Hello everyone. I have been doing a FTIR stup to measure the reflectance at grazing incidences by using parabolic mirrors. So I prepared a sketch of it, and attached. Could you tell me if this setup works or not? If you think this setup works, I need to learn what will be the beam diameter on the surface and what will be the angle range between sample and beam. This sketch is prepared for using parabolic mirrors (Thorlabs MPD249-M03 Off axis, RFL:101.6 mm ). I have setted the incident angle at 10 degree by taking consideration RFL, as a result X:100.0565mm and Y:17.64265mm. Could you give me advices on this setup please? I know I am not good at optics and I need some advices.
Comment: Dear Adem, One point I worry about is the direction of off-axis parabolic mirrors. The direction of parallel and focused lights might be opposite in this figure. Please check the shape of oap-mirror and the direction for focusing. Regards
In the special reflection of IR beam, the incident angle must be the same as the reflection angle. In your diagram, those two are very different as Dr. Irizawa properly pointed out.
You can calculate the size of the beam on the sample by knowing the initial beam diameter of the spectrometer, which is often 11 mm at the sample location in the case of transmission set-up, and using these beam size and the focal point of the mirror. The easiest way is to draw optics exactly the same way as the real system and measure it on the diagram.
The size of the mirror has two important consequences. When you use a large mirror, the angle spread is large and therefore, angular error is large. In case you are thinking of reflection-absorption spectroscopy to measure the molecular orientation, the orientation determined will have errors according to this angle spread. However, because of the large mirror, the energy throughput is also large, allowing the signal-to-noise ratio to be good. This can be important for surface studies (if you are interested in studying very thin film on a metallic substrate using reflection-absorption spectroscopy). So, there is a trade-off between these two quantities.
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