Re: [CAD_CAM_EDM_DRO] Camera-laser scanning

An easy way to visualize is that the subject plane, lens plane and film
plane all converge at "a" point, n.b. infinity is considered a point for
this discussion.

erie
(long time 'big negative' camera user)

Tom Hubin wrote:

>ibewgypsie wrote:
>
>
>>http://img.photobucket.com/albums/v85/ibewgypsie/Sscn0998.jpg
>>
>>This is how it works.. fuzzy picture, sorry.. You can get a general
>>ideal thou. Looks at that height and angle the resolution is approx
>>640 pixels for 3" As good as some probes but not as good as I'd like..
>>I can of course raise the table untill I got 1" FOV then reangle the
>>camera.
>>
>> Like others said, this is not as precise as a probe or distance scanner.
>>
>>
>
>*****************
>
>Hello ibewgypsie,
>
>Beware the Scheimpflug Condition! Details patented about 1904.
>
>I'll give you the short version then you can do a google search on
>"Scheimpflug: for technical details.
>
>The object for your camera is the plane containing the laser. That plane
>is to be imaged onto the camera detector or CCD plane. The problem is
>that the laser plane is not parallel to the lens plane. This results in
>a tilted image plane. The CCD array should be tilted for sharpest focus
>over the useful laser range.
>
>The math is:
>
>tan(beta) = m * tan(alpha)
>
>where:
>m is the magnification along the optical axis.
>alpha is the tilt of the object plane
>beta is the tilt of the image plan.
>
>If the camera has a very small aperture then the depth of field is large
>and the object tilt may not matter much. But if a large aperture lens
>(low fNumber) is used then a great deal of blurring will occur for
>points away from the center of the useful laser range.
>
>Another saving point is that the magnification is usually a fraction for
>laser triangulation. This means that the image plane tilt is much less
>than the object plane tilt.
>
>It is difficult to tilt the CCD array in a typical camera. In the late
>1980's I designed laser triangulation gages for a living. We designed
>and built the camera from scratch so I could specify and build the CCD
>tilt according to the math and verify with ray tracing software.
>
>Also, tilting the image this way results in a nonlinear mapping from
>object to image. Like the human eye, resolution will be best for points
>closest to the camera. Resolution will be worse for poitns far from the
>camera.
>
>There are optical methods that will allow a linear mapping (constant
>magnification) from object plane to image plane but you won't find that
>in a common camera. Ask if you want the details.
>
>Tom Hubin
>thubin@...
>
>
>
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