US 4,060,328 AGranted

System for measuring the modulation transfer function of an optical device

AssigneeVaro, Inc.

InventorWilliam E. Flynt

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Abstract

A slit image is projected through the optical device being measured and onto the optical input screen of a variable-gain photomultiplier tube. Before it is incident upon the input screen of the tube, the projected image is modulated to develop simultaneously a low-frequency image component and one or more higher-frequency image components. Electronic filters are employed to separate the electrical output signal of the photomultiplier tube into discrete electrical signals each representative of a different one of the image components, and one or more meters are used to indicate the relative amplitudes thereof. A phase-locked-loop detector and an amplifier are employed between each filter and meter to improve the signal-to-noise ratio of the system and increase the signal amplitude to a level suitable for driving the meter. In one embodiment, the overall gain of the system is manually adjusted until the meter reads 100 percent MTF for the low-frequency component. A normalized MTF measurement at each of the higher frequencies is indicated either on the same meter, by using a switch, or on a separate meter for each frequency. In another embodiment, a closed-loop feedback circuit comprising a comparator circuit which compares the amplitude of the low-frequency electrical signal with that of a reference signal and generates an error signal which is coupled to the photomultiplier tube to automatically adjust the system such that a 100 percent MTF reading is obtained for the low-frequency component throughout the entire time that the optical device is being measured.

Claims23

Dependent claims

A system for determining the image-forming quality of an optical device by projecting an image through said device and measuring the relative amount of light transmitted therethrough at a plurality of spatial frequencies, with said system determinations being normalized automatically and continuously throughout the duration of the evaluation of the optical devices under test, said system comprising:

photoelectric transducer means, including an optical input section and an electrical output section, for converting a light signal incident upon said input section into a corresponding electrical signal at said output section;
means for projecting said image along a projection path through said optical device and onto said optical input section;
means disposed in said projection path for modulating said projected image to produce simultaneously a low-frequency image component incident upon said optical input section and at least one higher-frequency image component incident upon said optical input section;
a plurality of filter means respectively tuned to the frequency of each of said image component and coupled to the electrical output section of said photoelectric transducer means for developing a corresponding plurality of discrete electrical signals each representative of a different one of said image components;
indicating means coupled to said filter means for indicating the amplitude of said discrete electrical signals;
and means for adjusting said system to cause said indicating means to indicate a predetermined amplitude for said low-frequency image component, said adjusting means comprising means for comparing the amplitude of said discrete low-frequency signal with the amplitude of a preselected reference signal and generating an error signal which automatically adjusts said system to substantially equalize the amplitude of said reference signal and said low-frequency signal, whereby the system is automatically and continuously set to a normalized level for the optical device being measured during the evaluation of the optical device under test and with the higher-frequency image components then capable of being measured under a standardized system for determining the image-forming quality of a series of optical devices at one or more higher spatial frequencies.

3 more elements

photoelectric transducer means, including an optical input screen, an electrical output terminal, and a gain control terminal, for converting a light signal incident upon said input screen into a corresponding electrical signal at said output terminal at an amplitude which varies systematically in accordance with an externally-applied gain control signal applied to said gain control terminal;
means for projecting said image along a path through said optical device and onto said optical input screen;
means, including a motor and a pattern wheel rotationally driven by said motor at a predetermined speed, disposed in said projection path for modulating said projected image to produce simultaneously a low-frequency image component incident upon said optical input screen and at least one higher-frequency image component incident upon said optical input screen;
a plurality of filter means respectively tuned to the frequency of each said image component and coupled to the electrical output section of said photoelectric transducer means for developing a corresponding plurality of discrete electrical signals each representative of a different one of said image components;
indicating means coupled to said filter means for indicating the amplitude of said discrete electrical signals;
means for generating said gain control signal;
and means for adjusting said gain control signal generating means to cause said photoelectric transducer means to develop at said output terminal an electrical signal corresponding to said low-frequency image component at an amplitude which causes said indicating means to give a predetermined reading, said adjusting means comprising means for comparing the amplitude of said discrete low-frequency signal with the amplitude of a preselected reference signal and generating an error signal which is applied to said gain control terminal of said photoelectric transducer means to automatically adjust said system to substantially equalize the amplitude of said reference signal and said low-frequency signal, whereby the system is automatically and continuously set to a normalized level for the optical device being measured during the evaluation of the optical device under test and so that the higher-frequency image components are then capable of being measured under a standardized system for determining the image-forming quality of a series of optical devices at one or more spatial frequencies.

4 more elements

Dependencies are inferred from the claim text.