Test of DTR's "LFL" Aspheric Lens.

[The_Doctor]

DTR gave me an 'LFL' aspheric lens to try when I bought some other things from him. I'll add to this thread as I get more detail. There is no documentation for the lens, so we have to figure it out as we go...

First, the lens is 6mm diameter, unlike 405-G-2's 6.35mm. Dave's barrels have an internal threaded bore diamter of 6.56mm, and a ledge bore diameter (aperture) of 5.58mm. This is a fragile fit for the lens, but usable if not under much pressure.

The lens thickness is 2.69~2.7mm, measured by caliper with digital display, using two bits of 0.1mm polyester drafting film to support the lens and protect it from the gentle clamping force needed to retain it during measurement. Other measurements are beyond this method, but a photograph allows good estimates of shape and dimensions based on known thickness and diameter.

The polyester film is important. It is washable in acetone and withstands high temperatures without deformation. The diffusing surface makes it slightly compressible, and the friction helps retain the lens in place. It is 0.1mm thick, which is ideal in this case.

A bore lining can be made from it with a coiled cylinder of three turns (strip length 58mm, allowing the bend in transition between the three layers without forcing concentricity error), to get 0.56mm under gentle pressure, a near perfect fit to adapt a 6mm OD lens to the 6.58mm bore. The stuff is slightly compressible, so will centre the lens accurately (likely within a few microns), and withstand enough heat to prevent later warping. (It's good enough to withstand laser printer toner fusing heat with no loss of accuracy in fine PCB artwork). Currently there is no secure clamp to retain the lens, but once tests are done to find the maximum beam width, it should be possible to know if a bit of thin modelling tube can run down the inside of the bore lining and be clamped by the barrel's original lens retainer without cutting into the beam. Trying to make the film strip just 1.2mm wide to fit well to the lens is likely impossible, by hand. I tried TEN times and could not do it once. Only a saint would have patience to make an Arctos-style array that way. The easiest way is to make the strip about a quarter inch wide for easy fitting, then gently press fit the lens into it using soft aluminium modelling tube until it reaches the back end, flush against the barrel's ledge. Not only is this easy, it's easily repeatable without damage to anything.

Next post, when ready, will be some measure (or demonstration) of focal length (and maybe of transmission efficiency). The point of this lens according to DTR is that it makes a beam profile more square from a red diode (specifically G71) without extra optics. I haven't seen that yet, I'm going very cautiously, documenting whatever seems important as I go. I'll do cooling tests too, but right now I have no TEC's, and no spare money, so that will have to wait unless someone wants to donate a 20~30mm TEC to the cause.

[olive13]

Maybe A390-A with Broadband coating.

[dnar]

Doc, what's your location? I have a collection of TECs of various size and power up to 66W and would be happy do send you one.

[The_Doctor]

Well done. Thanks, this will really be useful. I'll be buying at least ten cheap 40mm ones eventually, but for tests I could use one about 20W and no less than 16mm square. Smaller rather than bigger, otherwise I should have to use a heat spreader and mess up my designed height.

I'm in Bristol, UK. And I'd pay for postage. No problem if it's not so feasible, after all, I guess you're still in Australia right now, about as far as it gets..

[The_Doctor]

Maybe A390-A with Broadband coating.

Nope.. It's not very clear in the photo perhaps, but there's a good solid bump on the back set in a plane. The A390 variants I found diagrams for all have a shallow curve occupying the entire rear face. Also, from what I've seen today, the FL is a tad bit shorter, closer to 3.7 or so.

Incidentally, 'LFL' apparently means Longer Focal Length, I think in comparison to Dave's 2mm FL lens (it's definitely shorter than A390). I forgot to mention that in the first post.

[dnar]

Well done. Thanks, this will really be useful. I'll be buying at least ten cheap 40mm ones eventually, but for tests I could use one about 20W and no less than 16mm square. Smaller rather than bigger, otherwise I should have to use a heat spreader and mess up my designed height.

I'm in Bristol, UK. And I'd pay for postage. No problem if it's not so feasible, after all, I guess you're still in Australia right now, about as far as it gets..

I'll get it posted tomorrow. Please PM your full address. What is "too long" to wait?

Yep, the smallestbis around 15mm square, I'll check after this beer.

[The_Doctor]

No worries about time. I love procrastination and any excuse will do. PM coming up. As is a post here about FL and transmission ASAP. Can't procrastinate everything or nothing would happen at all...

[dnar]

As I say to my wife almost daily, why do today what can be done tomorrow? That's the good thing about unfinished projects, they are still there tomorrow.

Hows this for size? I don't have the power spec for it. It's the smallest I have, from here we jump to much larger @ 33W.

[The_Doctor]

Nice, that will fit well. It will be strong enough to manage close to zero °C I think, too. My test will likely stay north of that just to keep things dry, at least at first.


And to next stuff....

Using the end of the caliper, a depth gauge used with the barrel front end up on a polished stainless steel plate, the total length of one of Dave's barrels is 9.94 mm, and the depth to the ledge at the back is 9.41 mm (9.45 on a second try), so I'll assume 0.5mm as the nominal thickness. The bump on the back of the lens appears to stick out slightly less than this, so I assume a BFL of about 3mm max, not less than 2.75mm. What the EFL is I don't know without examining an accurate diagram, but I guess about 4mm, maybe as much as 4.5mm.

A very crude transmission test was based on a 1mV per mW reading by thermopile sensor and Fluke 79 multimeter, not zeroed because my circuit is too temperamental to do quick tests with. (Is accurate, calibrated by Lasercheck, but has poor thermal stability, and zeroing with a screwdriver upsets the zero reference while adjusting, so it's a hit-and-miss affair, time-consuming trial and error even in a very settled room...).

Instead, it was left to settle as best possible in a sealed room, reaching -77mV with low ambient light and no strong heat sources in the room. Light fog was used for visibility of focus. I stayed behind the sensor, and did not move during the test other than to take pictures and remove the lens from the beam. The laser is DPSS, with IR leakage, roughly 40% of its light is 532nm, the rest 1064nm. When it was stable enough to allow plenty of time for the photos, I took three shots with readings: 46, 47, 46 mV, then immediately removed the lens and took three more: 53, 52, 55 mV.

Averaging those, then adding the -77mV offset for 'zero' reference (no input beam) gives 123.333mW with lens, and 130.333mW without it. The test was arranged so that the spot formed by the lens had as closely as reasonably possible the same size and position as the beam without it formed on the thermopile.

While the test is crude, it strongly suggests that at least 95% of green light and IR gets through. Which suggests a broadband coating. A close look with bright light and magnifier seems to confirm that, showing a 'bloom', a bluish tint like MgF2.

And now I will procrastinate like there's no tomorrow. Ummm...

[logsquared]

Interesting. The g2 is around 4mm EFL. I figured these would be at least 6mm EFL or maybe even 8mm.