Fresnal Lens Focal Distance

ArkansasElkHunter

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I know that different sensors are using different focal distances (i.e ms20, RS, Amazon all different - from 5/8" to 1.2") and that Fresnel technologies sells lens for different focal distances.  Is this because of the sensor or does the design of the lens set the distance.  Has anyone tried a lens with a sensor designed for a  distance other than that recommended for that sensor.  I was wondering what the effect would be on the ms20 with a shorter focal distance.  I am starting the  design for a microcam using a s100.

Shrtcrkt,  are the lens you got from fresnel 5/8".  would you like to sell one for me to play with.  
 



Archilochus

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The optimum focal distance is defined by the characteristics of the specific Fresnel lens.  Moving the lens closer to the sensor element would cause a decrease in range.  So would moving the lens farther from the sensor.  This might be desirable in some cases.

Archilochus
 

davered1

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ArkansasElkHunter,

I have used the stock RS lense on the MS20 just to see if it would pick up the same.I moved the original distance(lens from PIR) to 1" as well.Seemed to work about the same. I end up with piles of these (MS20 lenses and RS) leftover because I use only half per camera.
 

ArkansasElkHunter

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Arch,  Thanks for the responce.

Are all sensors (cds?) about the same or are some more sensitive than others.  What sets the sensitivity/range of a particular sensor.   Is it all in the lens design?

Thanks
 

Archilochus

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Hi AE,
CdS cells are simply Light Dependant Resistors (sometimes called LDRs)  Their peak sensitivity is in the visible light range.  We use them to detect light levels for day/night disable circuits.  Inexpensive toys use CdS cells in crude circuits to detect "motion" - but those circuits are actually responding to visible light level changes.

Most motion detector sensor elements (Pyroelectric detectors) are sensitive to infra-red and beyond.  The sensitivity of the elements varies somewhat by manufacturer and specific device type.

Overall sensitivity of the complete motion sensor depends mainly on the electronics used (op-amp circuits, comparators, etc) and the Fresnel lens design.  Usually the manufacturers of motion detectors design the circuits so they operate at a reduced range.

Archilochus
 


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