From: boyd@apt0.sao.arizona.edu   
      
   charles wrote:   
   > Looking at a spot on a fluorescent rock.  Trying to check it with a   
   > spectrum analyzer.   
   >   
   > With a photographic spot meter, I read ASA 100, 1 second at f5.6.   
   >   
   > Can this be converted to some units that an engineer would understand.   
   >   
   > I am working close enough to the item that it is an extended source. I   
   > presume I cannot gather more light using a positive lens.   
      
   I'm an engineer and I understand what ASA 100, 1 second, f/5.6 means at   
   least enough to set a manual film camera.  Converting it to any other   
   units without a lot of additional information about the meter and the   
   light source  would be of little use for anything.   What is that   
   question about?   
      
   A lens can gather more light until you fill the area of the entrance   
   pupil and the  acceptance angle  of the instrument. For most   
   spectrometers the area of the entrance pupil is much smaller than on a   
   light meter.  The acceptance angle may  or may not be larger for the   
   spectrum analyzer compared to the light meter. Putting the randomly   
   emitting sample very close to the spectrometer entrance  pupil  is   
   usually as good as you can do.   
      
   So what is the problem?  Not enough light to get a good spectrum?   
   What kind of spectrum analyzer are you trying to use?  Rock fluorescence   
   is usually very low energy and a wide mix of narrow and wide emission   
   lines over and beyond the visible spectrum.   
   The solution is a low noise detector and long integration times. I'd   
   expect  CCD cooled to about -40 C and hour exposures should give results   
   similar to the website above.   
      
   I've never measure rocks, just stars.   
      
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