[Laser] sensor vs focal plane

[Jim Moss]

Note that the fresnel lenses usually do not focus quite as well as some of the
full size lenses. The result is a smearing of the focus, resulting in slightly
larger area.

Another attribute to the fresnel, is that they have a fair amount of chromatic
distortion. (focal point varies with light color or frequency) Be carefull to
focus the correct color to the sensor.

Jim
N9JIM/6

--- TWOSIG at ol.com wrote:

> In a previous post, Tim Toast  stated:  
>  
> ",,,I had read somewhere that under-illuminated diodes (when the focal spot  
> is smaller than the total area of the diode) would have more noise than if
> you 
>  used the whole surface area -..."
> 
> I am wondering if anyone has more information on the issue of trying to  
> illuminate a "large" area of a photo sensor.  Tim, if you can find a 
> reference on 
> under-illumination, I would like to check into it further.
>  
> As I see it, if you put the photo sensor at the focal plane, the incoming  
> signal forms a small dot that is limited by the apparent size of the source
> and  
> the quality of the optics in the collector.  If you move the sensor forward  
> or aft of the image plane, then the dot gets bigger.  As long as the  dot 
> still falls entirely on the sensor, you still capture all of the  photons.
>  
> The first question I have under those conditions is: does the photo sensor  
> work better, worse, or the same, when the same number of photons are
> clustered  
> together into a small dot, or if they are spread out over most of the surface
> 
> of  the sensor.
>  
> If your signal has neighboring noise sources in your field of view, there  
> may be a way to use the sensor off of the focal plane to capture all of the  
> signal photon, but reduce the noise photons.  Consider two types of noise  
> sources, point sources and area sources.  In what follows, consider an area 
> source 
> to be a large number of proportionately weaker point sources scattered  over 
> the area.
>  
> First take the case of a single point source.  Removing the noise  should be 
> as simple as shifting the direction of the optic path so that  the offender
> is 
> no longer in the field of view.  Shifting the sensor  out of the focal plane 
> is not needed.
>  
> That does not work if you have multiple point sources that come into the  
> field of view as you move some of them out of the field of view.  To deal 
> with 
> this problem, one thing that you can do is to reduce the field of view, 
> either 
> by increasing the effective focal length of the optical path, or masking  off
> 
> part of the sensor.  Either way the device will be harder to aim.   It may be
> 
> that if the photo sensor is more effective at converting the photons  into 
> electricity when they are bunched together, then it will be the better way 
> to 
> attack the problem of neighboring noise sources.
>  
> On the other hand, if the photo sensor is more effective when the photons  
> are dispersed over most of its surface, the there are two advantages placing
> the 
>  sensor further away from the objective lens.  If the signal source is  
> expanded to an area nearly equal to the sensor, then a noise source will be  
> expanded to a similar size, and unless it is very close to the desired
> signal,  some 
> of its photons will fall beyond the boundaries of the sensor.  Hence,  the 
> noise power will be reduced.  The increased distance from the objective  will
> 
> result in a proportional increase in the spacing of the signal source from 
> the 
> noise source on the sensor area.  That in turn will increase the spill  over 
> of the noise without increasing the loss of signal.  There will be a  small 
> narrowing of the field of view,, but it will not be as dramatic as masking 
> the 
> sensor or increasing the focal length.
>  
> There is another advantage to moving the sensor away from the focal  plane.  
> The larger dot will transition across the boundary of the sensor  slower.  If
> 
> the system has a sharp focus, the photons in the signal will be  more likely 
> to be completely on or completely off the sensor.  A large dot,  is more
> likely 
> to have some of the photons spill off the sensor, resulting in a  fade 
> instead of a sharp signal loss.
>  
> There is still one more way to make use of a sensor that is further away  
> from the objective.  That is to put an adjustable iris at the focal  plane. 
> To 
> acquire the signal you open it up so that you have maximum field  of view.  
> Once the system is locked onto the signal source, you stop down  the iris,
> which 
> will limit the field of view.  You can then use fine  adjustments to point
> the 
> system more accurately at the target, and perhaps stop  down the iris even 
> further, to eliminate more noise.  The noise that this  system will eliminate
> 
> probably will not cause problems finding and tracking the  signal, but would 
> limit the data transfer rate.
>  
> In all of this discussion, I would probably prefer to use a bigger  objective
> 
> lens (more signal received) and longer focal length (smaller  field of view, 
> less noise), and more accurate pointing (needed because the field  of view is
> 
> more limited), to improve signal to noise characteristics.   However, these 
> ideas may be able to squeeze a little more performance out of a  system that 
> cannot be modified in size and focal length.
>  
>  
> James
> N5GUI 
>  
>  
> 
> 
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