why infra-red and not visible led?

[dr_when]

Hi,

Someone threw this question at me and I could not come up with a definitive answer off the top of my head. They asked "why are almost all optical sensors, like photo interrupters, etc made with infra-red Leds and not visible (red) leds"?

My thinking was maybe less interference from ambient light...at least artificial ambient light. Sunlight is another story. Are the phototransistors typically more sensitive to IR or is that strictly based on specs i.e. some are and some aren't. I guess I am mostly sticking to the "less interference by artificial ambient light" as the primary reason. Any thoughts?

Bob

[sofaspud]

The only two reasonable answers I could come up with are 1) the ambient light theory and 2) it is easier/cheaper to achieve
a wavelength match in the infrared spectrum. The first just seems sensible, the second is more semiconductor implantation
physics that I've never really looked into. I wonder what wavelength the common optoisolators (i.e. 4N25) use? Never looked
into that either, but it could provide a clue.

[MrAl]

Hi,

There is also the chance that they dont want anyone to see the beam or what it strikes
or even where it strikes.

[CeaSaR]

Found the answer!
http://www.scte.org/documents/pdf/Issue%203-04.pdf
Page 3 explains it all.
Another possibility lies in paraqgraph 2 of http://www.everything2.com/?node_id=1931530.

Yahoo! search comes through again.

CeaSaR

[sofaspud]

I took a look at the 4N25 datasheet; it does indeed operate in the IR region. Curiosity then got the better of me. From
what I could come up with, infrared is used because it optimizes photosensitivity (e.g. wavelength matching, response
time, and all that good etc. stuff). This 'Use Of Phototransistors' paper states, "A key fact to remember in using
phototransistors is that all phototransistors are most sensitive in the infrared region and thus work best with infrared
emitting diodes (IRED). There is no requirement that the wavelength of the IRED exactly coincide with the
phototransistor peak response wavelength but the response factor for any deviation should be known and accounted
for in the design." Close enough for definitive?

[dr_when]

Thanks guys. I can see that in some sensor applications where the sensor is visible that red leds may not be preferred because they can be seen. I suspected the IR was better suited for the phototransistors but I don't think that is a real problem. I guess in the application that I am looking at aesthetics is one thing and really have to test the operation in different ambient light settings.