# vision FH omron-measure the size of an object

## 3 posts in this topic

```Hello everyone
I'm trying to measure the size of an object 20mm long with an error of +-0.05mm. then which camera of omron should i choose and how to calibrate it to get the most accurate measurement. What is the formula for calculating the measurement error of the camera?
Thanks```

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some vison systems can get sub pixel resolution as result of interpolating/averaging. but... i would not count on that.

i would want to get at least 2 pixels over the error value.

if camera is setup correctly, 20.00mm / 0.05mm = 400 counts

but as mentioned i would like at least 2x that so absolute minimum i would even consider is 800 pixels.

but that is to see the part itslf without anything around it. you also need to know how precisely part is placed and add some 20% if part is always in same place and features are easy to sense, then 800*1.2 = 960 pixels.

if part can move some 10 mm in the direction of measurement, then 20+10mm = 30mm therefore

30mm/0.05 = 600 counts, and 2x that is 1200 pixels. and add 20% is 1200*1.2=1440 pixels

again, this is my rule of thumb, other things could be a factor too.

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The formula is:

Measurement Resolution(distance units)  = Resolution (Distance units / pixel) * 3

you need to triple (at least) the resolution to get a good edge, doubling essentially rounds it off. I like to use a factor of 6 if possible.

if your FOV is 30mm you need at least 1800 horizontal pixels. ((30mm * 20ppmm) * 3)

3.2MP gives you 2048 horizontal pixels. Your resolution needs to be >= 1800 and this is your closest. The FHV7 line is probably the best bang for your buck and has good measurement tools. The F430 has a 5MP variant with 2592 horizontal pixels as well but fewer lens options.

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