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The MTF benches of OEG provide the fully automatic characterization of the imaging quality on the axis and in the field
of view for any azimuths and types of object/image conjugates by measurement of the modulation transfer function
(MTF).
The operational area covers almost all standard optics for the visible and NIR spectral range.
The modulation transfer function (MTF) is a recognized control criterion for the optical imaging quality. It characterizes the resolution of optical systems on the axis and in the image field. Beside the MTF
a lot of other parameters can be measured, e.g. the focal length, chromatic
 aberrations, distorsion,
astigmatism, field curvature, depth of field.
The MTF-benches of OEG are the result of research and development for many years in the field of MTF measurement and
working satisfactorily with many considerable optics manufacturers in the production control. In addition it is also outstanding suitable for the employment in teachings, research, laboratory as well as goods in and outgoing inspection.
(picture left hand side: MTF-MASTER for object / image conjugates infinite /
finite)
(picture right hand side: MTF-VARIANT for object / image conjugates infinite
/ finite, 150 mm free aperture)
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 The classical MTF measurement principle, with which the picture of an edge or a gap, produced by the test specimen, is photo-metrically scanned by a mechanical scanning movement, can be realized meanwhile more simply by the use of CCD cameras.
The use of CCD-cameras in connection with image processing systems provides real time data of the line- or edge-intensity distribution. From this data the MTF can be computed. The MTF-data can be stored or shown on the PC screen.
Additionally CCD-cameras provide the advantage, that both directions of MTF (sagittal, tangential) can be measured nearly at the same time. The MTF Variant uses analogue-CCD-cameras with frame grabbers for real time video digitizing. But also an interface to a high resolution digital camera (USB 2.0 or Fire Wire) is available. The live video of the edge or gap is shown on the PC screen. The measuring field is freely selectable. The efficient evaluation software adjusts the measuring window with automated measuring expirations according to the movement of the object. For interactive measurements the measuring field can be defined by the operator. The real time evaluation refers both to the MTF measurement and to line and edge display functions.
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The MTF test bench is adapted in coordination with the user to special setting of tasks.
The adaptions refer e.g. to the test specimen focal lengths and its f-number, the measuring wavelengths or the object and image plane situations.
A measurement setup for an optics for the path of rays finite/finite differs from the structure from a measuring instrument for the path of rays infinite/finite.
For test specimens with long focal lengths and large free aperture the focal length of the measuring collimator is differently to measure (as evident in the fig. MTF Variant 150) than for test specimen with very short focal length. For the conception of the equipment of the MTF test bench, necessary for the different measuring tasks, the OEG GmbH is available as competent partner with experience for many years.
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Despite the presence of sophisticated design and manufacturing techniques, lenses can still vary considerably in imaging quality because of manufacturing errors. Because of the growing demands in imaging performance of lenses their characterisation by help of the Modulation Transfer Function (MTF) as the premier parameter for objectively evaluating is more and more used.
The optical industry uses this parameter as meaningful quality function for the objective evaluation of optical systems.
The MTF describes how the image contrast varies with spatial frequencies. It is expressed as the ratio of contrast in the image to contrast in the object as a function of spatial frequency of a line grid with cosinus functional transmission. Spatial frequency is expressed in terms of line-pairs per millimeter (lp/mm). The MTF combines image resolution and contrast into a common representation.
Another feature of an MTF measuring instrument is, that it allows system testing in situations similar to actual applications. Field angle positions, spectral ranges, distances between object- and image plane can be replicated or simulated in the test of an optical system.
MTF measuring instruments are characterised by large versatility, there apart from the MTF numerous further parameters can be derived, e.g. field curvature, chromatic aberrations, effective focal length, back focal length or line spread function. The MTF measurement offers a method to engineer or test technician for the direct measurement of the picture characteristics and permits thereby conclusions on error causes in the manufacturing process. MTF measurement results can be compared with the associated optics calculation.
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The MTF measurement is based on the picture (provided by the lens under test) of an edge or a gap almost at the same time in meridional and sagittal direction.
The picture of the edge or line produced by the test specimen is seized and evaluated over an image processing system. In the case of use of the edge picture first the line spread function is computed through to differentiate the edge picture.
From the fourier transformation of of the line spread function the complex optical transfer function results. The MTF is the amount of the fourier transformation of the line spread function.
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For the measurement the interactive real time mode and the fully automatic measuring mode stand for order.
In the interactive real time mode can be represented the MTF as well as the edge- and the line spread function as live image on the PC screen. Thus MTF Variant can be used e.g. during the fine tuning with the objective assembly. The results of interactive measurements can be stored and represented together in diagrams. On the basis of the freely selectable designation of the curves these can be identified in the diagram.
In the fully automatic measuring mode measuring templates are processed automatically. These are programmed and stored generally once by the user. A measuring template contains information about the object angles (or field points) and azimuths which have to be measured. One or more templates can be assigned to a certain type of objective.
After the start the complete measurement runs off fully automatic.
As result first a quick report is generated. The quick report contains the focus curve as well as all measurements with clear marking, made in the measuring expiration. Additionally the numeric measured values are available for as many as desired freely selectable spatial frequencies.
The diagram contains the option to draw in expected values for the contrast as function of the spatial frequency, so that a comparison between measured value and desired value is very easy and fast possible.
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Advantages
On a view
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• real time calculation and representation of MTF, ESF (edge spread function) and LSF (line spread function) on the PC screen
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• fast switching between tangential and sagital MTF measurement
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• comparison with freely selectable desired MTF-values in the diagram
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• real time MTF for fine tuning during the objective assembly
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• simple change between different measuring wavelengths
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• fast change between measurement on the axis and in the image field
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• alternatively manual or automatic azimuth-dependent measurement
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• software controlled rotation of the specimen around 360° with automatic measurement of the contrast values for a given spatial frequency and a freely selectable number of azimuths
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• operation and controlling of the hardware by Joystick, mouse and PC keyboard
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• graphic and numeric representation of the measured values with drawn in desired values (optional)
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• small space requirement
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• automatic production of meaningful metrology records (quick report)
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• diagram of the focus curve (contrast for given spatial frequency)
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• software controlled or manual focusing
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• automatic focusing with a user selected spatial frequency
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• alternatively fully automatic or manual procedure for measuring functions up to 7 axis
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• measurement of chromatic aberrations
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• measuring procedure very easily programmable by user
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• measurement of focal length (infinite / finite) or magnification (finite / finite)
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• easy to adapt to different measuring tasks
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• software interface for digital cameras and frame grabber for analogue cameras
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• objective data base
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Technical Parameters
Parameter
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MTF Variant
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MTF MASTER |
MTF accuracy
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±0.02
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±0.02 |
MTF repeatability
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±0.02
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±0.02 |
frequency range
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0 … 200 c/mm (representable up to any resolution)
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0...200 c/mm (representable up to any resolution) |
max.
object height*
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< 100 mm, automatically adjustable
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<500mm |
max.
image height*
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30 mm, automatic, software controlled adjustment
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75mm, automatic, software controlled adjustment |
azimuthal measurements
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360°, manually / automatically
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360°, manually / automatically |
focal length range*
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0 … 200 mm
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0...600mm |
object/image conjugates types
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finity / finity, infinity / finity
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finity / finity, infinity / finity
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optical length*
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200 … 800 mm
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150..1200 |
spectral range
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420 … 900 nm
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420 … 900 nm
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spectral measurements
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yes, with filters, optional motorized filter wheel available
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yes, with filters; optional motorized filter wheel available |
maximum field angle sin(w)*
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60°
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80° |
maximum free aperture
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50 mm (MTF Variant Standard) / 150 mm (MTF Variant 150)
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70mm (larger on inquiry without problems possible) |
autofocus
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yes, for freely selectable spatial frequency
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yes, for freely selectable spatial frequency
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automatic measuring procedure
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yes, for up to 7 axis
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yes, for up to 7 axis
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maximum specimen diameter*
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80 mm (mechanically)
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80 mm (mechanically) |
measuring principle
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image evaluation of an edge or a line, tangential/sagital
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image evaluation of an edge or a line, tangential/sagital
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manual measuring procedure
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yes, by joystick
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yes, by joystick |
operating system
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Windows 2000/XP/7
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Windows 2000/XP/7 |
software
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32 Bit
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32 Bit |
PC*
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state of the art standard at delivery
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state of the art standard at delivery
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PC-screen*
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19" flat panel monitor
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*The indication refers to the mechanically maximally attainable field angle. The optically maximally meaningful field angle depends on the test specimen. The parameters refer to the standard equipment and can be adapted to special measuring tasks.
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+49 (0)335 5213894
+49 (0)335 5213896
CONTACT