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Applications

Effective Focal Length

A highly corrected collimator projects the image of a double slit 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 image of the double slit formed in the focal plane of the lens under test is collected by the head lens of the measuring head and focused on the high resolution CCD camera.

The measuring head with the CCD camera can be accurately moved up and down using the travel mechanism of the stand until the image of the double slit is sharply seen on the monitor. After pressing the MEASURE button, the measurement starts. The EFL is calculated, corrected by the factor contained in the calibration file and displayed on the monitor. When measuring negative focal length, the parallel beam emerging from the collimator enters the lens under test parallel to the optical axes and emerges from the lens as a divergent beam. In this case a virtual image is formed behind the lens under test. The head lens used in this case focuses in the virtual focal point of the lens under test. The head lens must have a back focal length exceeding the EFL of the sample by 10-20mm. The calibration of the instrument ensures the traceability to international standards. TRIOPTICS uses master lenses certified by National Standard Institutes.

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 (546.1nm) is mounted into the filter changer of the collimator.
OptiSpheric® can be customized to measure the MTF at other wavelengths or at several wavelengths. The MTF measurement can be performed on-axis or off-axis at different angles.
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 MTF calculated from the slit evaluation is more accurate and should be used to analyze the lens performance. 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. This procedure is very cost effective and recommended for production quantities when relative measurements in comparison with a master are acceptable. The relay lens focuses the image of the target on to the CCD-camera chip. The intensity profile of the target is scanned electronically in both the radial and tangential direction. The data is collected and, by using Fourier Transform techniques, the MTF is calculated and displayed on the PC-monitor in real-time. The software calculates and displays the MTF value at selected spatial frequencies, the theoretical MTF-graph and the effective MTF-graph.

Back Focal Length

In a first step the Measuring Head equipped with micro-objectives or achromats is focused on the vertex of the lens (sample) surface. Two procedures can be employed to determine the position of the lens vertex:

either the illuminated reticle of the measuring head (bright cross, Siemens Star, etc.) is projected on the surface vertex, reflected back and focused on the CCD-camera

or the measuring head - used as a telescope is directly viewing the lens vertex (or a mark, dust, etc. existing on the lens surface) via relay lens and CCD-camera.

In both cases the measuring head is finely moved until the image is sharply seen on the PC-monitor.

In a second step the accurate finding of the focal plane position of the lens (sample) is accomplished. The collimator projects the image of its reticle over the lens under test. The measuring head with the focusing head lens is moved up or down until the image of the collimator reticle is sharply focused on the CCD-chip. The best focusing positions for lens vertex and focal plane are found either visually or using a software feature displaying the contrast as a coloured plot. The BFL is defined as the distance between the surface vertex (step 1) and the lens focal plane (step 2). This distance is measured by means of a digital linear scale which is attached to the travel mechanism of the measuring head.

Radius of curvature

The measurement of radii of curvature is similar to the BFL measurement. The measurement is done in reflection only. Focusing is now performed on the lens vertex and in the center of curvature. The distance is measured with the same digital linear scale. The best focusing positions for lens vertex and center of curvature are found either visually or using a software feature displaying the contrast as a coloured plot.

Digital Linear Scale