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Old 07-09-2007, 08:07 PM #1
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Default Divergence measurements - question

I tried a divergence measurement with my DX 30mW green. I am fairly unfamiliar with doing this so I thought I would run it by you so see if it was done properly.

My target was 430.5 inches (10934.5mm) away from the front opening of the laser. The spot measured 7mm at the target and 2.5mm at the opening. I used a piece of paper and measured it from the backside.

If I understand it correctly, I am supposed to subtract the width of the outgoing beam from the spot size and divide that result by the distance to the spot.

(7-2.5)/10934.5 = 0.0004115 rad (0.41mrad)

Is the process correct and is it a good figure?

Thanks,

Paul


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Old 07-10-2007, 12:45 AM #2
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Default Re: Divergence measurements - question

Well... The figure is sure nice... but I'm not sure about that... I'll look into it for you a bit. I'm guessing there is some kind of discrepancy with the units or something :-?

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Old 07-10-2007, 01:01 AM #3
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Default Re: Divergence measurements - question

How did you measure the spot sizes? The math assumes that your spot size measurements are 100% accurate, which I guarantee they never will be without a profiler, but if you measure correctly you should get a decent approximation.

HOWEVER, your distance is a bit short. 20m+ would give you a much more accurate measurement and 50m+ is ideal for a good approximation.

and again, I stress its an approximation.

That said, doing the math on your data, saying a distance of 10.9347m, and a change in spot size in between 7mm-2.5mm and 7+2.5mm, I calculate that your divergence is roughly between 0.411 and 0.868mRad. That's a big range, but you can refine it by taking measurements further away.

I gotta say though, for a NewWish laser, those figures just don't sound right.. Too good to be true if you ask me. It would be great if they were true, but I'm left questioning the 7mm reading.
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Old 07-10-2007, 01:08 AM #4
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Default Re: Divergence measurements - question

Divergence can only really give you a TRUE figure if you have a *lot more distance than that.

I converted your figures to 36ft and 7mm dot size, which gives you a divergence of .64 Milliradians which is not a bad figure - but you really SHOULD go out at a further distance than 36ft. *Use this page right
HERE

That is Psuedonomen137's divergence calculator......
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Old 07-10-2007, 01:20 AM #5
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Default Re: Divergence measurements - question

That is exactly what I thought... The math is right and the units are right... it's just that the figure was too good to be true IMHO. I remember my DX200mW used to turn into a flashlight at a large distance... If you look from far away at the beam when it's pointing up to the sky, it looks like one of those starlights they use to promote a location from really far away. I also just measured my leadlight at a close distance and got very similar results. I will not believe it until I test it out myself at something around 50+ meters sometime.

This is a very good point though, thank you for bringing it up

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Old 07-10-2007, 01:36 AM #6
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Default Re: Divergence measurements - question

Good rule of thumb is - the longer the distance, the more accurate the measurement !
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Old 07-10-2007, 01:45 AM #7
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Default Re: Divergence measurements - question

OK, the first time, I used a start dot size of 2.5mm. *This time I shot the laser through a welders glass with a piece of paper on it. *That way the aperture dot was easier to measure. *

I now have an outgoing dot size of 1.0-1.2mm.
I put a target 16m away and shot the laser at it. *
I traced the dot on a piece of paper and measured the circle to be 10-12mm.
Using pseudonomen137's mRad calculator and the best/worst results are 0.550-0.688mrad.

I will try to test at a further distance when I get the time and I will re-post the results. *Thanks for the input!

Paul



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Old 07-10-2007, 02:12 AM #8
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Default Re: Divergence measurements - question

I just had another thought. Last week my wife and I were visiting her mother in Toronto. She has a 17th floor condo. I did a little evening playing with the DX30mW.

Approx 2520m away there is a plant with a large smoke stack (distance checked through Google Earth ). The laser dot was clearly visible on the smoke stack and narrower than the stack itself. The smoke stack is about 5m wide.

If I had to guess, I would put the dot size at well less than half the stack width. Using the mrad calc again with some guesses at the dot size

2.5m dot size at 2520m --> divergence ~ 0.9916 mrad
2.0m dot size at 2520m --> divergence ~ 0.7932 mrad
1.5m dot size at 2520m --> divergence ~ 0.5948 mrad

This seems to be in the same range of the short distance tests.
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Old 07-10-2007, 02:13 AM #9
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Default Re: Divergence measurements - question

Paul - that is fairly consistant with your first measurements - when plugged into Psuedo's div calc....Try a measurement at least 100' and see what you get - now, keep in mind - you CAN use mirrors to increase your distance, with minimal distortion to your readings.
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Old 07-10-2007, 03:30 AM #10
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Default Re: Divergence measurements - question

For short distance measurements it may be better to add the two spot sizes together instead of subtracting, because the laser likely converges first (though you won't notice this because of the rayleigh beam waist).

And just to add onto SenKat's tip: make sure they are "first surface mirrors". Regular second surface mirrors can add a 2nd ghost dot or distortions that mess with your readings.
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Old 07-10-2007, 03:40 AM #11
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Default Re: Divergence measurements - question

I have some first surface mirrors from an old scanner, perhaps I will try that - alignment will be a pain.

Any thoughts from the Toronto results I posted above? 2500m+ is a much longer distance. What would the expected mrad range for inexpensive lasers like the DXs and what are "respectable" figures for this type of laser?


Quote:
Originally Posted by pseudonomen137
For short distance measurements it may be better to add the two spot sizes together instead of subtracting, because the laser likely converges first (though you won't notice this because of the rayleigh beam waist).

And just to add onto SenKat's tip: make sure they are "first surface mirrors". Regular second surface mirrors can add a 2nd ghost dot or distortions that mess with your readings.
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Old 07-10-2007, 04:47 AM #12
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Default Re: Divergence measurements - question

Well, I'm not sure how accurate your measurements are but I guess if they're right, then you do have pretty dang low divergence on that laser. Something must be off though. If you have an initial diameter at 1.0-1.2mm, and even an mRad of 0.7, and the smallest diameter on the beam is right at aperture (highly unlikely), that's approaching the theoretical best... which doesn't really happen in a small pen portable. Something must be off somewhere in there.

You may have a good laser though with particularly low divergence. I know my NewWish lasers are more like 1.5-2.5mRad, and the initial spot is >1.2mm.

There are no "respectable" figures for mRad alone because mRad and minimum beam diameter can be traded off, so you need to know both. On the low end of mRads though, my RPL is somewhere in the 0.6-0.8mRad range I believe, whereas I've had some messed up greenies in the 3mRad+ range (there are other types of lasers like multimode diodes that are far worse than this too).
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Old 07-10-2007, 05:11 AM #13
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Default Re: Divergence measurements - question

Quote:
Originally Posted by pseudonomen137
(there are other types of lasers like multimode diodes that are far worse than this too).

+1 on that... My DX used to mode hop as well... see here http://www.laserpointerforums.com/fo...num=1183007151

The ray used to expand so bad once it hops... That's exactly how I found out.

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Old 07-10-2007, 05:40 AM #14
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Default Re: Divergence measurements - question

Eh, sorry to be a jerk but that's not really what I meant. Mode switching like what you're talking about is switching between the transverse modes in a DPSS laser.

multi-mode diodes are diodes with multiple longitudinal modes. You can find single-mode diodes up to a few hundred mW, but past that they're all multi-mode. While DPSS lasers can offer beams sometimes less than 20% worse than the perfect beam, multi-mode diodes are usually ~20x worse. That's why strong red diode portables are limited if you want half-decent beam specs.
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Old 07-10-2007, 05:51 AM #15
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Default Re: Divergence measurements - question

psuedo Can you take all that knowledge in that big brain of yours, download it onto a dvd-rw or 20 and send it on over to me for uploading please?
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Old 07-10-2007, 06:17 AM #16
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Default Re: Divergence measurements - question

HAHAHAHAHAHAHAHAHA... well... I'm not offended in anyway, and actually, I've only heard of that yet never understood why aside from the fact that diode lasers are simply not that efficient or stable as of yet (same thing happening with Blu-Ray now). But I'll chip in for a Brain-RW drive for us Kenom 8-)

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