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Category: New Answers

photoflood lamp

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July 27, 2006

This design is based on a standard incandescent tungsten lamp that is “over-run”. This means the filament is actually sized for about 100 volts. Thus the lamp is “over-run” 15% as standard household voltage averages 117 volts. When over-run, the lamp burns quite brightly but has a very short operating life. Worse, the filament is working very near the vaporization temperature of tungsten. Tungsten metal is evaporating all the time the lamp is on and migrates to the inside of the glass bulb. This action causes the lamp to quickly blacken. Also, the evaporation of the tungsten reduces the diameter of the filament. This increases the filaments electrical resistance. With higher resistance the filament burns brighter and brighter till burnout . All the time the color temperature shifts up scale toward blue. Bottom line is this lamp has a short life, changing output and color all the time.

Lamp makers introduced the tungsten halogen lamp as a countermeasure. These use a quarts glass envelope quite close to the filament. Quarts has a higher melting point thus allowing a higher operation temperature. The envelope is filled with one of the halogen gases usually iodine or bromide or a mix. Evaporating tungsten at this high temperature causes the tungsten to combine with the halogen. This gaseous solution steers the tungsten away from the quarts envelope keeping it quite clean. After shutdown the lamp cools and the tungsten is re-deposed to the filament. Thus the filament is renewed and the output remains quite uniform in light output and color from start to burnout.

All of these tungsten lamps fall short of the 5500ºK of daylight. Once upon a time the photoflood was available with a blue tint but I have not seen any for maybe 25 years or so. Anyway, you are better off using a color filter on the camera to adjust for the color of the exposing light. To convert 3200ºK to 5500ºK use 80A. To convert 3400ºK (movie lights) to 5500ºK use 80B. Some of the new compact florescent lamps advertise themselves as “full spectrum” or “true color” or the like. I have no experience using them therefore I am skeptical. Florescents outputs only UV light. A coating inside the tube glows upon exposure to UV thus the coating converts the output to visible light. The florescent, unlike the incandescent has a discontinuous spectrum and heretofore difficult if not impossible cause it to replicate daylight. Filters of every type only subtract selectivity, some light energy. Filters never add anything, they always remove something

Alan Marcus
ammarcus@earthlink.net

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July 28, 2006

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March 12, 2024