Using an Optical Bench to Measure Focal Length

Hi,

In a former life ('70 to '80) I have been using a collimating bench from ASKANIA (Germany) to measure many lens parameters like EFL - FFL - BFL and the like.

Their principle is simple:

- use a long reference collimator objective lens with good characteristics in a small NA and on a small FOV, equipped with a reference target in its focal plane, and a light source for illumination (generally filtered in the green, say 540 nm, for better resolution)

- positionning at infinity of the system by self focussing of this collimator is simple by reflection on a good mirror and looking at the reflected image on the test target

- calibration of the target (with engravings in um or mm) can be done by distance measurement techniques

- calibration of the collimator EFL can be done by theodolite or an autocillimator with angular measurement properties, or simply by measuring its long distance, or combined

- measuring any unknown lens EFL is done by imaging the target from the collimator through this lens, and reading the size of the obtained image with a calibrated microscope eyepiece

- the advantage of this large collimator is that you can change the working peak wavelength even with narrow filters, so measure chromatic aberration also (when the collimator lens characteristics are known, say by optical calculus)

- all of the required components can be purchased of the shelf from sources like Edmund

If this looks to long and to much overhead, any illuminated and calibrated (micronmeter reference) target imaged through your unknown lens by viewing through a theodolite (focussed at infinity) will do the job also; just read the angles D of a small distance h and use h= EFL * tan D; keep distances or angles small to avoid distortion and aberrations.