Thomas Duplex filters
Here's some info that I thought might be interesting. This may all be common knowledge, but hopefully useful to some.
The Thomas Duplex safelight was offered with various filters for different uses. Most of us at APUG are going to be interested in the versions "Type DUB" for black and white printing and ortho processing, or the "Type DUC" for color printing. To the best of my knowlege (and personal experience), the Type DUC is also fine for standard black and white printing (but not ortho film processing) .
The only differences are the filters fitted, and the information below explains the tape color codes that indicate the use for each filter.
This is excerpted, paraphrased, or interpreted from literature from Thomas Instrument Co.:
Black and White Printing (Type DUB)
Yellow filters in the housing and Red in the vanes (that refers to the color of the taped edge of the filter). Vanes can be fully open in a large darkroom (they indicate 12 x 14 feet with "standard" 9 1/2 foot ceiling height and the bottom of the safelight 7 feet high). Being able to use the vanes fully opened would obviously depend on how close the installed safelight is to areas where paper is handled. The vanes should be closed as much as necessary to prevent fogging for the exact darkroom size and conditions.
Ortho Processing (Type DUB)
"For Ortho Films, papers and most graphic arts materials, the vanes should be fully closed so that the light passes through the yellow and red filters. If the light level is much lower than desired, it may be raised substantially by removing the yellow filters and using only the red ones in the fully closed vanes. Make certain the vanes close tightly to avoid leakage of yellow light."
Color Printing (Type DUC)
Yellow filters in the housing and Black (black tape edge) filters in the vanes. "For color printing papers, the vanes should be fully closed, so that light passes through the yellow and black filters. The light level will be rather low, bu t still many times brighter than conventional safelights. ..."