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=1936Edmund Optics #62112, $45.00, center wavelength=650nm, bandwidth=10nm
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1903Mounting 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=1521This should be enough to get a discussion started for improving signal to noise ratio.
Tom Hubin
thubin@earthlink.net
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