I think I almost understand what domes on a LED die are, but what’s dedoming and why?
Dedoming means to remove the domes (which is easy for the Nichia 519A, hard for others and impossible for many more). It makes an LED a little more throwy, usually lowers CCT, duv and brightness.
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Thanks. So the main reason to dedome or to use a LED without a dome is to make it better interact with the reflector to concentrate the beam hotspot?
Sometimes. But mostly it is done to lower the duv (make it more rosy).
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Dedoming reduces the output by 10 to 15 percent. Maybe you should take this into account.
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Not really. I’d even say most reflectors are designed for domed leds and dedoming changes slightly the light distribution wich sometimes results in rings, artefacts or a hotspot with surounding ring. The cure for that may be playing with washer thickness (often 0,1mm makes a real diffrence).
Dedoming purpose is mostly to increase throw and sometimes it also reduces tint shift in corona/spill.
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Before/after dedoming a 519a:
You can see the hotspot became a tighter and CCT dropped from 4000K to about 2900K. As already mentioned, you will have 10-15% lumen loss and small artifacts in the beam, but not too noticeable in-person.
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And what you increase in throw more than makes up for the loss of lumens. It will appear a lot brighter.
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A reflector is technically only perfectly focused at one minute point. The smaller the light emitting area is, the better the LED will focus because more of the area is within the focal point.
The dome is a bit like a reading magnifier glass; removing the dome reduces the apparent size of the light emitting surface, so theoretically makes it “throw” better.
I understand this is counterintuitive - an XHP-70 with it’s massive light emitting surface and gobs of lumens logically throws further because it’s more lumens, right?
Wrong.
The lumens are higher but they’re “sprayed” all over the place instead of being concentrated into a narrow beam.
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I’ve only dedomed 519As and that was to make the tint rosy. They are very easy to dedome.
I’ve also sliced domes to make them less green or to make them slightly rosy. With 519As the slicing makes the tint slightly more rosy vs when they are dedomed, which for some can be too rosy. With SST-20s and LH351Ds, the slicing down to as much possible will help the tint to be less green, neutral, or slightly rosy.
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It’s possible to dedome an LH351D?
One can slice it to the same effect, but not dedome it without damage. It does not have an intermediate glass layer that separates cleanly from the silicone dome.
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It looks like there is an instructional video on slicing this very LED here:
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It’s a little fiddly, but slicing an LH351D can give some pretty good results. It lowers DUV & CCT and makes them much throwier. You can adjust how low you slice them to vary the results (doing it bit by bit works well). It can mess with the beam a bit though; with some reflectors it will create lots of rings etc
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It seems quite easy to do… I think I’ll try it on my FC11, now I see it almost like a rite of passage.
In the next few days, I’ll look for washers with the appropriate thickness.
LH351D:
The part not to be touched is the last 0.43mm, right?
You can also use two brad nails (or anything similarly shaped) of the correct size to get the height you want. Just put them on either side of the LED and they act as “rails” for the blade to slide on. Just make sure they don’t move as you slice.
An advantage with this method is they can fit between the LED and solder blobs on small MCPCBs that would otherwise require you to remove all the solder to get the washer to sit flat.
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It might be a good idea to leave the extra 0.20mm as well, there isn’t a clear demarcation between phosphor and silicone so you can very easily damage the LES
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Excellent question. Check out this fantastic post from DrJones that details the purpose of the dome and what dedoming accomplishes.
DrJones post is 12 years old, but it is still the definitive explanation of dedoming related to flashlights.
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Here more info on LH351D dedomed, in relation to slicing height
here is a good writeup: