Safelights for Holography
In order to see while you are working it is important to have some light. Fortunately if you are using film that is only sensitive to red you can make a green safelight. You should test your safelight before you use it.
I do a preliminary test of reflecting the light off of a diffraction grating, CD or DVD. Shine the safe light at the grating and bounce the reflection back to your eyes. If it looks the same then you are looking at the zero order reflection, rotate the grating or CD until you see a reflection that looks like a rainbow. This is a higher order reflection. If you can see red in this reflection then your light is not safe.
The next test is to get out a piece of your film. Find the uncoated side. Place a piece of electrical tape down one side to make a test patch that has not been exposed. Put the film about a foot from your safelight with the tape facing the light. Add one piece of tape at 15 seconds, 30 seconds, 60 seconds, 10 minutes, 30 minutes. At 60 minutes put the plate in developer. If your plate turns completely black then your safelight is not usable. If only the 60 minute or 30 minute lines develop you are probably OK.
If you are using long settle times it is wise to make sure no safelight hits the bench during the settle period.
The easiest light to use is a "Limelight" night light. It is very low power and mostly green. If you add a Rosco Gel #90 available from a theatrical supply shop it will be very good but very low power. Attach the gel with electrical tape. John Klayer uses a row of gelled Limelights above his bench. This is a good idea but I have not found a way to do this yet because of my space constraints.
Just using a piece of #90 over a conventional bulb is not enough. I like to use 2 layers of #90 inside my mini mag light. Lately I have found it better to have one layer of #90 and one layer of #95. #95 lets in too much deep red to use alone and #90 lets in too much yellow that the Slavich materials are sensitive to. I only use it to find something in a hurry and never shine it at the plate.
The main safelight in my studio is a 4' fluorescent shop light with two T40 bulbs. I have it covered with a sheet of Rosco #90 and have it aimed up to the ceiling to spread the light. Since I close my holography table for settling and exposure I can leave the light on all the time.
I also never open my box of plates/film in any light at all. I can perform all of the functions of manipulating the box in complete darkness. I am sure I am being over cautious but film is expensive.
I found a reference to a paper about using the T40 EncapSulite fluorescent bulb covers for holographic safelights. I did not read the paper so I am not sure what bulb he was suggesting or what film he tested with. Because of the expense I have not tried it yet.
S15ND2.25 Dark Green 535 to 560 nm $46.95
T20ND.75 Blue-Green 470 to 530 nm $46.95
T20ND1.05 Blue-Green 471 to 528 nm $46.95
T30ND1.05 Blue-Green 483 to 518 nm $46.95
For general information about Safelights see:
Kodak makes a #7B and #3 green saflelight filters (#3 is recomended by Shoebox Holography) suitable for red sensitive holographic films. Since I have so much theater gel lying around I have not tested it. Let me know if you try some.