Question on Surefire L4

Just received an L4 that I purchased on EBay.
Quite impressive since the only other Surefire I have is the 6P.
I was just playing around with it inside and was wondering about its beam at about 6 feet.
Exactly in the middle of the hot spot is a slightly darker area. Then around that is the hot spot which falls off quickly. Are these lights supposed to be slightly dimmer in the middle of the hot spot?
Excuse my ignorance but this is the first LED light I bought and really didn't know what to expect.
Thanks,
Lenny

Hi Lenny:

You are the proud owner of the L4 donut beam. Don't worry about it. It's a feature and not a bug.

Thanks.
But what is the purpose of this feature?
Or were you being sarcastic on the "feature" thing?
I've seen beam shots of the L4 elsewhere that didn't show the dark spot in the middle.
Maybe it's just the limitation of the camera taking the pic?
Anyway, thanks for the response.
Lenny

...actually it's there to help CPF members improve their typing skills! The 'black hole" generates a lot of comments and puzzlements! /ubbthreads/images/graemlins/crackup.gif

That "black hole" is normal for the L4 - and a few other lights using a 5watt Luxeon
mine also has the donut beam
it is due to the fact that the LuxV has four little 1.2 watt chips on the emitter - and there is a tiny little gap in the centre where the four meet - this little area and the fact that your surefire is perfectly in focus means that there is a slightly darker spot in the centre of the beam
have a look at the emitter in your L4 and you can see what i mean - this is an image that i have access to at the moment and it shows the dark area in the middle of the emitter.

hope this helps

the above is correct to my knowledge - if it is incorrect im sure someone will correct me later
Image by Shelby Chan
Mr Chan - if you have a problem with me using this image then please PM me and i will replace it with one of my own - the only reason im using it is that my image is on my computer, and im not using my computer at the moment

All my L2s have that slightly darker center area. Its only noticable on a white wall and at a certain range. In actual use, its not noticable.

You should worry if the little hole "IS NOT" there.....if it ain't there, your reflector is probably not set in well, or focused properly.

I am of the understanding that the partial null or area of less intense lightin the center of the beam is for a different reason than reported above. If you consider the area shown between the two red circles in the modified Shelby image below, you can see that most of this area is void of any illumination from the die. Any other smaller concentric ring sections, smaller in diameter will contain full illumination from the die. As I have been lead to understand, it is this outer perimeter that is missing full illumination area that causes the relative null in the center of the beam. If the 5W die were circular, it would not have the "donut" artifact. The donut is somewhat aleviated by the textured reflector which blends and blurs the projected image. Note, with a reflector, the projected image is Not a mapped image of the die like you can get from a properly focused optic. I guess you could say the image projected by the reflector is inside out with the center of the image projected to the perimiter and the perimiter of the image projected to the center.



Well anyway, this is *my* understanding of the physics involved. /ubbthreads/images/graemlins/icon3.gif

The "doughnut hole" in the center of the L2, L4, L5, and L6 beams is perfectly normal, and is nothing whatsoever to be concerned about. This is a perfectly normal configuration for any flashlight using a 5W LS LED and a reflector, and should be ignored.

McGizmo - you are probably right - however the way that it was explained to me was the way i described - i may have misunderstood however and your way of describing makes more sense to me - although it may be a combination of the factors that we both described
funny how we both used the same image though - it is probably the best image to use for describing what you are talking about for a LuxV

Ya learn something new every day!
Thanks again guys.
BTW, I absolutely love this light.
Lenny

It is kind of annoying, though, that the place where you want the light to be brightest is the place where it is darkest. That seems like a lot of money to spend on a flashlight that has a black hole smack dab in the center of the beam. I would think that with all their smart engineers and smart people and smart corporations working on this technology that someone wolud be able to fix this "feature that should be ignored". For that much money to be spent on their lights, I don't feel I should ignore any functional imperfection. BTW, I also have seen beam shots of 5W Luxeons in L5 and L6's that have no doughtnut holes.

socom - that's because the L5's and L6's have different-shaped reflectors from the L2 or L4, which minimize this design problem that's inherent to the LED. The issue is as Don has outlined above - the square-shaped 4-emitter array that the LuxV uses for its incredible output - you CAN get rid of that hole, as he has done in his own LuxV LE builds through the use of a shim against the reflector, but it costs you quite a bit of throw.

In other words, if you want a wall of light that throws fairly far (as the L4 and L2 are supposed to), then you'll have to give up something - at least until Lumileds perfects a method of making a huge emitter which acts like the LuxV's but without the problems created by that shape.

Echo63,
I was originally of the same understanding as you but with further comment from others and consideration, I have come to believe the case is as I have stated. Playing with older LED's that had distinct color differences in the center beam and corona with an optic, I noticed the color difference swapped when the same LED was coupled with a reflector. An optic can be used to provide a focused image of the die, just take an acrylic ball to a LED or look at the beam pattern of the X200 for instance. If you take a perfectly smooth reflector, you don't get a focused projected image of the die, that is to say, the beam is not square like the case of the optic.

Actually now that I think about it, I am changing my story to some speculation based solely on what makes sense to me! I am abandoning the projected null of my explaination above. /ubbthreads/images/graemlins/ohgeez.gif

I propose that there is a brighter ring around the center spot as opposed to thinking of the center of the spot being dimmer! Duh!

Look at the four bright corners of the die that are obviously off center and away from the focal point of the reflector. Where do you suppose light from these corners will end up in the projection? I propose that they will end up in a ring of light in the projected beam! If you add this ring of light to the beam that is projected from the balance of the die, I believe you will end up with a total concentration of light in the beam where there is a brighter ring about the center.

You can confirm this for yourself if you have a LED and loose reflector. If you get the LED centered and in the focal plane of the reflector and then move it off axis (off center) but still in the plane, you will notice that your projected beam will develope a null or donut. The further off axis, the greater the null.

Cool! I think I finally understand how the light source in a reflector is projected! Duh! /ubbthreads/images/graemlins/blush.gif

If a point of light (source) is in the focal plane but off axis, this point becomes a projected ring of light, not a projected spot.

So ... for the technically challenged ones ... it's the four corners of the LuxV that actually put a brighter ring of light around the center (because they are off-axis horizontally) ... resulting in a donut hole?
bernhard

Bernie,
Yes, this is my take now that I have really though about this and done some empherical testing.

My Aleph II head with 5w Luxeon arrived with a perfect beam, no donut hole at all. Once I removed the shim that produced this "perfect" beam there was the good old donut hole, and the beam was brighter and better. I've actually come to regard the donut hole (assuming it's fairly subtle and not glaringly abrupt) as a positive sign.

Brightnorm

Where is that shim installed and how does it help in removing the donut hole?? /ubbthreads/images/graemlins/thinking.gif

It's a metal ring between the reflector and LE that keeps the LED somewhat backed out. I don't know the scientific explanation of exactly why this works.


BN

Ok, got the idea. Thanks. /ubbthreads/images/graemlins/smile.gif