Operation of Optomatic - TRIOPTICS GmbH

Effective Focal Length

A highly corrected collimator projects the image of a special target to infinity. The parallel ray bundles enter the lens under test parallel to the optical axis and emerge from the lens as a convergent beam intersecting in the focal plane. The target image formed in the focal plane of the lens under test is collected by a microscope assembly and focussed on the high resolution CCD array accurately located in the focal plane of the microscope. The microscope assembly with the CCD array is mounted on a motorised linear stage driven by a stepper motor. A computer-controlled Auto-Focus System allows for automated and accurate finding of the focal plane of the lens under test. Simultaneously the size of the projected collimator target is precisely determined and thus the effective focal length (EFL) of the lens under test.

All aspects of the data acquisition starting with the powerful Auto-Focus System, scanning of the CCD array, self-calibrating and displaying the measured data are under software control and fully automated. The set up and measurement parameters for one lens type can be stored to a file and loaded again when necessary. This reduces considerably the set up time, so that an operator measuring a new batch of lenses starts producing reliable measuring data in 5-10 seconds.

Flange Focal Length

The Flange Focal Length (FFL) or the Flange Focal Distance (FFD) is the distance between the locating surface of the lens mount and the image plane. This parameter is especially important for the photographic lenses as distance between the locating surface of the lens mount and the film (image) plane.

The self-centering sample holder of the OPTOMATIC has three stable pins with a diameter step providing a precise mechanical stop for the lens mounts. The pins are made of hardened and grinded steel to ensure the necessary stability. The locating surface of the lens mount must be in ferm contact with the pin step as mechanical stop. To determine the position of the mechanical stop and thus of the locating surface of the lens mount, a special reticle in mount is positioned into the sample holder. The optical head of the OPTOMATIC will automatically focus and accurately ascertain the position of the the illuminated special reticle.

Since the thickness of the reticle mount (dimension X) is accurately measured before, the position of the mechanical stop is easily calculated. After this, the serial measurement of FFL can start: the image plane of the sample is measured and the FFL is automatically calculated and displayed. Besides the absolute measurement of FFL a relative measurement is also possible. In this case a master lens is measured as a reference. The data are stored and then compared with the results of the serial measurement.

Back Focal Length

The Back Focal Length (BFL) is the distance from the vertex of the lens to the second focal point. The optical head for BFL-measurement is designed as a special autocollimating device incorporating a special target, beamsplitter and imaging optics. A fiber optic illumination with condenser optics illuminates the target. The projected image of the target is automatically focussed on the vertex and in a second step in the focal plane of the lens under test. Each position of the two focussing planes (lens vertex and focal plane of the lens) is accurately measured. The distance between the two focussing positions is the BFL.

BFL Measurement with Optomatic

The collimated beam for the focal plane measurement is provided either by a tilt mirror located behind the lens under test or by the collimator included in the optical set up of the OPTOMATIC. When using the standard collimator, a second illumination unit with a shutter is needed. Each illumination unit is active when the corresponding focussing (on to the vertex or in focal point) is accomplished.

Centration Errors

The normal procedure to identify the centering errors is to rotate the sample in transmitted or reflected light. For the measurement in Transmission the standard optical head and the collimator is needed. The parallel beam emerging from the collimator is focussed in the focal plane of the sample to be measured.

Measurement of centering errors in
Transmission Mode with OPTOMATIC

For centration measurement in Reflection, the autocollimating optical head (same used for BFL measurement) is mounted on the measuring unit instead of the standard optical head. The Autocollimating Optical Head is focussing in the center of curvature of the lens. When rotating the lens under test - if centering errors are present - the image of the target formed on the CCD will move correspondingly.

The size of the displacement is proportional with the centering error. In order to be rotated, the lens is positioned in a precision self-centering holder featuring four indexing stops at 90°and rotation over 360°. During the holder rotation, measurement data are taken in any of the indexing stop positions. A special alghoritm allows for automatic calculation and display of the centering errors. The result of the measurement can be given as radius of the run out circle (in ľm) with an accuracy of +/-0.5ľm or as tilt of the surface or of the lens axis (in arcsec).

Measurement of centering errors in
Reflection Mode with OPTOMATIC MICRO

MTF-Measurement

The set up for MTF measurement includes the standard optical head equipped with a high resolution relay lens. A narrow bandwidth interference filter is mounted into the illumination unit of the collimator. Two basic measurement procedures are available: MTF-calculation based on the evaluation of the image of an edge target and a second procedure based on the analysis of the image of a slit target.

The edge image evaluation can be carried out with the standard collimator target i.e. a double slit, which is primarily used for EFL measurement. The advantage is the simplicity of the equipment and the simultaneous measurement of EFL and MTF. However, due to the measuring principle the accuracy is limited. This procedure is very cost effective and recommended for production quantities when relative measurements in comparison with a master are acceptable. The slit evaluation requires a special slit target to be mounted in the collimator. The MTF calculated from the slit evaluation is more accurate, however, it requires expensive hardware. The OPTOMATIC system includes a special illumination unit with an electronic shutter. The electronic shutter is software controlled and opens alternatively the double slit for EFL measurement or the narrow slit for MTF-measurement.

Centration and MTF Measurement with OPTOMATIC MICRO

Angle and Power of Wedges

The unique versatility of OPTOMATIC is underlined by the capability to measure not only parameters of lens systems but angles and power of wedges, windows, plane-parallel plates.A limited types of prisms (pentaprisms, 180°-prisms) can be measured as well. The set up includes an ancillary lens positioned in the sample holder. A reference measurement of this lens is taken. The position of the focal plane of the ancillary lens and the position of the target image on the CCD is accurately determined and saved. Placing a wedge between the collimator and the ancillary lens causes a displacement of the target image on the linear CCD array. The displacement is measured and the deviation angle calculated and displayed with a resolution better than 1 arcsec over a range of 1°-1.5°. Aberrations of the transmitted wave front through the wedge or window under test will result in modifications of the EFL of the ancillary lens. Thus the power of wedges and prisms can be simultaneously determined with an accuracy better 0.002 dpt.