Hi all,
I'd like to build myself a Long-Range, Low-Power IR Transceiver.
I know that the "long range" and "low power" may seem contraddictory,
but I thought, if I produce a very intense but very brief IR "flash",
the flash may be visible at a long distance, yet the average power
will be low. Rather than emitting 30mW for 5mS, I could emit 3000mW
for 50uS, with the same average power, but the intensity would be 100
times higher (and the range, IIRC, 10 times longer).
I intend to encode data by time-modulating these pulses, e.g. at a 1mS
rate transmit both clock and data (one clock and one data bit/pulse, in
sequence, continuosly.. clock pulse always present, data pulse present
if "1" and absent if "0").
The receiver would filter out DC, through a high-pass filter to detect
almost only those very brief pulses, and nothing else.

Does my design makes sense?

I'm now on the component identification phase. I examined the datasheets
of all the IR LEDs and photodioes/phototransistors that my favourite
online shop sells (small shop but excellent service and prices), and I
selected these devices:

tx:
TSHA6203 (875nM)
TSIP5201 (950nM)

rx:
BPW83    (875nM and 950nM)

The other devices I examined in detail were:
BPW21, BPW34, BPW40, BPW41, BPW42, BPW77N, TSTS7103

But I'd have some questions for you:

1) All the rest being equal, what wavelength is preferable, 875nM or
   950nM? I know that 875 would be better if there're glasses inbetween,
   but my application will be mostly free air.
2) What is in general preferable on the receiver side: photodiode or
   phototransistor? Why?
3) Do you have any better LED and photodiode/phototransistor to advice
   than the ones I selected? I'd have no problems to buy those from
   another shop.. if it's worth the extra shipping charge, etc.. of
   course (i.e. I'd have to make two orders instead of one).
4) How does the world look like, at those wavelengths? Is it generally
   "dark", or at 875-950nM reflections and absorbtions from objects is
   more or less like visible light?
5) At such high pulse currents, should I fear damage to my eyes if I
   accidentally look into the (invisible) beam?
6) To increase further the range, I'm thinking about making a 2m long
   tube, internally reflective (aluminium foil), diameter slightly more
   than the IR LED. This should decrease the angle of emission, but also
   make it more powerful and concentrated, right? (i.e. a bit like a
   poors' wannabe laser).
   For the receiver I'm thinking about using a small telescope, pointing
   it to the transmitter (hopefully 1+ Km away.. ehm). Will the lens be
   transparent for the incoming 875nM or 950nM light?

Thanks!
Andrea

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