Laser confocal microscope filter I wanted to mount a narrow bandpass filter which allows to transmit 405 nm wave, i. e. the laser wavelength. I wanted to cut off other light, cause my detector is also sensitive to other wavelengths.
Here is the similar setup(setup link): 
The question is, where should I mount the filter? Should I put it between beamsplitter and the lens which is focusing light into pinhole or should I put it between pinhole and detector?
 A: Is this for reflectance imaging ? The fact that you want to transmit laser light to the detector implies it is the case. Then you could put it anywhere that the detected signal goes through. (green or yellow path).
If this is for fluorescence microscopy, then you should use something like a LongPass420, put after the dichroic (yellow path). So in both case putting the filter between the lens and the pinhole would work.
In any case I would not put it between pinhole and detector for practical reason. Usually the pinhole is well aligned and less accessible... but this depends on your setup. You are also close to the imaging plane, while i would put the filter on a path with collimated light, where it functions better.
A: It is not for fluorescence microscopy.
I know that putting the filter between lens and pinhole is the mistake because in that case filter would act as second lens and it would disturb the measurment on the confocal pinhole during z-scan.
But from other way, I know that flat wavefront shouldn’t be directed towards flat surfaces (which are both of my filter).
