Seeing in the Dark

​I frequently find myself wanting to work on panchromatic film in the darkroom. I may need to re-spool it, cut it to a smaller size, process it by hand, and so on. In the past I have had to do this entirely by feel, which is possible but often tricky and frustrating. Photographer Christopher Creighton remarked that he uses an infra-red (IR) viewer to do such work. What a brilliant idea! The particular viewer that he uses is one originally made as an accessory to a video game, and it’s only occasionally available used on eBay in the UK; and quite expensive too. So I thought I had better start from scratch and see what I could find.
​
Illumination
We want illumination that doesn’t fog the film, but will register on the viewing apparatus. To plan that, we need at least an approximate understanding of the spectral sensitivities of film and viewer, and the emission spectrum of the illumination.

​The graph sketches the possible relations between the components of the IR viewing system. The range of human vision is approximately 400-700 nm (nanometres) and panchromatic film (blue line) matches that reasonably closely. The near infra-red (IR) extends from 700 to about 1200 nm, which is the region we want to use. The sensor in the viewer is likely to be CMOS (black line), which has good though declining sensitivity in this region. In the graph are the outputs of possible IR illumination. What we need is plenty of light emission in the region where the viewer’s sensor can operate, but none in the region where the film we are working with is sensitive, because that would fog the film. The commonest IR illuminators work at 850nm, but as you can see (yellow line) there is some overlap with the film sensitivity around 700 nm, so the film will be fogged to some extent. There are three possible solutions:

• 850 nm laser diode, as it has a much narrower distribution and doesn’t get near the film
• 940 nm LED which has a wide distribution but doesn’t overlap the film.
• 850 nm LED plus a sharp cut-off filter to remove the shorter wavelengths (a part of the left side of the curve).

Of these the cheapest is the 940 nm LED solution.
​
This is all a little simplified and a small amount of unwanted light may still exist, so the aim must also to be to limit the brightness of the illumination, which will maximise the length of time that film can be exposed before generating significant fog. The test I do is to try to get about the right brightness of the “lamp” (a little board with an array of 940 nm LEDs), as seen by my viewer, then leave a piece of film face up in the darkroom for 30 minutes with the IR lamp on, and a penny on the film so that fogging is distinguishable from any inherent fog from the film and processing. My LED board is for 12v DC, has 48 LEDs working at 940 nm, and is much too bright. However you don’t have to drive it at 12v, it will work with anything from about 7v upwards – 9v is convenient as suitable power supplies are common. I also mask off most of the LEDs leaving just 6 visible. I place this a metre or so above the workbench, reflecting off the white ceiling, and a 30-minute exposure doesn’t fog the film.

The 940 nm illuminator masked down with some thin black card to show just half a dozen of its 48 LEDs

I did also try an 850nm laser torch – as above, laser diodes have a much narrower bandwidth than LEDs and so should not emit wavelengths close to the limit of sensitivity of the film. This also worked, not fogging the film in 30 minutes; but it is a more expensive solution and is no better than the 940nm array.
A further option I didn’t try would be to get a piece of sharp-cut IR filter to subtract the tail of the emission from 850nm diodes. This material does exist, but so far I have only found suppliers of industrial quantities, with minimum orders which make it uneconomic.
​
Viewer
Thinking about the viewer, there are quite a few requirements for something that will actually be usable:

• Adequate sensitivity to the IR light produced by the illuminator. Most viewers have built-in IR lights but they usually work at 850nm, with significant overlap with the sensitivity of film. If so, they will need to be switched off or covered, with alternative lighting supplied separately.
• It should be head-mounted so that both hands are free for work.
• There should not be significant leakage of light from the screen when in use. These devices have an optical system working in IR, a sensor, and little video screen to display the scene using visible light. This visible light would fog the film if it escaped from the goggles in significant quantity.
• It should not magnify the view; most of these viewers (also called goggles or night vision binoculars) are intended for hunting or paintballing, working at a distance from the subject. In this case we are working close up and need a wide-angle view.
• It must be able to focus close enough to the subject (around 0.5 m).
• The electronics should be fast enough that the video doesn’t significantly lag behind real time.
• If you wear reading glasses, there needs to be enough room for them inside the rubber surround.
• Ideally the viewer would have two channels for the two eyes (stereo), so you could see depth. In practice we have to do without this, as in a sensible price range all the products I have found only have mono IR vision, even if they are styled as, or called, binoculars. The viewing screen can be watched by both eyes, but is mono.

The Bresser viewer, with the wide angle attachment in place and the 850 nm lights covered.

On that basis I found a viewer, binocular so-called but not really, made by a German firm called Bresser. It is one of the relatively few that don’t magnify, and there is almost no video lag. It is made for 850 nm and has two IR lights. It works in normal light too, and at first I thought that I just needed to turn off the built-in IR lights to use it with a separate illuminator. However that’s not the case; the IR switch not only turns on the lights, but also changes the settings of the sensor. To achieve IR sensitivity you have to turn on the built-in IR illumination, and for this application mask the lights with something opaque.

I made other adaptations too. It is 1x magnification, and focuses easily as close as necessary. The width of the scene is still insufficient though, so I bought a wide-angle adaptor x0.45, made to clip onto a mobile phone. I cut off most of the spring clip, and glued the part that the lens screws into, to the front of the Bresser. The Bresser’s rubber hood is too small to accommodate normal reading glasses, but the very narrow “pocket” ones do fit. I had to mask off one of the lenses of the reading glasses to avoid double vision. I did try a sheet of plastic Fresnel magnifier instead, but prefer the glasses.
​
IR light is safe in these quantities, there is plenty of it in ordinary daylight. IR torch lasers, like other lasers, need to be treated with care – don’t look into the beam. High-power IR lasers should not be used, and anyway are quite unnecessary for this purpose.

Costs
I’m trying to keep the costs down of course. I have spent, in late 2022:

• Bresser viewer £100 second-hand – would have been £175-£205 if I had bought new.
  www.bresseruk.com Product code 1877495.

• Wide angle adaptor £22. 
  ​https://smile.amazon.co.uk/dp/B07F1SMC3G?psc=1&ref=ppx_yo2ov_dt_b_product_details
• IR LED board £10
  https://smile.amazon.co.uk/gp/product/B09H72RDM7/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
• LED board power brick – I already had one, but a new one would be under £10

​So for under £150 I have a system that allows me to work for a good length of time manipulating panchromatic film, including aerial survey film with extended red sensitivity, and see what I’m doing. By the way, do not use this with actual IR film which would certainly be fogged. On the other hand I don’t see any reason why I couldn’t use it with normal colour negative and transparency materials.