Andy Ciddor /
08.04.2004 12:00 AM
Color Temperature: So Many Variables
In this, the third and final chapter in the war of the color temperatures, we deal with the weird stuff: The unnatural light that comes from sources that either don't emit the full range of the visible spectrum or where the balance of colors is way off the beaten track. Such non-black-body sources include commercial fluorescent tubes, most metal halide mixtures, and the high-tech wonder that is the "white" LED.
In the first part of this series, we met the humble fluorescent tube, which has fallen out of favor with interior lighting designers in recent years, and thus isn't seen very often in such places as fashionable stores. However, as it still features strongly in supermarkets and other large public spaces where news is gathered and public opinions are sought, the fluorescent continues to have an impact on our daily work.
Although there is a reasonably wide range of fluorescent-correction filters available to us, I have never been very successful at tweaking a useful result from them. While it would be easy to blame the filter manufacturers for not meeting our needs, the truth is much more complex and confusing.
HIRYLA, FOR EXAMPLE...
The reality is, there are literally dozens of variations of phosphors available. To begin with, there are more shades of white tube than there are Inuit words for snow.
You have white, cool white, daylight, soft white, natural white, warm white and deluxe warm white to name but a few. For any particular shade of white, there are variations in color temperature, color rendering and chromaticity coordinates between manufacturers, and in some cases, even between ranges of tubes from the same manufacturer. The change in permissible Mercury content in fluorescent tubes over the last decade has also necessitated changes to the chemistry of the phosphors used, and thereby introduced yet another factor into the variation in color.
Maintenance is another significant contributing factor to the variation between fluorescent sources. While some electrical maintenance contracts call for all of the lamps in an installation to be replaced in a single pass, this approach is still relatively uncommon for minimizing labor and fixture access costs. The most likely scenario is that once a noticeable number of tubes have started to flicker or refuse to start, a maintenance person, often one with no knowledge of luminaires and lamps, will be detailed to replace failed tubes. There is little likelihood that the replacement lamps will even vaguely match the color temperature and color rendering characteristics in the rest of the installation. After a couple of years of this haphazard lamp maintenance, there is virtually zero probability that the tubes in an installation will have similar color characteristics.
Given the vast number of possible tube colors, it not surprising that correction filters have not been made for every tube. Even if they were all available, the logistics of stocking, managing and correctly selecting such a collection of ever-so-slightly different shades of mauve and green filters, would require resources not usually available in production budgets.
The last decade has seen the introduction of a an entirely new variety of fluorescent tubes with phosphor mixes specifically designed to match closely with black-body light sources such as tungsten halogen lamps and sunlight.
Originally employed in the Kino Flo range of softlight fixtures, full-spectrum tubes are available in luminaires from a range of manufacturers. The tubes themselves have been used in place of the existing lamps in some common commercial fixtures, thus making it possible to have locations with fluorescent light sources that actually match the daylight or tungsten sources found elsewhere in the location.
As with the practice of placing a correction filter over every tube in a location, replacing all of the tubes is a time-consuming and labor-intensive practice, which may not even be an option for formats such as news and documentary production.
MIXING METAL HALIDE
When it comes to shooting video and film, metal halide discharge sources have many similarities to fluorescent sources, with the exception that there are even fewer correction filters available for them. The light in these sources is generated by passing a current through a cocktail of ionized metal halide vapors, each of which produces a narrow band of the spectrum. If you get the right mix of metal halides in your lamp, the result is something resembling white light, although usually not containing the full spectrum and rarely in the same proportions as a black-body light source.
The most efficient colors of light to produce with metal halides are up at the blue, green and violet end of the spectrum, resulting in the high-efficiency versions of the these lamps lacking severely in the red, orange and yellow department.
These are the light sources often encountered in sports arenas, shopping malls, car parks and similar large public spaces. There are no correction filters available for these sources, and any attempt at shooting full color shots of products or artwork is going to be troublesome, no matter how far you push your white balance controls.
There are, however, a variety of metal halide discharge sources that are designed for use with image production and can be used with little or no correction. Starting in the '70s with HMI, which was specifically tailored for film and television work, the major lamp manufacturers have continued to develop a range of metal halide discharge sources specifically for stage and studio work. These are generally a reasonable match for daylight, although some are slightly too magenta. There are a small number of correction filters available for use with HMI daylight and similar sources. These simply correct that slight magenta cast.
Some of the new high brightness (1 Watt and beyond) LEDs have found their way into photographic and cinematographic applications. However, the LEDs in this equipment are usually in clusters of red, green and blue, to create something resembling white light. White LEDs get their whiteness from a very clever piece of sleight-of-hand. The LED chip actually produces blue light, but some of the output is directed onto a phosphor that is stimulated by the blue light to emit yellow. Yellow plus blue equals white, except when the "white" light is shone onto a colored object, the holes in the output spectrum become apparent. When the engineers at Littlite (the folk who make those neat little gooseneck lamps that sprout from consoles) decided to make a long-life LED version of their little luminaires, they actually used a mix of white and red LEDs so that the color-coding on desks, scripts and cue sheets was still discernable.
The battle of the color temperature is one that we fight every day of our working lives. What experience has taught me, is that while I will rarely if ever win the battle outright, I can now begin to pick the battles that I may have some hope of surviving.