Lighting cavity diffuser plate efficiency. - Page 1
Forum section: electronics / projects

author: Kaptein QK
author_rank: Regular Contributor
timestamp: June 16, 2021, 11:49:06 am
content: Hi, I am making a lamp with LEDs.It is a lighting cavity with multiple LEDs covered with a diffuser plate to reduce glare and multiple shadows.I am trying to determine how much luminous power is lost in the diffuser plate (PLEXIGLAS LED 0M200 SC).I have found some information via google, but the math is over my head.Does somebody have experience with this and can give me a rough estimate?

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 16, 2021, 02:19:50 pm
content: You might be able to measure it using a solar cell in a dark room. As long as your diffuser does not appreciably modify the light spectrum, the sensitivity should be usably constant over a wide range of illumination. Use a single cell or a bunch of parallel cells that are big enough to capture the entire light field from the lamp, and measure the current into a low impedance like a multimeter in current mode or a low-value shunt resistor. As long as the voltage is kept significantly below the open-circuit voltage of the cell, current should track optical power pretty well.It's a bit more effort than a calculation, but if you happen to already have the equipment then there's less uncertainty compared to doing it on paper. Ronk on!

author: Terry Bites
author_rank: Super Contributor
timestamp: June 16, 2021, 03:07:03 pm
content: Well, Rohm claim 40% transmission (60% loss?)https://www.plexiglas.de/files/plexiglas-content/pdf/technische-informationen/222-28-PLEXIGLAS-LED-for-side-lighting-WH46-SC.pdfstandard-https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjfs7_5tZzxAhUQwKQKHU6OCqoQFjADegQIBhAE&url=https%3A%2F%2Fcdn.standards.iteh.ai%2Fsamples%2F73173%2F87b48805826b47a6b99a73880e9f6ec2%2FoSIST-prEN-ISO-13468-2-2021.pdf&usg=AOvVaw1z6MkOOuFnt3bhThcXCSXwDamn those Swiss!

author: jmelson
author_rank: Super Contributor
timestamp: June 16, 2021, 03:26:36 pm
content: Quote from: Terry Bites on June 16, 2021, 03:07:03 pmThis might be true for those translucent colors. I took plain clear Plexi and scratched it with the coarsest sanpaper I had (80 grit, I think) and it made a fine diffuser for an LED lamp. You can get molded diffusers for ceiling lights that have a sort of pyramid pattern molded in. They are "water clear" except for the lins-like effect of the pattern.These should be about 90% transmission.Jon

author: Siwastaja
author_rank: Super Contributor
timestamp: June 16, 2021, 03:41:31 pm
content: AFAIK there is no math involved; either you find the number (simply given in percentage!) in the diffuser material datasheet, or if they don't publish it, you have to guess or measure.Very good diffusers can indeed approach 90% transmission while still diffusing properly but that would be an expensive special product; this would also require special coatings since you lose almost 10% total already in the two air-plastic interfaces even with a perfect clear sheet with no absorption at all.Usually there is a tradeoff between the quality of mixing and transmission. Mixing the different color LEDs require that the material internally reflects the light in random directions number of times and this involves losses.A large part of the loss is due to diffuser doing it job, diffusing the light also back to the LED side. Adding reflective coating (aluminum tape, chrome paint, bright white paint...) on that side with just large enough holes for the LEDs help, but it's still the dominant loss source because there will be a mismatch in refractive index so each pass of photons to that reflective coating and back to the diffuser "core" loses energy.Sanding plain clear Plexiglas (PMMA) sheet with a coarse sandpaper is indeed in my experience surprisingly good, at least it's better than just using opal white PMMA, these do good job at diffusing but with the cost of being horribly lossy.

author: Kaptein QK
author_rank: Regular Contributor
timestamp: June 17, 2021, 11:41:22 am
content: Thanks all!I found the datasheet for the plexiglass and it says Light transmission τD65 = 72%, so I guess 72% of the light gets through.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 17, 2021, 04:38:57 pm
content: If you have tested it and know it diffuses i.e. mixes the colors well enough for you this is actually a very good number, you won't likely find anything much better.

author: xavier60
author_rank: Super Contributor
timestamp: June 17, 2021, 10:21:23 pm
content: I last bought the A10 option material from an ebay seller, no specs but it has a high transmissivity for an opal acrylic.https://www.ebay.com.au/itm/143888953637?ssPageName=STRK%3AMEBIDX%3AIT&var=443308696250&_trksid=p2060353.m2749.l2648