"Cool! keep an eye on the gels, if they fade enough you might get some fogging... glad it's working"

The Thomas Duplex saftelight does not have "gels." There are two sets of glass filters. For black and white paper use, no filters are necessary as it uses sodium vapor tubes. The spectral transmission is so narrow that it is in and of itself, within the correct wavelength for the "safe" spectral area of the paper. Secondly, the safelight is made to be reflected from the ceiling and walls which gives a greater overall perceived illumination level, but results in much less intense light reaching the paper because of the light falloff (1/d^2).

The second set of filters is for use with some color papers with the filter doors totally closed on the safelight, and the second filters stacked on top of the first ones. I have had one in my darkroom since 1982. I find it almost too brigh,t and use it with the doors only open about 3/4-inch - this is in a darkroom that is 10ft x 13ft.