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Comment by ThePhysicist | Hacker News Reader

It's pretty amazing how powerful small chip-on-board (COB) LED modules have become. I recently built a 6000 Lumen aquarium light with 7W - 850 Lumen COB modules. The active surface is just 5x5 mm, so the illuminance when held in front of your eye (1 cm² surface) is about 17.000.000 Lux, roughly 100 times brighter than direct sunlight on a summer's day at noon (so don't do it as it might be the last thing your see with that eye). And those modules are actually at the low end of the available power, you can get up to 35 W packaged into such a tiny COB, which is just crazy. Good thermal anchoring of the chips is therefore paramount, as they will quickly heat up to the point of being destroyed by electromigration if they're not properly cooled (and I imagine for the control electronics in the light bulb the high temperatures are also not beneficial). Anyway, it's still impressive how much light you can get out of such such tiny devices.

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This has impacted the video production business in a positive way over the last couple years. Previously, we had to put on gloves, find a 30 amp fuse and use hot tungsten lights to put together a production. Now folks are using high-quality LED-based lighting, running off battery power, and significantly smaller than a generation ago.

850 lumens per 5x5mm square isn't overly spooky. It's only about 1/50 as bright as the sun itself, and comfortably under the level where you risk burning spots on your retina.

I'm not really sure what happens when you hold it right up to your eye. I'd be worried about the total amount of heat my retina can dissipate more than the lumens, but I have no idea what that number actually is.

It depends on the distance. Even at 1 m an 850 Lumen source with 120 degree radiation angle is painful to look at, and at 1 cm distance the illuminance is 10.000 times higher again. The peak illuminance of the sun at the surface of the earth is around 120.000 Lux (Lumen /m2), significantly smaller than such an LED when viewed from a short distance.

I just don't think illuminance is the most important number here. Luminance is more relevant to eye damage in the vast majority of cases, because that measures the spot intensity on your retina. After all, the sun's light would be completely harmless to the eye if spread over a five degree circle, despite having exactly the same illuminance. (Well, you would want a UV filter, but the brightness would be completely neutered.)

The sun gets up to 1.6 billion candela per meter squared. 850 lumens in a 120 degree cone, from a 5x5mm surface, is about 11 million candela per square meter if I did the math right. With a retinal danger level around 100 million, you'd need some very strong secondary effects to damage yourself with an 11 million source.

Secondary effects like boiling your entire eyeball, or overloading the cornea while the retina is safe. And I don't have the knowledge here to say if that happens, or if your body can easily sink a spread-out watt of light and nothing bad happens.

I don't think that's right. If I hold this LED right in front of my eye the retina will be exposed to the entire 850 Lumen. Sunlight with an illuminance of 120.000 Lux on the other hand yields around 12 Lumen on the retina, assuming a surface of 1 cm². It's the relative distance to the light source that makes this so dangerous.

Even if you only let a pinhole of sunlight in, enough to illuminate a surface to 120 instead of 120k, it will burn your retina.

Normal retinal spot burns are not about total light coming into your eye. They're about incoming light per solid angle. A light that is bright enough per area is dangerous at any distance, until it's so so far away it looks like a single point.

I'm sure that there's some level at which total light is damaging, even if there are no super bright spots. But that level is going to be significantly higher than 120k. It's maybe a range this LED can reach, maybe not.

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