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Grid does nothing in IR?

The Lightning Stalker

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I got my ghetto spectrometer all set up tonight to do some measurements on some new IR
diodes to try and determine the approximate wavelength and have encountered a strange
problem. It appears that the light is just going straight through and not diffracting at
all. It works fine at around 660nm as you can see in the second picture, but for some reason
there is no diffraction happening at all with my mystery diodes.

The grid I'm using has 1000 lines/mm. The camera is a Sony DCR-SR62 in nightshot mode.
Panning reveals nothing. There is just nothing there.

I'm going to try a known 780nm diode from a 52x CD-ROM and see if that does anything.

Can anyone think of a reason for this? More importantly, is there a different grid I can use
that will work?


Mystery Wavelength (It's blurry because I had my finger over the camera's IR and autofocus to get more contrast.)

660nm
 

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The Lightning Stalker

Well-known member
Joined
Aug 14, 2013
Messages
2,655
Points
63
Okay, I figured out the problem. Diffraction grids/gratings have a maximum wavelength.
Apparently 1000 lines/mm is too many. The first order line would've come out at 70°. So it
was diffracting, but at such an angle that it was probably getting absorbed by the grating
itself. I switched to a "13500 lines/in" grid (531 lines/mm (actually, it's more like 204, or a
period of 4900 micron. Why it's so far off I don't know.)) and there it was - 940nm. Isn't she
cute? ;)

 

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