 Re: Having Problems With Exposure Times and Homemade Exposure Unit
If you cannot get the emulsion to completely wash out, sounds like you're overexposing, or have the light on too long. This could also be caused by your image medium (velum or transparency) not being dense (dark) enough in the image area. Also, emulsions have different "speeds" in relation to how quickly they expose to the light source.
First place I would begin is to go back to your emulsion supplier, and ask if they have a wedge exposure tester. This is a film positive that has varying degrees of density to replicate exposure times. You would expose this film, and wash out, and depending on which area washed out properly, would then give you an approximate exposure time.
Another method would be to take one of your image mediums, place it on your coated screen, and also place a piece of black poster board cardboard (same like the poster board you buy for your kid's school projects) cut to the same size as your medium, between the medium & screen, pulling it back enough to expose 10% or so of the image to the screen, and shoot it for 5 minutes. Then pull the poster board back to expose another 10% of the image, shoot it again for 5 minutes (now you have the first 10% image exposed for 10 minutes, and the 2nd 10% of the image exposed for 5 minutes), and then keep repeating this procedure.
When you have exposed all segments of your image, you will have 10% increments of the image exposed at 5 minutes intervals. Wash out, and see which segment washes out the best, and then you will get an idea of your exposure time. If the last 10% segment, which will have been exposed at 5 minutes still does not wash out properly, then that means your 5 minutes is still too long, and you'll have to repeat the test using perhaps 2 minute intervals. | 
