> Anyone know of a better way to sense photographic film in the dark
> than an array of IR diodes?
> I need to tell if its 8 16 or 35 mm wide. it moves a round side to
> side about 8mm where I have to sense it too.
> I currently plan to use 12 to 16 IR Led's about 4mm apart.

I find this question extremely interesting.

I used to work for a film processing company; and they required complete
darkness, except in the splicing room.  In the splicing room they had a
small IR light source in a corner of the room, and the splicers used IR
goggles in order to see what they were doing.

Spectral sensitivity curves for normal films show a lack of sensitivity
above about 700 nm. GaAs IR leds have emissions concentrated at 940 nm.
So it seems that using IR diodes would be ok.  However, I would still be
afraid of fogging the film.  If the exposure is long enough, normal
photographic film can be fogged by IR.

The first thought that comes to mind is what about using air?  There is
usually a supply of dry, plumbed air in a processing plant.  What about
air jets above the film, and an array of microswitches below the film?
The sensitivity of the microswitches can be increased by increasing the
surface area of the actuating levers.

Jim Albers N9CYL

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