Although the second attribute of light, color temperature, refers to its basic color, we're also speaking of a characteristic of light that goes beyond the obvious.
For example, in the photo on the left both sources of light (sunlight on the right; a standard light bulb on the left) normally appear as white light to the eye. It's only when we see them together as we do here that the difference in color temperature becomes obvious.
As we've noted, under normal conditions when we look at subject matter such as this, approximate color consistency comes into play and automatically makes a perceptual adjustments for these different sources of light.
Strangely, and maybe unfortunately, when we look at video or film, approximate color consistency doesn't work in the same way. Unless color corrections are made, we'll notice significant (and annoying) color shifts between scenes when they are cut together.
Although light can be any color between infrared and ultraviolet, there are two basic color standards: 3,200K (Kelvin) for incandescent lamps used in studios and 5,500K for average daylight. The latter can vary, as we will see.
Just for the record, film lighting directors and others who take such things very seriously insist that CCT, or Correlated Color Temperature is a more accurate and realistic reference than the traditional Kelvin scale. Even so, since they typically "slip" and talk in terms of Kelvin specifications, we'll continue to use this traditional designation.
while we are talking about such things, the "degree" designation is not
necessary when you use the term Kelvin. The Kelvin temperature scale is based on the centigrade or Celsius scale. By the way, in case you are ever asked the question on a quiz show, the Kelvin scale is named after Lord Kelvin, the father of thermodynamics.
Sunlight's Varying Color Temperature
The color of sunlight can vary greatly depending on the time of day, the amount of haze or smog in the air, and the geographic longitude and latitude of the area.
Because of its angle to the earth in the early morning and late afternoon, sunlight must travel through more of the earth's atmosphere.
Note the different lengths of the red lines on the left. The longer line in the drawing represents the sun's angle at sunrise or sunset.
The longer path results in more blue light being absorbed than red. (Shorter wavelengths of light are more readily absorbed.) Consequently, the color temperature of the sun is shifted toward red, which accounts for the red in sunrises and sunsets. (Note photo here.)
During midday, the sun's rays have less distance to travel through the atmosphere (the overhead sun in the above illustration) and the temperature of direct sunlight at noon equals about 5,500K. (Depending on conditions, this number can be from 5,400 to 6,000K.)
As the sun moves across the sky there are subtle color shifts. And, if the sun moves in and out of cloud cover, color temperatures (and light quality) will also dramatically change.
Color temperature also changes as a result of traveling through haze or an overcast sky. If a camera is not color-balanced under these conditions, this light will impart a cold, bluish look to skin tones.
The color temperature of average summer shade is rather blue -- about 8,000K. Midday skylight (no direct sun) can range from 9,500K to 30,000K. Why is the color temperature of shade or the light from the sky higher than direct sunlight? In both cases it's not the direct sunlight that predominates, but light from the blue sky.
Artificial Light Sources
We mentioned that 3,200K is the standard color temperature for TV lighting --considerably lower (redder) than average daylight. Note the high proportion of yellow and red in the incandescent area of this illustration.
This artificial type of lighting is commonly referred to as incandescent light, or tungsten light, after the coiled tungsten filament in these lamps.
In the ▲ the earlier photo daylight from a window is illuminating the subject on the right and the illumination on her left is from an incandescent light bulb. Since both types of illumination are present at the same time, the contrast between the two becomes evident.
Not all incandescent light is 3,200K. A common 100-watt light bulb, for example, is only about 2,850K. A candle flame (for those of you who have a need to shoot productions under candlelight!) is even redder-- about 1,900K.
Most of the differences in these sources can be handled by the built-in "indoor-outdoor" color correcting filters of your camera together with white balance circuitry.
Broken Spectrum Sources
You may have noticed that sometimes videos and still photos shot under standard fluorescent lights often exhibit a greenish-blue cast. Fluorescent lamps belong to the group of lighting devices known collectively as discharge lamps -- glass tubes filled with metal vapor with electrodes at each end.
Unlike tungsten type lights, standard fluorescent lamps have a broken spectrum. Instead of a relatively smooth mix of colors from infrared to ultraviolet, standard fluorescent light has sharp bands or spikes of color -- primarily in the blue-green areas. Even though the eye will not notice these spikes, color shifts can result with video.
Although a blue-green cast used to be rather obvious in video shot under fluorescent lights, recent improvements in CCD/CMOS sensor color response have reduced the problem.
CFL or Compact Fluorescent Lamps
With energy saving compact fluorescent lamps (CFLs) seemingly destined to replace most tungsten lamps in the coming years, the color temperature characteristics of these lamps are now significant in videography.
The majority of CFLs on the market are between 2700K and 3000K, which is comparable to an incandescent bulb. However, "bright white," "natural" or "daylight" CFLs are also available in higher Kelvin color temperatures: 3500K, 4100K, 5000K, and 6500K. The latter enhance cooler colors -- blue, green, and violet, and dull down reds and yellows.
Because they are essentially fluorescent lamps, they have a broken spectrum that can result in unexpected color shifts with film and video. (Note the discussion of fluorescent lamps below.) Before CFL lamps are used in critical color work the result should be checked on a good color monitor. If the color balance is not what you want, you should substitute lights of known color characteristics.
The Daylight Fluorescent Tube
Using a popular fluorescent tube, the daylight fluorescent, as an example, the average color temperature for this tube is 6,500K. Note that there are two "spikes" in the fluorescent spectrum (the green area in the illustration).
These spikes of high energy color cause the blue-green cast that we often see when we shoot under standard fluorescent illumination.
Correcting for "Standard" Fluorescent Color Temperatures
Although some video cameras have fluorescent filters included in their filter wheels, they can't completely or consistently solve the color balance problem of standard fluorescent lights. For one thing, there are about 30 different fluorescent tubes in use, each with slightly different color characteristics. In terms of color temperature they range from 6,500K to less than 3,000K.
The standard consumer-type fluorescent lamp that causes the least color temperature problem is the warm-white fluorescent, at 3,050K. Even though this type of fluorescent light can make subject matter look slightly pale and greenish, it will generally produce satisfactory results -- assuming the camera is white balanced on a card, and assuming perfect color fidelity isn't a goal.
As in the case of CFL lamps, to avoid the unpredictable effects videographers who want to accurately reproduce skin tones simply turn off standard fluorescents and set up their own lights. This not only solves the color temperature problem, but it brings the light up to a more acceptable level.
The discussion above has centered on standard fluorescent lights and their problems.
In recent years, at least two fluorescent tube manufacturers have started producing high-intensity fluorescent bulbs that use special chemical compounds to smooth out the spectrum spikes normally found in standard fluorescent tubes.
Banks of color-balanced fluorescent lights produce a soft, virtually shadowless light over a wide area.
This type of light (shown on the right) has been gaining popularity in many studio applications. Compared to traditional incandescent studio lighting, it generates much less heat and consumes much less energy.
However, since these fluorescent banks can't project light any great distance, their use is limited to subject matter that's relatively close to the lights. Often, color-balanced fluorescent banks are used to provide an over-all, even lighting, and more directional lighting instruments are then added as accent (key) lights.
Other Types of Discharge Lights
Other types of discharge lamps can cause much more severe color problems.
One type, the sodium vapor lamp, used primarily for street lighting, produces a brilliant yellowish-orange, broken spectrum light that will drastically (and hopelessly) skew color balance. (Note photo on the left.)
Operating at higher internal pressures are mercury vapor lamps, sometimes used for large interior areas such as gymnasiums. Although the basic color temperature ranges from 3,500K to 5,900K, depending on the lamp, because these lamps normally have a badly fractured spectrum, they can do strange things to color.
They are often mixed with other types of lamps to smooth out the color spectrum problem, especially when good video is a consideration.
Thanks to the human attribute of approximate color consistency, many of the color temperature problems we've discussed may not be obvious to the eye. But they can present major problems for video and film when you attempt to match successive scenes during editing. This represents just one type of technical continuity problem (scene-to-scene technical inconsistency) that you can encounter in video production.
TV Set and Video Monitor Color Balance
Before leaving the subject of color temperature, we need to mention one other factor that needs to be considered in reproducing video and film images -- the inherent color temperature of the TV sets and color monitors that display the pictures.
Professional TV monitors use a standardized set of SMPTE color phosphors that create "white" at 6,500K. Note that this is bluer than sunlight, and much bluer than incandescent light.
However, the color temperature of most home TV sets is considerably higher than 6,500K -- commonly 7,100K in the United States and 9,300K in Japan. TV sets sold in most European countries tend to be much closer to the 6,500K standard.
One of the reasons for the high color temperature in U.S. and Japanese sets is the consumer desire for bright, saturated colors, which are easier to create when there is a large blue component in the phosphor mix.
It's too early to tell about color standards with the new LCD, DLP, and plasma flat-screen TV displays. Right now they vary significantly. This is in part due to the fact that this technology has yet to be standardized.
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