 A Fluorescent Safelight
A Fluorescent Safelight
Background:
Over the past several months there has been a lot of discussion in the forums about home built safelights using LEDs or fluorescent lamps. When I built my
new darkroom, I decided to try to build my own safelight. For many years I have used a commercial safelight that is a 24 inch standard cool white fluorescent tube inside a special sleeve that makes it the equivalent of an OC safelight. This has been an excellent lamp, and it is safe for up to 15 minutes at five feet. Lamps like this are still commercially available with either OC or Number 1 filters; but they are now quite expensive.
For many years, the OC safelight filter has been the standard for black and white papers. But recently several papers from eastern Europe have become available that recommend a red safelight and warn against using the OC. For this reason, I decided to go with a red safelight in my new darkroom.
Design and Construction:
The safelight I built uses only commercially available components. The only special construction considerations are the usual concerns for hanging and wiring a standard fluorescent fixture.
The safelight is based on a standard fluorescent "Shop Light" that takes two 40 watt, 48 inch instant start fluorescent tubes. I chose to use red fluorescent lamps (F40T12/R). These tubes have a red emitting phosphor and a red coated tube that restricts emission of the mercury lines and phosphor fluorescence outside the red region. This means a brighter and safer light that can be used without special sleeves. The tubes I used carry the American "Industrial" brand name and are called "red ceramic" lamps.
Manufacturers can use any of several phosphors in their lamps, and they can use any of many filter coatings on the tubes. The phosphor and filter will affect the brightness and safety of the safelight. I checked my tubes with a hand spectroscope. They showed a pure band spectrum from mid-orange through mid-red with no trace of mercury lines or emissions at shorter wavelengths. This indicates one of the older silicate or germanate phosphors. Europium activated phosphors show a red line spectrum superimposed on a red band spectrum which is more confined to the red. They are also much brighter.
Although I could not see any short wave radiation from the fluorescent lamps, I decided to use sleeves to further attenuate those wavelengths just to be on the safe side. I chose inexpensive sleeves with Rosco number 27 medium red gel filters. These filters are designed for theatrical, not photographic, use, and you should not trust them alone for safelights. Rosco publishes transmission spectra for its products, and the number 47 filter shows an effective cutoff around 602 nm with a gradual rise into the red. In combination with these filters, my tubes show a very small amount of orange (around 600nm) and the same red band seen before. There was significant attenuation as well. The combination appeared to be quite suitable for safelights.
The light fixture was mounted to the ceiling of my darkroom. A permanent extension cord was installed from the lamp plug down to my enlarger area for use with the enlarger timer. The distance from the center of the light fixture to the center of my enlarging easel is a little over six feet.
Testing:
You should test any safelight. You must test any home built safelight. The safety of a safelight is a relative thing. The sensitivity of photographic papers does not cut off at any particular wavelength, but it diminishes toward the red. Bromide emulsions have a slight but very significant sensitivity to red light. What you can use as a safelight depends on the sensitivity of the photographic material, the wavelengths emitted by the safelight, the intensity of the light, and the duration of the exposure. With all these variables, only a test will give you guidelines on how to use the safelight without degrading your prints.
Overexposure to the safelight usually causes a very slight fogging of the print which is seen as a reduction in contrast or veiling of the highlights. Usually it can't be noticed except by direct comparison to an unaffected print. You need a test that will show these effects. The effects are additive to the normal print exposure, so they will degrade a print even though you can see no fog on a piece of paper that has just been exposed to the safelight.
The test I used involves making a standard print and then exposing bands of it to the safelight in a test strip. The effects are easiest to see in light areas, around Zone VI (sky, light rocks, skin tone). The idea of this test method is to expose a print on enlarging paper and then to add various amounts of safelight exposure to determine what is safe. To make evaluation easier, coins are used to block out the safelight in each band of safelight exposure.
Select a negative that give you plenty of middle highlights. Determine the exposure for a decent print. Make a print in the dark (no safelight). Place a number of coins in a row across the exposed paper; try to get them in the medium light areas of the print. Place an opaque card over the exposed paper and turn on the safelight. Now make a test strip using the safelight. Move the card to expose the first coin; expose for a time; move it to expose the second coin; expose for the same time; and continue in this pattern until all the coins have been exposed. Two, three, or five minutes per coin, depending on how confident you are, are reasonable times for each step. When done, turn off the safelight and process the paper in the dark. I used a tube processor to make this easier. Look at the print carefully. The exposure where the print shows the first hint of the outline of the coin tells you how long it takes before the safelight starts fogging your paper. As a rule of thumb, you can scale this time according to the paper speed for other papers.
My safelight is quite bright, and I expected to have limited working time. My test ran to 12 minutes, and I could not see any affect. Since that was a sufficient working time for me, I did not test any further. If the light is too bright, it is easy to reduce the illumination. Just run a piece of tape down the front side of each fluorescent tube. One inch blue painter's tape or black cloth tape should work fine. That will cut the light to about half.
Conclusion:
It is quite possible and economical to make an effective safelight using fluorescent lamps. Other designs are certainly possible, and this paper is just intended to give you some ideas. Testing a safelight is easy and gives you confidence about how safe your safelight is.
Last edited by nworth; 08-06-2008 at 11:23 PM.
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