colour

Sunrise and Sunset

The medium through which light passes can change its spectral distribution. Due to colour constancy, we normally tend to see colours as staying more or less the same. If, however, the colour change is relatively extreme, it will be noticed. The colour of sunrises and sunsets for example, can vary greatly depending on the time of day, latitude, season of the year, weather conditions, and air quality.

Rayleigh small-particle scattering of short-wavelength light causes the sky to be seen as blue. It also contributes to the vivid reds and oranges seen at sunrise and sunset. At dawn and dusk, light must travel through more of the atmosphere so short wavelengths like blue and violet are scattered early in the process. Light traveling along an extended atmosphere path is deficient in blue light and richer in long wavelength red light. As more and larger particles are encountered, the long wavelengths are also scattered. This effect is particularly dramatic when an unusually light concentration of dust or smoke particles in the air creates spectacular sunsets of red, orange, and yellow. The preponderance of red and yellow rays can change and objects apparent colour. For example, a yellow fence may appear orange; a blue or violet car may appear considerably darker in colour. Optimal conditions for viewing an objects' true colour is at mid-day under a slightly overcast sky with sunlight coming from the north. And that's why artists prefer to have windows in their studios faced to the north.

Light Sources and Photography

Incandescent and fluorescent lights are the two most common forms of artificial light. Incandescent light is created by passing an electrical current through a resistant tungsten filament. The heated filament produces photons, the spectral composition of which vary with the filament material and temperature. At lower temperatures and thus lower illumination levels, long wavelengths predominate. For example when the dimmer control is turned down, a white wall will appear redder. At higher levels, their is relative increase in the short wavelength emissions causing the same white wall to appear more yellow and then bluish-white. Incandescent lighting is commonly used in houses and restaurants where a "warm" relaxing environment is desired.

In a fluorescent light a voltage differential across the tube excites the gas molecules inside. As they return to an unexcited state, ultraviolet radiation is released and is absorbed by the phosphorus coating on the inside of the tube. More efficient than incandescent sources, relatively few red and yellow wavelengths are emitted by most fluorescent fixtures. Although the light from fluorescent sources usually appears to be "whitish-blue", by mixing different phosphors, light of different colours can be produced. Because it is much more efficient than incandescent light, fluorescent sources are popular for commercial and office settings.

The differing spectral output of different light sources creates some real challenges for photographers. Unlike the human visual system, camera film lacks colour constancy, and therefore its chromatic sensitivity must be matched to the spectral characteristics of the lighting used. The spectral sensitivity for daylight film for example, is matched to the spectral characteristics of sunlight. As you may have noticed when daylight film is used for taking a picture indoors under incandescent lights, it has a yellow/orange cast. This is due to the high proportion of long wavelength present in incandescent lights. Less colour distortion occurs when daylight film is used under fluorescent lighting because spectral characteristics are closer to that of sunlight. The effects of both incandescent and fluorescent lighting using daylight film can be compared below.

Scene under incandescent lighting

Same scene under fluorescent lighting

To get around the problem of matching their film to lighting, photographers use films of different "colour temperature". The colour temperature of film is indexed in degrees Kelvin (K). A log fire or candle has a colour temperature of about 1800 degrees K and a standard 100 W tungsten light bulb about 2845 degrees K. The colour temperature of photoflood lights often used for indoor photography and movies is 3200 degrees K and requires the use of a special colour matched film. Flash bulbs are typically rated at about 3500 degrees K and require a different film. Regular fluorescent lights have a colour temperature of about 6500 degrees K. To represent the north sky on an overcast day, a fluorescent light of 7500 degrees K would be needed.

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