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What is Calibration?
by Jonathan Ludlow on July 14, 2011
posted in Product Development
Microscan’s new AutoVISION products are bridging the world of Auto ID and Machine Vision. AutoVISION is machine vision simplified and represents a giant leap forward in ease of use at the boundary between image based symbology readers and general purpose machine vision.

This advance has been made possible by combining the abilities of the Microscan engineering teams in Renton and Nashua to produce products that combine the strengths and abilities of both. This “Vulcan Mind Meld” is an impressive organizational feat but it has left one small area where the customary terminology overlaps in a way that might lead to confusion.

This is the vexed question of “what is meant by calibration?

In the world of imager based symbology with products such as the Microscan MINI and QX Hawk, Calibration means the adjustment of focus and sensor parameter to produce an appropriate image.

With the QX Hawk and MINI this is achieved by using the EZ button to command the system to determine the best focal distance by using the auto-focus mechanism to measure the distance to the part and then adjusting the gain and exposure to produce an image with good contrast.

 

With the new AutoVISION products the same effect is either by using the new AutoVISION button or by selecting the Auto Calibration button in the Image View screen in the AutoVISION application.


In the world of general purpose machine vision where Visionscape products are used for gauging of one kind or another the term Calibration means the system for relating measurements made in pixels to some practical real world units while at the same time making sure that the resulting measurement is as accurately as possible. This is achieved relating the pixel values to real world units during a calibration procedure. In Visionscape FrontRunner this can be done either by specifying a distance corresponding to a specified number of pixels – referred to as “Quick Calibration”, or by specifying “Robust Calibration” which uses a special eight dot target and associated precision dot location information to generate a transformation between pixel space and real-world space which takes into account X and Y scale factors, perspective, and camera rotation.



So here we have a possible source of confusion


When a machine vision guy speaks of calibration he means one thing and when an Auto ID dude speaks of calibration he means something else.

Now you might ask – “well why don’t you stop using the term calibration for either MV or Auto ID and start calling it something else?” Well, we nearly did - but the fact is there are a lot of happy users of QX products and Visionscape systems who could be confused (and annoyed) by a change in what they see as a clear and well understood term. It seemed the best to leave things as they were.

So how do you keep the terms apart and to make sure that you are not confused?

Think of it this way…


In the world of Auto ID – Calibration is image Acquisition Calibration. This means it is to do with setting up the sensor in terms of focus, exposure and gain to deliver a good image to the decoding and analysis software.

In the world of General Purpose Machine Vision – Calibration is Dimensional Calibration. This means it is to do with transforming measurements made in pixels to accurate results in real world dimensional units such as millimeters or inches (or microns or parsecs).

So there you have it. 

        In Auto ID Calibration is ACAcquisition Calibration

        In Machine Vision Calibration is DCDimensional Calibration

AC and DC – do not get them mixed up!

Jonathan
Posted by Jonathan Ludlow,  Machine Vision Promoter
Jonathan Ludlow is Machine Vision Promoter at Microscan’s Technology Center in Nashua, NH. He has been active in machine vision product development for many years, has authored papers on the application of machine vision in semiconductor packaging and electronic assembly, holds several patents relating to inspection systems and is a regular speaker at machine vision symposia.

Comments

Bill Ross
July 18, 2011 13:27pm
I must thank you for defining calibration for this device. Part of our equipment validation process must identify calibration. Since dealing with auditors and quality people often, they tend to demand: "Is the unit calibrated?" Some equipment that is computer processed is difficult to apply or claim that it is or can be calibrated. Or even necessarily need to be calibrated!

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