Choosing Your Lights (...not what you think)

To the untrained eye, there is only color temperature between the various lights that we use in the motion picture industry. HMIs appear to be "blue" or daylight balanced; tungsten seems "warm" and "orange" resembling indoor lighting; fluorescent lighting seems to have a sickly green cast or sometimes a hint of magenta in them, etc. All of these observations are accurate, however there are a few other ideas you should think about when choosing which lights best fit your situation.

Before we talk about the actual units, let's talk about color temperature. Color temperature, in the film industry, is measured on the Kelvin scale, named after British physicist William Kelvin. The scale is based of of his findings when he heated a block of carbon. The block changed colors from a warm red glow to a bright yellow and eventually to a screaming hot blue color. Read about it here: http://www.3drender.com/glossary/colortemp.htm.
We use the Kelvin scale to measure the color temperature of a light. Film has two effect color temperature ratings, 3200ºK(tungsten/indoor film) and 5500ºK(daylight film). Now, the lower the number the "warmer" the color looks and the higher the number the "cooler" the color looks. Indoor lighting is estimated around 3200ºK and daylight is estimated somewhere around 5500ºK, hence the color ratings of the film stock. An important thing to note is that the Kelvin scale is NOT LINEAR. This means that a 200º change in color temperature at the higher end of the scale is not the same color shift in the lower end of the scale. The lower you go on the scale, the more dramatic each degree change is. I will address this idea when I talk about each light source.

HMIs:
HMI stand for Mercury, Medium Gap Arc, and Iodine. These are a daylight balanced light, so they are 5500ºK. There are many issues with HMIs, but I will only be addressing the color temperature and light quality issues. HMIs are gas discharge light, meaning an electrical arc is used within the bulb to pressurize the vapors inside the bulb which provide the light. There is, however, a shift in color temperature based on the life of the bulb and the integrity of the gasses inside... meaning the temperature will change over time. This is just one of the many problems with HMI units, however, there are steps you can take to compensate for these changes.

HMIs burn at about 70% light energy, 30% heat, translating into a lot of light per watt. They are very efficient lights with a lot output in contrast with a low(er) amperage draw. But there is often an unnatural cast to these lights. They often have a sharp specular reflectance on skin, causing an even bigger problem on oily skin. We have gotten used to seeing them light actors in movies, but there is still a subconscious disconnect to our eyes that we may or may not be aware of. This is the result of the electronically created light source. An HMI's light is the result of an electrical arc, an artificially created light source, which our eyes pick up on even if we don't recognize it. There is an inorganic characteristic to HMI light that has the potential to be exaggerated on skin tones.

Fluorescents:
Fluorescents are also gas discharge lamps created by an arc of electricity. Again, this inorganic light quality can be seen under these lamps. Now, they do come in a variety of color temperatures(3200ºK, 4700ºK, 5500ºK and more in between) but there is also a risk of a green or magenta shift in them. This is due to their interrupted nanometer spectrum( http://www.cameraguild.com/technology/kelvin.htm). This artificially created light source attributes to why people sometimes feel uncomfortable or even develop headaches with long exposure under fluorescent lighting. Again, power cycles and the hertz rate and voltage factor in to this problem as well.

Some people use fluorescent fixtures as fill light to raise the ambient light in the shadow areas, however, you run the risk of an extra shadow(any time you add a light, you add a set of shadows) as well as the risk of an artificial looking fill light. However, these lights can be used to great effect to simulate a computer screen glow, a monitor glow, overhead fluorescents or other types of artificial illumination.

LED:
LEDs are the latest addition to our lighting arsenal. They have have many of the same attributes that fluorescents so in terms of looking like an artificially created light source with the feeling of a heavy specular hit. For some reason these lights look sleep and "greasy" to me on skin tones due to the artificially created light. They have advantages much like fluorescents in terms of amperage draw and low heat, but they are not the most flattering lights on skin tones.

High Intensity Discharge Lights:
These lights include Sodium Vapor, Metal Halide and Mercury lamps used for lighting parking lots, buildings and other utility lighting situations. These lights vary in color temperature and often have other colors(greens and magentas) mixed in creating a generally unpleasing looking light. However, these lights work well for gritty lighting situations set in industrial locations. Again, these lights are pressurized vapors in an envelope stimulated by an electrical arc, so they also look a little artificial on skin tones. However, they do put out a lot of light and can achieve some incredible and unconventional looks.

Tungsten:
Tungsten in the oldest, and in my opinion best looking light used today. Tungsten is 3200ºK and used for indoor film. The word "tungsten" refers to the material used as the filament for electric light bulbs. If you want some more information on the mineral, please click here: http://periodic.lanl.gov/elements/74.html If you would like to learn about the electric light bulb, please visit the following links:
•http://inventors.about.com/library/inventors/bllight2.htm
•http://www.howitworks.net/how-light-bulbs-work.html
•http://home.howstuffworks.com/question151.htm

Why does tungsten look so good, in my opinion? It's simple, really... electricity is introduced to the tungsten filament and the filament glows as it burns, releasing heat and light. It's a naturally created light source, reminiscent of flame or fire light, and because of this it appeals to our base sense of illumination. We have gotten used to tungsten filaments for the 120+ years we have had electric illumination(tough a lot of people are opting for compact fluorescent bulbs in their homes now) but we also have an ingrained pre-history of naturally occurring sources of light with our use of fire and flame. Tungsten feels better to our eye on skin tones and faces and seems to have better wrapping ability without giving a harsh specular highlight on the surfaces being hit by the light.

I try to use tungsten as much as possible, though it is not always the most efficient, because I feel that I can get better results and can control the lights easier without having to worry about any light quality issues.

PhillM tip-of-the-day:
If you need to scrim down a softlite, just get a 1K scrim and lay it on top of the safety screen of the soft light lower the light level. However, be careful- because the softlite is a bounced light, the scrims won't have the exact same effect of your foot candles(there is only a slight discrepancy) as they would if you were using them in the traditional sense.