As a speckle free calibration reference in my rear projection measurements, i projected speckle free light source on to the intermediate screen and captured the image via monochrome cmos camera.When i calculate speckle contrast of the image, it is not zero it still gives .022. Why is it so?
A speckle free light source, does not have speckle and If this is not speckle contrast then what is this? Is it some kind of sensor noise?
For a speckle free light source, do you mean an LED or thermal source? Also, 0.022 for speckle contrast is very low. In a measurement, I wouldn't expect the speckle contrast to ever be zero, because of things like noise.
I have a couple other questions about your setup:
1) How large is the aperture of your light source?
2) How far is your light source from your intermediate screen?
@Milo Hyde
I mean Quartz Tungsten halogen lamp. When i change the aperture of the camera lense speckle goes higher to 0.09 also although i am using speckle free source.
So, you mean all this value is noise (Shot noise, Electron noise etc). But how to be sure that its noise and its not speckle although we calculate by speckle formula. I mean to differentiate or quantify them separately.
1) aperture of light source = 50mm
2) Light source to intermediate screen distance = 40mm, camera to screen distance = 287mm
Your statement about changing the camera pupil size got me thinking. Although the light from your Tungsten lamp is initially spatially incoherent, it gains spatial coherence as it propagates away from the filament, such that the light is actually partially coherent when it illuminates your intermediate screen which I assume is optically rough. Since you are imaging the light reflected from or transmitted through this screen, what matters for speckle contrast is the coherence area of the light illuminating the screen versus the equivalent area of the imaging point-spread function on the screen. I highly recommend that you look at Goodman's Speckle Phenomena in Optics book, in particular, pp. 178-181. He does a much better job of explaining what potentially is going on than I ever could.
Yes, you could determine whether what your measuring is speckle vice noise. The probability density functions of speckle are different than both shot (Poisson) and read noise (Gaussian). These PDFs are given in Goodman's book.
Milo Hyde Thanks for your brief and prompt answer.
Regarding changing camera pupil size, i have attached the bar graph with different f#.
Regarding the book "Speckle Phenomena in Optics: Theory and Applications" , Is it version 8th, 2006?
- Badges
- Science topic
- Similar topics
- Computer Science
- Images