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Wednesday, 01 June 2011


Worked on something similar back in the late 60's first digital light meter that was in the film plane and measured time+intensity giving you a good to go reading.--When one of the first epson ink jet came out, I put it to work making really bad inkjet prints. Boy, did I want to get out of the darkroom.
At the photo show that year in NY the Epson guys said -- It can print photo's, we never though of that...
Did a lot of work in the 3D TV area also -- their just getting there.

Now that was a good read, a larger window into all things Ctein.

I keep toying with how you could make a device on which each sensor receptor site would record the time that it took for x number of photons to strike it rather than count the number of photons that strike a receptor site in x amount of time.
Getting a clock register for each site would be tricky but then you could all sorts of neat things like have short exposure times in the highlights and long exposures in the shadows with no noise or clipping and an exposure range that would maybe not be unlimited but would be pretty big.
You could change the f opening during exposure so the the highlights had more depth of field than the shadows. All sorts of neat stuff.

Not intending to rain on Hugh's idea, (and certainly don't know anything about the engineering he's supposing) but wouldn't changing the aperture during exposure change the focal plane as well as depth of field?

Of course, since we're talking hypothetically, I suppose that could be accounted for in software.


Dear Patrick,

The position of the plane of sharpest focus only changes with aperture due to uncorrected spherical aberration. In a high-quality lens this focal plane shift can be ignored.


Dear Hugh,

That's an interesting idea. NASA has several designs for extreme-range sensor arrays (in excess of 20 stops exposure range) for use in exo-planet observing space telescopes. I don't think any of them gate the receptors using count rates rather than integrated flux, but that might be a failure of my recollection.

In any case, I think it's a rather interesting idea. Another application would be a way to build “characteristic curves” into the sensor response. You like a characteristic curve with brilliant midtones but a really long shoulder so that you don't blow out highlights (although they'll be very low in contrast)? Adjust the gating so that it's fairly minimal for lesser exposures and kicks in rapidly in the highlight regime.

Fun and games in the gedanken lab!

pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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Without using a locomotive or a flashlight -

Maybe instead of having a clock register for every photodetector you could read out the entire array very quickly but only one bit deep, sort of like the way one bit audio sampling works

Oh and before anyone say "Gotcha!" I'm well aware that the quantization error would be the greatest in the highlights and the lowest in the shadows, the opposite of the way it is in current detectors (no pun intended) but you would have a lot of time to over sample the highlights.

Better yet you could compensate by making the reverse bias higher during the exposure. At the beginning of the exposure the photodetectors would be relatively insensitive but a few clock ticks into the exposure you would increase the reverse bias so that by the end of the exposure you would
get avalanche breakdown in the photodetectors. (Unless that's what you mean by adjusting the gating.)

Oh by the way, the reason that I have been thinking about this is because I have always been fascinated by the way some photosensitive materials become less sensitive to light the more exposure they get*, printing out paper being the best known example. I tried mimicking that effect with various compensating and exhaustion development schemes when I was using film.

* yes, at a certain point all photosensitive materials become less sensitive to light the more exposure they get or suffer reciprocity failure, but not in their normal use.

Perhaps you (and several others) did come up with the same idea a few years too late, but you have to understand that at this point in time your claim comes off sounding a little like Al Gore's "I invented the internet" claim. I think sometimes it's more prudent to just accept that somebody else got there first and resist that urge to say, "Me too!"

dammit. I thought I invented the digital imaging device in 1977. You could take a 2K byte dynamic RAM in a ceramic package, (carefully) pop the metal top on it, focus an image on the die, write it with all 1's, stop the dynamic refresh, wait (a really long time) and read what was left. Voila' you had an, albeit poor, image. Used it for very early home-brew robotic vision.

Dear John,

A-- Al Gore never claimed that. Context matters.

B-- If that is how my "claim" sounds to you, then you have badly misread what I wrote. Try reading the paragraph following that sentence over again. Very carefully, and with thought.

Again, context matters.

C-- I'm not running for office, and therefore I do not have to worry about every little nuance and turn of phrase, for fear someone (other than me) will decide it isn't exactly what they want to hear.

Y'see, context... oh, you know what I'm gonna say [VBG].


Dear Gregg,

Heheh... and that's why you ain't rich, either. (Unless you are, for other reasons.)

I think one of the very mildly interesting things about the integrated circuit revolution is that it offered many different technical paths to imaging almost immediately. I know of a couple of other solid-state or hybrid schemes from that era.

pax / Ctein

I understand. A buddy and me "invented" the variable valve timing that's ubiquitous on cars today back around 1986 --- well before it showed up on production cars, I think--- over a couple of lunch breaks while bench racing ( the act of talking about how to make a car go faster ). Then again I'm sure VVT was already well underway by others before that.

Dear David,

Exactly! This column isn't about the primacy of invention, rather it's about the process.

You don't have to be first... or unique... to go through the process. You just have to be there.

There are a handful of things I've invented first. The process wasn't any different, just the chronology.

pax / Ctein

Regarding Hugh's comment, do multi-anode microchannel arrays provide a similar capability? I'm thinking in particular that the MAMA detector on the STIS instrument on Hubble is able to image in "time-tag" format. (Perhaps there are are many more with it that I'm not aware of—I know the Cosmic Origins Spectrograph on Hubble also has time-tag capability, but it's a pure spectrograph with no imaging mode.)

I've never worked with time-tag data, but from what I've gathered in the data handbooks, a time-tag data product is not an image, but an event stream, tagging each photon detected with a timestamp. According to the STIS data handbook, the time resolution is 125 microsec, or 1/8000 of a second. However, the MAMA detector on STIS is for fairly faint sources (> 50 cts/s/pixel), and I know next to nothing about the engineering behind it.

But when I first learned that this type of data product existed I was bowled over by the possibilities, as you've both mentioned them!

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