Intro to aspheric lenses (to help you choose the best one)

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snoopy
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Re: Intro to aspheric lenses (to help you choose the best on

Post by snoopy » 24 Jan 2013, 21:34

cant beet simplicity and engineering in one....
'who is that masked man? its the kemosabby der..the bloody injun tells you every week!'

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Re: Intro to aspheric lenses (to help you choose the best on

Post by Humpers » 25 Jan 2013, 22:27

OK, I've been doing some more thinking and further experiments in the field, and Phoenix (Bruce) has been helpfully sending me some really good thoughts too, so I've got an update.

I've realised that LED die size has even more of an influence on brightness of the beam from aspheric lenses than I first estimated (small is good). But of course things also APPEAR smaller when they're further away (no this hasn't turned into a philosophy lesson!). So being able to put an LED further away from the lens (by using a lens with a longer focal length) makes an LED with a small die appear even smaller to the lens.

Therefore I've updated my original post with the following, so that it's all in one place for anyone new reading this thread:


Effect on beam concentration/intensity of LED die size and distance from lens to LED

I’ve included this section because although at first glance the best lens would seem to be one which has a short focal length (so it can be placed closer to the LED where the LED’s beam is less spread out, and the lens doesn’t need to be as large in diameter to capture all/most of the LED’s output for long ranges). However, this is not necessarily the case…

Aspheric lenses produce the narrowest and hence most concentrated (brightest) beam of light when the light source is a single point source at the focal point of the lens. A point source just means an infinitesimally small pinprick of light. This should be at the precise point where the lens focuses; again this is a very small area (the focal point). In these circumstances the lens produces a parallel beam (pencil beam) of light of the maximum intensity for that lens/light source combination. If the light source is large some of the light is produced away from the focal point, and this will not be properly focused by the lens, so will be sent off at an angle by the lens rather than being concentrated into the parallel beam. So it is effectively wasted.

Therefore a very small LED die is much better than a large one as it’s more like a single point source, and therefore a higher proportion of its output will be concentrated into the parallel beam. However the size of the die as seen by the lens also varies depending upon how far away the LED is from the lens. As far as the lens is concerned the LED APPEARS to be smaller when it’s further away from the lens than when it’s closer. It’s all to do with what’s known as the solid angle, and is measured in steradians (I've included links if anyone wants to find out more).

So a lens with a LONGER focal length allows you to place the LED further away (more like a point source), and will produce a more tightly focused and hence a brighter beam at distance. There's a limit to what lenses can do, but generally the smaller the light source the tighter and therefore brighter the beam coming out of the lens. From my experiments in the field a small LED die is critical to getting decent illumination at range. Fortunately the Oslon Black IR LED is very good in this respect, with a 1mm x 1mm die.

Remember that to get the best results it is critical to mount the LED exactly centrally behind the lens, and not slightly to one side.


Lens quality

Basically a high quality lens will let through more light than a low quality one. Also the shape of a quality lens will be more precisely engineered to concentrate more of the light into the pencil beam. Unfortunately cost and reputation seem to be the only indicators of quality in aspheric lenses. Like so many things you probably get what you pay for.

I’m not aware of any coatings applied to aspheric lenses, similar to coatings applied to scope lenses to reduce reflections and hence to let more light through, but be aware that some coatings block IR light.


Summary

In my view the IDEAL setup would be:
  • A high output LED with narrow beam (small angle of half intensity) and a small die size.
  • An aspheric lens with long focal length so that the LED is a small point source, but with the lens being sufficiently wide to capture all of the LED’s output.
  • A zoomable illuminator to adjust distance from lens to LED, so that you can vary the intensity and width of the beam to suit the range you’re shooting at. Remember that if you’re using a zoomie to adjust from spot to flood, if possible always move the lens CLOSER to the LED rather than further away. This is to ensure that you’re capturing more of the LED’s beam rather than loosing some around the edge of the lens, due to the way the way the beam spreads out as it gets further from the LED.

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Re: Intro to aspheric lenses (to help you choose the best on

Post by Humpers » 25 Jan 2013, 23:15

And there's more...

Having thought about LED die size and solid angles I've tried to incorporate these factors into my spreadsheet. I think I've done it correctly but can't guarantee this as I'm getting towards the current limits of my understanding of the subject.

The table below also allows you to compare results for one LED with another as it takes into account to different outputs from each. I've not been able to get hold of any reliable information about the efficiency of light transmission of different lenses, so this may account for any discrepancies which don't match results you get in the field.

Please take the absolute numbers in the table with a pinch of salt, as I'm not sure that an LED/lens combination with a figure double that of another will be twice as good in the field; it may not be a linear relationship.
IR LED & aspheric lens comparison table, v3.jpg
IR LED & aspheric lens comparison table, v3.jpg (71.73 KiB) Viewed 917 times
As a small point source of light seems to be so important, I had the brainwave of trying my Odicforce 200mW laser behind an aspheric lens instead of an LED, as the laser beam diameter must be very small compared to the 1mm x 1mm Oslon LED die. These lasers come with what I believe is a very small collimating lens, which I first removed.

Behind an ahorton aspheric I got the narrowest, brightest beam I have ever seen when I got it at the focal point. However, although it was very bright it was so narrow that it probably wouldn't be usable for hunting (poor field of view). And although my laser is fairly good with relatively few aberations in the beam, they were also still visible (obviously).

But if you've got a laser and an asheric you might want to try it! Just make sure the laser is centred exactly behind the lens and that it's at the back focal length to get the maximum performance.

Have fun!

Humpers

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Re: Intro to aspheric lenses (to help you choose the best on

Post by snoopy » 26 Jan 2013, 01:43

it;s late, i'm a bit cabbaged so my philosify on it is get a huge bulb and crank up the volts till a pritty signal smoke cloud fluffilly waffs accross the bow..oh weight that's flash fotograghy.. time for bed.

hehehe :shock:
'who is that masked man? its the kemosabby der..the bloody injun tells you every week!'

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Re: Intro to aspheric lenses (to help you choose the best on

Post by Hartshot » 26 Jan 2013, 08:32

This is for specifically for Humpers :

Just spitballing here after reading the above - Would there 'potentially' be an advantage stacking 2 lenses (if they could be balanced) to magnify the beam ?
In other words, set the first lens at it's best focal length to the LED and then having adjustment on the second. Say a 28mm (or smaller) as the first lens and then a 66mm as the second !
Can't figure it out in my head so thought I'd ask you
In the cabbage :sick: that is my brain it would give max output and adjustment which is outweighed by the loss in transmission of putting an additional lens into the mix.

H
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Re: Intro to aspheric lenses (to help you choose the best on

Post by johnnyrebel » 26 Jan 2013, 09:19

Some very useful and interesting information there Humpers!.I too have tried the laser behind an aspheric but couldn't really make any significant difference to the beam, i was trying to spread the beam ,not concentrate it, so had the aspheric the other way round but i haven't got enough lenses to do an in depth experiment. I don't think it's a case of simply reversing the procedure of led's with aspherics and trying to apply that to lasers. Lasers are a different baby altogether, a few of the guys up at my local radio club have been experimenting with lasers and if i remember correctly,had managed to transmit a radio signal about 7 miles down a laser beam but i'm sure they had broadened the beam by electronics rather than using lenses. There's got to be some sort of connection between the two though because we're talking about wavelengths and frequencies here. I'm not clever enough to make the connection nor have i got the inclination to get back into all the radio stuff again ( had about 10 years of mind boggling radio theories/ experiments!).This thread has provided the information i needed and i'm sure it's been useful to a lot of other people going down the ir led illuminator path. Prior to this i was struggling getting over 40yds, mainly down to the quality of the camera, now, armed with watec 902h, oslon blacks, 50/66mm asherical lenses, night has changed to day and i'm a very happy man!.Thankyou lads for the information and all the time and effort you must have put in to achieve such a pool of knowledge. I take my hat off to you, well done.
john :D
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Re: Intro to aspheric lenses (to help you choose the best on

Post by some bloke » 26 Jan 2013, 09:52

This is incredibly in depth stuff - thanks Humpers.

The ahorton lens seems to have now switched from the best to the worst performer, I have never tried the kaidomain ones but it now looks like I should if I am reading your last results correctly? EDIT: Were I to feel I need more light which I don't right now but hey - guys like to play with torches lol..

I seem to have got something muddled because I think I read that thicker lenses are better than thinner ones. I'm slighly dyslexic and sometimes have trouble absorbing the written word when it's too long or deep so I've probably got that wong. :oops:
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Re: Intro to aspheric lenses (to help you choose the best on

Post by Humpers » 26 Jan 2013, 15:27

Hartshot wrote:This is for specifically for Humpers :

Just spitballing here after reading the above - Would there 'potentially' be an advantage stacking 2 lenses (if they could be balanced) to magnify the beam ?
In other words, set the first lens at it's best focal length to the LED and then having adjustment on the second. Say a 28mm (or smaller) as the first lens and then a 66mm as the second !
Can't figure it out in my head so thought I'd ask you
In the cabbage :sick: that is my brain it would give max output and adjustment which is outweighed by the loss in transmission of putting an additional lens into the mix.

H
It's an interesting thought, and I'd suggest it would be worth trying as you've got nothing to loose (assuming you have all the bits), but unfortunately I think it wouldn't work well.

Apart from the transmission losses, the light entering the 2nd lens would be coming from the full diameter of the 1st lens rather than a tiny spot which is what you want. So most of the light leaving the 2nd lens will NOT be in the parallel beam, but will be going off at all angles instead.

Sorry, but do give it a try and let us know how you get on.

Humpers

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Re: Intro to aspheric lenses (to help you choose the best on

Post by Humpers » 26 Jan 2013, 15:35

johnnyrebel wrote:Some very useful and interesting information there Humpers!.I too have tried the laser behind an aspheric but couldn't really make any significant difference to the beam, i was trying to spread the beam ,not concentrate it, so had the aspheric the other way round but i haven't got enough lenses to do an in depth experiment. I don't think it's a case of simply reversing the procedure of led's with aspherics and trying to apply that to lasers. Lasers are a different baby altogether, a few of the guys up at my local radio club have been experimenting with lasers and if i remember correctly,had managed to transmit a radio signal about 7 miles down a laser beam but i'm sure they had broadened the beam by electronics rather than using lenses. There's got to be some sort of connection between the two though because we're talking about wavelengths and frequencies here. I'm not clever enough to make the connection nor have i got the inclination to get back into all the radio stuff again ( had about 10 years of mind boggling radio theories/ experiments!).This thread has provided the information i needed and i'm sure it's been useful to a lot of other people going down the ir led illuminator path. Prior to this i was struggling getting over 40yds, mainly down to the quality of the camera, now, armed with watec 902h, oslon blacks, 50/66mm asherical lenses, night has changed to day and i'm a very happy man!.Thankyou lads for the information and all the time and effort you must have put in to achieve such a pool of knowledge. I take my hat off to you, well done.
john :D
Hi John,

I suspect the one of the reasons you're a very happy man is because you've got a Watec. They're much better than the 1/3" CCD board cameras, so you start out with a massive advantage even before you need to think about which illuminator to use.

An aspheric will work with a laser, as the laser is basically just a light source, albeit a very narrow one (and all the photons in phase, etc). To make the laser beam spread then use the aspheric the right way round (flat side towards the laser) but position the lens slightly nearer or further from the laser than the lens' back focal length. It should work fine.

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Re: Intro to aspheric lenses (to help you choose the best on

Post by terry1001 » 26 Jan 2013, 15:46

Humpers wrote:
Hartshot wrote:This is for specifically for Humpers :

Just spitballing here after reading the above - Would there 'potentially' be an advantage stacking 2 lenses (if they could be balanced) to magnify the beam ?
In other words, set the first lens at it's best focal length to the LED and then having adjustment on the second. Say a 28mm (or smaller) as the first lens and then a 66mm as the second !
Can't figure it out in my head so thought I'd ask you
In the cabbage :sick: that is my brain it would give max output and adjustment which is outweighed by the loss in transmission of putting an additional lens into the mix.

H
It's an interesting thought, and I'd suggest it would be worth trying as you've got nothing to loose (assuming you have all the bits), but unfortunately I think it wouldn't work well.

Apart from the transmission losses, the light entering the 2nd lens would be coming from the full diameter of the 1st lens rather than a tiny spot which is what you want. So most of the light leaving the 2nd lens will NOT be in the parallel beam, but will be going off at all angles instead.

Sorry, but do give it a try and let us know how you get on.

Humpers
I tried doing this using a Sony camera doubler lens in front of a 501b with dx lens etc but only inside the house and there did seem to be a significant improvement in the beam. It was only a brief and not very careful experiment but did show some promise but as I don't feel the need for more range at present I didn't go any further, it's just an idea for the future.

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