Measuring a dedomed MT-G2

Flashlight Modding and DIY Parts
1

I'm notoriously interested in the luminance of LEDs, because this value directly affects throw.

The MT-G2 is known to be difficult about dedoming, probably due to the bumpy surface and some adhesion of the posphor to the dome. Usually I'm more than happy with the dedoming results with 50/50 acetone/thinner, with Cree XPs and XMs. But I didn't want to repeat the efforts of others to see some phosphor go away.

Instead, I tried the cut-off method. I guess a basic requirement is to cut off the dome as close to the die as possible.

As I understand it from DrJones' post and some of my experiments, I guess that in this case the die luminance is increased by photons being reflected from the silicone/air boundary.

I found an appropriate washer as guidance for the scalpel. The dome is at the left. Although I tried to cut it off in one single smooth move, the surface was both slightly stepped and opaque. As attempt I tried to polish the cut-off half. It would work with the outer shell, but the inner part is too soft. So I just put some clear silicone grease on the surface to even it up. This worked very well. The photo doesn't show it properly, but to the eye it's the same effect like water on a sanded glass surface.

I have no device to measure high luminance (such devices are more than expensive) so I calculate it from the luminous intensity (candela, e.g. lux@1m), divided by the apparent die size. I determined the latter by counting pixels in these images:

I measured lumens with a small integrating sphere (PCE-LED1) and lux at one meter with an ExtechLT300.

He're the results:

The luminous intensity decreases to about 85%, that's a well known result for the XP and XM LEDs, too.

The luminance (lux@1m divided by apparent die size) increases by a factor of 2.2. Well, that's a lot!

With XPs and XMs (dedoming with solvent) it's a factor of about 1.9-1.95 at best.

I might as well have some errors in my procedure, however another guy in the german-lang TLF measured a factor of 1.93 after dedoming with the gasoline method, and he lost some phosphor. So I'm not too far off.

Here are the numbers:

MT-G2, P0, Cu-star, added: 5000K

 current [mA] lum. intensity [cd] lum. flux [lm] luminance [cd/mm²]
with dome 43mm²
1.1 258 791 6.0
5 21.1
without dome 15.7mm²
1.1 203.5 658 13.0
3 - 1548 30.8 (calc.)
5 725 2310 46.2
9 - 3180 64 (calc.)
10 - 3270 65 (calcl.)

What's the fuzz about this luminance? Max-Lux in the spot are determined by only

1. luminance (determined by the above, or indirectly by lumens, in connection with current and flux-bin)

2. reflector/lens diameter (or better apparent disk area showing the emitter) as seen from the location of the spot

3. losses (mirror reflectance, lens transmission)

It's quite some fun to predict throw (in terms of max-lux) from just reflector diameter and LED-current.

For completeness a graph with lumens (without dome)

I didn't want to find out about the current limit, especially as my switched-mode power supply unfortunately tends to spike when switching on [edit: rather meant increasing current, which is only possible stepwise]

Let alone that djozz and Match already have take some very fine measurements.

2

Trim away the phosphor that's not directly over the dice. It still gets excited somewhat and makes the apparent die size larger which is the very thing you're trying to minimize with de-doming.

3

Wouldn't dedoming decrease lumen output, but increase intensity?

In your case, it's more of partial dedome. I haven't heard any theory on what that does to total lumen output or intensity.

4

How did you attach the washer to the MPCB so it doesn’t move when cutting? Soldered, glued? I wonder why did you loose lumens if the dome is still there, you just cut a part of it off, did the cutting disturbed the phosphor layer?

5

I want to keep all phosphor, but cut away as much dome as possible. So I've cut off the dome almost directly above the phosphor. The die size is not increased anymore, because the surface is flat now (no magnifying-glass effect anymore).

I consider it almost as good as a full dedome. Hard to tell how moch more luminance could be achieved. But a factor of 2.2 even now seems a lot to me, that's why I think there's not much room left for improvement.

Yes, I glued (and partly cut) it for exactly this reason: to have all fixed tightly to get a proper cut-off. I believe I loose lumens for similar reasons as a complete dedome. (I can't tell if I also might loose a few lumens at angles very close to 90 degrees, missing the integrating sphere.)

Update: and afterwards I removed the washer and added a 2nd screw for tight thermal connection.

6

But the phosphor covers the entire square substrate, and sideways leakage causes it to glow. Which makes the light source larger than it would be if the phosphor only covered the die area. You don't need phosphor anywhere but over the die area.

7

I am wondering how do you take it off without disturbing the die, wires, etc. comfychair, I haven’t had any experience with a MT-G2 led…

8

Cut the straight lines with razor, then peel/scrape off with something non-scratching.

9

That is a nice increase and your dedome looks very good. You might be interested in this thread. Thanks for the report sma and welcome to the forum.

ComfyChair. I imagine the removing the surrounding phosphor cleans up the beam and helps reduce the yellow corona, but I imagine leaving the phosphor doesn't decrease the intensity of the hotspot or throw.

10

Thanks for coming back to me, sorry, just didn't get it at first. No I don't believe that a significant amount of light is "stolen", just because I can't "see" it.

In fact I saw that thread, thought it might be interesting to compare your results with calculations :-) , but wanted to post this and check what others think at first. After all, it's not a complete dedome, even though I guess it might be equivalent...

11

No, that's still not the point. The area emitting light is larger than it needs to be, it's not that in any way it's blocking anything or reducing output. The phosphor not directly over the die area is glowing, and the whole point of de-doming is to get a smaller light source. The extra unnecessary phosphor is working against that goal.

12

Ok, I got you. In fact I strongly believe this "smaller" is not the point.

I thought so, too, originally. The idea is just heck of attractive, but it doesn't pass a special experiment:

The result of "3. reason for increased luminance" in this article shows that reducing the apparent emitter size does not suffice.

Even better, the dome cut-off at half height slightly increases luminance. This supports the idea that it's about reflected photons, only a few in this half-height case. And cutting off the dome almost at the phosphor layer might be roughly equivalent to completely removing it by solvent.

13

That's a great article you linked to in the previous post. I admit, some of your work goes over my head still. I need to spend some quality time reading Dr Jones' and your articles at a slower pace.

Have you had a chance to get an impression on what impact your dedome had on tint and/or CRI?

14

I followed this advice back in November and got results. Whiter light, tighter beam profile, more throw.

15

Can you give us the before and after throw measurements?

I didn't mean to sound like I was knocking CC's idea above. I think it's a great idea. Just doesn't seem like it would improve throw. I agree it would probably make a more throwy-like beam profile.

Personally, I prefer warmer tint and don't mind a less focused appearing beam. I keep the surrounding phosphor because I figure it gives a few more white/yellow lumens. I'm not fond of the yellow corona when it hit nearby objects, but I think it may improve CRI down field.

I guess I should try it in one of my lights sometime. If I do, I will take before and after measurements.

16

This is a favorite light, the Solarforce M8, and it was done this way before Bucket made the big 8 ounce copper pill for it. I then traded the OEM pill with this emitter to a friend for his M8, so I don’t have it any more. But I should have those measurements, gotta get back to my own computer though….

17

No, unfortunately I had no colorimeter, yet, and I had not focused on the color difference.

I don't remember any unpleasent impression. The result looks ok to me.

5000K with dome is quite warm anyway, and

...oh wait, I hadn't noticed until now that I've only mentioned the flux-bin.

At the time of purchase, 5000K was the only variant for the P0 flux-bin. I'll add it above, too...

And I guess not only my piece, but 5000K in general (let alone even warmer bins) is uncritical.

18

Perhaps my favorite thing about dedoming MT-G2's is the tint shift and apparent improvement in CRI. I started doing it for throw, but feel in love with the resultant tint.