A photograph is a cumulative record of light. The light can either be reflected or transmitted. Furthermore, each thermal light source has a different color measured in Kelvin (K) degrees.
End easy part.
PJs must deal with whatever light they encounter and try to make it look as it would to the human eye. Since the camera records in absolute terms, this is frequently impossible and requires color correction or filtration.
Color correction ethics
Before anyone has a cow, understand all photographic processes are corrected to what the human eye sees. The use of color correction (cc) gels with light sources or correction filters on the lens is typically employed to make the scene appear as it would to the human eye.
Most modern films are panchromatic. This means they capture the color spectrums most humans see. Color negative film is on a brown substrate, which requires correction. Then, it must be corrected back to standard colors from their complementary color (i.e. cyan = red, magenta = green and yellow = blue).
Furthermore, each brand and type of film has a color bias. Films are selected specifically to exploit these biases. Since film is becoming less common, we won't discuss it much here. Positive film (chrome, slide) is the closest to neutral although each has its own bias as well.
In short, we're not talking about changing the color of someone's tie to please a design color pallet. We're discussing light and film balance to make sure Granny's hair doesn't turn green under a fluorescent kitchen light because it doesn't appear green when the PJ sees her sitting there.
What is normal light?
Daylight and artificial light (flash) are around 5000K degrees. Common films and digital cameras record everything at this color temperature unless corrected.
The PJ's goal is to record all light as if it were normal (5000K degrees). Sometimes this requires putting cc gels on light sources, putting cc filters on the camera lens or adjusting the color afterward via software. For film it's often all of the above.
Common light colors
In loose terms, the light sources encountered most by PJs are daylight, shadow, tungsten and fluorescent. Each has its own color and correction method. Although there is no "one" way to correct for these light colors, we'll discuss the most likely methods to be successful.
Here's a list of the commonly encountered light colors. Although all of them could be balanced and/or corrected, we'll only deal with the most common. However, it's important to understand where each light source falls within the Kelvin black body measurement spectrum.
1000K Candles; oil lamps; most open flames
2500K Household light bulbs (tungsten)
3200K Studio lights, photofloods (tungsten)
5000K Typical daylight; electronic flash; strobes
5500K Noontime sun
6000K Bright sunshine with clear sky
7000K Slightly overcast sky
8000K Hazy sky
9000K Open shade on clear days
10,000K Heavily overcast sky
20,000+K Open shade in mountains on clear days
Using cc gels
Once PJs start shooting color and move beyond standard daylight environments, it becomes necessary to use cc gels with a flash and/or use correction filters on the lens.
To gel a flash, simply cut a large sheet of gel material into a segment slightly larger than the flash head. Then use tape or Velcro to fasten it to the head. Softboxes, bounce cards and other light-diffusing devices can be added after the cc gel is attached.
An inexpensive way to manage this is to buy a Roscolux Swatch Book (about $2.50). The book has one sample of every gel they make. These gels are about the same size as most hand-held flashes.
Instead of buying the larger (and more expensive) individual sheets of cc gels or an entire cc kit, PJs can get the swatch book and have everything. However, I'd suggest getting two books because they get torn up with use and once PJs are past the learning curve, they can't live without these.
When purchasing the gels, make certain they are photographic quality and heat resistant. I've had several (expensive) gels evaporate into oblivion after one flash pop.
Daylight is fairly simple. It's already around 5000K. Use standard film or set digital cameras on daylight. Use flash/strobes without filtration.
Shadow color temperatures can vary widely. Basically, it's all shades of blue. Blue is considered a "cool" light temperature. It's a fine background color and actually helps lift the subject off the background if the subject is lit with standard flash or strobe (5000K-5500K). It allows the subject to reflect "warmer" tones and appear more alive than the background.
Correct for open shade with digital cameras by setting to shade. The overall scene will be warmed. It's important not to have natural daylight spilling into the scene or it'll appear grotesquely yellow.
For many years, I didn't correct for shadows while using a flash unless I saw a distracting difference. However, it makes a significant difference in the red and yellow portions of an image. Consequently, if the subject(s) has blonde or red hair or the scene is red or yellow, use the gel. It only takes a few seconds to attach the filter and it makes all the difference.
To correct for shadows, add a Cinegel Half Blue #3206 cc gel onto the flash and set digital cameras to shade. Film needs to be corrected in Photoshop or PJs can add a "warming filter" (81A) to the lens to correct in camera for chrome film.
Tungsten light is the light emitted from most household light bulbs and photofloods. The light is created when an electronically charged filament (wire) is heated and glows. It is a constant light source (as opposed to pulse). It measures around 3200K degrees on a black body light chart. To film and filtration, it's reddish orange.
In all cases where tungsten light is the prominent light source, it's best to gel flash and strobes to match available light. With flash, Rosco suggests Cinegel #3411: Roscosun 3/4 CTO. It converts 5500K daylight sources to 3200K to somewhat match ambient light.
Although household light bulbs are about 2500K, the gel correction leaves a touch of warmth in the room once color corrected. It's enough red to give the background a "home" feel without making it look like a furnace or disco.
There are both positive (chrome, slide) and negative tungsten-balanced films. Using available tungsten light, shoot as normal with T-type film.
With digital cameras, set the color balance for tungsten and shoot as normal with available light.
With standard negative film, shoot as normal for available light. Color needs to be corrected in Photoshop.
With standard positive film (chrome, slide) consider sacrificing two stops of light and use an 80A filter. It filters 3200K to 5500K.
If you noticed, fluorescent light doesn't appear on the chart above. It isn't part of the black body light spectrum. Instead, fluorescent lamps create light by electronically charging phosphors within a vacuum-sealed tube. Each fluorescent tube transmits a different range of colors depending on the age and composition of the tube. Generally, it transmits light in the yellowish green range.
Furthermore, it's a pulsating light source. It's regulated by alternating current (60 cycles per second in the U.S.). This means any shutter exposure faster than 1/50th is still unpredictable after all other variables are removed.
Most PJs start banging their head on the counter when asked how to correct for this light. There really isn't a sure-fire correction for fluorescent light. As stated above, each tube is different and unpredictable.
However, we must still do what we can to make it look somewhat "normal."
The best way is to custom white balance the light with a digital camera. Next would be to shoot with available light on negative film (try adding an FL-Day filter to the lens as well) and color correct in Photoshop. Then, it gets harder.
With flash, Rosco suggests Cinegel #3304: Tough PlusGreen gel. This gel "adds green to natural and artificial daylight sources to balance with U.S. Cool White or daylight type fluorescents. To be used with overall correction at the lens or in the lab."
Frequently, the gelled flash appears more cyan than the background. By the time it's corrected, the background shifts slightly toward red, which is good in moderation.
With positive (chrome, slide) or negative film, PJs can gel the flash and try to counter filter the color with a FL-Day correction filter on the lens. It frequently doesn't work, but it's still better than bug green.
Although I can't see a PJ doing it, I'll also mention cc gels can be purchased by the roll and sleeved around the light tube to restrict green range transmission. Supposedly this has adequate results, but it takes time, labor and permission to start ripping down lights.
My solution isn't for everyone, but it works. Blast the heck out of all available light with a really powerful strobe at a high synch speed, on a low ISO with a small aperture (F/22 is my favorite). Then, the color of available light isn't an issue. ;-}
How's this possible? A strobe can fire at about 1/8000th of a second. The camera's synch speed is determined by the speed at which the shutter is completely open. This means the strobe has the same effect at 1/500th of a second or several minutes. The only difference is the accumulation of available light.
If the available light is metered on 200 ISO at 1/30th on F/2.8, and the PJ sets the strobes on 200 ISO at 1/500th on F/16, the stop difference (using EV calculations) is nine stops. As we've determined before, the dynamic range is only five stops. We've effectively moved middle gray deep within the black range. We've also moved the highlights with detail to four stops below black with detail. Therefore, it's negligible. So, only the light from the strobe accumulates on the film or CCD. End color correction debate.
An extra bonus to this method is no blur. Since strobes actually fire somewhere around 1/8000th of a second. It becomes the de facto shutter speed. Angry bulls fly through the air and move in several directions simultaneously. But they are frozen in time at 1/8000th of a second. Any subject blur is a result of available light accumulation.
Now does everyone understand why I got so upset when the camera manufacturers decided to use a "new technology" and slowed the synch speed to 1/250th? Color correction and blur became problematic again.
Enough for now,
What's a Kelvin black body?
William Thomson was an Irish mathematical physicist, who specialized in thermodynamics. He created the absolute temperture scale. He was titled 1st Baron Kelvin for his work. The Kelvin River flows past Glasgow University in Scotland where he was a professor.
Gustav Kirchhoff actually introduced black body radiation theory into the mix. According to Wikipedia, "In physics, a black body is an object that absorbs all electromagnetic radiation that falls onto it. No radiation passes through it and none is reflected. It is this lack of both transmission and reflection to which the name refers. These properties make black bodies ideal sources of thermal radiation."
From there it becomes quantum physics and drool. Just consider the colors magma would radiate as it's heated. It starts black, warms from red to blue and then becomes invisible (again, due to your friend: quantum physics).