From: "Jeff Anderson-Lee" 
To: "Goran Hosinsky" , 
Reply-To: "Jeff Anderson-Lee" 


 "Goran Hosinsky"  wote:

> I want to get a simple (read cheap) webcam to document the Ronchi test
> when I get that far. I am doing a 10" f8 mirror. Do I need extra optics
> in front of the camera or do I just design the tester so that the camera
> lens can be positioned where I put my eye?

I've been playing with a Creative Webcam Pro ($40 at Fry's) which uses a
640x480 color CMOS imager giving an effective 320x240 monochrome image. 
With the supplied lens the image of an 8" f/6 mirror was way too
small.  However it was an easy matter to pop the case open and unscrew the
lens.

I have an old 35-105 zoom lens from a 35mm camera and was able to mount
that on a piece of 2x6 using some side braces that came up about 3/16in on
each side to make a "channel" and a piece of sheet metal
"strapping" with pre drilled holes that I picked up at Home Depot
bent over the top and screwed into the sides ox the 2x6.  The camera easily
mounted about 1in back from the lens using a single screw though the hole
in the body where the mounting post normally goes.  A piece of 1x6 on the
bottom makes an inverted "T" stand for the whole thing.

Next comes a piece of thin cardboard (thick paper?) bent in an inverted U
over the top between the camera and back of the zoom lens to form a light
shield to improve the contrast. That was simply stapled in place where it
met the 2x6. I had to add some black tape over that to improve matters
more.  If you can scrounge up some empty photographic paper holders from an
amature photographer and use that material instead it would help matters.

I used a super-bright blue LED ($4 at Radio Shack) with a 550Ohm resistor
and a DC-regulated 9V wall-wort as a power supply.  I printed out some 75
lpi grating using a 600dpi laser printer and laser-printer safe overhead
transparencies.  The quality is not as good as getting a pre-made grating,
but works for starters.

With the zoom lens I can easily frame and focus the image of the mirror to
fill the picture. All that said, the picture is quite noisy.  Even
averaging together 8 images in Photoshop shows a lot of speckle.  I'm
tempted to reassemble the webcam and return it, looking for something with
a CCD imager instead.  (It's probably good enough to document a Ronchi
test, but not great for automated testing.)

The IBM PC Camera Pro (Xirlink 33L4891B) ($31 on web) has been suggested.
However it is no longer manufactured so while you can still buy it on the
web you cannot find it in stores. Other possibilities might be the QuickCam
Pro 4000 ($100) or the Creative Webcam Pro EX ($70).

Happy imaging!

Jeff Anderson-Lee
Sacramento, CA

--- BBBS/NT v4.01 Flag-4