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!