KELVIN SCALE AND CRI LEVEL
Color temperature is a characteristic of visible light that has important applications in photography, video, publishing and other fields. Color temperature is a concept that was first defined by William Thomson., Lord Kelvin (1824-1907). Thomson, an Irish-born physicist, wrote over 600 papers on the laws of motion and the dissipation of energy.
Lord Kelvin’s name today is most associated with the Kelvin scale, a measure of molecular activity and energy emission. The color emission temperatures are based on the apparent color of a black body (a theoretical radiator and absorber of energy at all electromagnetic wavelengths.) heated to various temperatures on the scale. A theoretical black body begins to radiate light as its temperature increases; it glows red at 1000 K, yellow at 3000 K, blue-white at 6000 K, and intense blue at 60,000 K.
True black bodies exist only in theory, so to be practical, think of a piece of iron; as the iron is heated, it first begins to glow a dull red, then orange, then yellow, and finally a bluish-white. Each of these radiated colors is an accurate measure of the metal’s physical temperature – the activity of its molecules. These color temperatures also refer to different wave-lengths of light.
Because the Kelvin scale is based entirely on natural phenomena, it is regarded as an “absolute” scale, and temperatures on it are simply identified as “Kelvin” or “K”. The proper way to refer to the temperature of a quartz light is simply “3200 K” (for example).
In film and television production, there are a few benchmark color temperatures and standard methods for converting between them. The principal two are photographic daylight (usually thought of as 5600 K) and tungsten, or quartz incandescent (3200 K).
For video operations the relevant temperatures range from around 2,000K to 8,000K; these are common lighting conditions. In practical terms this usually means selecting lights, gels and filters which are most appropriate to the prevailing light or to create a particular color effect. For example, a camera operator will select a "5600K filter" to use outside in the middle of a sunny day.
C.R.I, or color rendering index, is another attribute of the light that a fluorescent bulb emits and it is as important as the color temperature in determining the quality of the light. This is a calculated measurement of how accurately a given lamp “renders” colors. A theoretically prefect score is 100. a lamp with a high CRI (anything above 90) makes colors look rich and deep, whereas a lamp with a much lower CRI makes the same colors in a set or costume seem washed-out and sickly.
The index rating is a measure of the average appearance of eight standardized colors of intermediate saturation spread throughout the range of hues. It measures what wavelengths are missing from a lamp when compared to a theoretically perfect light source.
CRI is an especially important rating for fluorescent tubes because they are often missing significant portions of the spectrum. Fluorescent tubes are a discontinuous light source that could be missing a chunk of red and have a huge spike in the green range. Fluorescent bulbs are tricky things, and some can have emission at only a few select wavelengths, almost as bad as the pure red, green, blue light. It takes work to smooth out the spectrum into a smooth distribution of all colors, so you'll pay more for a high CRI bulb.
Because the Kelvin scale is based on the balanced radiation of a theoretical black body, it’s technically incorrect to classify fluorescent's on the Kelvin scale. Kelvin ratings for tubes with a low CRI are less meaningful, unlike for tubes with high CRI which emit a more even spectrum.