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Looking backwards through a holographic diffuser

  1. Sep 6, 2010 #1
    I know a diffuser should be approximately bidirectional, so the "direction" isn't the important part of the question. Our application is illuminating some light sensors while we test their electrical characteristics. Like CMOS image sensors for cell phones. Long ago, we used some plastic holographic diffuser sheeting (unknown specs - but guessed to be 15-30 degree output.) However, the input wasn't collimated, but a simple disk full of various colored LEDs a couple of inches away. Think of it like a multicolored MagLite.

    Another designer (we're EEs, not optics experts) changed that to some opal glass to improve the uniformity, but I think all opal glass does is improve the wideness of the diffusion (not so important) while increasing the absorption HUGELY (suddenly an important thing to minimize).

    I'm recommending going back to the much less-absorptive holographic diffuser, perhaps 45-60 degree angle units, but he has an objection. He says when you look backwards through a single layer of the holographic material, you can make out the images of the point source LEDs. Since you can make out individual LEDs in the translucency of the diffuser material, it can't be a very good diffuser. He says the proof of diffusion is in the uniform milkiness of the opal glass. I claim it can be a great diffuser if the intensity of the image you see is uniform within the diffusion angle, even if you can make out the LEDs.

    Can anyone help me with an explanation of how holographic diffusers can be effective, if you can also discern items on the other side of them?

    For an example, see this Edmunds picture of a holographic diffuser, and how it's not totally milky like opal glass:

    http://www.edmundoptics.com/images/catalog/1001415.jpg

    Thanks from a first-timer!
     
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  3. Sep 6, 2010 #2

    Andy Resnick

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    You are asking a great question, unfortunately the answer is complicated.

    Based on your description, the opal glass does indeed homogenize the light better than the holographic diffuser. The critical point to address is what your requirement(s) is (are).

    One simple thing to try is to move the holographic diffuser close to the sensor and move the light source further away- this is the same principle that allows satellites to easily see details on the earth, but we can't easily see the satellites from the ground.

    But again, the critical point is what your specification is- how uniform (spatially, spectrally, etc) does the excitation need to be?
     
  4. Sep 6, 2010 #3
    Thanks for the suggestions. I was trying to avoid the term "homogenize", because in my limited research, it doesn't seem to be quantifiable, and seems like it was just made up to go along with "milky glass". But it certainly describes what opal glass does even more of than frosted glass, with the holographic diffuser doing the least amount of it.

    But I'm thinking along the terms of "a black hole has no hair". An eye can't reconstruct what's on the far side of a piece of opal glass, and it seems intuitive that it acts as a good homogenizer and diffuser.

    Holographic diffusers that allow the eye to reconstruct the position of point source LEDs on the far side, can be argued (by my colleague) to be less effective diffusers. I, on the other hand, believe their specs are true, and they are effective diffusers. They certainly absorb less, and right now I need intensity improvements to get more uW/cm^2 at the device under test (DUT) for the same LED current.

    I think the critical difference is that I'm looking at the holographic diffuser with my eye. It focuses and can discern an image. The device we're testing is essentially a photodiode with some UV and IR filtering, and has a relatively wide acceptance angle and can't see an image. Photons hit it from any angle, and it doesn't matter much if the source is diffused as long as it's relatively uniform and can be calibrated. Since we don't collimate the source (being comprised of many LEDs on a 2" disk) I'm not entirely sure what the intensity distribution is after the diffuser. I'm pretty sure the device under test doesn't care.

    You have a great point on the distance from the diffuser to the DUT. We are length-constrained to about 3", and we arbitrarily put the diffuser at 2/3 of the distance from the LEDs to the device. So if there were any hot spots on the diffuser from narrow-angle LEDs, they would be farther away from the DUT. But we also have an off-axis calibration sensor (light to voltage) that needs to see about the same intensity as the DUT, so I need its view of the diffuser to have the same approx. intensity as the DUT sees.

    If I had been the original designer, I probably would have made it out of a 3" integrating sphere, with LEDs poking through many holes in the interior. Just as an amateur optical person.
     
  5. Sep 7, 2010 #4

    Andy Resnick

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  6. Sep 7, 2010 #5
    Our published spec for uniformity is 2% over a 15mm diameter circle, placed about 10mm from the device under test. We're pretty sure the device doesn't notice any non-uniformity since it just accepts photons from about a 60 degree cone. And measuring uniformity is relatively difficult without an instrument with a small aperture placed close to the diffuser. Our calibration unit is a Coherent LaserCheck with 5% accuracy, and an 8mm circular silicon detector no optics. Absolute accuracy is not important for the application.

    But the application, and ultimately, the specifications aren't important to my political/social problem. This other engineer just has an intuitive feel that holographic diffusers don't diffuse very well, because of this "seeing the led through them" issue. I could take measurements for days with expensive test equipment we would need to purchase, and he could still object. I was hoping I was posting in the optimum forum, and I could find out why holographic diffusers behave this way while still meeting their specs as diffusers without being good homogenizers.

    I'm confident we can meet the requirements of testing the device by using one or maybe two layers of holographic diffusers (separated by a small distance), while absorbing a lot less light than the opal glass. I just don't want to have a debate about the diffuser without some idea why they have this optical appearance.
     
  7. Sep 8, 2010 #6

    Andy Resnick

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    Oh... I see. I'm not sure how much help I can be, if measured data is insufficient.

    Holgraphic diffusers (just like any diffusing screen) operate by multiple scattering- the more scattering, the more homogenization. Holographic diffusers are attempts to engineer a well-defined scattering medium, and trade-off loss and homogeneity. "Loss" meaning back scattering (or scattering out of the optical path in general), not absorption.

    But again, I'm unable to help with your political situation- I've had plenty of that nonsense contaminate my projects in the past, and it never ends well. Good luck...
     
  8. Sep 8, 2010 #7
    A piece of new data I didn't have earlier - the holographic diffuser used in the past was a 15 degree design, not a 50-80 degree design as the mechanical container should have demanded. Since the incoming light wasn't collimated, and the leds were distributed on a 2" disk about 2" from the diffuser, I'm not exactly cure what a 15 degree diffuser will do to such off-axis light. Probably just pass it through, and that's why we could make out the leds. I should just buy some 80 degree material and see what it looks like.

    Good thing about the other engineer - he's retiring in weeks, I inherit the product, and if I just delay a decision until he's gone he'll have no influence. Sometimes you just have to outlast 'em. Thanks for your assistance.
     
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