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According to DIN 3140 a centering error is present when the optical and reference axis of a lens do not coincide, respectively these are different in position and direction.
During the process of cementing, alignment and bonding of lenses in a mount, significant centering errors could result and add to the machining errors of a lens. Consequently, the requirements of high performance optical system can be fulfilled only when all the production steps from the centering tolerance measurement to the assembly of the lens in a mount are planned and designed as an integrated concept.
The TRIOPTICS comprehensive line of equipment – under the generic name of OptiCentric – has been specifically designed to cover applications related to any type of optical components and to respond to different accuracy degrees. The OptiCentric-System also covers all the production steps from the centration measurement to the alignment, cementing and assembly of optics. The OptiCentric includes valuable tools for any application, from simple and affordable visual instruments to complex, fully automated and PC-controlled production and laboratory stations.
For more information, download the OptiCentric brochure.
The normal procedure to identify the centering errors is to rotate the sample in transmitted or reflected light.
For the measurement, an autocollimator with additional optics is focused either in the center of curvature of the surface (Reflection Mode) or in the focal plane of the lens (Transmission Mode). For the measurement in transmission a collimator is additionally needed. The parallel beam emerging from the collimator is focused in the focal plane of the sample to be measured.
The images reflected from the lens surfaces (Reflection method) as well as the images projected in to the focus of the lens (Transmission method) are observed through the eyepiece of an autocollimator, of a telescope or of a microscope.
In Transmission method it is not possible to determine which of the two surfaces of a lens is producing the centration error. In some cases, a lens tested in transmission can reveal no centering error, although the lens is tilted in the mount. The images reflected from the lens surfaces, however, represent tilt and the individual centering errors.
The Reflection method is the only total and true method of measurement of centration errors. However, the Reflection method is in many cases difficult to manage. On the other hand, the Transmission method with some mechanical constraints gives in many cases satisfactory results. For a good economy of the optical manufacturing both methods should be considered.
When a centration error is present, the observed image describes a circle, while the sample is rotated around a reference or datum axis. The diameter of this circle is proportional with the size of the centering errors. The result of the measurement can be given as radius of the run out circle (in µm) or as tilt of the surface or of the lens axis ( in arcsec).
In this set up an autocollimator equipped with additional head lenses is focused in the image plane of the sample (transmission method) or in the center of curvature of the lens surface under test (reflection method). This set up is simple, the focusing procedure and the operation of the instrument can be easily carried out.
However, this set up requires the values of the focal length or radius of curvature of the sample to be known, otherwise the calculation of the centration errors is not possible. Difficulties are also experienced when measuring samples with longer focal lengths or radii of curvature. To accommodate longer focal lengths or longer radii of curvature either a large number of additional focusing optics or a long travel mechanism is necessary.
Since in the practice the values of the focal lengths and of the radii of curvature are not always known, TRIOPTICS developed instruments able to measure directly the centering errors.
This set up is more complex and includes two telescopes, a collimator and an optical assembly with beamsplitter and head lenses. The beam projected over the collimator is focused with the head lens in the image plane of he sample (transmission method) or at the half of the radius of the surface under test (reflection method). The resulting parallel beam enters one of the two telescopes and gives a direct reading of the centration error.
The values of the focal length and radius of curvature are not needed for the calculation of the centration error. Moreover, the errors resulting due to the difference of the nominal values of focal length and radius and theirs effective values are avoided. In comparison with the OptiCentric this set up allows for shorter travel mechanism or less additional focusing optics to cover the same range of focal lengths or radii of curvature.
The TRIOPTICS measurement systems described before are of modular design, so that the instruments are upgradeable and compatible with each other. To make the selection of the suitable equipment easier, in a first step the main instrument parts are presented:
Optical Sensing Head
The Optical Sensing Head is designed to view and
quantify the size of the centration error in single and multiple optical
components. The Optical Sensing
Head is also the basic unit used for alignment of single elements before
cementing or mounting in cells.
The Optical Sensing Head is basically an optical set up
(e.g. an autocollimator or
telescope) equipped with an appropriate reticle.
The illumination unit behind the reticle is adapted to the reflecting
properties of the sample. The
range of illumination units offered be TRIOPTICS are complete assemblies
with condenser optics and different light sources:
The reticles mounted in the measuring head allow for
direct reading of the radius of the run out circle described by the focused
or reflected images produced by the rotating sample. The eyepiece reticles (used for error reading) can be
selected from a large range of scales or tolerance circles. The illuminated reticles (projected onto the sample surface
or in its focus) are mostly bright crosses or pinholes.
The Optical Sensing Heads offered by TRIOPTICS are of
modular design so that the viewing and measuring of the centration errors or
lens alignment can be carried out:
The visual control using the eyepiece is the most simple and cheapest solution. However, the accuracy and repeatability is limited and very much depending on the qualification and the degree of fatigue of the operator.
The visual control using the eyepiece, CCD-camera,
CCD-adapter optics and video monitor increases the comfort in operation,
allows the operator to better concentrate on the rotation of sample,
however, has the same limitation concerning the accuracy and repeatability.
None of these equipment’s provides records of the centration errors or the accuracy achieved in alignment and cementing, so that optics rejected by customers could quickly annihilate the advantage of the low price of this equipment.
The use of a automated, PC-controlled Optical Sensing
Head increases dramatically the accuracy and repeatability, provides records
of the tolerances achieved and is not dependent on the qualification or
fatigue of the operator. The
CCD-camera is in this case connected to a frame grabber mounted in the PC. The software controls the measuring process, so that the
accuracy and repeatability are less operator dependent.
The direct measurement data as the radius of the run out circle are
automatically determined and used for calculation of the deviation
(transmission method) and of the tilt angle (reflection method).
The measuring certificate includes data presented
according to the standard DIN 3140, so that a direct comparison with the
tolerance values are possible. To
further increase the capabilities of the system and the efficiency in the
optical shop, a number of options as electronically generated scales,
tolerance circles or a Yes/No selection are available.
Each of the TRIOPTICS-Optical Sensing Heads can be
upgraded according to the changing customer needs.
Stands with Travel Mechanisms
The Optical Sensing Head is normally mounted on a stand having capabilities to move up and down the measuring head to adapt to a certain range of radii of curvature (for centration in reflection) or of focal length (for centration in transmission). The necessary movement of the measuring head is realized with different mechanisms and controllers. The measuring head movement can be done:
travel control, the TRIOPTICS stands
are equipped with coaxial rotating knobs for coarse and fine travel.
This solution is cheaper, however, the time needed for moving the
measuring head reduces to the efficiency.
Moreover, the travel is limited, since the height of the stand must
be selected so that the operator works in a seated position.
Consequently, to cover a certain range of radii/focal length a large
number of additional optics is necessary.
version uses stepper motor and controller to increase the speed of
movement and the travel of the measuring head.
Having a longer travel, the number of additional optics necessary to
cover a certain range of radii/focal length is reduced compared with the
PC-controlled version allows driving to the focusing positions for an
unlimited number of radii/focal lengths.
These positions are either known from the optical design of the lens
or can be found in teaching procedure and recorded for future use.
In this way, the alignment and assembly of multiple element lenses
become easier. In connection
with an automated measuring head, the motorized, PC-controlled stand can be
integrated into a fully automated measurement and production process.
The manual stands form TRIOPTICS are equipped with digital
slides showing at any time the position of the measuring head and
helping the operator to find the focusing positions.
In the motorized versions, the position of the measuring head is
given by the stepper motor controller.
All the stands can be used for measurement in Transmission and/or
Reflection. The equipment for Transmission additionally included a
collimator and a deflection mirror mounted in the stand base.
Lens Rotation Devices
Since the viewing and the measurement of the centration errors are possible only when the lens is rotated (except for extremely expensive errorless focusing devices), the availability of a lens holding and rotary device is crucial for completing a centration measurement.
The accuracy of the centration measurement and of the alignment/mounting of lenses is determined by the two basic components of a centering/mounting equipment:
Many optics manufacturers underestimate the importance of the Lens Rotation Devices in ensuring the required accuracy. Since the use of accurate CCD-cameras and complex image processing software provides a high accuracy of Optical Sensing Heads, the largest error source in lens centering and mounting is in many cases the Lens Rotation Device. Many of the Lens Rotation Devices used in the optical manufacturing do not provide a reference rotation axis which is enough accurate and stable. TRIOPTICS invested a large amount of work in designing a complete range of accurate devices for lens rotation:
Motorized Vacuum-Lens Rotation Device
The sample lays on a non rotating plastic chuck. The cylinder edge of the lens is in contact with a V-plate. The position of the V-plate can be finely adjusted using the two micrometers stages for x- and y- direction. The non rotating plastic chuck is adjustable in the height, so that samples of different thickness can be accommodated.
A friction drive with different friction wheels enables the lens rotation. The position of the lens on the non rotating plastic chuck is secured by vacuum. The vacuum adapter is equipped with a precision glass window which allows viewing of the projected image, while the lens is secured by vacuum. The vacuum pump and the electronics for speed and vacuum regulation are integrated in a control module. Due to the presence of the vacuum pump, a compressed air net is not necessary.
Precision Self Centering Holder (SCH)
The precision self centering holders are rotateable over 360 degrees. Three jaws precisely positioned at 120 degrees can be opened or closed by rotating a knurled ring. The lens lays on precision ring chucks made of hardened steel. In this device the lens rotates together with the ring chuck. A second knurled ring is used to rotate the holder manually. A motorized rotation realized by means of a friction drive is also available. The precision self centering holders are available in 4 sizes, each size corresponds to a certain diameter range of the samples.
PRH 02 2-100-023 13-34 2-4
A range of standard ring chucks is also available and can be selected in order to accommodate samples with different diameters or with different lens thickness. A quick lock mechanism allows the rapid exchange and a precise positioning of the holders.
Tip-Tilt Rotation Stage
This stage is rotateable over 360 degrees and features translation capabilities and two-axes tilt adjustment. This 5 axes tip-tilt rotation stage is normally manually rotated, however, a motor drive is available on request. The tilt and translation adjustment is made by precision micrometers screws. The adjustment sensitivity is ±2 arcsec and ± 1 µm respectively.
Ultra-Precision Air Bearing Table
This device combines the accuracy of an ultra-precision air bearing (run out less 0.05µm) with a stable tilt and translation stage. Due to the high stiffness and accuracy, the ultra-precision air bearing table is especially suitable for the assembly of high performance multi-lens objectives.
To select the suitable Lens Rotation Device some advantages and disadvantages of the different devices must be considered:
Motorized Vacuum-Lens Rotation Devices
When accurately manufactured and correctly operated, this device offers good results in accuracy and repeatability. Errors of the cylinder edge, when present, are averaged, so that the measurement results are reproducible. Main applications are the measurement of centering errors and cementing of achromats or lenses. Using plastic V-plates, even the assembly of some lenses in mount becomes possible. Except for some limitations in assembly work, the Motorized Vacuum-Lens Rotations Device features the best price/performance ration and is a reliable accessory for accurate, measurement of centration errors.
Precision Self Centering Holders (SCH)
The Rotary Holders are easy to use and the measuring procedure can be completed quickly. However, these holders require a large number of ring chucks to precisely match the diameter of the lens, so that risks of scratching the lens are completely eliminated. Furthermore the holders have unavoidable machining tolerances. The Trioptics holders have a total error of 2-4 µm including the rotation and self-centering errors. The repeatability of the measurement is also influenced by the errors of lenses edge, ovality or positioning on the ring chuck. However, for small lenses with diameter smaller than 4 mm this device is the only alternative. Applications: Measurement of centration errors (transmission or reflection), cementing of achromats, bonding of lenses in mount, measurement of endoscope lenses.
Tip-Tilt Rotation Stages
The main application of this device is related to the assembly of multi-component lenses and the bonding of lenses in mount. The measurement of centering errors is limited and only possible when using mechanical gages. The set up of this device requires that the lens chuck, the lens mount or the lens reference is accurately aligned to run true to the rotation axis. This can be accomplished by using the translation and tilt capabilities of the stage. With suitable extras as accurate optical tools (plane-parallel plates, glass balls) or mechanical gages the pre-alignment of the chucks or lens mounts become easier and the device delivers a good process accuracy.
Ultra-Precision Air Bearing Tables
This lens rotation device is similar with the Tip-Tilt Rotation Stages, however, the mechanical rotation stage is now replaced by an ultra-accurate air bearing. The accuracy is significantly higher: the axial and radial run-out errors are smaller than 0.05um. When the pre-alignment of the lens chuck or lens cell is accurately made, the Ultra-Precision Air Bearing Table gives the highest possible accuracy for assembly and potting of lenses in mounts. For pre-alignment, a ultra-stable and finely adjustable tilt and translation table with a diameter of 200mm us available. With this, not only tiny changes in tilt as small as 1 arcsec are achievable, but also the assembly of heavy, large, multi-lens objectives is possible. The rotation can be accomplished manually or by means of a smooth, stress free belt drive.
All the TRIOPTICS-Lens Rotation Devices described before have the same mechanical interface, a proprietary conical interface with quick locking mechanism. This fits to all TRIOPTICS-Stands and allows rapid exchange and precise positioning of different Lens Rotation Devices.
The advanced software is designed to work under all Windows operating systems. Windows 95/98/NR, OptiCentric uses a high performance 32-bit PCI-frame grabber for real time transfer of data to the PC RAM memory and the live display of images. The software fulfills the need of the optical shop for easy, intuitive operation and features a large number of options to accommodate a large variety of specific requirements. The software modules “Centration in Transmission” and “Centration in Reflection” allow for free rotation of the sample and different measurement principles:
To adapt to different hardware configurations, e.g. different reticle patterns (bright cross, dark cross, pinhole, etc.), the software features selectable options. Further options enable to select different measurement units: mm/inch, arcsec, microrad, etc. To increase the efficiency in production process, the software allows to save the optimized process parameter for future use. Further features of the software:
The calibration can be checked at customer site. The measurement certificate includes besides the data according to the standard DIN 3140 (surface tilt angle) the radius of the decentration circle. The statistic evaluation of the data and the records of the measurement parameters complete the information contained in the measuring certificate. For further processing the measurement data can be exported to text or spreadsheets processing programs.
Alignment, Cementing, and Bonding (Potting) of Optics
Each of the equipment’s described before facilitates the cementing of achromats and other multiple lenses.
The visual Optical Sensing Heads allow for alignment of the lenses before cementing in the classical way:
The automated PC-controlled Cementing Station is supplied with specific software which help the operator to align the lens.
Either a vector shows a magnified display of the size and the direction of the decentration and/or the displacement values in X and Y directions are made available with submicron accuracy for mechanical or motorized alignment. The center of the runout circle is indicated by a stationary cross.
The operator can concentrate on the alignment of the lens only. The PC-controlled cementing station enables continuous monitoring of the centration status even during the UV-curing. In addition, it provides the measurement and the record of the achieved centration tolerances after cementing. This results in a significant increase in accuracy and efficiency, making the PC-controlled cementing station indispensable for accurate and high class optics.
To align two lenses before cementing, either both optical axes have to be made to coincide or to be brought into the same position to a datum axis (e.g. concentric to the edge cylinder axis). In aligning a lens element in a cell it is necessary to arrange for the common axis of both lens surfaces to coincide with the datum axis of the lens cell.
To help the operator to align a lens, a modulated compressed air device or a piezo-electric positioning device is available. The accuracy featured by the modulated compressed air device is limited. The operation of this device requires experienced operators.
TRIOPTICS offers piezoelectric positioning devices which can be manually operated over a piezoelectric controller or integrated into a fully automatic alignment process with feedback from the Optical Sensing Head. The alignment devices with piezoelectric positioners feature a 0.1um resolution and a choice of three speed values.
The automatic alignment requires complex software and precise positioning of the Lens Rotation Device: after the centration error has been accurately measured and its size and direction are available, the Lens Rotation Device is stopped in a precisely determined position. The piezoelectric positioning device-in connection with a fast stepper motor serving as coarse positioning- is activated and the lens is moved into the required position. The Motorized Vacuum-Lens Rotation Device and the Precision Self Centering Holder (SCH) are more suitable for cementing of achromats, the Tip-Tilt Rotation Stages and Ultra-Precision Air Bearing Table are highly recommended for mounting of multiple element lenses.
Bonding (Potting) of Lenses in Cell
Due to the availability of UV-curing cements, the automated fixation of the lenses in mount using precision bonding, became the choice for efficient and accurate lens mounting. The lens cell is mounted by appropriate methods onto a Lens Rotation Device and the lens location is adjusted using the tilt and translation facilities until is running true on the rotation axis. When using the self centering rotary holders, the cell should have as reference the surface in contact with the jaws.
The PC-controlled Bonding Station offered by TRIOPTICS can be simply attached to any of OptiCentric measuring instruments, it includes automated cement dispensers, UV curing systems, stepper motor positioning of the dispenser and complex software for the control of all process parameters:
In connection with a piezoelectric positioning device, the entire production process including the Inspection, Alignment, Bonding and UV-Curing can be automated. The different process steps are individually programmable. A test mode allows to adjust and set up the process parameters for a lens type. Once the process parameters for a lens are defined, the automation mode or production mode can be started. The process parameters for one lens type can be stored to a set up file and loaded if necessary to repeat the procedure at a later stage.
As already mentioned, the instruments described before have a modular design. The components and accessories are compatible with each other, so that the instruments can be easily upgraded or configured for new applications. To make the selection of appropriate equipment easier, the main instrument versions are presented below:
The conventional series OptiCentric is based on a optical set up featuring measurement of centration errors in focal plane or in the center of curvature. The series OptiCentric is flexible, covers widespread applications and is conveniently priced. The measurement range is limited, however, this can be extended by means of additional achromats or lenses. The OptiCentric series is equipped with stands with manual travel control. The Optical Sensing Head are either of visual type or PC-controlled. The Lens Rotation Devices have to be ordered separately according to the specific customer applications.
The PC-controlled OptiCentric Station – depending on the Optical Sensing Head-achieves an accuracy in the range of one arcsec and can detect deviations or decentrations in the range of 2-3 µm up to 0.5 µm.
Accessories for OptiCentric and OptiCentric Direct
The OptiCentric-Direct series is based on a special optical set up which allows for direct reading of centration errors. The geometrical parameters of the sample (e.g. EFL or ROC) are not needed for the calculation of the centration errors. The basic measuring range is larger, so the number of additional optics required is significantly reduced. The measuring accuracy is similar with that achieved with OptiCentric series.
OptiCentric-MOT is the newest development of our comprehensive range of equipment for optical alignment and assembly. This Inspection and Production Stations is PC-controlled and equipped with an accurate stepper motor for positioning of the measuring head. The measuring head can be programmed to drive to several focus positions or centers of curvature of multi element lenses. In this way the centering, assembly, or bonding of multi-element lenses becomes easier.
OptiCentric-MOT can be integrated in a PC-controlled and automated production process.
In connection with an ultra-precision air bearing table, the OptiCentric-MOT features the highest accuracy and efficiency.
As all the other OptiCentric series it can be configured for any step of the centering and assembly process: for centration error measurement, for alignment, and cementing or for potting lenses in cell.
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