# HID Focal Point...



## eyesonfire (Nov 27, 2017)

What is the best focal point for a HID when used in a handheld light?
Is it best to align the bulb so the focal point is in between the two electrode hot spots, or better to pick just one of the hot spots?


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## FRITZHID (Nov 28, 2017)

On HID, dead center of the electrodes.


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## eyesonfire (Nov 28, 2017)

FRITZHID said:


> On HID, dead center of the electrodes.




Thanks...


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## The_Driver (Nov 28, 2017)

Focussing on one of the eletrode tips will get you more throw, but in a much smaller hotspot with a lot of corona around it. It will also require a rather precise reflector to make use of this (Lemax/XeVision do this with their lights). 

For a practical light I would put the center of the arc in the focus point.


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## bulbmogul (Nov 29, 2017)

The_Driver said:


> Focussing on one of the eletrode tips will get you more throw, but in a much smaller hotspot with a lot of corona around it. It will also require a rather precise reflector to make use of this (Lemax/XeVision do this with their lights).
> 
> For a practical light I would put the cente rof the arc in the focus point.



Surefire also should be having a precise reflector in the Arc Series lights too..


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## The_Driver (Nov 29, 2017)

bulbmogul said:


> Surefire also should be having a precise reflector in the Arc Series lights too..




Well yes, that should be expected considering the prices of those lights. 


*EDIT:* some more information regarding the luminance:
Here is a German thread on these lights where the topic was also discussed. 

I will use the Lemax/XeVision XV-LX70 as an example.
To calculate the luminance we need the luminous intensity (1,370,700cd), the area of the reflector, the reflectivity of the reflectors coating and the transmission rate of the lens. 

The large opening of the reflector has a diameter of 100mm (see here). The small opening has a diameter of around 15mm. 
The area (as seen from the hotspot) is thus 7854mm^2 - 177mm^2 = 7677mm^2

I have not found any information regarding the coating used for these reflectors. The reflecitivty could be anywhere between 75 and 98%. I will assume 90% of standard aluminium coatings.

The lenses are not ar-coated and made of Borofloat so they can tolerate the heat. Borofloat has a transmission rate of 94%.

So now the luminance of the part of the arc that is in the focal point of the reflector can be calculated.
Luminance = luminous_intensity / (area_of_reflector * reflectivity_of_reflector* transmission_rate_of _lens) = 211cd/mm^2

This is a very high value for such an HID bulb. Here one can find a detailed doctor thesis (in German) where the luminance of such bulbs was measured. 
At 35W this would only be possible by focussing on one of the electrode tips. At 70W things are different though. It's possible that the center of the arc reaches 200cd/mm^2 since the bulbs are more than twice as bright at these power levels.

So my statement above is not valid anymore. I don't know how they focus these lights.


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## eyesonfire (Nov 29, 2017)

Great info thank you...
When you say precise reflector, what are you referring to.


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## XeRay (Nov 29, 2017)

eyesonfire said:


> Great info thank you...
> When you say precise reflector, what are you referring to.



Electroformed reflectors are the precision way to make a reflector, A parabolic (or modified parabola) mandrel (male mold) is CNC precision machined to very high tolerances and then very highly polished to a mirrored finish. The mandrel is typically machined out of conductive S.S. metal. The finished reflector is typically made of electroplated nickel of less than 1 mm thickness, it is electrochemically plated onto the mandrel then removed from the mandrel (male mold).
The next step is high tech coatings which can be most commonly applied by vacuum deposition typically an aluminum or even gold coatings, sometimes Rhodium is used as well through electro deposition instead. Finally protective clear overcoats are often applied.


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## The_Driver (Dec 1, 2017)

How does the reflectivity change through the use of those protective overcoats?


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## XeRay (Dec 1, 2017)

The_Driver said:


> How does the reflectivity change through the use of those protective overcoats?



'
Thats a question I can't answer myself, I would need to ask my supplier. I would assume its very minimal or they would avoid it as much as possible


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## The_Driver (Dec 1, 2017)

That would be very interesting?
Can you tell us what actual coating you use or at least its specific features?


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## XeRay (Dec 1, 2017)

The_Driver said:


> That would be very interesting?
> Can you tell us what actual coating you use or at least its specific features?




You mean the aluminum coating or the clear coating if any ?


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## The_Driver (Dec 2, 2017)

Yes, the actual metal coating. Is it standard aluminium?


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## PolarLi (Dec 2, 2017)

The_Driver said:


> How does the reflectivity change through the use of those protective overcoats?



You can take a look here: https://www.optiforms.com/optical-coatings/ The most common coating seems to be silicon dioxide over aluminium, also known as AQ.


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## Timothybil (Dec 2, 2017)

PolarLi said:


> You can take a look here: https://www.optiforms.com/optical-coatings/ The most common coating seems to be silicon dioxide over aluminium, also known as AQ.


Or in other words, for all intents and purposes, glass.


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## The_Driver (Dec 3, 2017)

Timothybil said:


> Or in other words, for all intents and purposes, glass.



Seems like it :thumbsup: Pure silicon dioxide would mean that it is quartz glass.
So in theory there would be an additional 8% of light lost because it travels through the glass-air transition twice. Together with the reflectivity of aluminium (90%) this would cause the actual reflectivity to be 83%. 

Also factoring in the borosilicate lens would mean that around 78% of the inital lumens come out the front. 

If we want to be really precise we could calculate how much light actually hits the reflector and only subtract the 83% from this amount and 94% from it and the remaining light which hits the lens directly from the bulb.

I wonder how mechanically robust such a glass coated reflector is.


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## XeRay (Dec 4, 2017)

The_Driver said:


> Seems like it :thumbsup: Pure silicon dioxide would mean that it is quartz glass.
> So in theory there would be an additional 8% of light lost because it travels through the glass-air transition twice. Together with the reflectivity of aluminium (90%) this would cause the actual reflectivity to be 83%.
> 
> Also factoring in the borosilicate lens would mean that around 78% of the inital lumens come out the front.
> ...




Havent had a defective or failed reflector yet, Nickel is very hard and it is very well protected in the housing.


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## PolarLi (Dec 5, 2017)

The_Driver said:


> Seems like it :thumbsup: Pure silicon dioxide would mean that it is quartz glass.
> So in theory there would be an additional 8% of light lost because it travels through the glass-air transition twice. Together with the reflectivity of aluminium (90%) this would cause the actual reflectivity to be 83%.
> 
> Also factoring in the borosilicate lens would mean that around 78% of the inital lumens come out the front.
> ...



The common name is Aluminium Quartz or AQ. You can call it glass, 
then again, you can just as well call it a sand coating, because some of it contains a lot more Silicon Dioxide 
than normal glass.

The reflectivity for an AQ reflector in most of the visible spectrum is around 90%, as the graph show. 
Also, you do get special aluminium coatings with 95-99% reflectivity. I do expect these to cost more, and i suspect they may not be very durable, but you have to contact Optiform about that.

I have a couple of AQ reflectors, and mechanically, I have never had any problems. However, in a harsh environment with a lot of moisture or high UV levels, it's said they will deteriorate faster than Rhodium. But in a hobby application, I doubt you will have any issues.
Also, be aware that AQ has a temperature limit of only 120C. That can be a challenge with big arc lamps.


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## XeRay (Dec 5, 2017)

PolarLi said:


> I do expect these to cost more, and i suspect they may not be very durable, but you have to contact Optiforms about that.



We don't use Optiforms, we use Phoenix Electroformed Products, they "spun off" from Optiforms a long time ago.


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## sven_m (Dec 11, 2017)

The_Driver said:


> This is a very high value for such an HID bulb. Here one can find a detailed doctor thesis (in German) where the luminance of such bulbs was measured. At 35W this would only be possible by focussing on one of the electrode tips. At 70W things are different though. It's possible that the center of the arc reaches 200cd/mm^2 since the bulbs are more than twice as bright at these power levels. So my statement above is not valid anymore. I don't know how they focus these lights.


I once read (might take quite some time to find that again, though) that - given the same bulb - it's rather the arc dimension and not so much the luminance which grows with increasing power. It might have been a paper about real Xenon burners, and only apply to that type. But this sounds like an issue of general arc thermodynamics, as I understood it: The arc would get hotter at first, but then just expand laterally to get away the additional heat, why should it stay concentrated while it can expand easily? This way it sounds reasonable to me. And thus I also really wonderd if Lemax wanted and managed to focus near an electrode tip.

For 35W automotive Xenon: 

 The "Philips Automotive Lighting product catalogue 2008" mentions 60 cd/mm² at the center and 100 near the tip (in a diagram) 
 Osram "D2S / D2R XENARC (TM) High Intensity Discharge Lighting" mentions 65 cd/mm² avg for the whole arc, which matches the 60 above. 
 The thesis above mentions 100-100 cd/mm² in the center of the arc and roughly 250-300 cd/mm² for the very tip. That's much higher than the manufacturers. Thus I wonder if the manufacturers are just conservative (but why should they, max automotive luminous intensity is regulated at least in some countries), but also if the thesis guys might aim high..
All in all I really would guess that Lemax managed to center not at but near the tip. But what do I know :- )

I once refocussed a Polarion PH35 (indeed, not a PH40/50) step by step, through the whole arc, and made photos of the resulting spot, see here http://www.taschenlampen-forum.de/threads/polarion-ph35.20354/#post-266799 , post #14.
Thus it looks as if Polarion focusses rather on the center, for the reason you mentioned, beamprofile. But I would not rule out focussing near the tip for the Lemax.

Oh, I just didn't remember the PH35 photos at first, eyesonfire, my photos might answer your very question to some degree.


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