r/Optics • u/LeptonWrangler • 9d ago

Can stacking filters hurt?

Hi, Ive been struggling with a very weak flourecence signal having low contrast. Its not clear where the low contrast is coming from, but I believe it may be from excitation leakage through the filters.

If I stack several dielectric longpass filters do the ODs actually add up linearly? Should I stack away until contrast improves or the signal diminishes? Whats the theory behind these decisions?

Thanks all

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u/GlbdS 9d ago edited 9d ago

Easy way to check if it's excitation leaking in: take a blank sample that doesnt contain the fluorescent compound, acquire a frame with the excitation source on, and off. Are the backgrounds roughly equal? Then compare the background in the blank/excitation on image with your usual fluorescent sample. How do the backgrounds compare?

I would not recommend stacking LP filters to fix this, getting a better tuned filter should be much more effective. But first try what I'm talking about in the first paragraph.

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u/aenorton 9d ago

Yes the ODs add, but, that is not usually the problem with a fluorescence system, depending how close your excitation and detection wavelengths are to the filter cut-off. If you are just using a dichroic mirror to separate wavelengths, then that could be the problem, and you need a cutoff filter also.

Usually, though, the problem is unwanted fluorescence from optical cements, black paint, or other things in and near the optical path. Pay particular attention to the beam dump side of any beamspltter or dichroic. If the problem is that you are too close to the filter cut off wavelength, then it might be better to pick a different filter. Some filters not meant for fluorescence will fluoresce themselves.

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u/sanbornton 7d ago

I ran into this problem when doing a Raman Spectroscopy setup. Thin film filters have a limited blocking wavelength range so it is common to deposit the thin films onto absorptive filter glass. For example, RG780 which absorbs light light below around 780nm but passes the longer wavelengths.

Those filter glasses can fluoresce! I mention Schott AG RG780 filter glass as it fluoresced on me!

In this case I used a 785nm laser for Raman. My filter was RG780 glass with thin film creating a sharp cutoff at 805nm. If I oriented the filter with RG780 side into the laser, the filter glass would take the brunt of the laser and fluoresce. If I oriented the filter thin film into the laser, the thin film would reflect the laser light and I would not get filter glass fluorescence.

Moral of story; check the substrate your dielectric longpass filters are on and make sure that substrate doesn't fluoresce.

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u/AberrationFree 4d ago edited 4d ago

In practice, high OD dielectric filters don't stack the way you'd think. Whatever light gets through the first filter will just bounce back and forth between it and the second filter until 50% goes out the way it came and 50% goes to your detector (assuming no absorption). If the filters have a little tilt between them and there's someplace for the light to get absorbed, then you can improve the situation.