Paul said:
Thanks for that John. Just a couple of questions:
In order to cope with rain, fog etc, what kind of power would I need
to span 3 metres?
Am I better off with a shorter or longer wavelength for this
application, please?
Thanks again.
Paul.
I'm not sure which wavelength would be best. The shorter wavelength
is closer to the peak response of most silicon detectors (at 940 nm,
silicon is getting to be pretty transparent) and these are also happen
to be slightly more powerful emitters. But from a signal to noise
standpoint, you might get an advantage in using the longer wavelength
ones if you use a silicon detector filtered to exclude all light that
is much shorter wavelength than 900 nm, since the silicon has so
little response left longer than about 1000 nm. In other words, the
longer wavelength filtered silicon sees very little daylight between
the filter cutting off the short wavelengths and the silicon cutting
off the longer wavelengths. The longer wavelength should do a bit
better in fog, also. But I doubt there is a 2:1 advantage to either.
I would just run the LED at rated peak power, and possibly add a small
plano convex lens (perhaps a plastic fresnel) to more nearly collimate
the beam. If the whole thing operates in the dark, you may be able to
get by with a DC excitation, but if there is any significant
interfering light, you will need to modulate the emitter with some
carrier frequency (40 kHz is common) and detect energy only in that
band.
Does you design allow a synchronous detection scheme (electrical
signals shared by emitter and detector) or are you forced to a power
detector following a bandpass filter, only? The nice thing about the
latter is that they are available as an integrated optical long pass
filter, detector, amplifier, bandpass filter, rectifier and comparator
to produce a binary go/no go output from a modulated beam. (used for
infrared data communication) But the synchronous detector has an
advantage of higher ultimate signal to noise ratio capability if there
is interference near the modulation frequency.
Have you picked out a detector yet?