Smart Cameras

Resolution Needed for Solving a Vision System Application

  

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Choosing the proper smart camera resolution for measurement applications has come up in conversation a lot lately.  It is important to have enough resolution so your measuring method is repeatable. Below is an example that illustrates how to select the right camera resolution

Let’s say you are measuring the outside diameter of a machined ring.  The spec says

25  +/-  0.05 mm

The tolerance is +/- 0.05mm. This means the tolerance range is 2 x .05mm = 0.1mm. 

Many quality engineers will tell you that the measuring device should be 10x better than the tolerance range.  This means the range of measurements of n number of samples should be no greater than 0.1mm x (1/10^th) …which equals 0.01mm. 

I know from experience that PPT Vision’s Circle Gauge tool is repeatable to 1/10^th of a pixel if the presentation is perfect.  Let’s assume that the conditions are less than ideal and the circle gauge is repeatable to 1/4^th of a pixel. 

Since the ring is 25mm wide, assume the field of view will need to be 40mm to accommodate part movement, etc. 

Use this ratio:

40mm / X pixels = 0.01mm / 0.25 pixels

                Solve for X:

                                X = 1000 pixels

This means the resolution of your camera should be no less than 1000 pixels wide.  I would start further testing with a 1024x768 camera (such at the PPT VISION IMPACT T34, 7200 or M330).  Normally, I will gather 100 OD measurements of the same part.  From there, calculate the range of measurements (max measurement – min measurement).  If it is less than 0.01mm, then the camera is a repeatable measurement tool.  If it is greater than 0.01mm, then you will need to adjust the measurement method, lighting, etc.

There are differing points of views on this topic.  Here are few I run across frequently. 

1. Some will say that you cannot use the tolerance range in the calculation and say that you must use ½ of the range. 
2. Some will say that 10x better than the range is too much and this is only applicable for measuring devices requiring human interaction.  These people will typically say that 4x is fine for machine vision. 

Regardless of the differing points of views, the methodology above is essentially the same. 

This explanation gives a starting point for determining the proper resolution required to achieve a repeatable measurement system.   For more detailed information on optics selections and to download lens selection tools visit http://www.pptvision.com/downloads_optics_calculator.html.