Doughnut question...

Ok many of you probably laughed at this title as being a cop I should know all about doughnuts...

Actually I know most all 5 watters have the doughnut void in the beam center however I have a 3 watter that has a pretty pronounced deadspot from about a foot or more from the wall. Is the doughnut in a 3 watt caused by the reflector or is it the emitter itself? If I can somehow space the emitter farther or even closer from the reflector would it help any? I was wondering if the doughnut was like the luxeon lottery with tint in that some are better than others.

Robocop,

I haven't done much experimenting with Luxeons, but I'm pretty sure that a void i conjunction with a Lux III is caused by a defocusing issue. I don't know whether the Lux sits too far out of or to far into the reflector though. Actually I don't think this can determined without trying to move it, as the doughnut void can exist in either case AFAIK.

Jan

Robocop said:

Ok many of you probably laughed at this title as being a cop I should know all about doughnuts...

Click to expand...

:laughing:

I would think that this 'doughnut' is like what Jan said, caused by defocusing issue, since the same phenomenon is observed whenever I defocus my Minimag with Sammie and McR20/IMS20 reflector.

Also, if the reflector is originally designed for a incan light, the focusing point could be well above the opening of the reflector, and that could be a contributing factor for that.

One general principle I have found is whenever you look straight into the lux, and if the yellow reflection has filled up the whole reflector, then it is likely that your lux is in focus. If the yellow reflection could only cover a potion of the reflector, then I found that usually the yellow tightly surrounding the lux in the middle would yield the best result.

Hello Robocop,

It has to do with the relative position of the reflector to the LED.

I have observed that I get the highest lux readings if the null starts to show up at around 4 - 8 inches away from the wall.

An interesting exercise is to take a variable focus light, focus it to the brightest spot across the room, then hold it up to the wall to see how far away the null begins to show.

Tom

I like the chocolate iced glazed and milk!

Since we're asking about donuts:

I was wondering whether someone could explain to me how the "perfect" focusing of a LuxV works. I know it's harder to focus a LuxV because of the larger die, and that focusing on the "sweet spot" produces the "donut hole" (right?) Is the donut a projection of the space between the 4 die, then? Edit to add: how large of a reflector would be needed so the 4 die are reduced to a "spot" light source similar to the LuxI and LuxIII?

Kiessling just mentioned on a thread that the donut hole is a sign of properly-focused Lux (for maximum throw). Why wouldn't one get maximum throw out of a donut-less focus position (did that make sense)? Something doesn't make sense, especially considering that people seem to be getting different-sized donut holes.

yea, you'd think the 4 dies would act as just one big die for a large hotspot...

GreenLED,

Below is an image that I "borrowed" from Shelby and I drew in the red circles:



It is my suspicion that the donut is a result of off axis light that comes from the 4 corners of the large image of the 5W's die. If you were to mask the center section of the die out and just project the light from the source shown inbetween the two red circles above, I believe you would get a donut or circle beam as opposed to a spot or full circle. On the other hand, if you were to mask of the image portion within the two red circles and only project the round center image, I believe you would get a conventional spot beam without the donut. When you project the full or combined image of the center and the corners, you get the combined beam which does have a greater intensity. This used to be my take on the donut but with further consideration I would like to refine this even further and give creedence back to the fact that the reflector is truly closest to colimating the image that is dead center and coincident with the major axis; namely a dead space of the cross between the four dies.

If you were to turn off three of the four dies on the 5W and and check your beam, I believe you would find a donut because the center of "mass" or intensity of the image is basically in the center of the die and this center is significantly off the major axis of the reflector. If you then were to map the beams of each of the other dies individually, you would map out three additional donuts that partially overlapped eachother but not completely. There would be a net or sum "null" in the center of the combined beams.

If you take a single die LED and reflector and allign the image and focal point and then slide the die off axis but still in the focal plane, you will get a donut after displacing the die far enough off axis. I think the 5W in most small reflectors is the case of four images all sufficiently off axis to net the null or donut. Their projections are 4 circles or donuts and not 4 spots. As the focal length of the reflector is increased, these donuts would merge with eachother as well as have their center nulls diminish in size, I believe.

I think it would be very illustrative to have a light with a red, green and blue tiny smd LED mounted in a triangle and then take this image source and apply a reflector to it.

EDIT: I have seen a slight donut with a Luxeon III (single die) in a pairing with a small and very smooth parabolic reflector. I attribute the donut in that case to both the null line between the two innermost light bars of the die as well as the fact that the image is square so there is the four corners of light that do add their collective donuts to the projection. :shrug: Ultimately, I admit to this being conjecture and supposition for the most part as I have no real empirical or theoretical experience or knowledge that would be definitive.

You see donut holes when holdign a focused light up close to a wall because the beam has not converged at that focal point. My mag mods, focus very tight with no donut hole, unless you shine it close (inches) from a wall. On a foggy night you see a round donut of a beam in the air, then farther out it gets closer to it'self and converges into one solid beam.

Lux V is another issue and already good points being made here about it.

Issac,
If you had perfect collimation (parallel light streams), you would have a hole the size of the shadow cast by the image itself "package" itself. As you suggest, there is some convergence as well as divergence of light after it exits the optic. The 5W donut is not a result of this convergence or divergence to my way of thinking but rather due to the source of light relative to the reflector and outside of its infinitely small focal point.

Don, yup. well said. Also, from my experience to have a 5W focused tight you have donut hole, or you de-focus it some and loose some throw, but have a smooth beam. This is with smooth reflectors..

Don, thank you for taking the time to replying to my question. IsaacHayes, your comment about de-focusing a Mag helped a lot. I *think* I get it now. I'm starting to think it's not a donut at all, but a heavily diffused cross: die separation and square shape (?) in the LuxV and corner effect (sort of the negative image of a cross) on the LuxI-III. It looks like a donut because of the round reflector masks the cross? :thinking:

yup. Remember reflectors are better at making things round and smooth beam, so a cross void/whatever will just be a hole. Colored luxeons in mag reflectors let you see what's goign on a little better as whites have the phosphur that smooths it out a bit.

greenLED said:

I'm starting to think it's not a donut at all, but a heavily diffused cross

Click to expand...

I've thought the very same thing about the donut on my U2. One part of the cross is clearly more pronounced than the other. If you hold the light perpendicular to the wall, and rotate it about its long axis, you can see the cross turning.

If you go back to McGizmo's post, you'll see there is one section of the quad die arrangement that is much darker, right down the middle. There's another not quite as dark section across the center, from side to side. That's exactly how mine is. If you put it on low, and stare at it (while it's on, of course) your eyes will eventually adapt to the light and you'll be able to make out the detail. (Aside: maybe I ought to have been wearing sunglasses. The spot before my eyes took a while to disappear. )

I think it's the image of that cross shaped pattern that is getting focused and projected. Personally, I feel the U2 is a relatively floodly light anyway. I don't expect to get miles of throw from it. I wish they hadn't focused it quite so tight. I'd be happier with less throw and no donut.

What causes the hole in the beam?

Don, you must know. Why do some reflectors create a black hole in the hotspot at close range, and others always produce a bright hotspot at any range to the subject?

Is it the shape of the reflector, or the position of the die (front to back) in the reflector? Is is possible to adjust the reflector to eliminate the hole?

Re: What causes the hole in the beam?

Jdriller,
I don't have much more than a guess on this. For starters, the reflector has a hole in it for the LED and if all of the light coming from the reflector were collimated and parallel, you would have a shadow of the LED in the reflected beam. However the LED itself is putting out light directly and its lambertian distribution will be illuminating this hole to some extent. I believe fine adjustment of focus has much to do with this too. I have noticed that the jokers in non joker reflectors have this hole out further than non jokers will and the joker has its image slightly dropped below the focal point of the reflector. You can move the image or die up in the reflector and alieviate some of this close to target hole issue. In some cases though, you may find that you will get a hole then at greater distances. :shrug: The larger reflectors with longer focal lengths have a greater range in close where a nul is apparent. At least this is my observations as I recall. :thinking:

Re: What causes the hole in the beam?

jdriller, check this thread; we're discussing pretty much the same things, and Don gives a more detailed explanation there.

Re: What causes the hole in the beam?

McGizmo said:

Jdriller,
However the LED itself is putting out light directly and its lambertian distribution will be illuminating this hole to some extent. I believe fine adjustment of focus has much to do with this too.

Click to expand...

Since we are dealing with a 4 die flat emitter I think the cause is obvious if you plot out the distribution. Looking from the side crosssection, you have two dies. When you plot the two lambertian curves while respecting the space between then, you will notice a depression between the two peaks that appears similar to quartic equation plot(reall it is a bit different due to square dies but for simplicity...). This depression is likely the doughnut we see projected.

Just to support this a little bit:
My U2 has a bluish tint to it in the doughnut. If I take a magnifying glass and focus and project the image onto a white surface I can see the culprit. What can be seen is that a corner of one die at the point where the four meet, is blue (obviously the phosphor wasn't distributed over this portion of the die). You can actually seen how the shape of the die is projected as you use the glass to focus in and out. .

Re: What causes the hole in the beam?

Thanks to all for the responses. One of the best performing reflectors I have is from TNC Hyperlux III. No beamshot, but here is the description. Perfect fuzzy, round hotspot with bright corona. There is NO void at any distance. You can put the light less than 1mm from a wall and the hotspot is full. Chris had this reflector made for the light. It is truly impressive. Odd thing, is that when you look into the reflector, it appears to have rings (like a bullseye target) but upon close inspection, they disappear.

greenLED said:

Don, thank you for taking the time to replying to my question. IsaacHayes, your comment about de-focusing a Mag helped a lot. I *think* I get it now. I'm starting to think it's not a donut at all, but a heavily diffused cross: die separation and square shape (?) in the LuxV and corner effect (sort of the negative image of a cross) on the LuxI-III. It looks like a donut because of the round reflector masks the cross? :thinking:

Click to expand...

Here's a pic of my L4 up close. The cross is quite obvious in mine.