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I apologize if this question is stupid, but have been puzzled by it for a few days. Say I have a spectrometer measuring in the visible light range and we're looking to measure the absorbance on a solid material. In lab examples I've seen and read, the light source is placed on one end of the sample and the spectrometer on the other end. so the light passing through goes into the spectrometer. But what if you have a sample around 3 feet away from you. Could you place the light source next to the spectrometer and focus it on the sample then use a collimating lens in an attempt to see what was absorbed or not?

Once again, I apologize if this is a stupid question, I feel like I am missing some fundamental points of this process here.

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  • $\begingroup$ Let me clarify something. Do you just want to make the measurement on a solid, or do you need to make the measurement on a solid surface of some sort. More directly I guess is could the solid be in powder form? $\endgroup$ – MaxW Nov 3 '18 at 19:58
  • $\begingroup$ I just want to make the measurement on a solid, it would not be in powdered form. $\endgroup$ – Advert Nov 3 '18 at 20:02
  • $\begingroup$ Ok, that rules out photoacoustic spectroscopy. $\endgroup$ – MaxW Nov 3 '18 at 20:06
  • $\begingroup$ Is the sample something "small" that could be put inside an instrument? $\endgroup$ – MaxW Nov 3 '18 at 20:57
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Yes you could do what you propose provided that the light is properly collimated so that it all reaches the detector. It is quite common to place mirrors inside the sample housing of a spectrometer to take the light out into the lab, through a sample (usually a weakly absorbing gas ) and then use a mirrors to get it back into the spectrometer. Multi-pass cells can also be used to do this.

In atmospherical studies very long path lengths (100's metres) are sometimes used to identify trace amounts of various species. Of course if you do something like this you have to make sure extraneous light does not reach your detector.

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