EFL and Distortion

Relevant for: ImageMaster®

Distortion is the percentage of change in magnification between the center of the field of view and off-axis positions in the field of view. In order to determine the distortion a different method of measuring the EFL is used:

• A MTF measurement on axis (at a selected frequency) is carried out. The paraxial focal plane is determined using the MTF measurement at a frequency of interest. To achieve a high accuracy monochromatic light and MTF curve fitting are used while the aperture of the lens is reduced by a diaphragm.
  
  
• An angular field range and a number of measurement points is selected. The object generator will automatically move to the selected field positions.
  
  
• At each position in field the image height is measured. A table with value pairs image height h vs. field angles α is provided.
  
  
• The EFL is calculated with the formula h/(tan α) and drawn into a curve vs field angle. A polynomial fitting of the EFL close to α=0 allows the accurate and independent calculation of the paraxial focal length.
  
  
• The distortion is calculated as the ratio between paraxial and field focal length.

This procedure requires a comparatively large number of measurement points with accurately known object angles and field positions. For this reason ImageMaster® instruments with dedicated distortion measurement option feature a high precision angular encoder for the object generator (accuracy 5 arcsec) and a high precision linear encoder for the image field stage (accuracy 0.2 μm). This way, the EFL is calculated as an absolute quantity, not depending on calibration parameters.

Relevant for: ImageMaster® MTF Station