[photo-3d] colors of lights

[John Toeppen <toeppen@xxxxxxxx>]

General Electric lamp data sheets show that halogen cycle lamps produce
more blue light than normal incandecent bulbs for a given wattage.  Bulb
life is improved as well.  Krypton bulbs seem to be as white.  A small
ring of frosted tape surrounding (but not touching) the bulb also
distributes light more evenly on the diffuser behind the slide. 

"White" LEDs and fluorescent sources have many "bumps" in their emission
curves. Xenon arc is a blackbody source possessing some weak blue
spectral lines in the near UV. Xenon strobes have more of these atomic
spectral lines because they are rarely driven hard or long enough to get
"blackbody" light.  This is why strobes produce colors that you would
never see in nature.  Bulbs are the closest to sunlight in both color
and brightness. 

Fluorescent sources also contain a strong green mercury 546nm line. This
can dominate an indoor exposure but is unseen by the eye.  One uses a
FLD "purple" holium filter to correct for this in film when possible. 
Good camera filters are usually optically flat to 1 wave in
transmission, unlike Rosco films used in front of the sources.

The only really balanced source is sunlight.  I white balance my
computer with sunlit photowhite inkjet paper. The human eye is
subjective and will often account for the color balance of a scene. 
This allows one to "accurately" recognize colors even in the amber glow
of sunset. Film has its own zones of color sensitivity and will not
always match what one sees.

Knowing the general properties of sources does help one select the
appropriate light for any given application.  Calculations are
frequently misleading because many situations have too many variables to
accurately model.  Experience and intuition are usually developed buy
viewing results.

John Toeppen
http://www.members.home.net/toeppen/