Aspherical focal length

[snimmer]

<-----new member, first post. First of all, I ran across this site about 6 weeks ago and it inspired me to mod a gordon 3D halogne and offbrand 6v halogen into LED lights. Now I find myself looking at how I can squeeze a few funds out of future paychecks. How can I be 39 years old and just now running across a hidden addiction.

Enough about that, I read in a DX customer comment that you could shorten the focal length when using aspherical lenses by stacking 2 asphericals together. Anybody know if this works? I have two other lights with short heads that I would like to convert if this might fly. Thanks for any assistance you could provide.

 

[DM51]

Welcome to CPF, snimmer.

I'll move your thread to the H&M section, where modifications are discussed.

 

[gcbryan]

<-----new member, first post. First of all, I ran across this site about 6 weeks ago and it inspired me to mod a gordon 3D halogne and offbrand 6v halogen into LED lights. Now I find myself looking at how I can squeeze a few funds out of future paychecks. How can I be 39 years old and just now running across a hidden addiction.

Enough about that, I read in a DX customer comment that you could shorten the focal length when using aspherical lenses by stacking 2 asphericals together. Anybody know if this works? I have two other lights with short heads that I would like to convert if this might fly. Thanks for any assistance you could provide.

Welcome by the way! I can't quite picture that or how that would be useful. I have two aspherical lenses and I don't see how you could stack them in the first place given their shape but I guess you are saying that if place them in front of each other it has the effect of shortening their focal length?

So if I had two 10mm focal length aspherics you are saying that I could put one 10mm in front of the emitter and another one perhaps 5 mm in front of the emitter and the pair together would focus the emitter in the beam?

I don't know a lot about optics theory but it doesn't sound right to me (that doesn't mean much however). I don't have both of the aspherics where I can try this out right now or I would.

I'm sure you'll be more responses soon. By the way, the issue at DX would be even knowing what the focal length was of the aspheric you were looking at. They only seem to list diameter.

 

[TorchBoy]

I read in a DX customer comment that you could shorten the focal length when using aspherical lenses by stacking 2 asphericals together. Anybody know if this works?

It does.

In practice it might be a bit hard to get just right, but if the two lenses you use are identical your new focal length will be half that of using a single lens. The power of a lens is measured in dioptres, which is the inverse of the focal length in metres. So if you use two aspheric lenses each of 50 dioptres (20 mm focal length), together they'll be 100 dioptres (10 mm focal length).

FWIW the human eye is about 60 diopters, only about a third of which is the lens (the rest is the cornea). You could argue that both the lens and the cornea are aspheric; if adding them together didn't work we'd have a lot more car accidents.

:welcome:

 

[007adan]

No, you would lose too much light with 2 aspherical lenses. In order for an aspherical lens to be effective it needs alot of light that can be focused into a tight beam and it needs to be properly collimated with the led. I know that the DEFT uses two lenses the first is a pre-collimator pcx lens and the second is an aspherical lens.

 

[snimmer]

gcbryan,
to clarify, the DX comment was to use copper wire to build a bracket to hold the two aspherics close together at the end of the head (not where one aspheric was close to the led and the other at the end of the head.

007adan, I think you are right about losing too much light

Thanks for the responses everyone, guess I'll try to replace the current aspheric with an aspheric condenser to shorten the focal length

 

[TorchBoy]

Actually 007adan and snimmer it depends what you're doing with your two lenses. I've used two lenses to make a tighter beam than anything a small single lens is capable of, and even though there was much less total light in the resulting beam (the second lens was capturing less than 10% of the light from the first lens), because it was so tight it had a much better range.

As I said, in practice it can be hard to get right, but it's worth a try.

 

[gcbryan]

gcbryan,
to clarify, the DX comment was to use copper wire to build a bracket to hold the two aspherics close together at the end of the head (not where one aspheric was close to the led and the other at the end of the head.

007adan, I think you are right about losing too much light

Thanks for the responses everyone, guess I'll try to replace the current aspheric with an aspheric condenser to shorten the focal length

Where have you found to be the best place for sourcing your lenses? I'm new to this myself.

 

[snimmer]

I have only dabbled with a couple of glass aspherics from DX. Recently read about the aspheric condensers but having trouble finding them in 23mm diam. They supposedly have shorter focal length. Kaidomain has a few as well.

 

[007adan]

Dx has some really nice aspheric lenses, they call them glass optics however. The 66mm is about 2.5 inches which is the biggest lense they sell $4 what a steal. To get a 3 inch lense you need to go to ebay and buy a HID kit that come in pairs only.

 

[007adan]

Torch boy,

Any luck using a pre-collimator lense like saabluster uses on his deft? It consists of a pcx lense that must capture close to 100% of the led light before it hits the aspheric lens. It sits almost directly over the led. I think the eye lense from a pair of binoculars would be perfect for this.. Most binoculars have a 50mm lens which condenses down into the eye piece of a pair of binoculars. I'm guessing the reverse would be true in making a killer thrower. The smaller lens concentrates and captures the light from the led and the larger aspheric takes all of this focused bright intense light and makes a nice tight resolved beam.

 

[Walterk]

Anyone tried this yet? Should work.

Someone on CPF made a dive light with a Macro lens as aspheric.
Think that should work also. At least they have short fl.

 

[TorchBoy]

I have one of the DX 66 mm aspheric lenses on the way that I'll do some playing with. It might arrive by the end of the week.

 

[Nos]

I have one of the DX 66 mm aspheric lenses on the way that I'll do some playing with. It might arrive by the end of the week.

I threw this lens right away. It produced 7 hotspots of one emitter :shakehead

 

[gcbryan]

Just for what it's worth (not much) but in playing around with some aspherics today I noticed that they work when reversed at a different focal length.

I'm not sure if they are just as bright when reversed...it was somewhat difficult to tell.

I had one 28 mm aspheric that was focused in a 6P clone when it replaced the front lens. I had another 28 mm which has a shorter focal length and is therefore "overfocused" at that same point.

I reversed that aspheric so that the curved surface faced the emitter. It was now in focus.

I tried this with several other aspherics as well.

 

[TorchBoy]

I threw this lens right away. It produced 7 hotspots of one emitter :shakehead

I wonder if it's a bit of a variable quantity. After a small amount of testing I've found that mine (which did arrive on Friday) has what I presume is a very subtle nipple lens on the top, meaning the lens has two focal lengths. Apart from that it works very effectively for focussing a Cree XR-E; even with the LED run at 1 W it throws 200 metres easily. I've made a 2x D cell focusable torch with it that should last about three hours on a pair of Eneloop AAs in D cell adapters.

To belatedly sort of address 007adan's question: Using another smaller aspheric lens between the LED and the 66 mm DX aspheric lens (I actually used a C78 1x AA zoom torch on widest zoom) produces a very short focal length (the 66 mm lens is only ~4 mm off the front of the C78) and the projection of the LED die is larger and correspondingly dimmer.

 

[Paul Baldwin]

I wonder if it's a bit of a variable quantity. After a small amount of testing I've found that mine (which did arrive on Friday) has what I presume is a very subtle nipple lens on the top, meaning the lens has two focal lengths. Apart from that it works very effectively for focussing a Cree XR-E;

I've got one of these lenses and I just cant get it to focus properly, I put it down to it being a second/reject. Others seem to have recieved better ones but it's a lottery, you can't expect too much for the amount they cost. I took a picture of my mc-e with a dead die in my projector thread IIRC.

I'm sure my results would have been better if I had received a better lense? I played about with various dx lenses but their quality seemed to be a bit suspect too. In the end I found I got my best results using a 50mm lense as a pre-collimator.

This is still far from ideal as my best results were achieved with the larger lense touching the 50mm.
When funds allow I'll get some higher quality lenses from somewhere like Surplas Shed and try an SST-90 and have another play.

 

[Walterk]

Interesting setup!

What f-number have the lenses?
(or diameter and focal length for that matter)

For a narrow beam you need the largest lens to have a relatively long focal length. The first lens is there to catch light from the Led, the second determines the beamangle (and thus intensity) of the outgoing beam.
The f-number of the second lens better can be 4 then 0,5.

Also, I think the use of a pre-collimator is better once there is a substantial difference in diameter. Say 1:3 or 1:5 is great. If the diameters are to close together, the losses of rays going through two lenses is not made up with the tighter collimation.

I am working on a similar setup.

 

[Paul Baldwin]

Interesting setup! I am working on a similar setup.

I hope it's more professional than mine lol.

I'm afraid I haven't any information on the lenses apart from which ones they were. I didn't take any measurements of focal lengths etc as I was using a light meter on the projector output to measure the difference overall. This was my first play with lenses and I was just trying to get a watchable image from a projector with a dead bulb. I liked the fact the led could potentially last 50,000 hours instead of 2,000 for a bulb!
I'll have a look and see if I can find links to the lenses I used.

 

[Paul Baldwin]

http://www.dealextreme.com/details.dx/sku.13618
66mm-24mm 2-90 degree.

http://www.dealextreme.com/details.dx/sku.4584
22mm 5~80 Degrees (focal length 25mm according to the comments)

http://www.dealextreme.com/details.dx/sku.5297
28mm (someone in the comments reckons 16mm focal length to get an image of the die)

I wouldn't really recommend any of the above lenses for this sort of application.

http://www.kaidomain.com/ProductDetails.aspx?ProductId=1753
Specifications:
Focal length: 34mm
Diameter (overall): 52mm
Inner lens diameter inside the bezel: 46.8mm
Material: Optical glass, double sided Anti-reflection coating,
light transparency 98.8%+ (UCL grade)
Center thickness: 25mm
Back focus length: (I measured 22.5mm)
I think this lense is pretty good quality to be honest, it focuses the dies very clearly. I wish the 66mm had been equivalent as I'm sure my results with better smaller lenses would have improved my results even more.

I ordered all the above without really researching anything much tbh, they were just cheap. I did however find one of these which may be a slightly better holder for my MkII version? http://www.dealextreme.com/details.dx/sku.34509

I did find a usefull nugget of info here quite recently tho http://www.candlepowerforums.com/vb/showthread.php?t=248766 Check Saablusters post #3

electrodacus has also got some usefull info, photos and results in this thread http://lumenlab.com/forums/index.php?showtopic=24619&st=0 which got me started in the first place

I think I'd go for one of these next http://www.surplusshed.com/pages/item/l3788.html with an SST-90 and possibly a pre-collimator but they don't list international shipping costs and in my experience of firms that don't it's usually because they are totally unrealistic compared to the likes of KD,DX etc.