OPTISPHERIC | OPTOMATIC 2000 | OPTOMATIC | OPTOMATIC-MICRO

OPTISPHERIC
Universal Optical Test Station

OPTISPHERIC is a PC-based Optical Test Station designed to accurately measure the optical parameters of lenses and optical assemblies. OptiSpheric is modular and upgradeable. it covers a large range of applications and provides fast and reliable test results of almost all optical parameters:

• Effective Focal Length (EFL)
• Modulation Transfer Function (MTF)
• Back Focal Length (BFL)
• Radius of Curvature (R)
• Flange Focal Length (FFL)

Being a part of OptiTest - the world’s most comprehensive line of optical test instruments - OptiSpheric can be easily extended to cover more applications. The measurement of plano optics, for example, can be achieved by simply adding some mechanical fixtures and a software module. In this way the wedge angle, mirror tilt angle, parallelism of windows , filters and plates and many other parameters of plano optics can be measured quickly and accurately.

A further upgrade helps solving problems related to the alignment and cementing of optics or enables the non-contact measurement of centering errors of optical components and systems.

Covering a vast range of applications, OptiSpheric shows where the manufacturing process needs improvement and becomes an indispensable tool for quality assurance in the optical shop or for the research work in the laboratory.

SETUP

The Measuring Head is basically an optical set up (e.g. an autocollimator or telescope) equipped with a CCD-Camera and an illumination unit with an appropriate reticle. The illumination unit behind the reticle is adapted to the reflecting properties of the sample. The high resolution CCD-camera is mounted so that the focal plane of the measuring head precisely corresponds with the CCD-camera chip. The CCD-camera is connected to a frame grabber mounted in the PC.

The Stand has two travel mechanisms: a coarse travel and a coaxial ultra-fine focusing mechanism. The ultra-fine mechanism is especially useful in applications where a smooth, precise motion of optical instruments or subsystems are required.

The bottom body of the stand contains a clamp fixture for a Collimator and an adjustable mirror for beam deviation. The Collimator is equipped with a rotate-able reticle and filter changer. The reticle changer is normally fitted with following reticles:

• double slit-for focal length measurement
• single slit-for MTF measurement
• resolution test - for testing image quality
• bright cross-for radius or centration measurement.

The filter changer normally includes a green filter, an interference filter at 546.1nm and a white diffuser. OptiSpheric can be customized with reticle or filters to fit specific applications.

The position measuring Linear Encoder with LCD-display mounted on the stand is used for radius and BFL measurement.

The Sample Holder is normally a self-centering mount with a diameter range of 3-35 mm. Depending of the configuration of the instrument and applications, further holders can be supplied.

A Tilt Table is also included in the standard delivery kit. The self-centering mount can be mounted either directly on the base or on the tilt table which is then mounted on the instrument base.

OPERATING PRINCIPLE

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.

Manual Focusing or Autofocus

The Stand of OptiSpheric with the fast rack and pinion mechanism and the coaxial ultra-fine focusing mechanism responds optimally to the basic requirements of optical measurement: quick positioning of the measuring head for an efficient measurement and precise, smooth focusing for highest accuracy.

Combined with the powerful software providing highly accurate and repetitive measurement data, OptiSpheric is an extremely versatile and affordable instrument fulfilling the requirements of modern optical testing and of ISO 9000.

However, when testing optics in large quantities, an Autofocus system could be a justified alternative. Beside the software controlled measuring procedure, the Autofocus system will further reduce the dependence on the operators qualification or his tiredness.To respond to these specific needs. TRIOPTICS developed an Autofocus version of OptiSpheric which further increases the automation of the measurement process.

Software

The advanced software is designed to work under all Windows operating systems and fulfils the need of the optical shop for easy, intuitive operation. It offers a high level of speed and accuracy and provides consistent, reliable results. The software module of OptiSpheric is part of the TRIOPTICS’s concept of Integrated Optical Testing. capabilities can be simply fulfilled by enabling software features from related products (e.g. OptiAngle for angle measurements, OptiCentric for centering errors tests, etc.). This modular approach enables OptiSpheric to be configured or upgraded according to the operator’s requirements.

When measuring the EFL, only an approximate range of the expected EFL value has to be selected. The optimal head lens for the selected measurement is displayed. After the focusing is accomplished just a mouse click is necessary to perform the measurement.

The MTF measurement is based either on the edge or narrow slit evaluation. For quick tests in production, the MTF check based on edge evaluation might be the preferred choice. For accurate MTF measurement it is advisable to use the slit evaluation.The OptiSpheric can be configured to measure the MTF in sagittal and tangential direction or to perform tests on axis or off axis. The system may be manual or motorized, the focusing manual or automated.The spatial frequencies of interest can be selected prior to measurement. The real time measurement results can be compared with the calculated theoretical MTF values. In addition to the real time display of the camera image, a graphical display of the MTF and PSF as well as a table with the real time and theoretical MTF values are available. Further customized display of the data are available on request.

Measurement of Plano Optics

The addition of a software module-belonging to OptiAngle (another subsystem of OptiTest) -and the use of specific mechanical fixtures converts the OptiSpheric to a powerful tool for angle measurement of:

• wedges and filters
• 90°-prisms
• parallelism of plane plates
• deviation angle through wedges and prisms
• mirror tilt
• alignment of CD-optical heads
• wobble of rotating glass disks

The angle measurements of plano optics are made directly with the autocollimator of the measuring head (without additional head lenses). The mechanical fixtures includes three-ball supports, prism fixtures, etc. to be mounted on the instrument tilt table.

Measurement of centering errors

The unique performance can be further increased by adding software modules and accessories from OptiCentric (another subsystem of OptiTest).

Applications include measurement of centering errors in transmission and reflection. Using appropriate accessories even the alignment or cementing of optics can be carried out. With this extension of the measuring capabilities the OptiSpheric becomes a valuable and indispensable quality tool for any optical shop or laboratory. The largest range of accessories for accurate and efficient measurement of centering errors are available: precision rotary holders, lens rotation devices with vacuum, ultra precision air bearing tables, etc. Illumination devices have been specifically designed to provide reliable results even in difficult applications as surfaces with AR-coating. The software module provides fast and accurate results, shows the size and direction of the decentration and evaluates even extremely weak and blurred images.

Multi-Wavelength measurement capabilities

To respond to industry requirements for providing measurement capabilities at different wavelengths, TRIOPTICS developed a version of OptiSpheric able to measure almost of the optical parameters at different wavelengths: 400nm, 546nm, 650nm and 780nm.

A Filter Changer equipped with interference filters selects the required wavelength. A focusable telescope is calibrated for each wavelength. A colored mark on the telescope helps the operator to set the telescope optics at the selected wavelength. The high resolution CCD-camera mounted on the telescope provides via frame grabber and specific software fast and accurate measuring results.

Technical Data

EFL - Measurement

MTF - Measurement

BFL and Radius Measurement

Technical Data for Extension Modules

Centering Errors Measurement

Angle Measurement