What exactly causes bagel beams in luxeons?

[BlindedByTheLite]

I've noticed in my Streamlight 3AA Luxeon that there's a bit of a slightly dimmer spot in the center of the beam..

now it doesn't bother me at all unless i'm pointing it at white walls (which i do daily *lol*).. and i've heard other ppl mention similar beams in even L4s..

and upon inspecting the luxeon, i noticed an EVER SO SLIGHT dimple in the middle of it.. could this be the cause?

does everyone know except me? /ubbthreads/images/graemlins/confused.gif

 

[Rothrandir]

it's the reflector.

the way a reflector works, is by catching the light out of the emitter, and directing it forward in a beam. since light doesn't go straight out, the beamspread focuses differently at different points.
hold a light one foot away from a wall and you get a fairly thin beam, hold it ten feet away and you get a much larger beam.
usually at closer points, the hole is in the middle because you havn't reached the point where the light goes together. this happens with all reflectorized flashlights.

 

[BlindedByTheLite]

yeah i noticed that with my Pelican Mitylite..

however with this particular SL Luxeon the hole starts to show from about 10 feet.

but as you know, it doesn't hinder its use at all.. i was just trying to find out if there's a way to possibly get ride of it.. and have the light increase in the center of the beam like i've got used to over time.

 

[PhotonBoy]

Orange peel reflector perhaps?

 

[IsaacHayes]

You can try moving the reflector towards or away from the luxeon too.

 

[Ginseng]

A true parabolic reflector has no reflected light output at the vertex where the bulb protrudes. Thus, the dim spot.

Wilkey

 

[UnknownVT]

[ QUOTE ]
Ginseng said:
A true parabolic reflector has no reflected light output at the vertex where the bulb protrudes. Thus, the dim spot.


[/ QUOTE ]

I'm no expert - but doesn't the LED itself emit light forward along the central axis via its lens/body?
most normal LEDs' hot-spots are from that light.....
so shouldn't that central hot-spot still be brighter, and fill any "hole" caused by the light via the reflector?

 

[Ginseng]

Yes it does and no, the the central hotspot is not primarily from that light. The function of a parabolic reflector is to collimate the entire light output from a point source (in this case the LED die or an incandescent filament) into a roughly parallel beam. The majority of the central hotspot is built from off-axis light emission. You can prove this to yourself by carrying out the following experiment. Take a flashlight like an LED torch or perhaps easier, a Maglite. Aim the beam to the ceiling. Pretty bright hotspot right? Now put a quarter right in the center of the lens. If the majority of the hotspot were built from the on-axis direct emission, then the center should go dark. What actually happens?

High dome or Lambertian emission LEDs do output the highest luminous intensity on-axis, but still, the off-axis light, once collected and collimated will overwhelm the direct emission hotspot. You can prove this again by removing the reflector from either an LED light or a Mag. The central hospot almost completely disappears.

 

[UnknownVT]

[ QUOTE ]
Ginseng said:
Yes it does and no, the the central hotspot is not primarily from that light. The function of a parabolic reflector is to collimate the entire light output from a point source (in this case the LED die or an incandescent filament) into a roughly parallel beam. The majority of the central hotspot is built from off-axis light emission.

[/ QUOTE ]

yes....BUT....

this is surely true for a non-focussed "bulb" where the filament forms the "point of light".

But normal LEDs have the domed part of their body forming a focussing lens which gives rise to the much brighter central hot-spot on a bare non-reflector assisted beam.

Whereas your example of a MagLite bulb is a plain non-focussed bulb - which when bare and non-reflector assisted does not give any defined central hot-spot -

It's that LED focussed hot-spot I was asking about,
a reflector, parabolic or otherwise, will NOT eliminate that.....
unless of course Luxeon LEDs are non-focussed?

 

[Ginseng]

Just try the experiments I suggested. You might be surprised.

Wilkey

 

[evan9162]

[ QUOTE ]
unless of course Luxeon LEDs are non-focussed?

[/ QUOTE ]

Exactly. Luxeon LEDs are not focussed. The lambertian (high-dome) luxeon has an very wide viewing angle, approximately 160 degrees.

 

[BlindedByTheLite]

*jots things down*

 

[HunterSon]

The bezel on the 3AA streamlight can be focused like a M@G light. Screw the bezel in or out a couple of half turns to get rid of the spot.

 

[McGizmo]

It is my understanding that the null in the center is resultant from less light being produced at the perimeter of the light source. If the die were a circle, this null would not be there. Since the die is a square, at the perimeter you have 4 points of light (the die corners) along with four sections not producing light. As I have been led to believe, the light from the center of the source is evident in the outer portion of the beam. The light from the perimeter is evident in the center. If this is in fact the case. perhaps a small dot placed in the center of the LED lens, blocking that light, would show a reduced beam diameter??

 

[BlindedByTheLite]

HunterSon:
i don't really think this particular SL Luxeon is very focusable.. i noticed that when i attempted to focus the light, the more i had to turn the dimmer the beam is getting.. it's not terrible, but is it normal?

McGizmo:
i'll try a small dot on the Luxeon tomorrow.. i need to find something i can wipe of just in case. *lol*

 

[HunterSon]

My bezel cranked all the way down has a pretty big hotspot with a hole in it. When I back it off until the o-ring just starts to show the spot is very tight and bright. Any more and it gets dimmer because the emitter is not in the reflector any more. Must be a difference in manufacturing between the two lights I guess.

 

[BlindedByTheLite]

ahh, i was just being slow in the head!

i was aiming my beam at the wall and focusing while watching the beam..

i thought i had settled on a focus, but after shutting off all the lights in the house i got a MUCH clearer picture of the focusing, and i find mine to be the same as yours, right around where the o-ring starts showing..

thanks!

 

[HunterSon]

Glad it worked out. The SL TL3AA Luxeon is one of the best bargins out there but doesn't get much press.

 

[BlindedByTheLite]

*cosign*