Re: [ATM] Infrared Scope Design

[Dominic-Luc Webb <dlwebb@canit.se>]

The TV sensor is how I first get into IR, essentially for free.

Fortunately, there is a big difference between TV brands/models,
so some of the parameters like noise rejection are a bit flexible.
The idea that this was not really so hopeless came from my
observation that one older TV I had would sometimes spontaneously
change channel or some other setting when pointed towards the
Sun. I also noted that with a fresh battery, the remote worked
very well with indirect illumination of the TV sensor, such as
light bouncing off walls. My photometric grade CCD positioned
at the TV sensor only barely detected anything in the best case.
I pursued this further. I once found some info that at least
hinted how to disable the ~40 kHz modulation such that they
would not reject sunlight. Given the price and sensitivity of
these sensors, I think this really should be explored further
as a super cheap IR photometer. Of course, I know there are
others in here better than me at electronics, and I'll give you
the last word on this.

I went on to build my own DC sensors using low input bias
op-amps and PINs. While I have gotten some of these to work,
they were never anywhere near as sensitive as what I had in
my TV sensor. Several cheap webcams I tried also worked better,
so I gave up the fight and went for a CCD chip for lower IR,
like 900-1200 nm. Spatial resolution never hurts.

Dominic


On Sun, 14 Oct 2007, Bob May wrote:

> Dominic, the TV sensors come from the fact that they are
> detecting an AC signal out of the steady background.  Thus it
> doesn't matter so much how much ITR there is in a room  )and with
> incadescent light technology, there can be a real lot, especially
> if a bright bulb is very close to the TB) and the ultimate
> sensitivity depends upon the rejection of the line freq.  and the
> gain of the AC stages to the signal detector.  Ultimately thogh,
> those IR modules don't end up being good sensors because they
> first need a chopper of the incoming signal to even see the light
> as such and then they need to have that signal of a fairly low
> intensity in order to keep the gain stages in their linear
> region.  Most of them end up with a banb, bang output form rail
> to rail of the Power supply.  What is needed is a real
> commparison between two photosensors to eliminage the dark
> current of the photosensors and a multidecade log amp to handle
> the current difference between the two photosensorsfor the
> incoming signal.  chopping the signal would mean that the
> detector system will be more sensitive to accuracy which would be
> helpful.
> Please also note that such detectors are basically instant
> response rather than the timed response that a CCD array or
> bolometer array can provide.  In other words, there is no light
> collection time as with the CCD.
> Bob May
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