[Laser] variable field of view for noise elimination

[Glenn Thomas]

Hi all.

Interesting idea. However I think you will get some diffraction 
effects from your iris, especially when it is made small. Small holes 
act like lenses as you can see if you've ever used or visited a 
"camera obscura". A more common demonstration is available by looking 
at the shadow created by sunshine through the leaves of a tree. In 
many places the leaves combine to make a very small hole which 
appears on the ground as a round spot of light. Usually there are 
lots of them. These are a real images of the sun. It is a lot more 
obvious if you do this experiment during a partial solar eclipse and 
can watch all of the fuzzy balls of light in the shadow have a bite 
taken out of them!

A scheme that is similar in that it uses an iris and moves the sensor 
might be considered. The light collected by the optical system 
(lenses & mirrors & whatever) will create a true image of the scene. 
This is exactly what a camera or your eye does. Consider for a moment 
the entire image. Your desired signal will appear in that image 
wherever the transmitter would appear to be. Further, even the best 
optics aren't perfect, so even a point source (a star for example) 
will appear as a blob. The size of the blob is a function of how big 
(in steradians) the source is and how good your optics are.

If the size of the signal blob in the image is larger than the area 
of the detector, then it's simply a matter of physically moving the 
detector to the place in the image where the transmitter appears. If 
the size of the blob is smaller than the sensor (lucky you!), then 
place the sensor so that the entire blob is on it and use an iris 
with a blob sized and shape orifice directly in from of the sensor to 
block noise sources that are outside the blob. Very little in the way 
of diffraction effects here because the iris is right next to the sensor.

In the event that the blob is bigger than the sensor, no doubt 
additional optical processing can be done to further collimate the 
image so it will be sensor sized or smaller. The only problem with 
this is the need for additional specialized optics and their 
associated losses. In any case, bring money - lots of it!

73 de Glenn wb6w


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