De focused aspheric lenses using multi emitters.

[gav6280]

Anyone tried using a triple xpg star like this one with an aspheric yet?

Im wondering as the dies are so small and so close together if you would get away with a slightly out of focus aspheric lens and still get good throw and limit the side spill?

Does anyone have any pictures of de focused aspherics using multi emitter setups, Im guessing with an aspheric de focused to a point where for arguments sake 4 separate led's will come to form a nice spot, that it might still be better than a quad or even individual optics due to the light from the aspheric having less side spill? Maybe......

 

[gcbryan]

I think it will look odd but that's only from experimenting with "the ugly light" (MC-E) and an aspheric.

 

[Packhorse]

Probably much better off posting this question in the Homemade forum where you will get more people viewing it.

But my guess/ experience is that it would have to be very out of focus before all 3 beams converge into one clean beam.
You should be able to find plenty of threads in the homemade forum showing multi die LED's and aspherics. The triple XP-G will exaggerate the spaces between dies.
Probably only suitable for video lighting.

 

[gav6280]

Probably much better off posting this question in the Homemade forum where you will get more people viewing it.

But my guess/ experience is that it would have to be very out of focus before all 3 beams converge into one clean beam.
You should be able to find plenty of threads in the homemade forum showing multi die LED's and aspherics. The triple XP-G will exaggerate the spaces between dies.
Probably only suitable for video lighting.

Even though it would need to be very defocused, do you not think it might still give less side spill?

Ive got a whole load of different lenses on order but it will be a couple of weeks until i get them....

 

[Packhorse]

Less side spill than what?

By the time it converges into one spot the spot will be very wide. Very very wide. Think video light wide.

 

[gav6280]

Ah i see, thanks.

 

[Packhorse]

The above pic is a MC-E from this thread... http://www.candlepowerforums.com/vb/showthread.php?t=212132
Now imagine 3 squares instead of 4 and imagine them much furtherer apart. I doubt you would even see all 3 in the same sized frame as above.
You would also have efficiency issues since none/ not all of the dies are centred with the lens.

 

[archer6817j]

I tried this with a luxeon rebel tri star in very rough prototype form. I just held an aspheric in front of of the star, with and without secondary optics in place. Packhorse is right, it's like the MCE image he posted but worse. You can defocus but you still get three distinct (albeit defocused) spots with a lot of space in between. I was hoping the 27 degree frosted tri-optic would smooth is out a bit. A "bit" is the operative term. Not as good as I'd hoped.

Depending what your diving needs are you might be able to live with that. Some people dig the window pane I've considered putting something translucent in front of the star like a 1/8" white acrylic sheet but I imagine that would loose too many lumens to be effective. Although...might be good for diving since you don't need a beam that throws a half mile anyway.

 

[gcbryan]

I tried this with a luxeon rebel tri star in very rough prototype form. I just held an aspheric in front of of the star, with and without secondary optics in place. Packhorse is right, it's like the MCE image he posted but worse. You can defocus but you still get three distinct (albeit defocused) spots with a lot of space in between. I was hoping the 27 degree frosted tri-optic would smooth is out a bit. A "bit" is the operative term. Not as good as I'd hoped.

Depending what your diving needs are you might be able to live with that. Some people dig the window pane I've considered putting something translucent in front of the star like a 1/8" white acrylic sheet but I imagine that would loose too many lumens to be effective. Although...might be good for diving since you don't need a beam that throws a half mile anyway.

It wouldn't throw at all is the problem. Unless you're in the tropics and just want a flood light it wouldn't work.

My experience with a MC/E and an aspheric is exactly as you and Packhorse describe however.

 

[lucca brassi]

similar to that ?
http://forums.mtbr.com/showthread.php?t=591181

 

[jspeybro]

You would also have efficiency issues since none/ not all of the dies are centered with the lens.

I haven't tried it, but here's some info that might be useful.

Packhorse is partly right. efficiency is not really the issue, but the compromises that were taken into account when making the aspheric.
An aspheric lens works really well when the light originates from a point on the optical axis (line through the center of the lens). People design aspheric lenses to compensate for aberrations of higher orders but this only works reasonable well for small angles. when deviating too much from the optical axis, an aspheric will perform similar or worse, compared to a simple plano/double-convex lens.

Other than that, the image of packhorse already shows what is going to happen. When focussed at a certain distance, the optical system has a specific magnification. For example, if the lens has a focal length of 30mm, and the LED(s) is 31mm from the first principle plane of the lens, the LED(s) will be imaged (in focus) at 930mm (1/(1/30-1/31)) from the second principle plane of the lens.
The magnification of this optical system is given by -31mm/930mm=-30.
The negative sign tells you that the image will be inverted (not an issue here), but that the size of the LED(s) will be imaged with a magnification of 30.

for the mentionned LED board, the 3 XPG packages are in a triangle together. The XPG package is 3.45mm wide and the dome has a 2.6mm diameter. This means that the the 3 LEDs fit in the circle with diameter sqrt(3.45²+3.45²)+2.6=7.5mm. The magnified image will be 7.5*30=224mm in size (diameter of circle around the image). Since there is a gap between the LED dies, this gap will also be magnified.
Due to the fact that the aspheric lens isn't designed for off-center emitters, the edges will likely be softer due to aberrations. only the central gap will be imaged sharply, which you don't want in this case. This can in fact already be seen a little in the image of packhorse if you look at the dark line between 2 dies. It is dark in the middle but becomes lighter when going to the sides. also the center corners are sharper compared to the outer corners.

Defocussing the setup (actually focusing on another distance) might help a little, since the edges will be smoothed even more and the magnification will change, but you will most likely still have the dark spots in the middle and between the LEDs. (there is 0.85mm between the domes next to each other and a little more between the third one and the two next to each other, that's 2-3 cm in the image with magnification 30)

I hope this is not too technical. I tried to keep it simple.

Johan

 

[Jasonthephoneboy]

You could always frost the emitter or color it white. then you would have a full white head.

I know nothing of this sort of thing, but I think if you diffuse the light enough o take away the tri formation, you would in effect create a really large lamp.

 

[jspeybro]

You could always frost the emitter or color it white. then you would have a full white head.

not sure what you mean by this... put paint on the emitter(s)? what would this accomplish?

I know nothing of this sort of thing, but I think if you diffuse the light enough o take away the tri formation, you would in effect create a really large lamp.

that goes against what was intended in the original post. Taking away the tri formation would indeed solve the issue of the dark spots, but would also reduce the output to 1/3. The setup with multiple emitter is a possible way to increase the light output. The use of an aspheric lens is a way to get a nice focused beam without too much side spill as you have when using a reflector. if the two techniques could be combined, that would be nice.

Using a diffuser between the LEDs and the lens might work to some extend if you set up the optical system to image the diffuser rather than the LEDs, but diffusers will also cause quite some light not to reach the lens (absorption in diffuser, scattering under an angle that is too big to hit the lens). I'm not sure what the result would be. Maybe this could work.
I would place the diffuser close enough to the emitters to get most of the light in a small spot on the diffuser, but far enough to allow the angles of the different leds to overlap when hitting the diffuser.

I'm afraid the losses would still be quite big however.
Also, the object that would be imaged is a lot larger than the LEDs itself so the image would also be a lot larger which is probably not desired in this case.

 

[archer6817j]

I think we have already talked around the subject but I thought I would give another idea a shot. Clearly this would suffer from some of the drawbacks already mentioned but the end result might still be serviceable.

What if you took a length of acrylic rod with a diameter that would encompass all three leds. Bead blast (frost) one end and then polish the end that is closest to the emitters. The intent would be to gather as much light as possible inside the tube and then emit it as a single illuminated surface that the aspheric could focus. This might mean the acrylic tube would have to be relatively long? Then you would have to add the gap to the aspheric, which might mean your light is getting too long at this point.

Given all of the losses this would probably be less efficient that using a single led, even if was an MCE or P7 with four emitters. Of course we have the large die SST50/90 but they are major current hogs. However, I'm starting to think (in an optics/efficiency sense) that a single P7 (900 lumens) driven at 2.8 amps would still be a better option than three XRE's which would come out to about the same theoretical lumens. Of course you could series drive the XRE's at 1 amp and get three times the burn of a single P7 at 2.8 amps.

Of course this all assumes you are looking for a pretty tight spot in terms of beam profile. So, could you break it down and say that multiple led's could be best suited to more "floody" applications while a single led would be best suited to "spotty" applications?

 

[Packhorse]

I think we have already talked around the subject but I thought I would give another idea a shot. Clearly this would suffer from some of the drawbacks already mentioned but the end result might still be serviceable.

What if you took a length of acrylic rod with a diameter that would encompass all three leds. Bead blast (frost) one end and then polish the end that is closest to the emitters. The intent would be to gather as much light as possible inside the tube and then emit it as a single illuminated surface that the aspheric could focus. This might mean the acrylic tube would have to be relatively long? Then you would have to add the gap to the aspheric, which might mean your light is getting too long at this point.

Given all of the losses this would probably be less efficient that using a single led, even if was an MCE or P7 with four emitters. Of course we have the large die SST50/90 but they are major current hogs. However, I'm starting to think (in an optics/efficiency sense) that a single P7 (900 lumens) driven at 2.8 amps would still be a better option than three XRE's which would come out to about the same theoretical lumens. Of course you could series drive the XRE's at 1 amp and get three times the burn of a single P7 at 2.8 amps.

Of course this all assumes you are looking for a pretty tight spot in terms of beam profile. So, could you break it down and say that multiple led's could be best suited to more "floody" applications while a single led would be best suited to "spotty" applications?

Just use a SST-50 and be done with it.
Tighter beam than a P7 or 3 XP-G's. More efficient than trying to columate multiple dies. And best of all its simple!

 

[jspeybro]

What if you took a length of acrylic rod with a diameter that would encompass all three leds. Bead blast (frost) one end and then polish the end that is closest to the emitters. The intent would be to gather as much light as possible inside the tube and then emit it as a single illuminated surface that the aspheric could focus. This might mean the acrylic tube would have to be relatively long? Then you would have to add the gap to the aspheric, which might mean your light is getting too long at this point.

That will not work very well because you will only capture a certain angle of the light. All light above a certain angle will refract incorrectly in the rod and exit on the side of the rod. do a google search for total internal reflection to find out more about this.
This is also the reason why these TIR optics like the ones from Ledil and Carlco use a combination of a reflector (by using total internal reflection) and a lens in the center for which the angles are small enough to work with. For the larger angle, the light comes in the plastic and then reflects on the side of the plastic because there the angles are different.
For those 5mm leds, this can work fine because these have usually small angles, but high power leds have angles around 100°.