The There Are No Stupid Questions Thread

Hi did not think that holster would but had long lost that holster may pick one up

Hi. A noob/stew-pid question.

What do the acronyms XHP-50.2, and SST-20 stand for? “Extra High Performance/Power?
Super Standard Transmittance?”

Also, what significance are the numerical designators. Especially the .2?

Thought I’d ask. :student: not “Cree Extreme-High-Power XHP LEDs”

As for the SST… I have no idea. Maybe contact them and ask

SST - According to the datasheet it means “Surface Mount Package”. Though they used SFT for flat surface-mount LEDs too. By flat I mean that they didn’t have a dome.

.2 - this just denotes the second generation. In all Cree LEDs this means a completely different LED with much different properties. Sometimes better than the original, sometimes worse, often it depends on application.

Fielding a few more questions, please.

What does a dome on the LED do, protect, and desensitize? Is that why domes are
Shaved, to allow full Lumens to shine thru? :innocent: Allow more throw?

Is it a noobe possible task?

No, it improves light extraction.
That is: die emits some light, it enters silicone dome or a flat piece of silicone. Then travels across the silicone and hits silicone-air edge. At this edge some of the light passes but some reflects back.

With dome, light hits the silicone-air edge more perpendicularly. This reduces reflections and improves light yield.
Typically domed emitters give 10-20% higher light output.
But they also give less throw, much less.

See this to get a full explanation:

The dome basically acts as a lens that enlarges the size of the LED’s phosphor relative to the reflector.

How easy removing the dome is depends on the particular LED. Some can be sliced easily, some should have the dome removed with a chemical solvent as physical cutting will damage it.

Do many people here really examine those highly technical graphs (like CCT comparisons over runtime) that the most technical reviewers put in their content? I am impressed with the scientific and mathematical minds who endeavor to produce all of that work, but… I often wonder if it makes sense to only a very select few of BLF members. It often feels like such highly technical reviews require far more effort to produce than the very light given is even worth.

I do appreciate some graphs, like runtime on various cells, but in the end the aggregate performance numbers are my main interest (total runtime on each brightness level).

Data are always good.

If people are willing to make those graphs, I’m always happy to have them. I’m an engineer and usually know how to interpret the graphs. If not, I can always ask :slight_smile:

Bear in mind that those graphs also supply the information that the “select few” can use to answer questions accurately and give good advice to the rest of us.

A lot of that info is relevant to choosing what emitters/drivers/optics/etc. work best in combination. For example seeing that the Nichia 219C has a low forward voltage means you do NOT want to put it in a single-emitter FET light. Some of the other info shows that new emitters render old ones obsolete (or that they don’t) based on things like color quality too.

Furthermore it shows how LED tech is advancing and just what kind of improvements/disadvantages things like the jump from the design of the XHP50/70 to the .2 versions offer.

I have both the Astrolux ft03 versions. (sst40 & xhp50.2) I’m looking to dedome one of them. Which would be the best to dedome?

Keep the sst 40 dome on the ft03 I cannot speak for the 50.2

The XHP50.2 will be an easier dedome job since it’s a flip chip (no bond wires in top), on the SST-40 you’ll have to be cautions of the 2 bond wires plus the ESD protection diode.

Also, if the sst-40 in your FT03 is anything other than cool white it’ll be impossible to replace if you mess it up.

Wouldn’t dedoming the sst-40 gain more throw than the xhp50.2?

Well yes, but like I tried to explain its a notoriously hard LED to dedome and the supply of non CW SST-40s is literally non existent. If you mess it up on your first try you can not get another.

The dome may also serve as a chemical seal to protect the phosphor from eventual contamination by volatiles coming from the soldering and heat transfer goop.

If you remove the domes you can buy a paint-on liquid sealer and apply that. I used to have a link for that stuff but can’t find it now.

There’s a lot written about failure modes for LEDs, problems that will start to appear over time (only after 10,000 hours, of course, trust the sellers on that).

That makes perfect sense. What about a partial dedome? Not going too low as to harm the bond wires, yet still leaving a protective layer to protect the led?

The word “dedome” implies removing the whole thing. With certain leds you can just slice off the top leaving a thin layer. This has almost the same effect. Candela increases a lot. Some leds are harder to slice than others and the results can vary. With the 50.2 folks recommend a “slice and dice”. You slice off the top and then slice off the sides. This is supposed to help remove some of the yellow corona and or spill, but when I tried the spill light stayed yellow.

It’s always best to have a spare led in case you don’t like the results or accidentally damage it.

Thank you for the info!

That’s good to know. I guess that’s why the Nichia 219C has been popular for use in triple and quad emitter designs?