The only real problem with continuous lights is the heat that some kinds of bulbs throw off—specifically tungsten and quartz-halogen bulbs. Newer daylight balanced fluorescent bulbs have eliminated this problem. There are three parts of these lights to consider: stands, reflectors, and bulbs.
StandsStands come in a variety of styles and prices. Their purpose is to hold lights and other lighting devices in a fixed position. They are usually collapsible for easy storage and have sections so their height can be adjusted. You can add a boom to hold lights, reflectors, diffusers, gobos, or other objects out at a distance. In tabletop photography, stands need not be too tall—6 to 8 feet should suffice.
Reflectors vary from those found in hardware stores to expensive professional units. When using them be sure they are neutral in color so they don't add a color cast. However, there are gold reflectors and make colors look warmer.
The bulb is the most critical part of the continuous lighting system.
- Tungsten lamps, especially photofloods, throw off a lot of heat. Some also have an unusually short life span—as low as 3 hours. Because these were the only bulbs available when continuous lights were last popular, they account for the alternate name for continuous lighting "hot lights".
- HMI (Halide Metal Oxide) lamps are small, very expensive arc lamps that generate four or more times the light of tungsten bulbs with less heat. The light is also perfectly daylight balanced.
- Fluorescent bulbs are inexpensive, cooler, require 90% less power, and last 100 times longer than tungsten lights—up to 10,000 hours. They can also be dimmed to 3% of their full power and provide a more consistent color temperature. A new type of fluorescent bulb, called a compact fluorescent (CFL) comes in a variety of color temperatures. The 6500°K bulb emits white light commonly called "Cool Daylight" and 5000°K bulbs match midday light. Because these bulbs have so many good features, they are the authors' number one choice for digital desktop photography.
Although a digital camera's white balance control can capture normal colors under a wide variety of lighting conditions, you should be aware that different types of lights cast different colors on a setup. This is why when shooting in homes, photos often have a warm reddish cast, while those taken under some florescent lights look greenish. When choosing studio lights, especially continuous lights, you should investigate two color-related terms used to describe them—color temperature and color rendering index.
- Color temperature describes how cool or warm the light source appears. For example, incandescent lamps have a warmer, more reddish appearance than colder, bluer HMI lamps. Color Temperature is expressed in degrees Kelvin (K). Daylight on a clear day is about 6500° Kelvin—a mix of direct sun at 5500°K and skylight at 9500°K. Lights with lower color temperatures look red; those with higher color temperatures look blue. To picture this imagine a blacksmith heating an iron bar. It first gets red hot, then as its temperature increases, it becomes white hot, and finally, blue white hot. To measure the color temperature of a light, you can use a color meter. These are fairly expensive and although crucial with film photography, they are not as critical in digital photography because of white balance control.
- The Color Rendering Index (CRI) is a relative measure of how colors shift when illuminated by a particular lamp as compared to a reference source such as daylight. Daylight has a CRI of 100, the highest possible CRI. The closer the CRI of a light source is to 100, the "truer" it renders color
- Watts describe the power consumed, not the light emitted. For example, there are many different lighting fixtures that use 100 watt lamps, but the output efficiency of these lamps will vary by 100% or more.
- Lumens indicates the lighting intensity of continuous lighting. It's a measure of the total light output of the lamp. A 27 watt CFL lamp has 1750 lumens, the same as a 100 watt tungsten bulb.
- Reflector efficiency ensures that the available light will be focused on the subject to be photographed and not on areas outside the camera's field-of view.