Some Wratten Photo Filters
Ideal for the Viewer Box

First check out the only region of all Wratten Filter curves below that we're interested in, just the middle section, where the human eye senses light and color. The above graph is a simplified diagram of what we see: the light colored central area. We don't sense light in the gray areas to each side (but CCD's and film can frequently sense parts of it, that's why this data is usually shown).

The deepest red filters generally available (here, a #29) allow light right above 600 nanometers to pass through. This can stimulate the green cones of the eye if the red light is set high enough. We can do better than that. Look at the red portion of any violet filter's curves, like the next one:

#35 is a very deep purple filter, quite visibly intense (look through it!). It blocks all the cyan through green, yellow and orange-red portions of the spectrum, allowing only the blues and far reds through. The red portion looks ideal for our viewer, but we have to remove that blue region.

This orange filter, #21, will do that job nicely. Many other similar filters from deep yellow through light red will serve as well. We're concerned that everything above 650 nm or so gets through, but certainly not below 480 nm, where the purple filter passes blue light.

This overlap of the two curves above shows us that a sandwich of the purple #35 with the orange #21 does the job ideally well. We're left with only the very far red region above 650 nanometers, as one or the other filter blocks everything below that. Inelegant, perhaps, but it works! (I've also just noticed that there is one reasonably adequate single filter that can be used, a #92, and one that's almost identical to our sandwich, #70. For some reason the #70 was unavailable when I built the viewer, and my sandwich is slightly better than #92 alone, besides being "irresistibly clever..." :^)

In keeping with the purity of the deep red filter, here's an ideal deep green filter for the other light. #99 is a near-monochromat (don't worry about that right hand "infrared" region -- we can't see there, remember?), much less likely to trigger any of our eye's cones, except the green ones. But we'll keep the light source down at a level too dim for that, and we'll be in good shape.
(Note: the main problem with using broader filters is that we risk triggering the blue and green cones -- narrower filters like these permit somewhat higher light levels, within these restrictions.)
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