I want to detect IR light in spectrometer but using different photodetector than used in equipment. I'm using Bruker v70 and i want to collect light in sample compartment. The "spot" inside it is to big, so i wonted to focuse it but most of mirrors changes shape so there will be problems in this field. I wonted to use something like aluminium funnel to make spot smaller. Does anyone have experience how to solve problem without lossing too much signall?
Usually size of the spot and its angular divergence are related in inverse way: smaller spot will have bigger divergence. Your spectrometer has given value of divergence, so your incident beam must correspond to it, otherwise you will have only increasing of the parasite light level. This is the case of aluminium funnel: you can estimate increasing of divergence till returning part of light backward by simple paper drawing.
You can model your situation, using visual light, its focusing will be very close to IR, which you have to register.
It is difficult to propose you something more detailed because it is necessary to know your optical layout.
A general principle that one may consider in cases when lenses used may not provide the needed beam size due to wavelength being used is to switch the type of optics used to focus the beam. For example sometime using mirrors (e.g., off-axis elliptical mirrors) instead of lenses can help enhance the situation in terms of spot size of the focused beam. However, in he absence of sufficient knowledge about your set up I am not sure how easy would it be to incorporate a mirror based optical setup in your experiment.
Which spectral range you are interestd in ? If the part below ca 600 cm-1 is not needed you can use ZnSe lens.
You can use aluminum or gold coated off-axis parabolic mirror to focus IR light. You can choose different off axis angle to minimize shape change. Edmundoptics supply these.
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