Hello Guest it is March 28, 2024, 04:24:48 AM

Show Posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.


Messages - TomHubin

Pages: « 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 »
131
Video P*r*o*b*i*n*g / Re: Improving Signal to Noise Ratio
« on: December 28, 2007, 12:28:34 PM »
Just a couple more thoughts on color filtering to improve SNR.

Assuming a color camera is used, can the blue and green data be ignored and only the red component used? This might be comparable to using a broadband red filter, like a red cellophane candy wrapper, on the camera.

Better yet, is it possible to digitally filter the picture for a narrow band of red around the laser color?

Yellow or blue or green workspace lighting might make it easier to isolate the red laser in the picture.

I have no experience with these methods. Just wondering out loud.

Tom Hubin
thubin@earthlink.net

132
Video P*r*o*b*i*n*g / Re: Just purchased a laser line pointer
« on: December 27, 2007, 06:55:52 PM »
Has anybody tried powering a 5 volt laser module from a USB port...either the same one that powers the webcam or a second USB port?

Tom Hubin
thubin@earthlink.net

133
Video P*r*o*b*i*n*g / Improving Signal to Noise Ratio
« on: December 27, 2007, 01:21:47 PM »
Hello,

This forum has been way too quiet over the holidays so I thought it time to wake everybody up.

My Background.

I am a self-under-employed optical engineer and electrical engineer. These days I design laser delivery optics for laser micro-machining. These are basically 3 axis CNC mills with a tightly-focused high-energy laser spot doing the milling or drilling. Designing optics for a UV 193nm excimer laser focused to a 1um diameter spot to engrave diamond jewelry was my introduction to laser micro-machining.

Between 1986 and 1992 I designed optics and electronics for several commercial laser triangulation gauges. We usually used a Sharp 780nm near infrared laser diode and a Fairchild CCD 133 line array with 1000 pixels clocked at 1MHz, as I recall. I wrote diagnostic and testing software in c under DOS. I left the writing for Windows user software to those who like programming for a living.

The largest range I did was for a space docking gauge. The near point was 6 inches and the far point was 30 inches. Ultrasound is usually preferred for that range but ultrasound does not work in the vacuum of space. I do not know if it ever made it into space.

The smallest I did was nicknamed "pencil gauge" because the 3 inch long glass probe was about the diameter of a pencil. The range on that one was about 0.1 inch with the near point about 0.1 inch from the glass tip. This probe looked sideways and was used to inspect between the blades of a torpedo propeller. 

We always located and oriented detectors to take advantage of the Scheimpflug condition (http://www.multires.caltech.edu/pubs/DGPCourse/CurlessChapter.pdf pages 4 and 5). This results in a detector surface which is not perpendicular to the lens optical axis. Because of packaging, this is not convenient for consumer grade webcams...but it is possible.

Improving Signal to Noise Ratio.

The subjects of improving signal and/or reducing noise have come up in several postings. So I thought it appropriate to tackle the subject.

Signal is the result of laser light hitting a surface which scatters light over a large solid angle (think hemisphere), some of which is collected by the camera lens and focused (or at least concentrated) onto the camera detector. The detector for WebCams is usually a CCD area array.

Noise is just about anything else that will produce a signal from the camera. Noise can be electrical or optical, random or fixed.

An example of fixed optical noise is the image of other things in the area (aka background). One way to reduce that is to turn off AGC and grab one frame with the laser off and grab another frame with the laser on. Subtract one frame from the other, pixel by pixel. This will subtract the fixed optical noise from the picture and leave only the laser.

Another method is to use a color filter on the camera. The filter should pass the laser wavelength but as little else as possible. For a red laser, a red cellophane candy wrapper over the camera lens will help reduce background while having very little effect on the laser. This filter eliminates most of the blues and yellows but not much of the broad spectrum reds.

You can also work in the dark but this is often inconvenient or unsafe. Try using a red filter over the camera lens and a yellow light bulb (aka bug light) to see your workspace. Very little of the yellow light will get past the red filter into the camera.

A better solution is to use a laser line filter like those listed below. If you hold one of these up to your eye and look at the laser spot on the surface you will see the laser spot but not much else. Try looking at a light bulb through one of these filters to see how little white light passes.

Where to get color filters.

Camera stores have color filters. I have not found enough data yet to make any recommendations.

Theatrical supply shops have gelatin filters, often just called gels, for a many combinations of colors. Graphs exist for transmission spectra but I have not been able to find that data on the internet.

Edmund Optics has a few filters for 650nm but not much for 635nm, although the first one below should be usable for either. For the same money I would choose the 10nm bandwidth. I would choose the 80nm bandwidth if I happened to get a much better price or if I wanted to experiment with 635nm and 650nm.

Edmund Optics #46153, $46.30, center wavelength=650nm, bandwidth=80nm

http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1936

Edmund Optics #62112, $45.00, center wavelength=650nm, bandwidth=10nm

http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1903

Mounting Filters.

I like Thorlabs tubes for mounting optics. They also sell taps for their tube threads.

Thorlabs SM05 series of stackable tubes are suitable for mounting half inch diameter optics like lenses and filters.

http://www.thorlabs.com/newgrouppage9.cfm?objectGroup_ID=1521

This should be enough to get a discussion started for improving signal to noise ratio.
 
Tom Hubin
thubin@earthlink.net

134
Video P*r*o*b*i*n*g / Re: Just purchased a laser line pointer
« on: December 12, 2007, 06:21:35 AM »
What's the difference between the 650 nm and the 635 nm line generators?

The human eye sensitivity to 635nm is about twice that of 650nm. This is shown in the data table at

http://heelspurs.com/a/led/tech_details.html

So for a human visual response, all other things being equal, I would choose a 635nm laser.

http://www.fairchildimaging.com/main/documents/CCD133ADataSheetRevA.pdf page 6, lower right corner, shows the spectral response of the Fairchild CCD133 1000 pixel line array. Peak responsivity is in the near infrared at about 780nm. 650nm responsivity is better than 635nm but not by much. This is typical of ccd arrays.

So for a CCD camera response, all other things being equal, I would choose a 650nm laser.

This is fortunate for laser triangulation since longer wavelengths are often easier to manufacture so less expensive.

Tom Hubin
thubin@earthlink.net

135
Video P*r*o*b*i*n*g / Re: setting up cammera and laser
« on: December 04, 2007, 06:43:22 PM »
The example art shows is a line laser not a point laser. I have used David, and triangle and both use a line laser. I also own a nextengine 3d laser scanner. I think your confused and should find out what is correct before giving advice.
 I could not resist
 Don

Hello Don,

My apologies for directing my previous post to you. You are correct. Your query was specific about setting up a line laser and my response was meant to be a general tutorial to demostrate the principle of laser triangulation with minimal equipment.

So, although I stand by the accuracy and usefulness of my response, it did not answer your question and I should not have quoted your query.

This is only my second day on this forum and I am just getting the hang of how to post. Perhaps I should have started a new thread for discussion of laser triangulation basics.

Tom Hubin
thubin@earthlink.net

136
Video P*r*o*b*i*n*g / Re: setting up cammera and laser
« on: December 04, 2007, 10:20:13 AM »
How is the camera to laser setup. Is the camera and laser running parallel to the X axis (X axis moving front to back (gantry)). What should the angle be of the camera and the laser. Is there any setup notes yet?
 Don

Hello Don,

Try this experiment to demonstrate the concept of laser triangulation.

Install a simple laser pointer (not a line generator) in the spindle. Use a drill chuck or endmill holder or collet. Whatever works. Battery operated pointer might be easier to avoid wiring difficulties. Otherwise, figure out how to route laser pointer wiring. DO NOT RUN THE SPINDLE MOTOR.

The laser is pointing downward and hitting a piece of white paper on the XY table. You see a bright red spot where the laser hits the paper.

Attach the camera somewhere on the Z axis and pointed down at the bright red spot where the laser hits the paper. Use tape or rubber bands to lash the camera into place. You should see the bright spot somewhere on the camera monitor screen.

Move the Z axis up and down and observe that the bright spot on the monitor moves. For every Z position the spot appears in a unique location on the monitor.

Tom Hubin
thubin@earthlink.net

You probably also see the table and whatever happens to be in the field of view of the camera. Turn off all of the nearby lighting. This will make the table and other stuff disappear but the laser spot is still visible.

Again, move the Z axis up and down and observe that the spot moves on the monitor. Other than the laser spot, the screen should be pretty dark.

Tom Hubin
thubin@earthlink.net

137
Video P*r*o*b*i*n*g / Re: Driver Installation question
« on: December 04, 2007, 09:59:52 AM »
Graham,
Thanks, that fixed it. Now, I'm off to figure out a mount for my webcam and laser.

Also, I realize that my camera appears to be B/W. I wonder if that will be an issue.
Dave

Hello graham,

All other things being equal, B/W is better than color. Usually B/W cameras have better characteristics like sensitivity, noise, contrast etc. There is no useful color info to be had with laser triangulation. The laser is about as monochrome as light gets and all other sources of light should not be seen by the camera.

More on how to eliminate lighted background later.

Tom Hubin
thubin@earthlink.net

138
Video P*r*o*b*i*n*g / Re: MachCloud point viewer program
« on: December 03, 2007, 08:06:56 PM »
I would like to see something related to signal strength for each data point. Label it with the letter "I" for intensity.

That would then be a qualifier for how credible that data point is.

This could be the sum of the pixel values in the group or it could be the peak value.

Adding intensity for each point also allows scanning a flat part where the Z value is nearly constant.
Then you could display (X,Y,I) data as the cloud of points.

Intensity also shows surface texture details like roughness or machining marks or the angle of the surface.

Tom Hubin
thubin@earthlink.net

139
Video P*r*o*b*i*n*g / Re: setting up cammera and laser
« on: December 03, 2007, 06:11:27 PM »
Hello Don,

If you arrange the laser so that it is on the left end of the table with laser light traveling from left to right then you will be locating points on the left side of your part. Not what most folks want.

However, I think that what you are trying to do is make the laser plane parallel to the detector plane. This is the way the camera was meant to be used. However, getting data on the left side of a part is not popular.

It would make more sense to have the laser pointed down so that the light is in the YZ plane. Then locate the camera on the left end of the table looking to the right and focused on the laser YZ plane. This would allow you to measure the height of points on a surface. However, most points would be occluded from the camera. That is, laser light hits the part at some point but the light scattered from that point often cannot get to the camera.

So you raise the camera above the surface then rotate it clockwise so that it is pointing in the neighborhood where the laser light hits the surface. This works fine for a pinhole camera but not so well for a camera lens of significant diameter.

As the camera lens diameter increases it becomes apparent that the plane of the laser is not imaged on the detector unless the detector is also rotated an appropriate amount. I have formulas and graphical methods for all of that but I want to avoid the math until the principle of operation is established.

Some of this is explained here:
http://www.multires.caltech.edu/pubs/DGPCourse/CurlessChapter.pdf pages 4 and 5 show the correct orientation of the detector as not perpendicular to the lens optical axis. This is known as the Scheimpflug condition.

Tom Hubin
thubin@earthlink.net

140
Video P*r*o*b*i*n*g / Re: setting up cammera and laser
« on: December 03, 2007, 02:52:02 PM »
Hello Dave,

You have to move a small amount (say 0.010 inch) in the X direction, stop, then grab the frame, then find the spike in each horizontal camera line. This produces a set of (V,H) camera coordinates which will uniquely map to (Y,Z) machine coordinates. The X value is taken from the machine.

When you have completed the X scan you move back to the start of X and over to the next Y value. The Y increment is the width of the track that you just took. That could be a quarter inch or several inches depending on your setup.

You are raster scanning your part in X and Y with small steps in X and large steps in Y.

In order to create the cloud of points in XYZ coordinates you need to know how to map the (V,H) camera coordinates to (Y,Z) machine coordinates. This requires some calibration of the camera and laser combination.

One simple method is to move the head of a pin in the YZ plane of the laser. The pin head should image to a small area on the camera. You could move 0.1 inch steps in Y and Z and find the center of the hot area on the camera for each (Y,Z). This is probably not the most accurate method but it is easy to demonstrate.

Tom Hubin
thubin@earthlink.net   

 

Pages: « 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 »