JPH0145861B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an MTF measuring device for optical systems such as lenses.

MTF used as an indicator of lens performance
To measure the modulation transfer function, contrast transfer rate, and response function, a linear pattern provided as a slit on a test chart is imaged with a test lens, and the light intensity distribution of the obtained line image is measured using a Fourier method. The quality of the lens is determined by comparing the measured MTF value with a predetermined standard value.

The basic configuration of the MTF measuring machine will be explained with reference to FIG. The light intensity distribution of the obtained line image is formed on the imaging plane 5 by a light receiving element array (broadly speaking, a charge coupled device (charge coupled device)).
Detected by coupled device (hereinafter abbreviated as CCD) 6,
The signal is input to the computer 7 and Fourier transform is performed to obtain the MTF.

A plurality of slits 2 are provided on the test chart 3 at various image height positions and azimuth directions of the lens, and a CCD array 6 corresponds to the image forming surface 5 so as to be orthogonal to each line image of these slits 2. It will be established. FIG. 2 shows an example of the test chart 3, in which slits 2 in the radial and circumferential directions are provided at five locations, four on the axis and on the periphery.
as shown in Figure 3, in the corresponding position.
A CCD array 6 is provided.

Now, in MTF measurement with a finite magnification in the MTF measurement device having the above configuration, the optical path length is generally measured at a constant value in order to avoid the complexity of the device and the troublesome operation.

In this case, even lenses of the same design have variations in focal length due to manufacturing errors, resulting in different image magnifications. With conventional MTF measuring instruments, the magnification used to calculate MTF is calculated using the design values of both optical path length and focal length, so errors can be avoided in the MTF value obtained by measurement calculation. Nakatsuta.

In addition, as a result of keeping the optical path length constant in one MTF measurement device, the magnification term in the MTF derivation formula is fixed, so different types of lenses with different focal length design values can be used.
MTF cannot be measured with one measuring device,
In many cases, only lenses with a single focal length could be measured.

The present invention eliminates the above-mentioned drawbacks of the conventional MTF measuring device configured as described above, can calculate an accurate MTF value corresponding to the actual magnification of the optical system to be tested, such as a lens, and can calculate an accurate MTF value corresponding to the actual magnification of the optical system to be tested, such as a lens. iso-optical system
The purpose of the present invention is to provide an MTF measurement device that can measure MTF without adjusting the optical path length.

EMBODIMENT OF THE INVENTION Hereinafter, the present invention will be described in detail based on drawings showing embodiments thereof.

FIG. 4 shows an embodiment of the basic configuration of the MTF measuring device of the present invention, in which a test chart 3 provided with a slit 2 is illuminated by a light source 1, and a line image 2' is formed by a test lens 4. The method of detecting the intensity distribution using the CCD 6 installed perpendicularly to the image, performing Fourier transform calculations in the calculation section, and outputting the MTF is the conventional method explained in Fig. 1.
It is similar to the MTF measuring machine.

In the conventional device, only one slit was provided in one direction at one measurement point on the test chart 3, as illustrated in FIG.
In the test chart 3 of this embodiment, two or more parallel slits are provided at one measurement point. For simplicity, FIG. 4 shows only one measuring point on the test chart 3, in which two parallel slits 2a and 2b are provided. Therefore, there are two parallel line images 2a' and 2b' on the imaging plane.
In the output distribution diagram formed across the CCD array 6, two peaks appear at a certain distance D, as shown in FIG. 5b. By dividing this D by the known distance d between the slits 2a and 2b on the test chart, the magnification m can be determined accurately.

By the way, in the conventional MTF measuring machine, only one slit was provided in one direction at one measurement point, so the output waveform detected by the CCD array only showed one peak, as shown in Figure 5a. It was not possible to accurately measure the magnification. In conventional measuring machines, focusing is usually done by keeping the optical path length constant and turning the distance ring on the lens to maximize the size of the peak of the slit image. However, the actual focal lengths of camera lenses, etc. vary widely, even for the same type of lenses with the same design, and due to the above-mentioned focusing, the magnification can vary greatly from the magnification when designing the optical system of the MTF measuring machine. The conventional MTF measurement method, which calculates using a constant magnification, causes errors in MTF values.

To explain this using a calculation formula, Here m: Projection magnification L: Optical path length f: Focal length Here, K: Proportionality constant Ex: CCD output A: 2πU/m×Δx …(3) U: Spatial frequency Conventionally, the MTF was calculated with the magnification m constant and A as a constant in equation (2). However, in reality, according to equation (1), if the focal length f changes, the optical path length L
When m is kept constant, the magnification m also changes. It is clear from equation (3) that if the magnification m changes, A will change, which will affect the MTF.

Therefore, in the MTF measurement device of the present invention, the magnification at the time of measurement is always detected, and this is used in the calculation using equation (2) to calculate an accurate MTF value.

As mentioned earlier, the MTF value calculation in the arithmetic section is performed using a magnification m that is accurately calculated by dividing the distance D between the two peaks of the CCD output by the known distance d between the two parallel slits on the test chart. However, at this time, the image 2 formed by the auxiliary slit 2b is
b′ is treated as ignored.

In another embodiment shown in FIG. 6, the test chart 3 has a large number of parallel slits 2 of the same thickness.
are provided at intervals of Therefore, a large number of parallel line images 2' are formed on the imaging plane by the lens 4 to be tested, and a line image 2' is formed in a direction perpendicular to this.
The output of the CCD array 6 has a waveform in which many peaks appear at regular intervals D, as shown in FIG.

By providing a large number of slits in this way, even if the lens is decentered, the slit image will remain unchanged.
This prevents the slit image from protruding outside the range of the CCD array, making it possible to measure the MTF value of a lens with a large amount of eccentricity, which was previously impossible to measure due to the slit image protruding outside the CCD array. In this case, two line images projected onto the center of the CCD array may be used to measure the magnification m.

Additionally, by averaging several line images projected onto the CCD array during MTF calculation, errors during chart manufacturing, CCD noise, etc. can be removed.

As described above, according to the present invention, the MTF is calculated with a magnification that corresponds to each lens, so it is possible to calculate the MTF with high accuracy.
Since the MTF value can be determined and the focal length can be calculated backwards from the magnification and optical path length, it can also be used as a focal length measuring device. Furthermore, the averaging process provides various effects such as removing noise and increasing the accuracy of measuring the MTF value.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing an example of the basic configuration of an MTF measuring machine, Fig. 2 is a front view showing an example of a conventional test chart, and Fig. 3 is a corresponding image plane.
FIG. 4 is an explanatory diagram showing an embodiment of the present invention, and FIG. 5a is a layout diagram of a CCD array.
A diagram showing a CCD output waveform, FIG. 5b is a diagram showing a CCD output waveform in the apparatus of the above embodiment of the present invention,
Fig. 6 is a diagram showing an embodiment of the present invention, and Fig. 7 is a diagram showing an embodiment of the present invention.
FIG. 3 is a diagram showing a CCD output waveform. 2, 2a, 2b...slit, 3...test chart, 4...lens, 2', 2a', 2b'...line image, 6...light receiving element array.

Claims (1)

[Claims] 1. The image of the optical system to be tested is determined by Fourier transforming the intensity distribution of the line image obtained by imaging the slit on the test chart with the optical system to be tested, such as a lens.
In an MTF measuring machine that measures MTF, multiple slits are placed parallel to each other on the test chart.
An MTF measurement device characterized in that the distance between adjacent line images among a plurality of corresponding line images is measured, the magnification of the optical system to be tested is determined, and this is used to calculate MTF. 2. Obtain the MTF for each of the multiple line images mentioned above and average them to determine the value of the optical system under test.
The MTF measurement device according to claim 1, characterized in that the MTF is determined.