LED tint variation between hotpot and spill, what's the deal?

[Theorem29]

I suppose you could call me a tint snob, as I tend to be obsessed with it in general and often find most LED tints (neutral, warm or otherwise) not entirely ideal for one reason or another.

One of the biggest issues I have with most LED lights is the large variance in tint between the hotspot and spill. Most notable are the new XM-L lights I have owned and currently own, they seem to be the worst in this regard.

An example of this is a cool-white XM-L having a distinctly yellow hotspot with bright blue spill (two nearly opposite tints on the spectrum of color), and this becomes very obvious if you hold the flashlight flat against a wall and turn it on. So far of all the XM-L lights I have owned, they all had this problem of having major tint inconsistency throughout the entire beam profile (spill, hotspot and corona). Everytime I use one of my XM-L lights, my eyes immediately pick up on the contrast between the two, and it tends to bother me quite a bit.

The XP-G lights I own also have this problem, but it is much less significant. However on some old surefire LED's (of unknown emitters) and random incandescent lights I have lying around, they all have very consistent spill and hotspot tinting. So perhaps this is mostly limited to Cree's recent LED's (XP-G and XM-L).


So this begs the question, why? I tried searching on the subject and came up empty handed, but I would really like to know what causes this. I personally think that this is one the biggest handicaps for LED's, alongside lower CRI and tint, when comparing to incandescent lighting in flashlights.

 

[Imon]

The yellow corona you describe that separates the hotspot and spillbeam I believe is actually a product of how the beam is shaped. If you were to take off the reflector and/or lens of that XM-L flashlight and shine it again against a flat wall I'm pretty sure you'll get a consistent cool white to light bluish tint.

I think this is an issue that does bother some people but I think the emphasis of the flashlight designer wasn't really tint consistency but rather reflector and/or lens performance.

 

[GaAslamp]

The yellow corona you describe that separates the hotspot and spillbeam I believe is actually a product of how the beam is shaped. If you were to take off the reflector and/or lens of that XM-L flashlight and shine it again against a flat wall I'm pretty sure you'll get a consistent cool white to light bluish tint.

Perhaps what is happening is that the blue light that is emitted toward the sides of the LED die--from its front surface--are effectively passing through a greater thickness of phosphor, which converts more of the blue light than usual into other spectral colors, thereby lowering the color temperature of this portion of the light, making it appear yellower. This is sort of analogous to how the Sun looks a lot warmer in tint than it really is when it's close to the horizon and its light has to effectively pass through a greater thickness of atmosphere. The relatively warm side emission of the LED is then reflected by the reflector (of all things 😉) into what becomes the hotspot, making it appear somewhat warmer and yellower than the spill overall.

As for why this effect is more pronounced in some flashlights than in others, it may have something to do with the radiation patterns of their LEDs, the packaging of the LEDs, and the design of the flashlights' optics. In general, LEDs that emit in a wider pattern and more brightly to the side will more strongly exhibit the effect. In addition, some LEDs are more exposed to the side, while others may line the perimeter of the die with a diffuse reflector that blends the warmer side emission with all of the other light and directs it out the front, making for a more even and consistent color temperature. Most of the latest LEDs used in high-end flashlights these days tend to emit in a wide pattern that is fully exposed, which is why variations in tint within beams are common. That's my guess, anyway.

 

[jrk]

I recently received an XM-L Neutral headlamp where the effect was so pronounced that I returned it. The emitter was housed in an LOP reflector with a 12 / 80 degree spot-spill. In that case, the spot was quite warm while the spill was nicely neutral. This occurred at all lumen levels. I think that my perception could have lived with either tint, but not both in the same beam. I have other XM-L based reflector lights with perfect mono-tint beams. I'm looking forward to this discussion.

 

[the.Mtn.Man]

...this becomes very obvious if you hold the flashlight flat against a wall and turn it on.

This reminds me of the old joke:

"Doctor, it hurts when I raise my arm above my head!"
"Then don't raise your arm above your head."

 

[Theorem29]

This reminds me of the old joke:

"Doctor, it hurts when I raise my arm above my head!"
"Then don't raise your arm above your head."

Ah, a grand example of fixing a problem without actually fixing the problem. Touche Mtn.man 🙂

All kidding aside, the contrast between the two is simply exaggerated when doing the flat wall test, it is not a requirement to see the difference, even looking for something behind a dresser or using the light in a confined area make it very apparent.

@GaAslamp: Interesting theory, you may be onto something there. Because from what I gather by looking at my LED lights, is that the spill is when you are looking directly at the emitter with no reflector enhancement, and the hotspot is the reflection off the reflector from the sides of the emitter.

It is still strange though that the tint can change so drastically. It's as if I were to get a red LED, stick it in a reflector, and somehow the hotspot is blue.

As suggested by Imon I have confirmed his theory, as I have a MXDL DX light that can have its LED be exposed with no reflector. The weird thing is when I look at the LED from the side, it is yellow, but if I look at it from above, it is blue. Now it makes sense to me why the hotspot would be more yellow, since it is primary comprised of light from the side of the LED, whereas spill is from looking at an LED from above.

Still though, would be nice if LED manufacturers (looking at you Cree) could offer an LED with 100% consistent tint from all angles of the LED, because I do have LED's capable of doing this so I know it can be done.

 

[Norm]

Moved to LED - Norm

 

[Harold_B]

Regarding your search results, I'm surprised these didn't show up:

http://www.candlepowerforums.com/vb/showthread.php?309724-XM-L-Color-Separation

Thread lost from the crash:

http://www.candlepowerforums.com/vb/showthread.php?307909-XM-L-tint-variation-across-the-beam

There are probably more but the first one explains a lot.