Re: Measure distances w 1mm precision

Hi

Before you start you have to some estimation about the focal length of your camera. Let's say you have a measurement accuracy of 1/10 Pixel in your image (maybe a little bit optimistic) and your distance to the camera is 2 m. The you get with intercept theorem:

f=focal length
p=distance on the sensor (e.g 1/10 Pixel of 6 um (typical pixel size on the sensor)
d=distance camera object (here = 2 m)
x=distance on the surface (you want to measure at least = 0.1mm)

f/p =d/x

then you need at least a focal length of 12 mm. Be careful this approximation is only valid in the mid of the image and it's really optimistic! At the border the accuracy is not so high. With the given sensor size you can calculate the field of view (Same formula as above p=length of the sensor)

To get such a high accuracy you need a very good camera calibration. Maybe you can use the approach from Tsai (or the one which is implemented in OpenCV) But don't use Zoom lenses you don't get this high accuracy with them.

It must be also clear that the distance camera and your objects must be well known and stable or you must calibrate the system before each measurement.

But with all of them you will get such a high accuracy.


Regards
Jürgen





googlegroups.9.multiversum4@xxxxxxxxxxxxxxx schrieb:
Hi all!

I'd like to measure the distance between two objects (one meter sized
thin edge and one centermeter sized roundish object). The objects are
normally about 0 to 20 mm away from each other. I could place a camera
two meters away. I would like to measure the distance with a precision
of 1 mm or so. Light conditions are okey, always same angle, b/w should
do fine.

Do you think it would be feasible to use digital image processing for
this purpose? What makes me uncertain is the precision. What kind of
camera would I need to measure 1 mm difference on 2 meters distance?
What precision would be reasonable?

Anyone with experiences or ideas is most welcome to comment!

.

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