[Laser] Photo sensor, optics, and mask

[laser@mailman.qth.net]

I recently got some photo sensors, supposedly PIN diodes with an active
area 3.6mm by 3.6mm.  I want to mount them so that I can use them in any
one of three optical tubes with lenses 50X300, 100X200, and 140X340.
The mount I am working on uses 2" plastic pipe reducer and I am
wondering about the effect of using a mask with a 2mm ( 5/64 inch)
diameter hole to narrow the effective field of view.

I have questions about how photo sensors work, but let me
describe a hypothetical situation before I try to ask the specific
questions.  

Assume that I have a 100mm X 200mm lens and I mount a photo sensor 10mm
in diameter at the focal plane.  I then have a field of view of 10mm
divided by 200mm or 50mR.  Assume that my optics and total system allow
a distant modulated light source to form an image 1mm in diameter on the
sensor.  All of the energy of the desired signal falls on only 1 percent
of the surface of the sensor.  The geometry of the system allows me to
move the sensor further from the lens by 20mm, then the image of the
light source will expand to 10mm in diameter, which if the system is
perfectly aligned, will spread all of the light energy over the entire
sensor area.

  Now the question, will I get better conversion of the light energy
into electricity if I get all of the received light at a small area of
the sensor, or if I spread the light over a larger area of the sensor,
or will I get the same conversion regardless of the area that the
received light hits.



If that question is not far enough off the wall, then what happens when
I add noise sources within the field of view, either from general
background light (like daylight) or more point sources similar to my
desired "signal".  If I want to eliminate some of the noise from
reaching the photo sensor, I can put an opaque mask with a 5mm hole in
front of my sensor.  In effect, I have reduced the area of the sensor to
one fourth of its original value, so I should get one fourth the noise,
with no change in the signal as long as it falls within the 5mm unmasked
region.  ( I do need to aim and hold  the system more precisely.  I do
not gain the advantage of a smaller sensor which have additional
benefits, but I can add a mask without having to obtain a different
sensor. )  So if I leave the sensor at the focal plane, I gain signal to
noise ratio.  What happens if I move the sensor back 20mm so that I
spread the signal energy over more of the sensor.  Now the noise is
spread to a non-uniformly illuminated spot that is 15mm in diameter.  (
I know there is vingietting, but I am not sure that it is significant to
the noise reduction in this example.  Without the mask, an uniform noise
background would "fill in" the noise that would miss the sensor from
points on the focal plane that are now blocked by the mask. )  Now for a
really interesting question: would this improve the signal to noise
ratio?  At first thought, it would be minor, but if the photo sensor has
a threshold that the noise must overcome, or a non-linear response at
low intensity, would sensor convert the signal more efficiently than the
noise.  Or is this process degrading the signal to noise ratio of the
output?



In designing a line of sight optical system, my "gut" tells me that
longer focal length and smaller sensors are better, if I can keep
everything pointed on target.  The parts that I have seem to have more
field of view than I should need, so I am considering a removable mask.
Should this improve system signal to noise performance and should I
consider moving the sensor beyond the focal plane, with or without a
mask?


James
N5GUI


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