylight at noon||5400 K||white|
|electronic flashlight||5500 K||white|
|unclouded, daytime||6500 K|
|unclouded, summertime||> 8000 K|
How can a colour cast be handled ?
- Professional photographers employ a meter to measure the colour temperature. When taking the photograph a filter is used to remove the colour cast. Such a meter is too costly for the amateur photographer. However, when using artificial light sources one can easily estimate the colour temperature of the light emitted and apply appropriate filters to remove any colour bias.
- If you want the colour rendition to meet high requirements, you can use a colour control map and take a photograph of it under identical conditions as the photograph of the scene of interest. Kodak Color Control Patches are available through retailers. Professional laboratories can use the standardized colour set to make the necessary adjustments in print. This approach is less costly than purchasing a colour temperature meter and a set of colour filters; in addition it overcomes differences in colour rendition of film or digital devices.
- The more sophisticated digital cameras have facilities to adjust the colour temperature at the time of taking the photograph.
- Colour corrections can be made retrospectively by comparing the photograph with the original object. This is within anyone's reach who is capable of manipulating an image by computer, using software such as Photoshop®. Store digital images in RAW format, use software such as Lightroom® and Photoshop® and adjust the colour temperature then.
Another unit often used to express colour temperature is the mired, for “micro reciprocal degrees”.
To obtain the mired value for a colour temperature divide 1 million by the colour temperature. Thus 5500 K is the same as 182 mired, since 1 000 000 / 5500 = 182.
Mireds are commonly used for converting light from one colour temperature to another using a colour conversion filter. The decamired system is often used by continental European manufacturers in lieu of the arbitrarily numbered Wratten system applied to colour conversion filters.