Hello Mark,
Thanx for commenting and questioning. I work best in an interactive mode.
Accuracy depends on being able to find the center of the image of the laser where it encounters the target surface. Noise, like illuminated background objects, can be a serious problem. Part of the solution is to make the laser diameter very small so that it is 100 to 10000 times brighter than any other light on the target surface. Another part is to be sure that the laser is to be imaged small on the camera ccd array while anything closer or farther is blurred.
That means that the laser plane must be imaged onto the ccd plane. A short depth of focus will permit a fine image of the laser while blurring any sources not originating in the laser plane.
Most of the drawings I have seen that explain the principle of laser triangulation show a single line being traced from the intersection of laser and target, through the center of the lens, to the ccd array. This is valid for a pinhole camera that does not have a lens but uses a tiny pinhole instead. Unfortunately a pinhole camera has low resoultion, low sensitivity, and high depth of focus...all undesirable characteristics.
When you use a lens you need to consider the point where the laser encounters the surface as a small source scattering light in all directions. A cone of that light passes through the lens and is imaged to a small area. If you trace several such cones you will find out that if the ccd array plane is parallel to the lens plane then the laser plane must also be parallel to the lens plane. If this is not so then most of the points in the laser plane will not be imaged onto the ccd array. Putting it another way, the "points" on the ccd array will be large blurs.
To use an inexpensive camera like a webcam, with the lens built in so that the lens plane is parallel to the ccd plane, it is necessary to arrange the camera and laser so that the laser plane is also parallel to the ccd plane.
Ideally the laser would point straight down the z axis and the camera would be off to the left and looking to the right. However, the camera's view would be blocked by the target.
A more practical solution would be for the assembled gauge to be rotated. Suppose the setup is rotated 30 degrees CW. Then the laser plane is 30 degrees CW of the z axis and the camera axis is 60 degrees CCW of the z axis. The ccd plane and lens plane are parallel to each other and to the laser plane. This also results in very little distortion since the only source of distortion is the lens itself.l
Certainly it is desirable to package in other ways. Like maybe have the laser pointed straight down the z axis and have the camera axis 30 degrees CCW from the z axis. This is the type that I used to design. The problem is that the laser plane is not parallel to the ccd plane. In order to form an image onto the ccd array it is necessary to separate the lens from the camera and mount it at an appropriate angle.
We can talk about those calculations if there is interest. But to use a prepackaged camera and lens it is essential that the laser plane be parallel to the ccd and lens planes. The lens should have the lowest fNumber you can arrange so that only the laser plane is imaged onto the array while background and foreground are blurred as much as possible.
If you do this then you can turn off the camera AGC and set the exposure time for 1ms or less. All of the foreground and background will be black since the exposure time is so short. The laser scattered from the target will be bright enough to be detected by the camera and it will only cover a few pixels. This allows for faster capture of frames and faster processing of the small data set.
I don't know what hardware arrangement Art and others are using. The written descriptions that I have seen have been vague and I have seen no drawings.
Tom Hubin
thubin@earthlink.net