Painted collar

Yes, if you’re chasing the last percent, painted sides should work.
Actually it could allow you to avoid dicing at all. Just paint sides and you’ll eliminate side emissions. Maybe not the whole tint shift (because side phosphor may generate photons that go a bit upwards).

Collar tint (and CRI) issues aside, couldn't we also attain increased intensity/output by hitting the die off-angle with one or more blue lasers focused onto it? Without collar, of course. Plenty of available blue laser emitters with the same frequency as the emitter's blue die, there would be no tint shift and no CRI crapping (I hope).

Cheers ^:)

I forgot to say that before:
A very nice idea! Would really like to see it implemented with XHP70.2.
I wonder if painting the lower part of reflector would work?

I don’t expect any of this will be very successful. I expect The larger the difference in collar and die size the greater the return of side emissions directly back onto the phospohors.

A spherical reflector tries to reflect photons back onto the phospohors for recycling. The smaller the reflector the greater the change in incident angle across the die. Same reason a big lense is better then a small one. The smaller the dome the more likely the light hits off angle and ends up either getting lost due to being absorbed in the dome material, heading out the opening at an odd angle, or not hitting the phosphors at all.

It should but I was reluctant to mess with reflector in any way (I know better), the emitter on the other hand will withstand more handling.

Plus when I first did this, I figured if it was a disaster, I could simply de-dome. With a reflector I’d have no way of removing the tint without handling which would leave marks.

I posted a questing about the same thing. I too am very interested to see if the leds themselves can be laser pumped.

That said, if it was only the blue light from the led being bounced back and reexciting the phospohors then we wouldn’t see a doubling of intensity with a RLT collar, there just isn’t as much blue phospohors in the already converted white light.

I’m starting to believe the best way to think of the led surface as a mirror. If you can return all the stray light you get a much higher chance of it reflecting in the right direction. A laser just tries to get more light from the led, that doesn’t happen. If you can return the lost light it can have a chance of going the right direction the next time, it may take hundreds of tries, but it will eventually. Laser pumping still loses the stray light and doesn’t make the led make “more” light. You may get “reflections” of your laser, so you added that power, but it too goes random directions.
I expect the phosphors are like inductors, they can become saturated and not release more light, but maybe they can “reflect” the light coming back on them.

Once concern I have is how reflective is the paint going to be that you put on the dome. What color paint would you use? White? Silver? Mirror spray paint?

If you use white, the paint will reflect light in all directions… not just back to the source. If you try silver mirror spray paint… that stuff produces a mirror surface on glass, but isn’t designed for spraying on silicone. Not sure what result you’ll get.

And in any event, I doubt these paints are designed for high heat. Is your paint going to cook and catch on fire? Someone should try it and find out.

Engine block paint would work fine, heat wise. They may even have a metallic paint.

I don’t think a metallic paint would be any better than just using white paint. In both cases the light will scatter everywhere and a darker metallic color will probably absorb more than white.

What you really want is a mirror finish on the inside surface where the paint sits on the dome so that light coming out of the dome is reflected directly back to the emitter and nowhere else.

There is some special spray paint available that creates a mirror-like reflective surface when sprayed onto glass. Not so sure it would on Silicone and I doubt it could take the heat.

What about silver foil?

Howbow that new Nichia with the glass dome?

Well, there probably wouldn’t even be a need to paint, since even without the paint there is light reflecting off the surface of the dome from the TIR effect. But even if we could apply a reflective coating, I’m not sure how much reflected light would get back on the die. First, because the LES is not a small point in relation to the dome, the light may or may not reflect back to where it can leave the dome. Second, the perimeter of the LES is transcribed on a circle (the base of the dome) which accounts for at least a 57% larger surface area than the LES. So if the coating and silicone were 100% efficient and the light is randomly scattered, it would have a 64% chance of hitting the LES. On some emitters that area is coated with phosphor, but that area would be under paint. So selection of LED would be important - maybe a Nichia with its hexagonal LES may be a good candidate. Third, since the LES is at least as large if not larger than the unpainted aperture, the light may or may not leave that aperture with a defined angle the way it does with an RLT. But I could be wrong, and this would be fairly easy and inexpensive to test.

Djozz has seen 20% improvement with a silver mirror which he claims to be better than mirror paint which in turn would be much better than white paint. I trust that djozz is right so I think that his result is an upper bound for what can be gained. Or close to an upper bound.

That’s not exactly apples-to-apples because he didn’t dedome. I’m not sure if the expected improvement would be higher with or without dedoming.

Yes, I share the concern that a mirror paint wouldn’t work for some reason. But performane of white one would be way worse. F.e. with SST-20 die area is 2 mm² while under-the-dome area is 6.9 mm². Furthermore some light reflected off white collar would escape up through silicone - at wrong angle. So I see only marginal potential for improvement unless we can make a mirror.

ADDED: another small loss on a white dome: light reflected towards another part od the dome would risk being absorbed again.

It could be the opposite - it may work better with an SST-20, since the emitter is a lot smaller than the dome, the light reflected off the mirrored portion would reflect back a little more like you’d expect it to. But then, you would have to rely on the accuracy of the dome itself.

You can simulate this on a larger scale - get a cheap COB LED on eBay or Amazon then make a RLT collar that’s slightly bigger than the LES of the COB, then you can get an idea as to what the light does as it leaves the aperture. I have a hunch that the beam angle will not coincide with the aperture angle, but I could be wrong.

I’m not sure if I understand your comment correctly. Do you mean that white paint might work better with smaller LEDs?

I think the idea in general may actually work better if the LES is considerably smaller than the dome. Otherwise the angle of the beam would be considerably wider than your aperture would suggest, which would indicate more light lost than necessary when focusing. But if the ratio of LES to dome is too small it’s also possible the dome is too inaccurate to be of benefit. So there’s probably a formula to determine an optimal trade-off.

I see. Think that the optimum for reflector collar is much larger than any factory dome though.
BTW, I see some AR coated hemispherical glass lenses available in China. :wink:

White paint won’t work, it scatters light.
You need a coating that reflects light in the direction it came from.
You don’t see your reflection when you look at a white wall.

Best is glas

No polished metal gives the high result

and white colour diffuse .

saabluster probably used this technology in the DEFT EDC LR flashlight he offered years ago.