I like to experiment, and was thinking of trying this for quite some time. A few weeks ago I did the actual experimentation, but the results were a bit disappointing so I hesitated doing the write-up. But I believe it is just fair to not just report your successes, but also the failures, and it involved quite some work, so here we go :-)
The idea was to deposit a silver layer directly onto a led-dome, to accomplish that the light going sideways (and is lost when the led is used in a aspheric lens flashlight) is reflected back to the die and so increase the illuminance. This has been succesfully done before with a ring shaped globular reflector outside the led, it is even patented, with a fancy name. Depositing the reflecting layer directly on the dome has been mentioned once on CPF a few years ago (I looked it up), but as far as what I can find, no one has actually tried it.
The chemical reaction that I used is the so-called Tollens reaction, used traditionally to make mirrors until somewhere in the 20th century, then they started making the now common aluminium mirrors that do not have that slight yellow colour. I demonstrate the reaction every now and then in the seconday school where I work as a technician, this is my first attempt of trying it on something that is not glass. Briefly: the substrate is cleaned with soap and then with 67% nitric acid, a silver nitrate solution is added, then a 25% ammonia solution, this gives a brown deposition, keep adding until clear , add a potassium hydroxide solution, brown stuff is deposited again, add again 25% ammonia until clear, then add a glucose solution and keep stirring until the silver layer is formed.
The first attempt was on a XM-L on a 20mm star. The result:
So to be usefull the top of the dome had to be cleaned. Further, the led had to be reflowed on another board, the silver gives shorts everywhere. I scraped the silver off th top of the dome with a tooth pick:
The cleared area of the dome was not crystal clear after that, but it was not too bad. I reflowed the led on a Sinkpad and tested it in series with an untreated XM-L to see any difference:
The treated led had a slightly warmer colour, but it was nothing obvious, and on the picture there is no visible difference leftover. It seemed that the treated led had a bit less illuminace than the untrated one, this was not promising for the method.
Second attempt:
A XM-L2 3C was used, and I wanted the clear part of the dome untreated, so this time I covered the top bit of the dome with a piece of 'parafilm' , which is thin parafin sheet. It adheres a bit to the dome without really sticking to it:
After the silver plating (the solder pads and lettering did not like the nitric acid treatment :evil: ):
That looks much better than the first attempt! Here it is compared to a untreated one:
But reflowed on Noctigons this time. The tints compared did again not look much different (led tester at 2mA):
I soldered the boards in a Cofly 18650 zoomie with a Qlite revA driver to test the numbers, first the untreated led board, then the treated one:
XM-L2 3B XM-L2 3B with silver ring attempt 2
current: 2.98A 2.97A
zoomed out: 586 lumen 533 lumen
zoomed in: 306 lumen 305 lumen
throw zoomed in: 13.1 klux at 1meter 14.4 klux at 1meter
So that is not all that impressive, is it? A slight improvement of throw, but nothing like doubling the number, what I hoped for of course :-(
It is impossible to see the inside of the silver layer from outside the led, so I dedomed the treated led to see how the reflective layer looked like from the underside of the dome. And it looked a bit golden, not good. Perhaps the silver deposition had been too fast, and some brown dirt had come into the layer.
Here's a stereo picture of the dome from the underside. (and it is a difficult one: the removed dome is cut in half and put under water to be able to see the layer, hence the air bubbles everywhere)
So I went a ahead with attempt nr. 3, I had the untreated XM-L2 3C leftover, it was already on a Noctigon, so this time I wrapped the entire board in parafilm, put a the piece on the top of the dome again, leaving only the area to be silver plated exposed. This time I forgot the nitric acid claeaning, but that did not seem to matter in the end. Usually I heat up the reaction mix a bit to help the speed, I did it now at room temperature, letting the silver layer deposit much more slowly.
Looking even better than the last time. but the numbers still disappoint: current 3A, zoomout: 585lm, zoom in: 330lm (= slightly increased), throw at 1meter: 15.7 klux. So the numbers are a bit better again, but still not impressive at all, compared to the untreated led, the throw went up by only 20%.
I had the dedomed led leftover from the second attempt, I measured that one as well: current 2.94A, zoom out 512lm, zoom in 266lm, throw at 1meter 25.4 klux. Now that is what I call a real improvement. The silver plated led did not do anything close to what dedoming does.
At this point I gave up. I can not see how I can do a fourth attempt that is any better than this. And I ran out of spare XM-L2's.
Still I got way further than I expected beforehand, I even was not sure if the silver would deposit at all on a silicon dome, and if the parafilm would prevent the silver plating on the covered bit, and if the 67% nitric acid would destroy the led (it did not, but also appeared not be be neccessary in the end). It was fun doing all this, but quite a disappointment that I did not succeed, especially not because I have many ideas why it failed but no idea what is the right one.
I hope you enjoyed reading about these wanderings outside the commonly done mods, even though it failed :-)