Color and Temperature of Light and White Balance in Digital Photography

The knowledge of the color of light is indispensable to obtain satisfactory results. A good source of light emits determined proportions of the visible spectrum. Depending on the type of energy released, the light can be predominantly lacking red, green, yellow, blue, etc. Each type of light has a spectrum and, without the eye's adapting mechanism, we'd see how variable the color of light can be. Film provides an optimal fidelity to the color when the source of the light is that of the characteristics for which the emulsion has been designed (4.7.2). It's necessary to recognize these variations in the color of the light to be able to foresee it's effect on the film. On the other hand, the color sensitivity of digital cameras is variable , because we can electronically correct the signal according to the amount of light available (an be able to see the results on the screen). The characteristics of the spectrum of a type of light can be examined on a graph, were the differences jump to view. We can distinguish two types of spectrum: continuous (daylight, incandescent light), were practically all the wavelengths are present, and discontinuous (fluorescent light, low consuming domestic lighting, mercury or sodium gas lamps for industrial or road lighting) were only determined radiations appear.

Temperature of Color

The composition of a light of continuous spectrum can have a relative predominance to one of the extremes of the spectrum (red or blue). To refer to the different types, we can use spectral curves or adopt a simplified method: the proportions of red, green and blue of the light that can be defined using the concept of temperature of color. The system is based on the color of the light emitted by a black body at a given temperature, which is measured on the absolute Kelvin scale, similar to the centigrade, whose absolute minimum zero is (-273 ºC). Each temperature is associated with a spectral distribution of energy, at determined proportions of red, green and blue. Independently of the real

temperature of the source of light, the color temperature informs us of the composition of the light it's referring to. The instrument with which to make these precise color of light measurements is a colorimeter. These are some color temperatures as reference: Light is redder the lower it's color temperature is , although in visual terms it is the contrary: we associate a red light with a warm atmosphere and a blue light with a generally cold tone. Through these definitions, the manufacturers of lighting equipment can define the characteristics of the light emitted by an incandescent lamp or by a flash and the manufacturers of film can indicate which is the light that provides a correct color balance. Two standard temperatures are used to define tungsten light, which provide the color balance with film for artificial light:

• tungsten type A: 3,400ºK
• tungsten type B: 3,200ºK

And a standard temperature for daylight , which is around 5,500ºK can produce visible changes in the color of the final image. The effect of photographing with tungsten light using film for daylight is that the image will be completely dominated by orange, and with the inverse we obtain an image with a strong dominance of blue (4.7.2). When the color temperature of light doesn't coincide with the temperature that gives an optimal color balance, it's necessary to alter the composition of the light with filters (8.2). The concept of color temperature is not applicable to discontinuous spectrum lights. The response of the film depends on the spectral distribution, which can be superabundant in green (fluorescent), yellow (sodium) or any other color. The correction of the dominants with filters is a lot more uncertain, because, unless the light is normalized for use with color photography, it's spectrum lacks some of the wavelength and a certain distortion of tones is inevitable.

White Balance

All digital cameras have the possibility to choose pre-established color sensitivity, precisely according to the same pattern as with film: daylight and tungsten. But also we can have other different color sensitivities when the light doesn't adjust to one of the sensitivities pre-established (when photographing with fluorescents or house lighting, at dusk or in the night, etc.). This is achieved with a device called white balance . To memorize a determined sensitivity to colors (doing the white balance) we simply have to show the camera a white surface lit with the available light and press the corresponding button, which lets the camera reproduce neutral tones like whites or grays. As with film, if the available light spectrum is lacking a color, it's inevitable that come tones will be affected, but the overall result will be the best possible.