JPH09145544A - Method for measuring mtf - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure an MTF immediately and adjust synchronously with the measurement of the MTF, by obtaining the MTF of an object to be detected, by hardware in real time. SOLUTION: An output data of a solid scanning element 12 to an effective part of a chart is sent to a maximum value detection circuit 16 and a minimum value detection circuit 17. A clock-counting part 14 makes the maximum value detection circuit 16 and the minimum value detection circuit 17 output a maximum value and a minimum value of the output data every predetermined time. The maximum and minimum values are converted to an MTF by an MTF conversion part 22. Accordingly, the MTF of an object to be detected is obtained in real time by hardware.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MTF measuring method for obtaining an MTF of a test object by using a chart having a pattern of a predetermined spatial frequency.

[0002]

2. Description of the Related Art MTF measurement methods include an MTF measurement method that uses 0 spatial frequency and an MT method that does not use 0 spatial frequency.
There is an F measurement method. In the MTF measurement method that does not use 0 spatial frequency, an image of a chart having a pattern of a predetermined spatial frequency is imaged on a CCD (charge coupled device) through an object to be photoelectrically converted, and the maximum value MA of output data of this CCD is obtained.
From X, minimum value MIN and dummy bit data D, MTF = (MAX-MIN) / (MAX + MIN-2 ×)
The MTF of the test object defined in D) is calculated.

Further, JP-A-56-2516 discloses that
MTF using 0 spatial frequency white and black levels
The measuring method is described. In this MTF measuring method,
An image of a chart having a pattern of a predetermined spatial frequency is formed on a CCD through a test object and photoelectrically converted, and a time-series signal from the CCD is digitized and input by an A / D converter 1 as shown in FIG. It is stored in the RAM 3 via the output port 2. The micro CPU 4 performs a digital calculation based on the data in the RAM 3 to obtain the white level a of the necessary chart portion.
And the black level b, that is, the average value a of the signal of the maximum value several bits in the white level area, the average value b of the signal of the several bits near the minimum value of the black level, the white level A and the black level B close to 0 spatial frequency. MTF = {(a−b) / (a + b) ÷ (A−B) / (A
+ B)} × 100% calculation to obtain the MTF of the test object. If a plurality of bits of a, b, A, and B are detected and their respective average values are substituted into the above equation, the MTF calculation accuracy is improved.

Further, in the MTF measuring method using the white level and black level of 0 spatial frequency, when the MTF measurement of the entire image height is performed, the white level and black level of 0 spatial frequency and a predetermined MTF measurement are shown as a chart. Three types of frequency charts are required, and these charts are used by switching.

Further, three colors of the object to be inspected, for example, red (hereinafter referred to as R), blue (hereinafter referred to as B), and green (hereinafter referred to as G) M
When measuring TF, in the above MTF measuring method, a 3-line CCD that photoelectrically converts each color component of the chart image is used as the CCD, and the MTF is separately obtained for output data of each color of the 3-line CCD.

[0006]

The MTF measuring method using a chart having a pattern of a predetermined spatial frequency may be constructed by hardware like the MTF measuring method described in Japanese Patent Laid-Open No. 56-2516. Software is used for MTF calculation. For this reason, it is difficult to perform MTF calculation at high speed, which has been a bottleneck in measuring MTF at high speed.

Further, since the MTF measuring method using the chart having the pattern of the predetermined spatial frequency does not use the white level and the black level of the 0 spatial frequency, the 3-line CC is used.
MTF measurement using D cannot perform highly accurate MTF measurement. Further, in the MTF measuring method using the white level and the black level of 0 spatial frequency, there are problems that the charts are switched and the MTF of the entire image height cannot be obtained. An object of the present invention is to provide an MTF measuring method capable of realizing high speed and highly accurate MTF measurement.

[0008]

In order to achieve the above object, the invention according to claim 1 forms an image of a chart having a pattern of a predetermined spatial frequency on a solid-state scanning element through an object to be photoelectrically converted. In the MTF measuring method for obtaining the MTF of an object from the output data of the solid-state scanning element, the output data of the solid-state scanning element for the effective part of the chart is sent to a maximum value detection circuit and a minimum value detection circuit, and these maximum values are output. By causing the detection circuit and the minimum value detection circuit to output the maximum value and the minimum value of the output data at regular intervals by the clock counting unit, and converting the maximum value and the minimum value into the MTF by the MTF conversion unit. The MTF of the test object is calculated in real time by hardware.

The invention according to claim 2 is the M according to claim 1.
In the TF measuring method, solid color scanning elements for three colors that perform photoelectric conversion of three colors to the effective portion of the chart are used as the solid state scanning elements, and each color is determined from output data of the solid color scanning elements for these three colors. The maximum value detection circuit, the minimum value detection circuit, and the clock count unit obtain the maximum value and the minimum value of the output data at regular time intervals, and the MTF conversion unit determines the maximum value and the minimum value for the three colors for each color. It is converted into MTF with the conversion coefficient set separately for each.

[0010]

1 shows an embodiment of an MTF measuring apparatus to which the invention according to claim 1 is applied, FIG. 2 shows its concrete configuration, and FIG. 4 shows an overall hardware processing. The flow is shown. A chart 11 having a pattern of only a predetermined spatial frequency, which does not have a white level and a black level of 0 spatial frequency as shown in FIG. 3, is illuminated by a light source, and an image of the chart 11 is an object formed by an optical system such as a lens to be inspected. An image is formed on the solid-state scanning element 12 composed of a CCD through the inspection object. The CCD 12 repeatedly scans the image of the chart 11 to perform photoelectric conversion and outputs it in time series.

By the way, in the MTF measuring method using the white level and the black level of 0 spatial frequency, the maximum and minimum values of the necessary chart portion in the CCD output data for the chart having the pattern of the predetermined spatial frequency and the 0 spatial frequency From the white level and the black level, MTF = (maximum value−minimum value) / {(white level of 0 spatial frequency−black level of 0 spatial frequency)} ... (1) MTF of the test object defined by There is a way.

Here, there is a correlation between the data of the white level of 0 spatial frequency and the data of (maximum value + minimum value) / 2 <median of width>, and the absolute value of the data of black level of 0 spatial frequency is Since it is very small and its contribution to equation (1) is low,
Expression (1) can be replaced with MTF = (maximum value-minimum value) / {(maximum value + minimum value) / 2} (2).

Therefore, in this embodiment, the chart 11 having a pattern of only a predetermined spatial frequency, which does not have the white level and the black level of 0 spatial frequency, is used, and the maximum output data of the CCD 12 with respect to the effective portion of the chart 11 is used. The MTF of the test object is obtained from the value and the minimum value by the MTF calculation of the equation (2). That is, the CCD 12 is driven by the clock from the clock generator (Clock) 13 at the set capture time, captures the image of the chart 11 and performs photoelectric conversion. The clock counting unit 14 counts the clocks from the clock generating unit 13 and generates an output timing gate signal.

The clock counting unit 14 uses a counter circuit 15 composed of a counter IC, and the counter IC 15 is, for example, TTLIC. If the counter IC 15 is a TTL, it is more effective to use a high speed type such as an F type. The maximum value detection circuit 16 and the minimum value detection circuit 17 are a maximum value detection comparator circuit 18 and a minimum value detection comparator circuit 19 each including a comparator IC, a maximum value latch flip-flop circuit 20, and a minimum value latch flip-flop circuit 21. Is used.

For the maximum value detecting comparator circuit 18 and the minimum value detecting comparator circuit 19, it is effective to use a ROM, a RAM or the like instead of the comparator IC. If the maximum value latch flip-flop circuit 20 and the minimum value latch flip-flop circuit 21 are TTL, it is more effective to use a high-speed type such as F type. The output data of the CCD 12 for the effective part of the chart 11 is A / D converted by an A / D converter (not shown), and the maximum value detection circuit 16 and the minimum value detection circuit 17 are clocked by the output timing gate signal from the clock count part 14. The maximum value and the minimum value of the output data of the A / D converter within the set time (time that can be arbitrarily set) set by the counting unit 14 are detected.

Maximum value latch flip-flop circuit 20
The minimum value latch flip-flop circuit 21 determines the maximum value and the minimum value within the set time detected by the maximum value detection circuit 16 and the minimum value detection circuit 17, respectively.
It is held until the next set time ends by the output timing gate signal from 4. The maximum value and the minimum value held by the maximum value latch flip-flop circuit 20 and the minimum value latch flip-flop circuit 21 are MTF.
It is sent to the conversion unit 22.

As the MTF conversion unit 22, an MTF conversion ROM 23 having a table storing the MTF calculation formula of the above formula (2) is used, but a RAM or the like is also effective in addition to the ROM. The MTF conversion ROM 23 immediately converts the maximum value and the minimum value held by the maximum value latching flip-flop circuit 20 and the minimum value latching flip-flop circuit 21 into the MTF by the hardware described above. Such an operation is repeatedly performed at the cycle of the set time by the clock counting unit 14, and this is repeatedly performed for each scan of the CCD 12 to perform the MTF of the test object.
Is required.

Therefore, since all the output data of the CCD 12 is processed by hardware, MTF measurement can be performed in real time for each scan. Further, since the chart 11 does not have the white level and the black level of 0 spatial frequency and the clock count unit 14 for setting the time is provided, the MTF measurement at an arbitrary image height of the entire image height can be performed.

As described above, this embodiment is characterized by claim 1.
11 is an example of an embodiment of an MTF measuring apparatus to which the described invention is applied, which is a chart 11 having a pattern of a predetermined spatial frequency.
CCD image as a solid-state scanning device
In the MTF measuring device for obtaining the MTF of the test object from the output data of the solid-state scanning element 12 by forming an image on and photoelectrically converting the output data of the solid-state scanning element 12 with respect to the effective portion of the chart 11, the maximum value detection circuit 16 and Minimum value detection circuit 1
7, the clock count unit 14 outputs the maximum value and the minimum value of the output data to the maximum value detection circuit 16 and the minimum value detection circuit 17 at regular intervals,
By converting the maximum value and the minimum value into the MTF by the MTF conversion unit 22, the MTF of the test object is obtained in real time by hardware, so that high-speed MTF measurement can be realized. Further, since the MTF measurement can be performed in real time, high-speed adjustment synchronized with the MTF measurement becomes possible.

In an embodiment of the MTF measuring apparatus to which the invention described in claim 2 is applied, an example in which the MTFs of three colors, for example, R, B, and G, of the test object are measured in the above embodiment The chart 11 is illuminated by the light source, and the image of the chart 11 is formed on the solid-state scanning device composed of a 3-line CCD through a test object including an optical system such as a test lens.

This three-line CCD has a clock generator 13
Each color component of the image of the chart 11 is photo-electrically converted by being driven by the clock from, and the maximum value and the minimum value for each color are separately obtained from the output data of each color of the 3-line CCD. Minimum value detection circuit 17
3 sets of the maximum value detection circuit and the minimum value detection circuit and the clock count unit 14 are detected in the same manner as described above.
It is input to each of the MTF converters.

These three MTF converters use R, which is obtained by not using the white level and the black level of the 0 spatial frequency,
MT with higher accuracy by preventing errors between MTFs of B and G
In order to perform F measurement, the MTF calculation formula of the above formula (2) has a table in which different correction factors for R, B, and G are respectively stored, and a table is stored, and the maximum value of the above three sets is detected. The maximum value and the minimum value input from the circuit and the minimum value detection circuit are converted into MTF and corrected.

That is, since the slope between the data of the white level of 0 spatial frequency and the data of (maximum value + minimum value) / 2 <median value of width> is slightly different for each color of R, B and G, The slope is obtained in advance by an experiment, and the correction coefficient is determined, and MTF = (maximum value−minimum value) / {(maximum value + minimum value) / 2} × correction instead of the MTF calculation formula of the above formula (2). Coefficients ···· (3) The table storing the MTF calculation formula is the above three MTs.
The F converter has it. These MTF converters convert the input maximum value and minimum value into the MTF of the test object by the expression (3).

As described above, in the embodiment of the MTF measuring apparatus to which the invention according to claim 2 is applied, photoelectric conversions of three colors are performed on the effective portion of the chart 11 as the solid-state scanning element in the embodiment described above. 3 as a solid-state scanning device
Using a line CCD, the maximum value detection circuit and the minimum value detection circuit for each color are obtained from the output data of the solid-state scanning element, and the maximum value and the minimum value of the output data are obtained at fixed time intervals by a clock counting unit, and these three colors are obtained. The maximum and minimum values of MT are converted by the MTF converter using the conversion coefficient set separately for each color.
Since it is converted into F, the MTF measurement of the three colors of the test object is performed to correct between the respective colors, thereby making it unnecessary to switch the chart when using the white level and the black level of the 0 spatial frequency, and the entire image is obtained. High MTF measurement can be performed with high accuracy.

The present invention is not limited to the above embodiment, and for example, the above embodiment is an example of a digital system in which the output data of the CCD is A / D converted and processed. An analog method may be used in which the output data is processed as it is as an analog value.

[0026]

As described above, according to the first aspect of the invention, the image of the chart having the pattern of the predetermined spatial frequency is formed on the solid-state scanning element through the object to be photoelectrically converted, and the solid-state scanning element is formed. In the MTF measuring method for obtaining the MTF of a test object from the output data of the solid state scanning element, the output data of the solid scanning element for the effective part of the chart is sent to a maximum value detection circuit and a minimum value detection circuit, and these maximum value detection circuit and minimum value are detected. The clock counting unit outputs the maximum value and the minimum value of the output data to the detection circuit at regular time intervals, and the MTF conversion unit converts the maximum value and the minimum value into MTF. Since the MTF of the test object is obtained, high-speed MTF measurement can be realized, and high-speed adjustment synchronized with the MTF measurement becomes possible.

According to a second aspect of the present invention, in the MTF measuring method according to the first aspect, a solid-state scanning element that performs photoelectric conversion of three colors on the effective portion of the chart is used as the solid-state scanning element, From the output data of the solid-state scanning device, the maximum value detection circuit and the minimum value detection circuit for each color, and the clock count unit obtain the maximum value and the minimum value of the output data at regular intervals, and the maximum value and the minimum value for the three colors are obtained. The value is M with the conversion coefficient set separately for each color in the MTF converter.
Since it is converted to TF, by performing MTF measurement of the three colors of the test object between each color, it is not necessary to switch the chart when using the white level and the black level of the 0 spatial frequency, and the whole image is obtained. High MTF measurement can be performed with high accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an MTF measuring device to which the invention according to claim 1 is applied.

FIG. 2 is a block diagram showing a specific configuration of the embodiment example.

FIG. 3 is a diagram showing a chart and CCD output data of the embodiment.

FIG. 4 is a flowchart showing an overall hardware processing flow of the embodiment example.

FIG. 5 is a block diagram showing an example of a conventional MTF measuring device.

[Explanation of symbols]

 11 chart 12 CCD 13 clock generator 14 clock counter 16 maximum value detection circuit 17 minimum value detection circuit 22 MTF converter

Claims (2)

[Claims] 1. An image of a chart having a pattern of a predetermined spatial frequency is formed on a solid-state scanning element through a test object and photoelectrically converted, and MT of the test object is detected from output data of the solid-state scanning element.
In the MTF measuring method for obtaining F, the output data of the solid-state scanning element for the effective part of the chart is sent to a maximum value detection circuit and a minimum value detection circuit, and a clock count part is supplied to these maximum value detection circuit and minimum value detection circuit. The maximum value and the minimum value of the output data are output at regular time intervals by using the
The MTF of the object to be measured is calculated in real time by hardware by converting to MTF.
Measuring method. 2. The MTF measuring method according to claim 1,
As the solid-state scanning element, a solid-state scanning element that performs photoelectric conversion of three colors on the effective portion of the chart is used, and a maximum value detection circuit, a minimum value detection circuit, and a clock count are output for each color from the output data of the solid-state scanning element. The maximum value and the minimum value of the output data are calculated at regular intervals by
An MTF measuring method characterized in that the maximum value and the minimum value of color components are converted into MTF by a conversion coefficient set separately for each color in an MTF conversion unit.