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I am experimenting with absorbance measurements. My optical configuration is shown in the image below. The lens after the fibre is used to collimate the light and the lens after the sample is used to focus light on the detector. The light has a wavelength of 620nm. The detector is a photodiode.

I am now using reference standards to measure the accuracy of my optical configuration. I have two sets of absorbance standards - natural density (non-reflective) and quartz-on-glass (reflective) standards. I have good results using natural density standards. The measurement error is 0.5%. But I have problems measuring reflective standards, the measurement error is around 4%.

I think I have a problem with reflections between a reflective sample and a collimating lens. What are the options to eliminate the problems with reflections? The only solution I could find is maybe to use a lens with anti-reflective coatings. Could this solution work?

Thank you!

enter image description here

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1 Answer 1

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There are probably a few things that could be done, but if you are taking a reference spectrum first, then calculating the absorption spectrum using the reference spectrum it shouldn't matter a lot if the lenses are coated or not.

Since your neutral density filters are about right, I think the issue might be that you are using a reflectance standard that is calculated for reflectance, and not for absorption. By that I mean your reflectance standard is the reflection from the first air/material interface, and when you are measuring the absorption you are also measuring the loss from the back material/air interface. If that last interface is glass that would be about a 4% error.

It actually can be a little tricky to make highly precise absorption measurements, 1% or 0.5 % is not so hard, 0.1% or 0.01% can be quite a bit harder and depends on how absorptive the sample is.

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  • $\begingroup$ 1. Yes, I am taking reference (baseline) measurements of air/empty standard. I am calculating the absorption using the formula Abs = log10(I0/Imeas) 2. The reflectance standards are calculated for absorption. I checked both standards using a high-end spectrometer and the measured absorption value is correct. There is a problem with my optical setup. $\endgroup$
    – Eugen
    Aug 30, 2022 at 9:39
  • $\begingroup$ Hmmm....What percent transmission are you seeing? Is it highly absorptive or highly transmissive? Is the error high or low? When you say reflective standard what is the stack of materials with thicknesses? Is the spot on the detector smaller than the detector? If the sample shifted the spot you could lose some light. Do you have any polarizers in the path? At normal incidence polarization should not matter a lot, but you mentioned quartz if sample rotated the polarization some and there was a polarizer in front of the detector I suppose it could make a small change. $\endgroup$
    – UVphoton
    Aug 30, 2022 at 12:02
  • $\begingroup$ If you measure a glass microscope slide is that correct in your spectrophotomer and your set up, or do you see the same error? $\endgroup$
    – UVphoton
    Aug 30, 2022 at 12:06

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