Formula for calculating throw using aspheric lens

[gcbryan]

I think it's hard to find a host for the larger diameter lenses. I don't know what is considered a longer focal length but most aren't long enough to mount as a simple replacement for the front glass lens.

The hosts are all (of course) made for reflectors and therefore tend to be deep as well as wide. For an aspheric wide and shallow would be good.

There's always cat food cans...right Dr!

 

[Dr.Jones]

So, using two lenses as beam-expander or reversed telescope, results in:
- different f-number (only affecting Lumen as the viewingangle changes )
- different diameter ( affecting throw as surfacearea changes )
- different beam angle ( only affecting the spotsize )

Not sure if I understand you completely, but yes, a beam expander increases the diameter (and thus throw) and decreases the beam divergence (giving a smaller spot), while keeping the same flux (minus losses).
The two lenses of a beam expander should have the same F#.

Using a precollimator then is great for making use of the large lenses.

Again I'm not sure if I understood you correctly...
Without a precollimator the lens is already completely used, since the LED beam is much wider than the lens. But only a fraction of the beam flux is used. The pre-collimator helps getting more flux (lumen) into the main lens, resulting in a wider beam (bigger spot) with the same intensity (same throw).

There's always cat food cans...

 

[ma_sha1]

Well, as long as you think focal length has influence on throw, you don't have enough experience, I'm afraid.

Dr. or Ra,

Would it be fair to add a qualifier "Assume the same lens efficiency" before the "Focal length doesn't matter" theory?

Ra's exp. is pretty convincing, however, it's done with long EFL lens.
Most Aspheric lens folks used in flashlight has a small focal length, for example 35mm-38mm EFL on a 52" lens, (F number in the .7,-.8 range) where the Led is about an inch away from the flat side.

It makes sense to me when you get "Too close" to the lens, the angle from led to the outside section is getting more wide, which will cause more reflection loss & therefore less light going through the lens.

People always thought led "too close to the lens" cause the lens to reduce "efficiency" due to the colimating angle being too wide, by Ra's formular, reducing lens efficiency = less throw.

So, Is it possible that there's a minimum Focal lens limit where the "Focal length doesn't matter" theory holds true? where the further reduction of Focal length with reduce throw?

Would love to see some experiment like Ra's but with same diameter lens that has small F numbers, really small like 0.5., 0.7, 0.8, 1.0 etc.

Small F numbers is preferred to keep the flashlight compact, especially when trying to fit 3"-4" large diameter Aspheric lens into a flashlight.

Thanks for your contributions, I find your input enlightening, in a place most people are limited by "try & error" approaches.

 

[Dr.Jones]

Would it be fair to add a qualifier "Assume the same lens efficiency" before the "Focal length doesn't matter" theory?

It makes sense to me when you get "Too close" to the lens, the angle from led to the outside section is getting more wide, which will cause more reflection loss & therefore less light going through the lens.

So, Is it possible that there's a minimum Focal lens limit where the "Focal length doesn't matter" theory holds true? where the further reduction of Focal length with reduce throw?

Yes, indeed. However, that reflection loss depends on the polarisation. Up to a certain angle (brewster angle, ~56° for glass), light polarized radially actually suffers less reflection loss, while the losses for light polarized tangentially always increases. That gain and loss cancel each other out to some extent, the losses prevail though. That brewser angle however corresponds to an F# of 0.34. Lower F# would be very bad, since both reflection losses increase a lot then.

 

[Ra]

A pre-collimator creates a magnified virtual image of the die (just like the LED dome); that image has a smaller viewing angle, and that virtual light source's luminance is the same as the die's, so there's no increase in throw. The bigger size of the virtual image results in a wider beam and thus a bigger spot.

You catch more of the LED flux with a pre-collimator, but again it just increases spot size, not throw.

I hope I'm allowed to add the remark that this means that a pre collimating lens is only for getting more torchlumens from the source. By narrowing the beam angle from the led, the pre collimating lens alowes the aspheric lens to grab more lumens within the main beam, causing the wider beam.


Regards,

Ra.

 

[Walterk]

The two lenses of a beam expander should have the same F#.

Sorry Doc, I happen to think to knnow there is more to this then just that !

For beam-expanders / telescope / eyepieces (working all the same ) there are several principles based on plain convex lenses, all effective.
Maybe one more then another but if you have several lenses around then probably you can find a combination that is working.

Especially look for the Kepler, Ramsden and Huygens eyepiece on Wiki.

I see Ra posted, so for clarity:
I like throwing lights, preferable portable, not necessarily compact and pocketsize.
For that I found me a couple of big lenses, bigger then the average flashlight will fit, among them 75 and 100mm.
But having long focal lengths, it's to little Lumen to build me a light from those lenses alone.
But using a pre-collimator lens, I can put the full diameter of the lenses in function.
They give more throw then whatever you carry in your every day.

 

[Ra]

And regarding Saablusters posts #69 and #71:

I do not have the time, nor the need to react on that right now (or ever)!!!
And even if I did, it would have no use I think..

But those posts can bring some unnessesary doubts for some members, and that is not a good thing, as this already is quite difficult matter for some..(at least for one apparently..)

Regards,

Ra.

 

[Ra]

I see Ra posted, so for clarity:
I like throwing lights, preferable portable, not necessarily compact and pocketsize.
For that I found me a couple of big lenses, bigger then the average flashlight will fit, among them 75 and 100mm.
But having long focal lengths, it's to little Lumen to build me a light from those lenses alone.
But using a pre-collimator lens, I can put the full diameter of the lenses in function.
They give more throw then whatever you carry in your every day.

To certain extend, yes.. But there is a limit to the usefullness of such a setup: a 75mm lens with a focal length of 300mm already is not very practical for this.. But you definitely will have a very long torch...

 

[saabluster]

With all due respect, that's getting a little old as well.

Why are you in this thread if that's the case? You have your unfinished thread (sticky) and you have time to state, yet again, that RA makes no sense (to you?).

It's disruptive to interrupt a thread to only criticize and then to become "busy" when the questions become more detailed. If you understand the subject then it shouldn't take any more time to post a paragraph regarding the subject matter than to post a paragraph disagreeing with RA's point.

And regarding Saablusters posts #69 and #71:

I do not have the time, nor the need to react on that right now (or ever)!!!

So the question gcbryan is are you going to apply the same standard to Ra as you are to me? I have stated my arguments and they have yet to be addressed. Don't try and put this all on me. And I find it funny this so-called physics professor(although I have yet to see him claim that) has just shown up here regurgitating things both myself and Ra have posted and he is now suddenly the expert in your eyes.

 

[Walterk]

To all: please keep the discussion to Led's, light, lenses and if you can't resist reflectors.

 

[Ra]

Dr. or Ra,

Would it be fair to add a qualifier "Assume the same lens efficiency" before the "Focal length doesn't matter" theory?

Ra's exp. is pretty convincing, however, it's done with long EFL lens.
Most Aspheric lens folks used in flashlight has a small focal length, for example 35mm-38mm EFL on a 52" lens, (F number in the .7,-.8 range) where the Led is about an inch away from the flat side.

It makes sense to me when you get "Too close" to the lens, the angle from led to the outside section is getting more wide, which will cause more reflection loss & therefore less light going through the lens.

People always thought led "too close to the lens" cause the lens to reduce "efficiency" due to the colimating angle being too wide, by Ra's formular, reducing lens efficiency = less throw.

So, Is it possible that there's a minimum Focal lens limit where the "Focal length doesn't matter" theory holds true? where the further reduction of Focal length with reduce throw?

Would love to see some experiment like Ra's but with same diameter lens that has small F numbers, really small like 0.5., 0.7, 0.8, 1.0 etc.

Small F numbers is preferred to keep the flashlight compact, especially when trying to fit 3"-4" large diameter Aspheric lens into a flashlight.

Thanks for your contributions, I find your input enlightening, in a place most people are limited by "try & error" approaches.

My lens experiments have enough variance in focal length to prove the theory right.. Lenses with shorter focal length act the same, as long as the entire surface plays along, like said earlier. Lens distance to the source does not matter (at least, within the focal length's I used during my experiments..). Apparent surface brightness is not affected by source-lens-distance..

Extremely short focal length's bring difficulties for the shape of the lens: Apart from the fact that they need to be extremely aspheric, the extreme radius of curvature brings a limit as well.. The shorter the focal length, the harder it is to make the entire surface play along.. (needed for throw..)


Regards,

Ra.

 

[pepko]

Hi all,

can you help me ?
What should I do to get more throw in my setup ?
https://www.candlepowerforums.com/threads/280018

I use 3" aspheric lens 74(70)mm diameter, 28mm height. Distance from led is about 40mm.

Thanks.

 

[Dr.Jones]

Walterk, you're right, ... but I can't resist (Ill keep it short):
Saabluster, I'm not a professor (no one said so (except you)), but I have a doctor's degree in experimental physics (related to optics). I bring up arguments from Ra, because they are correct, also arguments I learned in university, for the same reasons. All that stuff isn't actually new. Furthermore, gcbrian and I know each other already from another forum.

Back on topic, beam expanders:

For beam-expanders / telescope / eyepieces (working all the same ) there are several principles based on plain convex lenses, all effective.

Yes, eye pieces can be very complex, mostly for two reasons: avoiding chromatic aberration and getting a good field of view with low image distortion. For expanding a collimated beam, the latter is often unimportant, because the beams have rather small angles to the optical axis already, so a simple setup often suffices, although the use of achromatic lenses (duplets) is advisable for white light.


Back to pre-collimating:

I present my newest super-thrower, the Sloppy270:

I guess you can see why it got that name.... and I didn't even apply a battery yet (and probably never will; on the other hand, I might take it out for some field test...)

There are three versions:
ver1: no pre-collimator,
ver2: low-NA (high-f#) pre-collimator, f=150mm
ver3: higher-NA (lower-f#) pre-collimator, f=150mm

The spot brightness, measured at 18.3m, is nearly the same for all three, it's a bit lower for ver3 because the pre-collimator is only a spheric lens with quite some aberrations at that NA (or f#).
While the spot brightness is roughly the same, with the pre-collimator the spot size increases (5cm/10cm/15cm).
And of course a bigger spot with the same illuminance (lux) means more flux (lumen).

 

[Dr.Jones]

What should I do to get more throw in my setup ?

Really nice light... Would like to have one

Other than getting a bigger lens or a LED with higher luminance (surface brightness) - none, as far as the optics basics go.
Then there's anti-reflex-coating the lens, using a lens with better quality (if yours isn't already very good), better heat sinking for less thermal sag, ...

Regarding basic optics, there are ways to increase the spot size and total flux (lumen) though.

 

[saabluster]

Saabluster, I'm not a professor (no one said so (except you))

Dr. Jones (as I recall) has a Phd in Physics.

Back to pre-collimating:

I present my newest super-thrower, the Sloppy270:



I guess you can see why it got that name.... and I didn't even apply a battery yet (and probably never will; on the other hand, I might take it out for some field test...)

There are three versions:
ver1: no pre-collimator,
ver2: low-NA (high-f#) pre-collimator, f=150mm
ver3: higher-NA (lower-f#) pre-collimator, f=150mm

The spot brightness, measured at 18.3m, is nearly the same for all three, it's a bit lower for ver3 because the pre-collimator is only a spheric lens with quite some aberrations at that NA (or f#).
While the spot brightness is roughly the same, with the pre-collimator the spot size increases (5cm/10cm/15cm).
And of course a bigger spot with the same illuminance (lux) means more flux (lumen).

Agree with everything you show in your experiment and it falls right in line with my own.

Benefits of additional optic
(1)With the additional optic there was 60% more light getting out.
(2)There is a huge reduction in chromatic aberration.
(3)It now throws slightly farther.
(4)The field of view is bigger due to a larger projected die size.

 

[ma_sha1]

Bravo, Dr.

First of all, the 116mm lens with Q5 @0.9A converts to 274,000 Lux at 1 meter, beating DEFT FTP 135Lux at 1 meter by 200%. The New King of LED thrower, if you can pack them into a flashlight.
(Please check my math 820x18.3x18.3 =~270K lux)

Size does matter, no matter how you work it :devil:

Second of all, the only thing that the pre-colinmator did that could be of benefit to a flashlight is to reduce the system EFL, thus, allow led to move closer to the lens & make it more compact.

So if a single lens could fit in a flashlight, there's no "throw advantage" on introducing a pre-lens,
if anything, it'll reduce lux by failing to transmit 100% of the light through. But it does make a much bigger spot.

This is a myth buster on pre-collimation will increase throw, nice job!
.

 

[Dr.Jones]

Having a PhD does not imply being a professor, at least not around here.

Benefits of additional optic
(1)With the additional optic there was 60% more light getting out.
(2)There is a huge reduction in chromatic aberration.
(3)It now throws slightly farther.
(4)The field of view is bigger due to a larger projected die size.

(1) Actually ver2 has ~300% more light; ver3 ~700% more light (flux)
(2) No, it actually gets worse, especially ver3.
(3) No, ver2 slightly less, ver3 even worse.
(4) Yes.

ma_sha1 said:
(Please check my math 820x18.3x18.3 =~270K lux)

That's what I mean with 270kcd.
1 cd is equivalent to 1 lux at 1 m - if the light source size is negligible. With well collimated throwers it isn't, so it should be measured at a bigger distance.
I measured it at 11.9m and 18.3m and got consistent results.

BTW... ver1 has a very narrow beam... good laser pointers have 1 mrad, bad laser pointers have 2 mrad, ver1 has 3 mrad... A 'light pointer'?

 

[Ra]

Really nice light... Would like to have one

Other than getting a bigger lens or a LED with higher luminance (surface brightness) - none, as far as the optics basics go.
Then there's anti-reflex-coating the lens, using a lens with better quality (if yours isn't already very good), better heat sinking for less thermal sag, ...

Regarding basic optics, there are ways to increase the spot size and total flux (lumen) though.

I hope you don't mind Dr.Jones.. But: To complete this answer..

When you can create better heatsinking, you can increase power to the led (somewhat..), which brings higher surface brightness, and therefore better throw..


Regards,

Ra.

 

[saabluster]

Second of all, the only thing that the pre-colinmator did that could be of benefit to a flashlight is to reduce the system EFL, thus, allow led to move closer to the lens & make it more compact.

So if a single lens could fit in a flashlight, there's no "throw advantage" on introducing a pre-lens,
if anything, it'll reduce lux by failing to transmit 100% of the light through. But it does make a much bigger spot.

This is a myth buster on pre-collimation will increase throw, nice job!
.

Yeah it is not quite busting any myths. It did and does increase throw on the DEFT as it corrects for aberration in the lenses I make. As I have mentioned before the biggest effect is more throughput and broader beam. Given perfectly made lenses then yes the throw will not increase. However it is also folly to say there is no benefit for flashlights other than a reduced FL. Since the beam becomes broader that means your field of view is larger. Given an aspheric's propensity to have an extremely narrow beam this is a huge benefit in a flashlight application.

Having a PhD does not imply being a professor, at least not around here.

It is true that the word professor can be taken more than one way. One of those refers to someone who has had and graduated from a school of higher learning and is now no longer a student. That is the sense I called you a professor and it is because of comments made not by myself.

(1) Actually ver2 has ~300% more light; ver3 ~700% more light (flux)
(2) No, it actually gets worse, especially ver3.
(3) No, ver2 slightly less, ver3 even worse.
(4) Yes.

You totally missed the point of that part. That was a link back to tests I did of my light. I got 60% more throughput. I had mine throw slightly farther.

 

[ma_sha1]

That's what I mean with 270kcd. 1 cd is equivalent to 1 lux at 1 m

Thanks, I didn't know what a cd was