> >> Hi all,   
   > >>   
   > >> In most of the optical layout I can see for photodetection, light arrives   
   as a focused beam onto the photodetector. I'm wondering if it would equally   
   work with a collimated beam (parallel) onto the detector (without any focusing   
   lens in front of    
   detector). For large area photodetectors, like the photocathode of a PMT, it   
   seems to me that a collimated beam would generate a more repeatable response   
   compared to a beam focused onto one single point, notably in case of   
   photocathode response non-   
   uniformity.   
   > >> Of course, for imaging detectors, it's clear to me that detector plane   
   must be optically conjugated to sample plane. However, for photometric   
   measurements, where image is not needed, I don't really understand the   
   interest of optical conjugation    
   between sample and detector.   
   > >>   
   > >> Thanks,   
   > >    
   > > For laser sources I rarely use a lens.  Just send the beam onto    
   > > a photodiode, align from maximum signal.     
   > >    
   >    
   > Yup.  Focusing light down on the detector is a win if you're in a low   
   > light or high speed situation, because you can use a smaller detector   
   > that is faster and has lower capacitance and less area to pick up   
   > background light.  It doesn't help anything if the detector is big   
   > enough to handle the whole beam.   
   >    
   > The drawbacks are nonlinearity and possible detector damage.   
   >    
   > If the beam is mildly too large for the detector, then you can use a   
   > lens well out of focus to adjust the spot size to the detector area.   
   >    
   > Cheers   
   >    
   > Phil Hobbs   
   >    
   > --    
   > Dr Philip C D Hobbs   
   > Principal Consultant   
   > ElectroOptical Innovations LLC   
   > Optics, Electro-optics, Photonics, Analog Electronics   
   >    
   > 160 North State Road #203   
   > Briarcliff Manor NY 10510   
   >    
   > hobbs at electrooptical dot net   
   > http://electrooptical.net   
      
   Hi,   
      
   Thanks for the explanation. Now, I'm a bit puzzled. It's often said that   
   photodetector like photodiode gives an output (photocurrent) propotional to   
   irradiance (W/m²). But irradiance is not conserved and is notably not the   
   same for a collimated beam or    
   a focused beam. I would be more at ease if photodetector would response to   
   radiance (W/m²/steradian), because of radiance conservation principle. There   
   is something wrong somewhere ! Can you help ?   
      
   Thanks   
      
   Eric   
      
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