Last time, I broke down the elements of tungsten light sources; for this second installment of Adventures in Lighting, we’ll take a look at fluorescent fixtures.
|A compact fluorescent lamp (CFL)|
Once upon a time, fluorescent light was the bane of the cinematographer’s existence. The light produced by the ubiquitous lamps was greenish, ugly and unflattering. However, modern advances in multi-phosphor mixes in fluorescent tubes have made them feasible production lighting tools.
Fluorescent lighting has a lot to recommend it: it’s naturally soft, consumes very little electricity and puts off nearly no heat. These are all strong pros in the world of digital video.
The cons: the inconsistent mixture of different types of tubes you’re likely to encounter in any given location, the high green content of the light, the non-continuous and false color spectrum, and the flicker associated with shooting at very fast shutter speeds or high frame rates.
A fluorescent tube is made up of five primary parts:
- Mercury gas
- Phosphor crystals
- Glass envelope (bulb itself)
- Metal terminals
Fluorescent light is derived from electrons that are emitted by an electrode at one end of the tube colliding with a free electron in the outer orbit of an atom of mercury (a metal/conductor in gas form) inside the tube. This collision, which happens at high speed, produces ultraviolet radiation. The radiation in turn excites the phosphors coating the glass tube, which then emit light.
Fluorescent light is unlike incandescent light and doesn’t produce a full spectrum of color. Although fluorescent lighting is energy efficient and doesn’t create the heat that standard incandescent lighting does, its lack of a full color spectrum causes digital videographers a number of headaches. Instead of providing a smooth, even distribution of color from the full spectrum, fluorescent light provides spikes of specific color wavelengths. Although fluorescent bulbs may simulate a specific color temperature (called CCT or correlated color temperature) equivalent to incandescence, it is never perfect. That being said, with careful selection of your lamps, you can use them very efficiently to light your scenes.
Parts of a fluorescent lamp
There are many types of fluorescent lamps, from the popular CFL (compact fluorescent lamps) to the Slimline to the ubiquitous T-12 tubes. The most important aspect of the different shapes and styles is that you find the right lamps for the fixture you’re working with.
By the nature of their operation, fluorescent lamps require precise control over the electrical current delivered to the lamp. Without this control, the lamp would nearly immediately die, possibly in a volatile manner.
A fluorescent lamp is a negative differential resistance device; as more current flows through the tube, the electrical resistance within the tube actually drops. If the lamp were connected to a constant supply of electricity, it would most likely explode, or at least quickly burn out the electrode.
Electricity is moderated by a ballast, which provides an initial high burst of current to cause the electrode to spark, and then carefully regulates the flow to prevent overheating. A ballast is designed to work with a particular size and wattage of fluorescent lamp. If you ever need to replace a ballast, make sure it’s designed for the lamp in your fixture.
Most important to digital cinematographers are the CCT and CRI ratings of the lamps they work with. CRI (a somewhat controversial topic beyond the scope of this story) stands for color rendering index; it is a scale from 1 to 100 by which noncontinuous spectrum light sources are measured to denote how accurately they simulate a full spectrum of color and, in turn, allow objects to be seen in their “real” colors. In other words, it represents the accuracy of colors in that light as compared to a natural light source.
A CRI of 100 represents natural daylight, which renders objects’ true colors. Light with a CRI of 50 might render reds more maroon or greens more grey than their “true” colors. You can white-balance under low CRI lamps, but you’re not going to get true colors in your scene because the wavelengths of light just aren’t there, regardless of how neutral your camera makes the whites.
Low CRI lamps are often used where inexpensive, long-lasting illumination is needed and where color fidelity is not a priority. The worst-case real-world example is the low pressure sodium vapor street lamps that pervade many cities. These fixtures emit a very narrow wavelength of yellow/orange light (about 589 nm) and have an effective CRI of 0.
Commercial and residential fluorescent lighting normally has a moderate CRI in the 70-85 range to make the light more comfortable for people to work and live in, but it’s not photographically precise. These commercial fixtures are moderately accurate in representing natural colors and can have a high concentration of green/blue in them.
|CRI color information|
Advancements in phosphor combinations and in fluorescent lamps have led to some really great tools available pretty much anywhere. Look for lamps with high CRI ratings—in the 85 to 95 range. Look for lamps with a CCT that most closely matches the lighting you’ll be mixing the fluorescent source with.
In the late 1980s, gaffer Frieder Hochheim and best boy Gary Swink created a high-output fluorescent fixture with remote ballast for cinematographer Robby Müller during the production of the film Barfly. This invention evolved into a whole new career for Hochheim as he founded Kino Flo Inc., a prominent manufacturer of lighting equipment for film and television. These fixtures set the standard in the industry and brought fluorescent technology out of taboo and into vogue. Research and development into specific recipes of phosphor blends, first with third-party vendors and then through Kino Flo’s own creation, resulted in color-correct tubes with very high CRIs of 95.
In addition to getting the phosphor mixes perfected to work seamlessly alongside tungsten and daylight color temperatures, Hochheim incorporated several advances into the Kino Flo fixtures to solve the other problems of fluorescent lighting in motion pictures. One of the primary differences is Kino Flo’s high-frequency ballast, which cycles the lamp 25,000 times a second (as opposed to a standard ballast, which cycles a lamp 60 times a second). This ballast makes Kino Flo units virtually flicker-free at any shutter speed or frame rate.
Today there are many additional manufacturers making fluorescent fixtures, including ARRI, Lowel and Mole-Richardson.
In the last few years, especially with the emphasis on conservation and environmental preservation, compact fluorescent lamps have become very popular. Their efficiency is twice to four times that of incandescent bulbs, which lose 90 percent of their energy to heat. Since fluorescent fixtures get their brightness primarily from the surface area of the glass, manufacturers designed a way to twist the glass into a tight spiral to compact it and get a lot more surface area in a smaller space. CFLs incorporate their ballast in the base of the bulb to create a single device that can be screwed into any standard light bulb socket. The CFL’s smaller size and all-in-one construction meant that people could replace their tungsten bulbs with fluorescent ones without having to change their fixtures.
Fluorescents are very efficient sources, and they’re fairly inexpensive. (Although they are much more expensive than the equivalent wattage in tungsten bulbs—about five to 10 times.) They are not manufactured for use in photography, however. Most CFL globes do not have published CRI ratings, and there are considerable differences between manufacturers and even between individual product lines and shapes from a single manufacturer. You can certainly use CFLs to light with, of course; if you use them, it’s best to use them exclusively and stick to one manufacturer and a specific model of CFL. In that case, you can white-balance away any green cast and achieve some good results. If you intend to combine with any other sources, however, I’d recommend staying away from CFLs altogether.