Comparing Fresnal Lenses

gizz

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In building a new cam recently with the Lamson, i noticed the sensitivity was less on the Lamson than what I'm accustomed to with the MS20. Not necessarily in distance but in "detection zones". I compared the frensal lenses (they are very similar in size) and found the following:

They both had 3 rows of the "little circles" - (focus points)
On the top row the MS20 had 12 circles, the Lamson had 8
On the middle row  MS20 - 11, Lamson - 4
On the bottom row  MS20 - 5, Lamson - 3
I tested the MS20 lense with the Lamson PIR and it made the Lamson a lot more sensitive. Also using the Lamson on the MS20 reduced the sensitivity of the MS20 as well.
I didn't check the distance - guess i should've.
Maybe a lot of the sensitivity concerns are due to the Fresnals lenses more so than the circuit?

Can anyone count the focus points on the RS PIR? Also the dimension of that PIR would help cause I think it may be a bit bigger than the MS20 and Lamson.

Does anyone know the configuration of the Fresnals available from fresnal technologies?

(Edited by gizz at 11:56 am on Sep. 8, 2001)
 



Passthru

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 I put together a camera last night useing the RS PIR,when walking i noticed that the range is short and has a hard time with certian angles,should i flatten out the fresnal or alter its shape or distance from the unit?will this maybe help me get more range?
 

Archilochus

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Hi gizz,
Fresnel Tech offers lenses in many configurations.  The lens you described even sounds like one of theirs.  The lenses they make do not have their name on them - just a series of numbers describing the lens characteristics.
Unfortunately they don't have lens descriptions on-line.  I have their whole catalog with part #'s and complete descriptions. If you ever wanted to see some of their lenses I could always FAX them or scan them and send them as pics.

Archilochus
 

Richard Webb

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Hi Passthru - The RS PIR has a slow sticky response, and it helps eliminate those piles of false pictures with no animal in them. It works great out in the field, and a warm blooded mammal will not pass by undetected if the distance from the PIR Can to the lens is 5/8", and the diameter of the opening is 5/8". I center one of the vertical lines in the fresnal lens inside the 5/8" hole giving equal zones on each side of center. The focal length is very important with that PIR, so a foam mounting is not practical. I only use that PIR in Jon's mini cam.
 

gizz

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Thanks Arch, I may take you up on that sometime. I've had 2 cams attacked by bears recently and have had to replace the fresnals as a bear tooth has managed to find it's way through each of them. I had a couple lying around but at this rate I may have to get a stockpile of spares.
 

Passthru

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 What i done is cut the units speaker out right beside the lens,leaving the whole lens to stick out the front of my case,is what your say that I need the lens to not be as wide as it was made, tape each side so it only has a small hole to look through,so that it wont detect stuff from so far to the sides?
 

Richard Webb

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Passthru - Sorry for the delay. It's Saturday night, and time for a little socializing. I built Jon Bain's version with the RS PIR, and the picture of that cam is on my home page. The fresnal lens is glued flat on the enclosure, and it works great with the 5/8" dimensions that I described. Coal30 and a few others have built up cams with the RS PIR housing glued on enclosure like what you are building. It should work as-is if the board is in the side slots of the housing, and pushed all the way forward.

One note: If you have been used to the MS20 with its snappy response the RS PIR will seem sluggish during hot weather. I had to use a thermostat when the temperature was over 85 degrees.
 

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