BEOS outtakes: Photographic Film

From the cutting room floor at Building Electro-Optical Systems, Third Edition:

Photographic Film
 Okay, okay.  Photographic film isn't a detector of the sort we've been discussing.  Film is so out of fashion, so inconvenient.  It needs messy chemicals.  Getting it to be highly sensitive requires all sorts of 1960s alchemy such as pre-flashing and hypersensitizing in a forming gas or hot hydrogen atmosphere.  Why do we care about it at all, in these days of 4k x 4k CMOS imagers?

 There are two reasons.  The first is that a telescope has a lot more than 4k x 4k resolvable spots.  The Palomar Schmidt has a 6.6\degrees\ square field.  At 1 arcsecond resolution, its plates were digitized at 23040 pixels square for the Digital Palomar Observatory Sky Survey (DPOSS).  That's 530 Mpel, which is a lot of imager chips, but just one photographic plate.  The plate can be digitized later on a scanning microdensitometer that also has many more than 4k x 4k resolvable spots.  Even an ordinary 35-mm camera produces images equivalent to 30 Mpel--and that's real pixels, not Marketing Megapixels (TM)  (see Section 3.9.14). The defect density in photographic film is lower than in IC imagers, too, and it makes a nice archival record that is guaranteed to represent the measurement data well. 

Photographic film has a power-law response over a huge range of signals.  The contrast exponent gamma can be anywhere from 4 down to 0.5, which compresses the dynamic range and makes bright and dim objects visible simultaneously.  Using low-contrast developers such as POTA, photographic film can record images whose dynamic range approaches 106 :1, optical, e.g. a bomb flash and its surroundings, which is a task beyond any silicon imaging sensor whatever.(1)

Film has two sorts of noise: grain, which is analogous to the digital nibblies from CCD pixels, and fog which is analogous to dark current.  Fog is due to a few grains being rendered developable by a few loose electron/hole pairs in the emulsion, and contributes random noise in the same way.

The second reason to talk about film is that some modern alchemy has got photographic film up to a QE of 1.0, and a multiplication gain of 2, so that a single photon can expose a grain of silver halide.  This is the quantum efficiency of the best CCDs, so there's no waste of photons any more.  The trick is to add formate ions to the emulsion to scavenge all the excess holes without increasing the fog.  Unlike other hypersensitizing tricks, this one works at room temperature and is stable indefinitely.

(1) POTA was invented by Marilyn Levy of the Army's Photo-Optical Technical Area at Ft. Monmouth NJ (hence the name).  She was working on improving aerial reconnaissance photography, where there's often lots of light but very deep shadows.

Next: The Hurter-Driffield Curve