Need help understanding LED optics

[ccoutts]

Hi,

I'm new to this forum, so first up HI to everyone.

I'm in the process of designing a homemade mountain bike light, using 6 cree Q5's, a MaxFlex controller, and some optics sourced from DealExtreme.

It was hard finding optics which were spec'd for the right mix of flood and spot I wanted, so I bought a whole bunch of different ones. The ones I think will work are actually designed for SSC led's (bigger holes in the base) so I'm trying to work out how to mount them so they work with cree's.

I'd like to understand where the light is designed to go in these optics. They seem to have a hollow bit where the lens of the LED fits. I guess I'm trying to work out if the point source of light needs to be at a specific place in that hollow section (to focus properly) or can it be anywhere up there? IE, does the placement of the point source (ie led) relative to the optic determine the focus of the light (like in the case of a conventional reflector), or does it not matter?

Thanks for any info, or links that might help explain it (can't seem to find much, or not looking in the right places).
Chris.

 

[LukeA]

It matters very much. You have to position teh die correctly in three dimensions to get the right pattern. It will be better to either buy SSCs to fit your optics or to buy optics to fit your Crees.

 

[ccoutts]

Thanks LukeA. Yeah, I realise that now... after reading this http://www.ekspoled.lt/files/File/Designing with lenses and secondary optics.pdf

Last night, I jammed the clear optic (no holder) down over the metal ring of the cree (fit almost perfectly, hence I thought it was meant to work that way). Light obviously came out the front, but there was tons of light escaping out the sides (blinding!). So I guess the internal angles were all wrong, and passing through the optic, without refracting.

Anyway, I'll play around some more, and try to minimise side loses.

The problem is that I've found DealExtreme... the downside being I'm very reluctant to pay "normal" prices for things again! And DX don't have much in the way of wide angle single Cree optics.

 

[Lighthouse one]

May I suggest you also check out the sections on bikes. THere are lots of bikers who make and or use stock lights. I just read one yesterday where the biker mounted 2 MTE P7D lights to a bike. Talk about powerful! I have the MTE P7d from DX. It's a lot less hassle than trying to reinvent the wheel!

 

[mdocod]

As Lighthouse one says, a couple of P7s is a lot less hassle and in a textured reflector will probably provide about the perfect beam profile for what you want to do...

Nothing wrong with re-inventing wheels though IMO

I've played around with placing various TIRs on various LEDs with various degrees of success... When you choose an Optic specifically designed for an LED, then it's usually a simple installation as the optic is designed to seat onto the LED with just the right focus achieved.

One thing I have found, is that when you use a optic for say, a Seoul or Luxeon, on a Cree, it results in a big flood, when you use it back on the cree, it's often a tight spot.... The emission pattern of the LED and how it fits into the TIR plays into all of this.

I like the idea of a custom heat sink and separate battery pack for bike lights as you can guarantee that you will have enough runtime on tap, and enough heat sinking to protect the LEDs, not all handheld flashlights will meet every demand in this department.

 

[ccoutts]

Yeah, I agree about the p7 (or MC-E) being less hassle, but I'm kinda committed to using Q5's unfortunately (bought them a while back when the NZ$ was strong against the USD, and now have a lot of optics to test.) Anyway, you gotta draw a line in the sand, as these things will always change by the time you finish building "the perfect light".

Ta for the comment re matching optics to emitters. From some of the Carclo diagrams, it looks like the optic is meant to sit about 0.5mm from the metal ring of the led. I might try that, and see if it reduces spill.

Yeah, a detached model is best for mount biking I think, cos you can have a big liion pack in your bag, the controller/switch/levelGauge on the cable, and the light on your helmet.

 

[yellow]

even when You use a perfectly fitting optic for that specific led You use, there will a good amount of light escape through the sides and illuminate the area around the led.
Optics are transparent and reflection is never 100 %. With reflectors the light non reflected does not get out through the sides and so they seem to "waste" less light.

What kind is more/less efficient lies in the eyes of the user, for me its the (quality) reflector that is better, but ...



PS: wide degree optics are a waste of effort, better use tight ones (around 8 deg.) but have the beams of the individual led not running parallel --> but a few degrees to the sides + up/down.
f.e. when You use two 16 deg. optics, You illuminate a wide area. If instead when using two 8 degrees but aligned side by side to also receive a beam as wide as the 16 deg. horizontally, You do not illuminate the very near ground and the sky with the main beam.
Means You have much more main beam on the trail, while spill alone is enough for near distance.

(of course the best were an offset of about 3-6 deg., giving a bright overlapped supermain, the remaining main beams and spill from all the led used. With Your 5 individual emitters --> perfect)

 

[ccoutts]

Thanks Yellow, thats a mean idea! Adds a bit more complexity to the construction, but as you say, would be much more efficient than using eliptical optics to acheive a East-West spread beam.

The design I'm tending towards (with weight and cent of grav in mind, as this is a helmet mount) is 6 waterproof individual cylinder pods of emiter + optic, in a row, all backing on a common long finned heatsink.Maybe I could put slight bends in the heatsink between each pod, to get them to point diff directions. Wouldn't look as smooth as your typical shiny lathed 1-reflector light though, but I guess I could get some sort of retro style going on.

Thanks
Chris

 

[LukeA]

even when You use a perfectly fitting optic for that specific led You use, there will a good amount of light escape through the sides and illuminate the area around the led.
Optics are transparent and reflection is never 100 %. With reflectors the light non reflected does not get out through the sides and so they seem to "waste" less light.

Reflectors waste much more light than optics. Reflectors are about 75% efficient, while optics are typically 85% efficient or better. It's a fallacy to say that reflectors waste less light because you can't see light going in the wrong direction. Reflectors waste light by absorbing it and heating up slightly. Of course you can't see that.

 

[K_Labs]

what kind of beam would this have? http://www.dealextreme.com/details.dx/sku.13983

 

[Gunner12]

My guess, based on the bumps(wrong term) on the optic, I'd say a smooth floody beam.