GB1598648A - Method of determining a characteristic of an optical component - Google Patents

Description

(54) METHOD OF DETERMINING A CHARACTERISTIC OF AN OPTICAL COMPONENT (71) We, SIRA INSTITUTE LIMITED, a British Company, of South Hill, Chislehurst, Kent BR7 5EH, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to methods of determining a characteristic of an optical component.

Current instruments for determining the characteristic of an optical component such as modulation transfer function (MTF) employ dynamic systems such as frequency scanning, grating scanning, line spread function scanning or edge spread function scanning. All of these require movement of one sort or another.

The present invention provides a method of determining a characteristic of an optical component comprising focusing a pattern on to an array of photo detectors by means of an optical system including the optical component under test, electronically scanning the array to produce an output signal and processing the output signal to produce the characteristic. The photodetectors are preferably photosensitive diodes. Thus the method according to the invention is static as the scanning of the diode array is carried out electronically with no moving parts.

The arrangement may be used to measure such parameters as MTF, distortion, field illuminance, scatter and lateral chromatic aberration.

A preferred embodiment of the invention will now be described by way of example only.

The apparatus for carrying out the method of the invention comprises, spaced sequentially along an optical axis, a light source, condenser lenses including a filter, a screen either carrying one or more slits to comprise a line pattern or pinholes, the optical component under test, which may be a lens or a mirror, and a diode array. The light source, condenser lenses and screen form an object generator. The output from the diode array is fed to a signal processing unit.

Depending on the type of optical component to be evaluated several such object generators can be used at defined field positions.

The diode array is used to detect the image produced by the screen of the object generator. If the size of the image on the screen does not cover a sufficient number of diodes then auxiliary optics or other means are used to activate a large number of diodes. Furthermore, to overcome the obvious difficulties in instances where a slit is used and the array of diodes are arranged in lines, the lines of diodes and the slit are arranged to be at an angle to one another.

Depending upon the optical component under evaluation, a series of diode arrays may be used at predetermined positions. The diode array can be in the form of a linear array or a matrix.

The diode array comprises a semiconductor device having on a front face an array of light sensitive diodes the outputs of which are fed to an initial signal processor unit which is arranged, by a series of logic circuits, to scan the array in sequence. The diodes and the initial signal processing unit may be in the form of integrated circuit from which there may be a single signal output comprising in sequence the outputs of each diode of the array.

This signal may be fed to an externalsignal processing unit which may process the signal in a simple manner, generally comparing the output signals for the optical component under test with those of an ideal similar optical component.

In order to measure the value of the MTF which is given by the Fourier transform of the line spread function for a slit, the value of the line spread function is obtained directly from the array and hence the MTF can readily be calculated. Since several slit sources can be used to image on to one or more arrays, the MTF can be evaluated at several field positions simultaneously. The azimuth of the measurement is given by the orientation of the array.

In order to measure the distortion, the slits (or pinholes) of the object generator are at accurately known locations. Therefore the images are also at known positions for a lens with zero distortion. The peaks of the spread function of these objects are detected by the diode array. Since the position of each diode is known the position of the peaks of the spread function is determined. The distortion is measured- by the deviation of a peak from its known true position.

Field illuminance may also be measured.

The field illuminance is a measure of the light intensity across the image plane. The diodes are positioned across the image plane and detect light intensity. Therefore the field illuminance is measured.

In order to measure scatter, light from the slits or pinholes is focused by the optical component under evaluation to well defined areas. Outside of these areas the light detected is due to scatter. Readings are taken from these diodes under normal testing conditions and also with the object generator masked or switched off. The difference between the two readings is an indication of the scatter produced by the lens. Lateral chromatic aberration may also be calculated.

The position of the peaks of the spread functions is determined by the array. Various colour filters are placed in the object generator in sequence. The shift of the peaks of the spread functions, if any, as detected by the array is the measure of the lateral chromatic aberration.

It will be seen that with the exception of the last measurement, which requires an interchange of filters, all of these results can be processed for one initial setting and without the use of mechanical scanning.

It is foreseen that the method of the invention is particularly useful in a workshop environment where lenses can be tested frequently and without difficulty.

Throughout the specification reference to optical components includes reference to infrared and ultra-violet components with suitable radiation detectors in place of the light sensitive diodes.

WHAT WE CLAIM IS: 1. A method of determining a characteristic of an optical component comprising focusing a pattern on to an array of photodetectors by means of an optical system including the optical component under test, electronically scanning the array to produce an output signal and processing the output signal to produce the characteristic.

2. A method as claimed in claim 1 in which the photodetectors are photosensitive diodes.

3. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the modulation transfer function and the pattern comprises a line, the array detecting the line spread function.

4. A method as claimed in claim 3 in which a plurality of lines are used, so as to produce the modulation transfer function for several field positions.

5. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the distortion and the pattern comprises slits or pin holes, the correct position of the image of which relative to the array is measured whereby the distortion may be determined.

6. A method as claimed in claim 1 or claim 2 in which the characteristic determined comprises the field illuminance.

7. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the scatter and the pattern comprises lines or pin holes focused by the component to well defined areas on the array.

8. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the lateral chromatic aberration, the pattern being masked by various colour filters in sequence to determine shift of the image on the array.

9. A method of determining a characteristic of an optical component substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. MTF can be evaluated at several field positions simultaneously. The azimuth of the measurement is given by the orientation of the array. In order to measure the distortion, the slits (or pinholes) of the object generator are at accurately known locations. Therefore the images are also at known positions for a lens with zero distortion. The peaks of the spread function of these objects are detected by the diode array. Since the position of each diode is known the position of the peaks of the spread function is determined. The distortion is measured- by the deviation of a peak from its known true position. Field illuminance may also be measured. The field illuminance is a measure of the light intensity across the image plane. The diodes are positioned across the image plane and detect light intensity. Therefore the field illuminance is measured. In order to measure scatter, light from the slits or pinholes is focused by the optical component under evaluation to well defined areas. Outside of these areas the light detected is due to scatter. Readings are taken from these diodes under normal testing conditions and also with the object generator masked or switched off. The difference between the two readings is an indication of the scatter produced by the lens. Lateral chromatic aberration may also be calculated. The position of the peaks of the spread functions is determined by the array. Various colour filters are placed in the object generator in sequence. The shift of the peaks of the spread functions, if any, as detected by the array is the measure of the lateral chromatic aberration. It will be seen that with the exception of the last measurement, which requires an interchange of filters, all of these results can be processed for one initial setting and without the use of mechanical scanning. It is foreseen that the method of the invention is particularly useful in a workshop environment where lenses can be tested frequently and without difficulty. Throughout the specification reference to optical components includes reference to infrared and ultra-violet components with suitable radiation detectors in place of the light sensitive diodes. WHAT WE CLAIM IS:

1. A method of determining a characteristic of an optical component comprising focusing a pattern on to an array of photodetectors by means of an optical system including the optical component under test, electronically scanning the array to produce an output signal and processing the output signal to produce the characteristic.

2. A method as claimed in claim 1 in which the photodetectors are photosensitive diodes.

3. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the modulation transfer function and the pattern comprises a line, the array detecting the line spread function.

4. A method as claimed in claim 3 in which a plurality of lines are used, so as to produce the modulation transfer function for several field positions.

5. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the distortion and the pattern comprises slits or pin holes, the correct position of the image of which relative to the array is measured whereby the distortion may be determined.

6. A method as claimed in claim 1 or claim 2 in which the characteristic determined comprises the field illuminance.

7. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the scatter and the pattern comprises lines or pin holes focused by the component to well defined areas on the array.

8. A method as claimed in claim 1 or claim 2 in which the characteristic determined is the lateral chromatic aberration, the pattern being masked by various colour filters in sequence to determine shift of the image on the array.

9. A method of determining a characteristic of an optical component substantially as hereinbefore described.