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I am looking at the product specs for an IR light and see that it has:

Beam Angle: 50 deg

Viewing Angle: 25 deg

What is the difference between viewing and beam angle?

enter image description here

What is the difference between the two?

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@JYelton thanks for pointing that out – Vader Jul 11 '14 at 21:19
If the beamis 50 degrees wide, you can be 25 degrees off axis and still see it. So I'm guessing viewing angle is the half angle. Be nice to know that's what the datasheet authors meant though. – Brian Drummond Jul 11 '14 at 21:24
@Brian, possibly but I am not sure that would really have to be given on the fact sheet. Here is the LED by the way mouser.com/ProductDetail/Optek-TT-electronics/OP290A/… – Vader Jul 11 '14 at 21:25
up vote 5 down vote accepted

Don't trust the summary specs given by the distributor. The real spec is what's in the original datasheet issued by the manufacturer.

AFAICT, the datasheet gives only an "emission angle" spec, with a typical radiant intensity vs angle curve on page 6:

enter image description here

Even the "typical" values and curves in the datasheet are not hard promises. Only the minimum and maximum value specs are actual promises from the manufacturer about the device performance.

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