Formula for Resolution

The formula for resolution is:

where:

= resolution (height of one pixel)

= height of field-of-view (same units as )

Table C-1. Camera Scale Factors

Camera	Scale Factor
Panasonic GP-CD 40	4.8
Panasonic GP-CD 602
Panasonic GP-MF 702	6.6
Sony XC-77	6.6

When choosing the size of the field-of-view, there is always a trade-off between image size and image resolution. When the image is large, more objects or features can be captured in each picture, which reduces the number of pictures required for the application. However, image resolution is reduced as the image size is increased, and resolution is the key to accurately locating and measuring image features. On the other hand, processing time increases as larger areas of the image are processed.

The following steps will help you decide on a lens and camera-to-object distance:

1. Determine the minimum required resolution (smallest feature that must be resolved accurately). We recommend that a factor of 5 to 10 be applied to this minimum resolution to guarantee consistent results.1

2. Based on the required resolution, determine the maximum field-of-view size. If the maximum available field-of-view is too small to view the entire object you are inspecting, you will have to:

a. Use more than one camera to make the inspection.

b. Move different areas of the part into the field-of-view and make multiple inspections.

c. Accept a lower minimum resolution.

3. Using the upper and lower limits of the camera-to-object distances, determine the range of possible lenses.

For example, suppose you are using a medium-resolution camera. Your measurement accuracy needs to be within 0.1 mm, and the camera can be mounted 60 to 100 mm away from the object.

1. Using 1/4 pixel accuracy, apply a factor of 10 to the desired resolution and calculate the available field-of-view height:

2. Calculate the lens focal length at the minimum distance:

3. Calculate the lens focal length at the maximum distance:

4. Your application will achieve the desired resolution using lenses with focal lengths between 15 and 25 mm. If a 16 mm lens is selected, the proper viewing distance is:

NOTE: The effective focal length of a given lens can be lengthened by adding extension tubes. However, extension tubes may introduce image distortion.

These calculations do not take into account any error introduced by manufacturing inaccuracies in the camera. In general, higher resolution cameras are better constructed and should be used when resolution tolerances are tight.

1 This factor is sometimes referred to as the Part Tolerance Measurement Ratio (PTMR).