SU1597653A1 - Method of measuring modulation transfer function of optical system - Google Patents

Abstract

The invention relates to optical instrumentation and can be used in image quality control of optical systems, namely, in devices for measuring the modulation transfer function of optical systems. The method consists in forming an image of a periodic object, converting the image into an electrical signal and graduating by lowering the frequency of electrical pulses by M times and extrapolating the obtained values to a unit of modulation gain for zero spatial frequency. The proposed method allows graduation for lower than spatial frequencies known to be known. 2 Il.

Description

The invention relates to the field of optical instrumentation and can be used in the image quality control of optical systems, namely in devices for measuring the modulation transfer function of optical systems.

The purpose of the invention is to provide the possibility of calibrating the installation at low spatial frequencies

The figure shows a block diagram of a device for implementing the method; pa - time diagrams of processes in the electronic path of the device

A device for measuring the transmission function of the modulation of optical systems consists of a source of light 1, a capacitor 2, a slit test object 3 s.

drive, collimator 4, test lens -5, analyzing slit diagram 6, photodetector 7, amplifier 8, driver 9 pulses, one-shot 10, analog switch 11, frequency divider I2 and measuring device 13,

The purpose of the main units of the device is as follows: Source 1 of light illuminates the slit test object 3 using a capacitor 2, Collimator 4 and the test object 11 in 5 form an image of the test object in the plane of the analyzing slit diaphragm 6o Photo receiver 7 converts the illumination in the plane, the image into an electrical signal which is set by an amplifier 8. An electrical signal is then applied to the driver.

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9 rectangular pulses and an analog switch. According to the Odinbrator-.10 cl:, pkit l get 1 rectangular pulses of the required duration. Frequency divider 12 divides the pulse frequency of the input pulses by a specified number of times with. Analog switch 11 allows you to connect the input signal to the measuring device 3 by means of air.

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The device with which the method can be realized works as follows.

The image of the test object 3 is moved. according to the analyzing target b Due to this, the spatial distribution of the materials in the image is transformed into a temporary one, the Too is scanned 20 images that accept 7 transforms this distribution into an electrical signal

The input electrical signal U. in the form of pulses, in the form of identical to the distribution of illumination in the image of the test object, after amplifier 8 enters the shaper 9 pulses Shape :. 9 pulses converts INPUT S 3 pulses into a rectangular Of the pulses Uj the one-vibrator forms the pulses U

with a duration approximately equal to half the period of the following impulses and. Divider 12 frequencies per each M pulses-.U produces one pulse and. i t „eo divides the frequency by M times (in Fig. 2, the division is 4 times as an example) The pulses are equal in duration to the Q period of the pulse U, but are shifted

relative to them for half the period o The received signal opens the analog key 11, which permits cooling to the measuring device 13 each

 input voltage pulse Voltage at the input.

device 3 is the input voltage U, from each M pulses of which exclude gamma from M-1 liM- 2Q pulses. This is antagonistic to lowering the spatial frequency of the test object by M times

In this way, hailstones can be conducted on inferior spaces. After graduation, the analog key is switched on continuously and the measurements are made in the usual order with the difference that the amplitude

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the first harmonic, by adjusting the transmission coefficient of the electron path, is set equal to s amplitude of the harmonic with the number M in the graduation

The application of the proposed method makes it possible to increase the measurement accuracy of the FGS by an amount of 0.03-0.05 units of the transmission coefficient of the module for lenses with average image quality (for example, lenses for amateur and professional photo and film equipment). the range of spatial frequencies, whose Pia is possible to measure an MTF, expands through the use of longer-focus collimators. For example, in the conventional measurement method, it is recommended that the focal distance of the collimator be chosen equal to 3-5 focal lengths of the tested optical system. If a test object is used with a spatial frequency of 0.5 mm (normal value), the image frequency will be 0.5 mg g 3 1.5 MM l The maximum frequency at which be monitored, equal to 3 mm. If we take the ratio of the focal lengths of the collimator and the lens to 10-20, then the first harmonic of the image will be equal to 5-10 measurements can not be carried out because of the resulting large error in the calibration

Using the proposed measurement method, the spatial frequencies during calibration can be reduced to quite acceptable values of 0.5-1 mm by dividing the signal frequency by 10-20 times. At the same time, the maximum spatial frequency where measurement is possible

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on the ratio of the focal lengths 5 of the collimator and the test lens. The maximum possible ratio of the foci of the collimator and the test lens depends on the relative aperture of the latter. For a relative aperture of 1: 2 (with an average quality of the lens), it is 45-. 50, for 1: 5 - 20-25o For high quality lenses, this ratio can be even greater

From the above reasoning it follows that it is also possible to reduce the nomenclature of high-quality expensive collimator lenses by testing short-focus systems.

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Claims (1)

Claim A method of measuring the modulation transmission function of optical systems ^ consisting in forming an image of a periodic slit object, converting the spatial distribution of illumination in the image into an electrical signal, calibrating by measuring the amplitudes of the first two or three harmonic components of the signal and graphically extrapolating the obtained values to a unit of transmission coefficient modulations for zero spatial frequency, measure the amplitudes of the harmonic components of the signal, and measured To these values, the modulation transmission coefficients are determined at the corresponding spatial frequencies, 'o t' is characterized by the fact that, in order to enable the calibration at low spatial frequencies, the frequency θ of the electric signal is reduced by a factor of M by eliminating the source electric signal from each M pulses groups of M-1 pulses, carry out a calibration for the received signal and set5 the amplitude of the first harmonic component of the original signal equal to the amplitude M of the harmonic component signal with a reduced frequency, and M is an integer and In M> 2, 1 ^ 2 П ППППП ΠΠΙ Τ L £ L__ PPPPPG “ t M GL t No. A Λ