COVID has forced online delivery of my courses for which some of my content lectures/office hours I now will do on a chalkboard with a video stream. But I get a bright glare from the lighting that makes parts of the board hard to read and removes the ability to use half the board. I have invested in a cheap umbrella light which youtube advice convinced me was better lighting, but the effect remains.

What are the likely causes of glare on a chalkboard? Bad lighting, bad lighting placement? Bad camera placement?

What is an inexpensive/low effort solution?


Bad lighting and/or bad camera placement are the cause. If you can't move the lights or the chalkboard, such as in many typical classrooms, the first thing you should try is moving the camera.

You can think of the chalkboard surface like a mirror. In fact, if you hung a mirror flat against the chalkboard, you'd see that the position of the chalkboard "hot spots" corresponds exactly to the position of the reflection of light sources in a mirror. The difference is that microscopic abnormalities in the surface of the chalkboard scatter the light more randomly, which effectively "blurs" the reflection.

If you can't avoid the glare by moving the camera, or by moving/dimming the lights, then there is one other way to fight glare: Use a polarized lens filter. These work the same way that polarized sunglasses work to cut glare. There are screw-on variants made for photography cameras called CPL's or "Circular Polarizing Lenses," which let you adjust the polarization angle for best effect. Polarized filters don't work for all reflections though, and their efficacy depends on the polarization of both the light source and the reflective material.

CPL's are fairly cheap. You can probably pick up a used one for $15-ish, and new ones are usually below $100. Just make sure you buy the right size to fit your lens. Either inside the camera's lens cap, or written on the lens, you'll see a number next to the symbol "ø". This is the filter size, and should match whatever you buy.

If you wanted to go totally shittyrigs cheap, you could tape one of the lenses of a pair of disposable movie theater 3D glasses in front of your camera lens. The 3D variant filter polarized light like any other camera filter or sunglasses on the market. The difference is that the polarization of the left and right eye is offset by 90º.

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    The side effect of adding a polarizer is the 2-stop light loss. (In other words, the filter reduces the amount of light coming in to the camera by 1/4.) You have to watch out for the vicious cycle of adding more light, causing more reflections.... Our (joke) solution to all reflection issues back in school was, "Hit it with dulling spray!" That included the bald talent's head. So that's an alternate, poor shittyrigs option. – Michael Liebman Jul 16 '20 at 1:11
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    Another method, following the shittyrigs philosophy, could be to use a shower-curtain in front of the light to soften it significantly, until the hotspot disappears. – Florian Claaßen Jul 16 '20 at 7:12
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    Thx. Daylight through window was big source of excess light, easily "dimmed" with blinds. Added CPL brightness changed momentarily then camera self adjusts and looks the same though slightly sharper image. Main help was to raise camera to look slightly down on the board instead of straight on. Each trick added its own nudge towards a good enough solution. Happy! – Algeboy Jul 20 '20 at 2:39

Did you try bouncing the light off of the ceiling? Make sure none of the light is visible in a direct line to the blackboard. You will need more light to get the same exposure, but, if you don't have more lights, more exposure might still work.

  • Will give it a try, it is harder than expected. I will never watch a Good Will Hunting in the same way again, amazing how hard it is to pull off good light of such a simple object! – Algeboy Jul 21 '20 at 21:41

Large area soft lights. Bouncing lights off the ceiling is a good idea. If you don't have soft lights, you can soften them by placing a diffuser in front of them. Shower curtains work well for this. (Be aware of fire risks if you're using hot lights). Good explanation of hard vs soft light here:

Shower curtain diffuser: https://nofilmschool.com/2014/09/super-cheap-versatile-diy-diffuser-shower-curtain

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