JPH0368334B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus display device in an MTF measuring device.

Conventionally, MTF measurement equipment generally converts the width direction light intensity distribution of a light beam from a projection chart having a specific spatial frequency into an electrical signal using an image sensor through a test lens, and calculates its maximum value MAX. From the minimum value MIN, calculate the MTF of the lens under test (MAX-
MIN) / (MAX + MIN) x 100 (%) = MTF A contrast measurement type MTF measuring device calculates the widthwise light intensity distribution of the light flux that has passed through an extremely narrow slit, and the imaging device uses the test lens to measure the light intensity distribution in the width direction. There is an MTF measuring device using a digital Fourier transform method, which calculates the MTF of a lens to be tested by converting it into an electric signal and discretely Fourier transforming it.

Contrast measurement type MTF measuring devices are used to some extent for testing mass-produced lenses.
However, since we use a projection chart with a specific spatial frequency and obtain MTF at that spatial frequency,
We will find MTF at one spatial frequency,
There is a problem in that the projection chart must be replaced when determining the MTF at a different spatial frequency. Therefore, the MTF can only be determined at a spatial frequency that corresponds to the type of projection chart, and it is not possible to determine the MTF at an arbitrary spatial frequency. In addition, it is necessary to adjust the focus of the test lens before measuring MTF, but this is done by projecting a resolution chart through the test lens and decoding the projected chart image with the human eye. It's summery. In addition, the MTF of the digital Fourier transform method
The measuring device samples and measures the widthwise light intensity distribution (light intensity distribution of a line image) of the slit image projected by the lens to be inspected, and discretely performs Fourier transformation. With this MTF measuring device, when the light intensity distribution (LSF) of a line image is sampled with the number of samplings n and the sampling interval Δl and is Fourier transformed, the amplitude part becomes the MTF, so the MTF M (corresponding to the spatial frequency u) u) is given by the following equation.

Here, L _k is the sampled line image and light intensity, m is the projection magnification, and a is the slit width.
The first term on the right side represents the correction coefficient for the slit width, and if the slit width is infinitely small, it will be a negligible value, but it is generally specified because the slit width is about 10 μm to 20 μm. This is a value that should be calculated in advance corresponding to the spatial frequency u.

FIG. 1 shows an optical system in an example of an MTF measurement device using the digital Fourier transform method. Chart 1
1, a slit consisting of an extremely thin opening with a width of 5 μm to 20 μm is formed corresponding to each measurement point, and is illuminated by a light source consisting of a lamp 12 and a diffuser plate 13. The light flux passing through each slit of the chart 11 is passed through the test lens 14 to a one-dimensional self-scanning solid-state imaging device such as a charge-coupled device, photodiode array, bucket brigade device, etc. installed at a position corresponding to each measurement point. The light is received by the elements 15 ₁ to 15 _i and converted into a time-series electrical signal corresponding to the light intensity distribution. The lens 14 to be tested is mounted on a lens holder 16, and is moved up and down by the focusing mechanism of the lens 14 itself or the focusing mechanism of the lens holder 16 to perform focus adjustment.

Figure 2 shows the above digital Fourier transform method.
The electrical circuit in the MTF measuring device is shown.

Values of a, u, m, △l in equation (1) and MTF
The test standard values of the values are input by the teletype 17 to the computer 19 via the input/output interface 18, and these values are stored on a floppy disk by the floppy disk device 21 via the input/output interface 20. In addition, the computer 19 sequentially calculates the values of sin and cos according to u and stores them on the floppy disk by the floppy disk device 21, and also calculates the correction coefficient according to the slit width of equation (1) and stores them on the floppy disk. Device 21
This will cause it to be stored on the floppy disk. Since these values remain unchanged during testing, they are stored on the floppy disk by the floppy disk device 21 during setting.

During inspection, the values stored on the floppy disk are transferred by the floppy disk device 21 to a dynamic random access memory (hereinafter referred to as RAM) built in the computer 19.
Among these, the values of sin and cos are RAM22 for cos and RAM22 for sin.
It is stored in RAM23. In this case, an initial address is preset in the address counter 26 by the computer 19 through the input/output interface 24 and the data bus, and the address counter 26 is incremented by a clock pulse generated by the timing generation circuit 27, and then the floppy disk device 21 outputs the address to the computer. Dynamics within 19
The sin and cos values transferred to RAM are RAM22,2
3 are sequentially stored. The operations up to this point are performed in advance as test preparation.

Once the above operations are completed, the solid-state image sensor 15 ₁
The photoelectric conversion signal corresponding to the linear light intensity distribution from ~15i is selected by the multiplex 28 according to the address instructed by the computer 19, sampled and held by the sample and hold circuit 29, and sent to the analog/digital converter 30. The images are digitized and sequentially stored in the line image RAM 31. This RAM31 is the RAM22, 23 mentioned above.
The address is specified in the same way.

When the photoelectric conversion signals from the solid-state image sensors 15 ₁ to 15i are stored in the RAM 31, a read end signal is sent from the address counter 26 to the computer 19.
The computer 19 decodes this signal and sends out an instruction signal to start calculation corresponding to equation (1). That is, the computer 19 first sets the start address of the line image RAM 31 and the constant RAM 32, and the data from these RAMs 31 and 32 is incremented by the address counter 26 by the clock pulse from the timing generation circuit 27. The signals are sequentially sent out and input to the multiplier 33. A multiplier 33 multiplies these data, and the results are sequentially added by an accumulator 34. These operations are controlled by clock pulses applied from the timing generation circuit 35 to the multiplier 33 and the accumulator 34. Here, binary signals of 1 and 0 are stored in the constant RAM 32, and _〓Lk in equation (1) is calculated by the multiplier 33 and stored in the accumulator 34. This is repeated for the line image data from each of the solid-state image sensors 15 ₁ to 15 _i , and each time the calculation result of 〓L _k is taken into the dynamic RAM of the computer 19 through the data bus 25.

Next, in the same way, computer 19 uses RAM 2 for COS.
2. Set the start address of the line image RAM 31, calculate 〓L _k cosθ _k (θ _k =2πu△lk/m), and load it into the dynamic RAM of the computer 19 through the data bus for each solid-state image sensor. be included.
Next, the calculation of 〓L _k sin θ _k is performed in exactly the same manner and is loaded into the dynamic RAM of the computer 19. And computer 19 is dynamic
The MTF is calculated for each solid-state image sensor according to equation (1) from 〓L _k , 〓Lksinθ _k , 〓L _k cosθ _k taken into the RAM, and this MTF is compared with the inspection standard value in the dynamic RAM. , the test results are displayed on the cathode ray tube of the teletype 17. Note that the solid-state image sensor 15
₁ to 15 _i are driven by a drive circuit 36.

In the digital Fourier transform type MTF measuring device mentioned above, it is necessary to adjust the focus of the test lens before measuring MTF, and the test lens must be adjusted so that the MTF value on the axis is maximized. Must be. Conventionally, therefore, the axial MTF value is displayed with an analog meter, and the measurer adjusts the focus of the lens under test while looking at the analog meter. However, with this method, the MTF on the axis
Since a huge amount of calculation is required to obtain the value, the operator repeats the process of checking the MTF value on the analog meter and adjusting the focus of the lens to be tested to achieve the focused state of the lens to be tested. It takes quite a long time to do so. In addition, because the measurer was reading the MTF value on the axis using an analog meter, the reading error of the measurer was large, and the resolution of the analog meter was limited, resulting in low accuracy and problems with repeatability.

For this reason, MTF measuring devices using the digital Fourier transform method are extremely rarely used for inspecting mass-produced lenses;
It is used in lens research and development to find the However, even in the inspection of mass-produced lenses, it is desired to determine the MTF at an arbitrary spatial frequency. In order to make it usable for inspecting mass-produced lenses, it is necessary to eliminate the above drawbacks and, in particular, to be able to measure the focusing state in real time. If it takes a long time to measure the in-focus state, it will be useless for inspecting mass-produced lenses.

SUMMARY OF THE INVENTION An object of the present invention is to provide a focus display device that can improve the above-mentioned drawbacks and improve operability and reproducibility.

In order to achieve this objective, the first claim
The invention described in 1. includes a member having a plurality of slits having a width of 20 μm or less for measuring the MTF of a lens to be tested, a light source for illuminating this member, and each light beam passing through the plurality of slits being tested. It is equipped with a plurality of image sensors including an axial image sensor that receives light through a lens and converts the width direction light intensity distribution into an electrical signal, and performs discrete Fourier transform on the output signals of the plurality of image sensors. of the lens to be tested.
MTF of digital Fourier transform method to find MTF
The focus display device of the measuring device includes a sample and hold means for sampling and holding the output signal of the image sensor on the axis using a clock pulse, a comparator for comparing the output signal of the sample and hold means with a reference voltage, and a comparator for comparing the output signal of the sample and hold means with a reference voltage. A reference voltage applying means for applying a voltage, a counting means for measuring the width of the output pulse of the comparator, and a display device for displaying the focusing state of the MTF measuring device in accordance with the count value of the counting means. The invention as set forth in claim 2 is the focus display device as set forth in claim 1, comprising means for adjusting the reference voltage. The invention described in item 3 includes a member having a plurality of slits having a width of 20 μm or less for measuring the MTF of a test lens, a light source for illuminating this member, and a light source for each light beam passing through the plurality of slits. It is equipped with a plurality of image sensors including an axial image sensor that receives light through a detection lens and converts the width direction light intensity distribution into an electrical signal, and discretely converts the output signals of the plurality of image sensors into a Fourier image sensor. A focus display device for an MTF measuring device using a digital Fourier transform method for converting the MTF of a lens under test, includes a sample hold means for sample-holding the output signal of the image sensor on the axis using a clock pulse, and a sample hold means for holding the output signal of the image sensor on the axis using a clock pulse. a comparator that compares the output signal with a reference voltage; a peak detection circuit that detects the peak of the output signal of the sample and hold means; and a voltage divider circuit that divides the output voltage of the peak detection circuit and applies it to the comparator as the reference voltage. and a counting means for measuring the width of the output pulse of the comparator, and a display device for displaying the focusing state of the MTF measuring device in accordance with the count value of the counting means, The invention set forth in claim 4 is the focusing device set forth in claim 3, which includes means for adjusting the reference voltage.

Next, examples of the present invention will be described.

FIG. 3 shows an embodiment of the present invention, and FIG. 4 is a timing chart thereof. This embodiment is an example of what is attached to the MTF measuring device using the digital Fourier transform method, in which a photoelectric conversion signal from an on-axis solid-state image sensor (charge-coupled device in this example) ₁₅₁ is amplified by an amplifier 37 and sampled. The hold circuit 29 samples and holds the sample. That is, during focus adjustment, the multiplexer 28 selects the electro-optical conversion signal from the solid-state image pickup device ₁₅₁ based on a signal from the computer 19, and the amplifier 37 is omitted in FIG.

The signal Video from the sample and hold circuit 29 has a waveform similar to the light intensity distribution of the slit image as shown in FIG. 4, and is input to the comparator 38. In addition, the comparator 38 has a constant DC voltage Vs
is divided by a voltage dividing circuit 39 and inputted as a reference voltage V _Ref , these input Video and V _Ref are compared, and a pulse W is output when the input Video is higher than the reference voltage V _Ref . The counter 40 is the drive circuit 36
The transport clock φ _T outputted from the frequency divider circuit 41 divides the frequency and inputs it as a clock pulse CLK, and when the pulse W is input from the comparator 38, the width of the pulse W is determined by counting the clock pulse CLK. Measure.
The count value of the counter 40 is decoded by a decoder 42, and a display corresponding to the count value is displayed on a display device 43. The control pulse generating circuit 44 is a circuit that generates a control pulse from the output pulse W of the comparator 38, and generates a start pulse CS and an end pulse CR according to the leading edge and trailing edge of the pulse W, as shown in FIG. The counter 40 starts counting from zero in response to this start pulse CS, and continues counting until the end pulse CR is applied. In addition, the voltage divider circuit 39 has a reference voltage
It has a terminal (for example, a sliding terminal of a variable resistor) that can adjust V _Ref according to the level of the signal Video, and is adjusted to a desired value in advance.

By the way, at the time of focusing, the slit image is at its sharpest point, so the width of the pulse W is at its minimum, and as the test lens is defocused, the sharpness of the slit image decreases and the width of the pulse W increases. .
Therefore, when focusing, the count value of the counter 40 becomes the minimum, and as the lens to be examined is brought into focus, the count value of the counter 40 increases. Therefore, the measurer can read the displayed value corresponding to the count value of the counter 40 on the display device 43 and adjust the focus of the lens to be tested. By changing the reference voltage V _Ref , it is possible to perform focusing corresponding to the spatial frequency component.
The lower the reference voltage V _Ref is, the higher the spatial frequency component will be focused.

Figure 6 is an example in which the spatial frequency is 30c/mm.

Note that the display device 43 may be a level meter,
A digital display may also be used.

FIG. 5 shows another embodiment of the invention. In this embodiment, the reference voltage V _Ref is dynamically changed in accordance with the signal Video. In the above embodiment, the peak value of the output signal of the sample and hold circuit 29 is sequentially detected by the peak detection circuit 45, and the peak value is divided by the voltage dividing circuit 39 and inputted to the comparator 38 as the reference voltage V _Ref .
If the reference voltage V _Ref is dynamically changed in accordance with the peak value of the output signal of the sample-and-hold circuit 29 in this way, even more precise focusing adjustment can be performed according to the embodiment described above.

As described above, according to the present invention, the in-focus state is detected and displayed without performing any Fourier transform in an MTF measuring device using the digital Fourier transform method. This makes it possible to adjust the focus in real time, making it possible to adjust the focus in real time. Focus adjustment time can be significantly shortened. Furthermore, since a plurality of axial slits for measuring the MTF of the lens to be tested and an image sensor are used, the configuration becomes simple. In addition, after sampling and holding the output signal of the light receiving element on the axis, a comparator compares it with a reference voltage, the output pulse width is measured by a counting means, and the in-focus state is displayed on a display device in accordance with the value of this counting means. , it is possible to improve the reproducibility accuracy. This makes it possible to use a digital Fourier transform MTF measuring device to inspect mass-produced lenses.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an optical system in an example of a conventional MTF measuring device, FIG. 2 is a block diagram showing an electric circuit in the same MTF measuring device, and FIG. 3 is a block diagram showing an embodiment of the present invention. FIG. 4 is a timing chart of the same embodiment, FIG. 5 is a block diagram showing another embodiment of the present invention, and FIG. 6 is a diagram for explaining the above embodiment. 15... Light receiving element on axis, 29... Sample hold circuit, 38... Comparator, 39... Voltage dividing circuit, 40... Counter, 43... Display device,
45...Peak detection circuit.

Claims (1)

[Claims] 1. 20 μm for measuring MTF of test lens
A member having a plurality of slits having the following widths, a light source that illuminates this member, and receiving each light beam passing through the plurality of slits through a test lens to generate an electric signal to measure the light intensity distribution in the width direction. A digital Fourier transform type MTF measurement device, which is equipped with a plurality of image sensors including an on-axis image sensor that converts the image into a digital image sensor, and obtains the MTF of the lens to be tested by discretely Fourier transforming the output signals of the plurality of image sensors. The focus display device includes a sample and hold means for sampling and holding the output signal of the image sensor on the axis using a clock pulse, a comparator for comparing the output signal of the sample and hold means with a reference voltage, and a comparator for applying the reference voltage to the comparator. A reference voltage applying means for applying the reference voltage, a counting means for measuring the width of the output pulse of the comparator, and a display device for displaying the focusing state of the MTF measuring device in accordance with the count value of the counting means. Features a focus display device. 2. The focus display device according to claim 1, further comprising means for adjusting the reference voltage. 3. 20μm for measuring MTF of test lens
A member having a plurality of slits having the following widths, a light source that illuminates this member, and receiving each light beam passing through the plurality of slits through a test lens to generate an electric signal to measure the light intensity distribution in the width direction. A digital Fourier transform type MTF measuring device that is equipped with a plurality of image sensors including an on-axis image sensor that converts the image into a digital Fourier transform method that discretely transforms the output signals of the plural image sensors to obtain the MTF of the lens to be tested. In the focusing display device, a sample and hold means samples and holds the output signal of the image sensor on the axis using clock pulses, a comparator that compares the output signal of the sample and hold means with a reference voltage, and a sample and hold means that samples and holds the output signal of the image sensor on the axis, and a voltage dividing circuit that divides the output voltage of the peak detection circuit and applies it to the comparator as the reference voltage; a counting means that measures the width of the output pulse of the comparator; A focus display device comprising: a display device that displays a focus state of the MTF measuring device in accordance with a count value of the device. 4. The focus display device according to claim 3, further comprising means for adjusting the reference voltage.