DIY Laser levelling using webcam and laser level

[devmonkey]

Hi John,

Let me just sanity check your methodology. You have the sensor or wire on a precision positioning system. You move the system in steps based based on your dial indicator and click measure at each step? Is that correct? Just wanted to check you aren't moving the system until the measure value increases by one step then clicking measure as this would always produce a straight line, I don't think you are doing this but just want to be 100% sure.

You have the umPerPixel set to 1 therefore the numbers you are recording are in the units of pixels, this means anything to the right of the decimal point is to a sub-pixel accuracy. This ok to an extent (say 1 decimal place) but will soon deteriorate into noise. The gaussian fit will be afffected by a single intensity point change of a single pixel that is why you get jitter at extreme sub-pixel resolution. To remove all jitter you would probably have to perform the test under lab conditions in a vacuum to avoid dust particles and with a lab quality camera with high precision pixel exposure times. The exposure times (and therefore the intensity readouts) of these cheap sensors with rolling shutters are not perfect, when a pixel/row is exposed for slightly longer its intensity values will naturally be higher. Couple this with the onboard image processing pipeline that is probably trying to do all sorts or AGC and white balance adjustment and noise is inevitable.The image is then compressed onboard to a jpeg adding all sorts of artifacts before being shipped to the PC.

All that said your 0.01mm test run shows your residuals are within +/- 0.4 pixels or 0.0016mm (think you sensor is ~4um per pixel) of a straight line, this is pretty good isn't it? Shimming or scraping in the 1 micron range is most likely impossible for us and probably completely overwhelmed by the thermal movement of the frame.

The 0.0016mm could as you say be down to trying to position the sensor using a 0.01mm dial indicator, quite difficult to judge 1/10th of one division...and outside the tolerance of the indicator.

I can add warping for the catenary sag if you have the equations?