Metalizing luxeon optics

[evan9162]

We've had the topic of metalizing the back of a luxeon optic with some kind of chroming come up many times here on CPF. The generally accepted conclusion is that it would do more harm than good, as the optics are designed specifically around the index of refraction between the optic material and air. Any change in the optical properties from modifying the surface (i.e., buy metalizing it) would result in a change in how the optic works.

Larry (tvodrd) sent me a couple of optics to do a comparative test. One is a bone stock 30mm optic, while the other has had the backside metalized. The metalized optic also had the outer rim cut off, but that won't affect the optical properties at all.

Here are the contenders (normal on the left, metalized on the right):

Here's the mounting system I used:

The LED was a Luxeon III star, bin TY0L, run at 700mA.


Some quick beamshots - these were done by placing a white board about 24" away.


These shots at ISO50, daylight While balance, F/3.5
Normal on the left, metalized on the right


Shutter speed: 1/50

Shutter speed: 1/250

Shutter speed: 1/1000

Finally, useful "across the garage" shots (distance of 15 feet) (same camera settings for both shots)

It looks like the metalized optic has a dimmer hotspot and slightly wider beam angle.

Center beam measurements (taken at a distance of 1 meter with a MeterMan LM631 light meter):

Normal: 2860 lux
Metalized: 1147 lux

I used my beam angle measurement test rig to characterize the beam from both optics. Here are the results:

So indeed, the metalized does spread the beam out somewhat, but does so at a severe weakening of the hotspot. The wider beam angle is more noticable in actual viewing than the graph would indicate.

 

[HarryN]

Nice analysis and presentation work Evan. Thanks for having that metalization work performed Larry. I think many of us have wondered if that would work out or not.

 

[Kiessling]

Does the beam of the metallized one really look like a reflectored beam or is it just my tired eyes suggestings things that are not there?

Nice comparison! :thumbsup:

bernie

 

[chimo]

Very well presented results! Any speculation where most of the lost light energy went (out the back, sidespill, heat)?

Paul

 

[evan9162]

I think the metalization has cause some losses from it, but I think the majority of the light from the center spot went into brightening the side spill.

The brightness of the two optics cross over between 5 and 6 degrees.

In the first 5 degrees, the normal optic is outputting approx. 27 lumens, whereas the metalized only puts about 13 lumens in the first 5 degrees. But, from 6-25 degrees, the normal optic puts out 10.6 lumens, while the metalized, 19.8 - so the metalization has diverted some light from the center of the beam into the sidespill.

 

[tvodrd]

Thank you very much Evan!!! :bow: I guess I can toss the CR2 optics I had done along with it.

Larry

 

[pbarrette]

Hi Larry,

I wouldn't toss those CR2 optics just yet. I assume that they have been cut down to fit in a CR2 form factor light and as we all know, cutting an optic down changes its beam characteristics.

Instead, perhaps you could send a sample of each (metalized and bare) to Evan for another test. It's possible that at the reduced diameter, the metalized optic could have better beam characteristics than the bare. So it may still be worthwile to have Evan test one of those as well.

pb

 

[tvodrd]

Pb,
That design evolved to using a reflector (See avatar. )

Larry

 

[Ledean]

Thanks for the results evan . Till i saw your well presented article i was convinced in my mind that using a metal reflector and thus trapping the escaping light , we can cartainly geta better beam .
In the first thre photos was the camera the same distance from the wall where teh beam was projected ?

 

[evan9162]

Ledean,

The first 3 sets of photos (different exposures) were all taken at the same distance, with the LED about 2' from the target, and the camera in the same position for all.

The target was a piece of white shelving board, about 1' across.

The last photos (across the garage) were at a distance of about 15', with the camera positioned right next to the LED.

Putting a reflector behind the optic will yield much different results than what's shown here - when putting a reflector behind the optic, you're preserving the air-optic interface, so you're not changing the index of refraction - and thus, the optic should work as it did before.

The addition of a reflector will cause some of the spill light that escapes the optic out the sides to be reflected back in. However, I suspect that the dynamics of the light that escapes are such that the reflector will not likely yield much if any improvement in spot brightness, rather the reflected light would be scattered into the spill beam (not focused in any way)

 

[TrueBlue]

I'm actually disappointed in the optic with the reflector on it. I really would have thought that the optic with a reflector on the back would give a brighter beam because there would be less light loss thought the sides of the optic. Instead it looks more like a scatter beam. Apparently the reflector adds another layer to the skim light effect to scatter more light than it reflects. Would putting the reflector on the optic have caused the optic outside edge to distort giving it an internal 'satin finish' to scatter more light?

I do have awesome airbrush equipment so a thought occurred to me. What would happen if I sealed the edge of the optic with acrylic spray then airbrushed a very fine enamel based aluminum coating on the side of the optic? The aluminum paint is called Testors Metalizer plastic model paint and I have lots of it that I use for modeling. The paint is made to be airbrushed on a model then rubbed for a glossy look. I do have a spare optic that I could give my thought a try. I was thinking the acrylic sealer would prevent the enamel paint from eating into the side of the optic. Then the enamel paint that can be sprayed very fine would have a smoother surface to reflect off and project a finer beam forward where it should be going.

 

[nemul]

nicely done evan

i wonder how one would work with a LOP texture molded on the back.
not metalized just molded into the optic when they make it...
.............
*holds a can of clear gloss spray paint and a optic* hmmmmm. naw n/m

 

[jashhash]

Perhaps through the metalization process a microscopic layer of the optic may have been melted which could compromise the optic's refractive properties.

It seems to me that putting a reflector on the back of the optic may preserve a slight amount of light but then again it would increase the cost of the optic. So its realy not a cost efficient solution.

 

[tvodrd]

My theory was the portion of the optic's internal reflection could go truely "total" with the rear cone metallized. Optics work through combining refraction and reflection. A metallized layer is typically less than .0001" thick, and I doubt any melting occurred as the aluminum atoms condensed on the plastic. :shrug:

Larry

 

[IsaacHayes]

Hmm. Interesting. At first I was thinking "wow, looks like a carloco mirror lens" and thought maybe it would work.. But nope! I guess the lens has to be designed to work with a metal finish... I also thought the topic said "mentalizing luxeon optics". I need to go to bed!

 

[Burnt_Retinas]

Thanks for the research and actually trying this. I thought it would help capture those lost photons we see escaping the rear along with others and it's surprising how the effect is actually so obviously negative.

So, I doesn't work, at least when the metalised surface contacts the optic.

Now my thoughts turn to what if a reflector is air spaced (to maintain the refractive index of the plastic to air relationship) but angled to coincide with the refractive index of the plastic, taking into consideration the curvature of the optic, such that the reflected light can be sent back through the optic such that it refracts forwards? I guess it can be possible, but I have a feeling the gain would be perhaps hardly noticeable and certainly not worth the effort/cost.

Thanks again for actually doing this, it has had me wondering 'what if' for well over a year. I can sleep easy now....

Chris

 

[NewBie]

My theory was the portion of the optic's internal reflection could go truely "total" with the rear cone metallized. Optics work through combining refraction and reflection. A metallized layer is typically less than .0001" thick, and I doubt any melting occurred as the aluminum atoms condensed on the plastic. :shrug:

Larry

Consider 100% reflection efficiencies of TIR (total internal reflection) if the optic was perfectly smooth and all the angles in the design were right to allow this.

Then consider the reflectivity of a reflector surface (much lower).

Then consider the loss of reflectivity when the reflector material is put on an optic, and the even lower reflectivity of the material when the index of refraction difference is less on the coated optic, plastic to metal, vs. metal and air.

 

[VidPro]

Nice test, so its kinda like that idea just made a reflector with a hunk of plastic in it, messing it up. i would have NEVER thought that.

i put reflection behind optics, just so nothing goes backwards, where it has no value, that seems to work, and i tested putting a reflector behind an optic, and had so much trouble getting it fit, and correct that i tossed the idea.
but i still wonder about it now, even more so, now that you brought it up.
the prolight reflector fits the back of the optics very nicely.

i sure am happy i didnt try to chrome paint 3 optics now.