Calibration method of variable focal length lens and variable focal length lens device

A calibration method of a variable focal length lens whose focal length is periodically varied in response to a periodic drive signal by using a variable focal length lens device,

• the variable focal length lens device comprising:
• the variable focal length lens;
• an illuminator configured to illuminate a measurement target;
• an image detector configured to detect an image of the measurement target through the variable focal length lens;
• and
• a lens controller configured to output the drive signal to the variable focal length lens and control the illuminator and the image detector,
• the calibration method comprising:
• setting the measurement target in a form of a calibration tool having a plurality of height-different parts on a surface of the calibration tool;
• subsequently, preparing a calibration table by repeating outputting from the lens controller the drive signal having a predetermined voltage to the variable focal length lens, detecting a surface image of the calibration tool using the image detector, detecting two points whose image contrast is maximum in the surface image, calculating a focal depth from a difference in a focal length between the two points, and recording the focal depth and the voltage of the drive signal in a corresponding combination;
• and
• retrieving a value of the voltage corresponding to a desired focal depth from the calibration table to set the variable focal length lens at the desired focal depth of the focal depth, and adjusting the voltage of the drive signal to be outputted to the variable focal length lens on a basis of the retrieved value.