# Dome VS. no dome



## SmurfTacular (Jul 5, 2010)

OK, so _naturally_ LED's tend to project light forward and throw the light. And the dome is there to refract the light and bounce it off of the reflector to give the flashlight more reflector-throw (as opposed to natural LED-throw that is pretty much flood).

So say I would want to build an aspheric Mag mod, I would rather have no dome than have a dome, right? Naturally the SST-90 has a dome, and the dome-less equivalent SBT-90W has no dome. So would I rather have the SBT-90W than the SST-90 because the LED naturally throws more toward the aspheric lens? It would seem getting the SST-90 would be silly as it would waste light.


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## saabluster (Jul 5, 2010)

SmurfTacular said:


> OK, so _naturally_ LED's tend to project light forward and throw the light. And the dome is there to refract the light and bounce it off of the reflector to give the flashlight more reflector-throw (as opposed to natural LED-throw that is pretty much flood).
> 
> So say I would want to build an aspheric Mag mod, I would rather have no dome than have a dome, right? Naturally the SST-90 has a dome, and the dome-less equivalent SBT-90W has no dome. So would I rather have the SBT-90W than the SST-90 because the LED naturally throws more toward the aspheric lens? It would seem getting the SST-90 would be silly as it would waste light.



The dome is not there to "refract the light and bounce it off of the reflector". It is there to increase the light extraction of the package. In doing so it focuses more light forward. So actually less light is being wasted in an aspheric setup with the dome in place. When you remove the dome the output drops on the order of 25-30% and on top of that now the beam is not being directed forwards as much so there are additional losses. The only thing that improves by removing the dome is the intensity of the collimated beam. The beam will be smaller however. 

So no the SBT-90 does not throw more light towards the lens. Quite the opposite.


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## zzonbi (Jul 6, 2010)

Why does the intensity improve?


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## znomit (Jul 6, 2010)

zzonbi said:


> Why does the intensity improve?



The dome magnifies the image of the die. 

Removing the dome makes it look smaller, hence the surface brightness increases. This offsets the reduction in lumens due to poorer light extraction.


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## aurum (Jul 6, 2010)

> output drops on the order of 25-30% and on top



I don't really understand *why *the output drops ... if somebody removes the dome there is less material which is able to absorb the light?! So the output should increase.


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## bigchelis (Jul 6, 2010)

Lambdalights_______SST-50*DOME*_________3 NiMH C Tenergy__________1057.7_____ 1 sec_____899________22,475___
2D Mag _____________Direct Drive_____________*___________*__________901.5_____30 sec________________________
*_________*__________________________________________________ ____ 873.8______1 min________________________
________________________________________________4. 23A at tail_____ 836.2______2 min________________________
__________________________________________________ ______________ 821.5______3 min________________________
Lambdalights_____SST-50*DOMELESS*_______3 NiMH C Tenergy__________ 785.4_____ 1 sec___2,300________57,500____
2D Mag _____________Direct Drive_____________*___________*__________684.6______30 sec________________________
*_________*__________________________________________________ ____ 646.2______1 min_________________________
________________________________________________4V input_________ 634.6______2 min_________________________
__________________________________________________ ______________ 629.2______3 min_________________________




In addition to the loosing lumens, the light got hotter alot faster and the vF seemed to drop. With the dome I was able to use 4.2V input, then without the dome at 4V input the light was blue for about 2 seconds then back to normal. 


I can't explain it myself, but I should get a VARA2000 Copper model today that I will do the same tests.


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## aurum (Jul 6, 2010)

> Lambdalights_______SST-50*DOME*_________3 NiMH C Tenergy__________1057.7_____ 1 sec_____899________22,475___
> 2D Mag _____________Direct Drive_____________*___________*__________901.5_____30 sec________________________
> *_________*__________________________________________________ ____ 873.8______1 min________________________
> ________________________________________________4. 23A at tail_____ 836.2______2 min________________________
> ...



ok ... that's what everbody says. But *why *does the Led behave like this?


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## znomit (Jul 6, 2010)

aurum said:


> I don't really understand *why *the output drops ... if somebody removes the dome there is less material which is able to absorb the light?! So the output should increase.



LED has a high refractive index. When passing light to air there is a lot of reflection at the boundary. This is minimised by using an intermediary medium with a refractive index between the LED and air. 

http://en.wikipedia.org/wiki/Light_extraction_in_LEDs


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## saabluster (Jul 6, 2010)

bigchelis said:


> In addition to the loosing lumens, the light got hotter alot faster and the vF seemed to drop. With the dome I was able to use 4.2V input, then without the dome at 4V input the light was blue for about 2 seconds then back to normal.
> 
> 
> I can't explain it myself, but I should get a VARA2000 Copper model today that I will do the same tests.



It makes perfect sense. The vF is lower because the die is hotter. The die is hotter because less light is escaping the package and dying as heat. Less light is escaping because without the dome there is more light being reflected back down towards the die. It is reflecting back down because of the angle of incidence for the light leaving the die and its interaction with the outer refracting surface. 

You know how a TIR lens works right? In this case the domeless LED is using the same process just without the T. So it's not TIR but IR.


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## aurum (Jul 6, 2010)

> http://en.wikipedia.org/wiki/Light_extraction_in_LEDs




:twothumbs


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## jtr1962 (Jul 6, 2010)

I tested light output of a Cree XR-E with and without the dome. See here. Light output without the dome was 24% less, and the beam angle was wider as well.


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## sfca (Jul 6, 2010)

jtr1962 said:


> I tested light output of a Cree XR-E with and without the dome. See here. Light output without the dome was 24% less, and the beam angle was wider as well.



From the light intensity graph should I interpret 0 degrees as standing directly in front, and as we move away from the centre the domeless XR-E would be more intense (lux), even with a bigger hotspot?


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## jwyj (Jul 6, 2010)

sfca said:


> From the light intensity graph should I interpret 0 degrees as standing directly in front, and as we move away from the centre the domeless XR-E would be more intense (lux), even with a bigger hotspot?


 I suppose dome or domeless would make no difference in throw if we use it in a Deerelight body with an aspherical lens.


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## zzonbi (Jul 7, 2010)

looks like some domes shift the image axially, depending on viewing angle, detrimental to throw; going domeless takes care of that
with an aspheric, with a smaller angle range, that's less an issue


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