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DAVID-Laserscanner Forum • View topic - Measurement of the Line Width
Dear DAVID Community,

In 2016, DAVID and this forum have been integrated into HP Inc.
This forum has been read-only since then, and will be switched off by then end of 2019.

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Measurement of the Line Width

Which camera works? Where can you buy an adequate laser? And more...

Measurement of the Line Width

Postby WalterMo » Fri Nov 16, 2007 4:36 pm

Hello,
The knowledge of the laser line width is very important for accurate scanning. Manufacturers of lasers promise us values which we must trust or not. By means of this setup it is now possible to measure the line width at different distances by the moving-edge method.
The photo shows the optical arrangement. The laser radiates its horizontally orientated line onto a “large” photodiode of 6.2mm x 6.2mm which output signal is given to a current meter and from here to a computer which is also controlling a translation stage with a resolution of 5 µm / step.
A blade at the translation stage is shifted vertically in increments of 20 µm and covers so the sensitive element more and more. In each position the current of the photodiode is measured and stored. Note:
It is important not to measure the output voltage but the output current because the relation between light intensity and voltage is logarithmic. Between light and short circuit current it is strictly linear.
After getting the light intensity curve (red in the attached diagram) it is differentiated with respect to the movement of the blade. This (green) curve shows the beam intensity profile of the laser line. Normally the intensity profile of a laser beam is Gaussian and has no sharp margins. Therefore the beam width is defined where the intensity falls down to 1/e² (=13.5%). Markers are set at this level.
Five different line lasers were to my disposal.

A) LC 650-5-3-F(14x45)
B) LC 532-5-3-F(16x65)
C) LFL 650-5-12(9x20)60
D) LDA-700-1-M2 with attached cylindrical lens (used by Simon and Sven at the very beginning of their 3d-scanning)
E) LaserLevel from the home-improvement market Praktika

A and B were focusable ones. C was equipped with a built-in line generator lens. This laser was measured without and with additional cylindrical lenses of f = 200mm and 300mm. The focussing gives of course a thinner line. All measurements were made in a completely dark room.

Results:
A @ 0.5 m = 0.23 mm and @ 1 m = 0.39 mm
B @ 0.5 m = 0.177 mm (new); 0.184 mm (old) and @ 1 m = 0.38 mm
C @ 1 m = 0.78 mm
C @ 0.2 m with add. lens (f = 200 mm) = 0.35 mm
C @ 0.3 m with add. lens (f = 300 mm) = 0.48 mm
D @ 1 m = 0.87 mm
E @ 1m = 2.04 mm

The very thin line of the green laser at 0.5 m is really surprising in relation to the red one at the same distance. Therefore I will repeat the measurement of the red laser to try if it will be possible to get also 0.184 mm.

Here are some general informations about the differences between red and green semiconductor lasers:
They consists of completely different systems whereby the green one is much more complicated. A red laser has „only“ a laserdiode and one or two lenses built in. The wavelength of the emitting diode determines the colour of the light.
The active light source of a green laser is mainly a IR-laserdiode (? = 808nm). This light pumps a resonator (Nd:YVO4-crystal) which itself radiates light of a wavelength of 1064 nm. A second crystal (KTP) devides the wavelength by 2. So we have green 532 nm light. This bigger effort is the reason for a price more as double as hight as for a red one.


Walter
Attachments
Set-up.jpg
Used optical set-up
LaserA05.PDF
Diagram of the measured LC650-5-3-F(14x45) at 0.5m as an example
(13.66 KiB) Downloaded 597 times
Last edited by WalterMo on Thu Jan 29, 2009 1:37 pm, edited 5 times in total.
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Postby Simon » Fri Nov 16, 2007 8:37 pm

Hello Walter,
thanks a lot for your sophisticated analysis!
0.184 mm sound really great. I was wondering whether there is a varying quality for different lasers of the same type or not.
But this can only be answered by testing more than one laser of one type. :?
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Postby WalterMo » Sat Nov 17, 2007 9:59 pm

Hello Simon,
You are right, several measurements with the same laser are necessary to determine the line width of a special laser type correctly to reduce the uncertainty of the specification.
But also with three or more devices of the same type.

Nevertheless I will repeat the measurement with the red laser @ 0.5m.

Walter
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Postby johnrpm » Wed Nov 21, 2007 10:18 am

Excellent Walter, science in action.
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Postby WalterMo » Wed Nov 21, 2007 1:07 pm

Hello,
In the meantime I have repeated several measurements with the 2 focusable lasers @ 0.5 m. Each time I tried to adjust the focus as precisely as possible by using a magnifying glass for my eyes. The results for the red laser could be reconfirmed and for the green one could be improved to 0.177 mm.
The table in my report above is already renewed.

Walter
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Postby cadcoke3 » Thu Dec 20, 2007 6:46 pm

I am not certain what method the David software uses to locate the laser line... but am guessing that it is trying to find the center of the line. However, there have been other programs, such as the "Shadow Scanner" which uses the leading edge of the shadow created by passing a stick in front of a light source (such as a desk lamp).

In the shadow example, the width of the beam is not an issue. If the same approach for a laser scan, the issue would be how consistant the width of the beam is. If the beam gets narrower as it spreads out, then the leading edge is actually a slight curve, and not a line.\

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Postby MagWeb » Thu Dec 20, 2007 8:51 pm

Hey Joe,

you're right, David tries to find the center of the laserline.

Sven tried to use both edges of the line instead, but he dropped it, for the result was too bad.

see


bye
Gunter
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Postby Spiyda » Sat Dec 22, 2007 12:25 am

As long as the laser is bright enough, I imagine the line can be narrowed by means of a slit mask, positioned a reasonable distance from the output. Provided the mount is rigid, dialled in correctly and doesn't reflect too much it should enable even narrower lines, or will it create some kind of diffraction ?

Anyone care to try it ?

Chris
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simple measurement method

Postby PZsolt » Mon Mar 10, 2008 11:01 pm

Attachments
line_measurement.pdf
(748.15 KiB) Downloaded 1198 times
Iron: Intel Core2 6400 + ASUS P5B-E / 2 GB RAM / Asus NVidia 6600 + dual 19" TFT
WebCam: Logitech QuickCam Pro 9000
Laser: Picotronic LC635-5-3-F
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Postby Bertho » Fri Sep 19, 2008 1:41 pm

I love Walters measurement setup and data and I also like Zsolt's cleaver and simple way. By the way, if 5.7 degrees are used as the angle it would be a 10 to 1 beam expansion which is easier to convert.

Independently of that I have been thinking if there is a way to get a quality index or display from the camera itself. Even if the laser beam is perfect but too powerful, it will saturate the camera pixels and bloom into other pixels. What we really want is the best quality line as seen by the camera and the software.

As an example, look at Zolt's secondary measurement of the line on the paper.

We should not be changing any camera settings to evaluate the beam or we will not have the true scanning condition.

For example a 10:1 or more digital magnification would be great for looking at the laser beam as seen by the camera.
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Postby Bertho » Fri Sep 19, 2008 2:07 pm

Hi Zsolt,
What software did you use to make the "Surface Maps"? If it was an expensive package, can you suggest a low-cost/free one? The graphs look very useful.
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Postby yesterdayon » Fri Oct 24, 2008 6:25 am

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Re: Measurement of the Line Width

Postby moog » Wed Nov 25, 2009 9:36 am

I've recently recieved my LC532-5-3-F green focussable laser, but when focussed about 50cm from the panel the beam looks to be in the region of 2 or 3mm wide, is this normal or should i see a very thin beam in the order of the width's you state.
Or are your figures more related to the ligth intensity, and therefore the effective width detected by david?
LC532-5-3-F(16x65), BenQ PB8220 DLP Projector, logitec quickcam PRO9001, Casio EX-FH20
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Re: Measurement of the Line Width

Postby WalterMo » Wed Nov 25, 2009 4:56 pm

Often the line width seems to be too wide. It can be a visual impression by our human eyes for the brightness appears too high.
Therefore test the line using sunglasses and/or radiate onto gray paper. If you still have the impression that the line is wider than 1mm at a distance of 50cm, so send the laser back.

In my report above I have measured the light intensity. Have you opened the attached pdf file?

And yes, DAVID calculate a centerline of the detected optical pixel line.
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