DIY Laser levelling using webcam and laser level

[John McNamara]

Hi Michael :)

I am glad you have popped in here to shed a little light on the optical side of these experiments.

Regards
John

[Michael Gilligan]

I think it worth sharing this FAQ answer from Edmund Optics:

Frequently Asked Question
Question

What is the difference between refractive and diffractive line generators?

Answer

A refractive line generator uses a cylinder or rod lens to focus a laser in only one axis only (drop-moved placement) in order to create a line of light. A diffractive line generator uses a flat optic with an etched microstructure that breaks apart a laser beam and forms an interference pattern in the shape of a laser line. Refractive optics do not correct for the inherent Gaussian profile of a laser beam and form a line with a "hot spot" in the center and fading edges. Diffractive optics will create a line that is uniform in thickness over its length, but is segmented. Diffractive line generators also cause a small portion of the light to be redirected into different diffractive orders, which causes additional faint lines to appear. Some laser line generators use a unique patented Powell glass lens design in order to achieve a continuous (not segmented) line with an even distribution along the length of the line.

Here is Powell’s patent:
https://worldwide.espacenet.com/publ...C&locale=en_EP

and a more comprehensible description:
https://www.laserlineoptics.com/powell_primer.html

MichaelG.

[John McNamara]

Hi Michael

Buying A Powell lens from Laserline, Edmund or other big optical equipment suppliers is pretty expensive starting at around $250 USD. I did a little google search and found a few from Ebay etc. at a more reasonable cost for an amateur experimenter. The question is which one?

I have not yet got my head around what would be the ideal angle for the Powell lens assuming that the l operating range is from 100 to say 3000mm.

The laser driving the unit also is also a consideration I nice clean spot is required.

Regards
John

https://www.google.com/search?q=powe...l%20lens#spd=0

[Michael Gilligan]

You’re quite right, John ... they are pricey, ‘though It’s encouraging to see better prices on ebay

My post was really just for technical information.

For what you are are doing [with the wire shadow and very small displacements], I don’t think you need worry about laser lines anyway: Unless I am very much mistaken, you could use any collimated light-source that filled the area of the sensor.

As for laser spot quality ... the classic solution is to use ‘Spatial Filtering’ ... but that’s another potentially costly and bulky solution to a problem that might not exist.

My impression is that this project is all about leveraging the precision of mass-market sensors with ingenious software.

MichaelG.

[devmonkey]

Hi Michael,

Do you have a view on how flat the projected plane of a cylindrical lens laser line is likely to be? This is something I still haven't measured as I have no large enough reference plane.
Along a given ray it should be absolutely straight but will it undulate across 'rays'? By ray I mean a line from laser source to sensor.

My latest thoughts on how to measure this was to place four pillars in a square on a reasonably flat stable surface (concrete floor). Place the laser so that it projects across the tops of the four pillars and use height readings from 3 of them (as any 3 points will be planar) to bring the fourth into plane with some shimming. Now move the laser around the room and have it cast a different area of the beam over the pillars and measure again, check if all 4 pillars are still planar, repeat this a few times.

Thanks, Joe

[Michael Gilligan]

That’s a tricky question, Joe ... but here’s the best I can do for the moment:

In an ideal world, a perfect laser beam would would be refracted by a perfectly cylindrical lens to produce a perfect line on a cylindrical surface.

In the real world, however: Our ‘laser diode’ is an imperfect source and will be refracted by imperfect optics ... with the inevitable [but practically impossible to estimate] consequence that the line will be imperfect.

That said: Your proposed test seems very appropriate.

MichaelG.
.

Edit: This is a little laboured, and is intended for an audience of ophthalmic opticians, but it may be helpful to anyone struggling with the general concept of cylindrical lenses: https://youtu.be/vHugCo2md_o
.
Edit: Note, however, that for the laser line generator we are using a negative cylindrical lens
... See Application 8, here: https://www.newport.com/n/beam-shapi...ndrical-lenses
... and for convenience, here’s a direct link to Application 1: https://www.newport.com/n/focusing-and-collimating

[devmonkey]

Ok, but would the line just be noisy or would/could it be bent? I'm not sure I fully grasp how the source is refracted by the cylinder, specifically is it true that a given arc segment of the cylinder is generating a corresponding section of the line or does the entire arc of the cylinder contribute to every part of the line? If the latter then cylinder imperfections would produce nose whereas if former then the line could be bent.

[Michael Gilligan]

I'm not sure I fully grasp how the source is refracted by the cylinder, […].

Ray-tracing would help you understand ... but be warned ... it’s tedious !!

If I can find an easy and convincing answer to your question, I will post it [but don’t hold your breath]

MichaelG.

[devmonkey]

Don't get me wrong I understand how refraction works, what we need to understand is given the light source is not perfect nor perfectly collimated and cylinder is not perfect can these imperfections add up to a line that is bent, or just a line that is noisy (which is fine for our purposes). Presuming that the ideal model of refraction we get taught is not what actually occurs in the real world.

[Michael Gilligan]

For the present purposes, I’m pretty sure that we can take the line as being straight but fuzzy [i.e. noisy]
... Like so many things though: The closer you look, the more difficult it gets to describe.

This is what I find so impressive about this project ... the software is effectively using a ‘best fit’ definition of the line.

MichaelG.

.

P.S. for what it’s worth ... here are some numbers from the specification of a ‘metrology-grade’ unit:

Beam Profile Line
Power Distribution over 16 mm ±10 %
Fan Angle 18 °
Focal Distance (factory preset) 180 (±2) mm
Line Length (@ focal distance) ~ 45 mm
Line Width (@ focal distance) 50 (±10) µm
Alignment of Optical to Mechanical Axis ≤ 0.6 mrad
Line Straightness (max. deviation from ideal line) ±2 µm (over 16 mm)
Depth of field (laser line < 60 µm) min. ± 5 mm