JPH0350513A - Automatic focusing system - Google Patents

Abstract

PURPOSE:To accurately detect the best focus position by calculating the variance of data regarding the brightness of a subject by focus positions and regarding the maximum value of the variance as the best focus position. CONSTITUTION:A control circuit 6 calculates data of a signal (e.g. brightness signal) regarding the brightness of the subject stored on a memory 3 at intervals of a time (e.g. 100 ms) which is preset and inputted to a memory 9 to find its variance. When the best focus position is obtained, the widest distribution range of brightness is obtained, so the position where the variance becomes maximum is the best focus position. Namely, the variance of the data regarding the brightness of an image is calculated by the focus positions and the maximum value of the variance is regarded as a proper focus position. Consequently, the proper focus position is accurately detected without being affected by local brightness, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an autofocus system suitable for use in controlling the focus state of a device for photographing and inspecting a subject using a video camera. [Prior Art] Conventionally, when an object such as a C wafer or a printed circuit board is photographed using a video camera and the presence or absence of foreign matter is inspected, the focus control is performed using the brightness Ofil1.
This was done by finding the maximum value of the signal or the maximum value of its differential value.

When the subject is out of focus, the image becomes unclear, the bright parts of each pixel become blurred, the dark parts become brighter, and the difference between the two becomes smaller. On the other hand, when the object is in focus, the image is clearest. At this time,
Among each pixel, the brightness of the bright part is the brightest,
The brightness of the sunny part will be the darkest, and the difference between the two will be the largest.

For example, consider an image where the left half is black and the right half is white, as shown in Figure 2. If the brightness and frequency of each pixel that makes up this image are expressed in a histogram, when it is in focus, it will be as shown in 3rd V4 (a), and as the focus shifts, it will be as shown in (b) and (c) of the same figure. It will now show. As is clear from comparing Figures 3(a) to (c),
When the image is in focus, the brightness of the pixel is maximum. Therefore, by detecting the maximum brightness value, the optimal focus position can be detected. Also, the slope of brightness near the boundary between the black and white parts is greatest when the image is in focus. Therefore, the ta focus position can be detected from the maximum value of the differential value of brightness.

[Problem to be solved by the invention]

In this way, in the conventional method, the optimal focus position is detected from the maximum value of brightness or the maximum value of its differential value. Therefore, it was difficult to accurately detect the optimal focus position. As a result, for example, if there is an object on the subject, such as glass, which is significantly brighter than other parts, the brightness will be affected and the image will be out of focus. This invention was made in view of this situation.
This makes it possible to accurately detect the optimal focus position. [Means for Solving the Problems] The autofocus method of the present invention photographs a subject by sequentially changing the focus position within a predetermined range, and calculates the variance of data regarding the brightness of the subject for each focus position. , find the maximum value of the variance, and select the focus position corresponding to the maximum value as the optimal focus position. [Operation] For example, a subject is photographed sequentially by a television camera while changing the focus position from a predetermined focus position to another focus position. For example, the variance of data related to brightness, such as a luminance signal, is calculated for each predetermined focus position. The focus position where this variance is maximum is considered the optimal focus position. Therefore, it is possible to accurately detect the optimal focus position without being affected by local brightness. [Embodiment] FIG. 1 is a block diagram showing the configuration of an embodiment of an autofocus device to which the autofocus method of the present invention is applied. In the figure, 1 is a video camera, and an IC (not shown)
Photographing objects such as wafers and printed circuit boards, 2 is A
A/D converter converts the video signal output from the video camera 1 into A/D and supplies it to the memory 3 for storage. Reference numeral 5 denotes a display device such as a CRT, which displays an image corresponding to a signal obtained by superimposing one or both of the output of the video camera 1 and the output of the RAM 4. Reference numeral 6 denotes a control circuit consisting of, for example, a microcomputer, which controls the shadow operations. Reference numeral 7 denotes input means consisting of a keyboard and a switch for inputting predetermined commands to the control circuit 6. 8 and 9 are memories that store predetermined data and the like. 10 is a driving means such as a motor, which drives the table 11 (or the video camera 1) on which the object to be shown is placed. Next, we will explain its operation. When a predetermined switch constituting the input means 7 is operated to command the start of autofocus, the control circuit 6 drives the drive means 10 to move the table 11 from a predetermined focus position to another predetermined focus position. Move at a constant speed of 1.

Of course, in this case, it is also possible to stop the table 11 and move the video camera 1. By operating a predetermined switch of the input means 7, the memory 9 stores
Predetermined data such as the first speed necessary for this movement is stored in advance.
! It is remembered. The control circuit 6 operates the table 11 in accordance with the data tQ recorded in the memory 9.

A video camera 1 photographs a subject placed on a table 11 and outputs a video signal 6. This video signal 9 is A/D converted by an A/D conversion word 2 and written to a memory 3 It will be done.

Further, the output of the video camera 1 is provided to the display 8l>5. As a result, the image of the subject at each focus position is displayed on the display screen 5.

The control circuit 6 inputs the data of V4 F'+, (for example, brightness brightness) related to the brightness of the subject stored in the memory 3 to the time (for example, Looms) that is preset and input to the memory 9. The calculation is performed, and the variance thereof is determined. That is, the level x. ．． For X2,...x2, the variance V
is calculated from the following formula.

Here, i is an average value, which is obtained from the following equation.

In addition, fi is the number of pixels at each level xi, A is the total number of pixels (A
= 埜fi). 1st Dispersion (2) calculated at each predetermined time period is stored in the memory 8 together with the corresponding focus position.

As mentioned above, the calculation of variance is a predetermined ■. Between v (100
ms). Therefore, for example, Fig. 4(a)
As shown in Table 1, what is the J-th movement rate? When the setting is slow, as shown in FIG. 13(b), the sampling position for determining the variance with respect to the movement position (focus position) of the table 11 shown in FIG. This roughness is adjusted by setting the first speed to an appropriate value.

Hiss the variance stored in memory 8! When expressed in one gram, it becomes as shown in Figure 5. In other words, ideally, the shapes are distributed symmetrically on the left and right with a predetermined maximum value as the center. As shown in FIGS. 3(a) to 3(c), at the optimal focus position, the brightness distribution range 1111 is widest. Therefore, the position where the dispersion is maximum becomes the optimal focus position.

When a predetermined switch of the input means 7 is operated, ? The control circuit 6 generates a histogram as shown in FIG. ＠Indication y indicating the current focus position (for example, a straight line) is stored in RAM.
4 and displayed on the display 5 together with the image of the subject. This allows the user to check the dispersion distribution state and the corresponding image.

When the variance at a predetermined focus position has been calculated in this way, the control circuit 6 then searches for its maximum value. First, a search is performed in the focus direction where the maximum value exists. Therefore, the dispersion between the end (right end in FIG. 6) in a predetermined direction (for example, the rear bin direction) and the range IJIID (direction size) recorded in the memory 9 in advance. The difference is calculated.

For example, the dispersion at the end of this range T) in the rear pin direction is V, the dispersion at the end in the front pin direction is Vn, the variance between them is V, .
When V, . . . Vn-■, {direct S} is calculated from the following equation.

S = (Vt-V2)”(Vz-Vj)”・(Vn-
t-Vn)=Δ■+Δ2+...+Δn-■
...(3)? When S is positive, the maximum value is repositioned from that position toward the rear pin, and when f''L, the front pin direction li'd
It is determined that the location is located at . Each value Δi is stored in the memory 8 and is also used for peak determination calculations. By setting this range D to a certain degree, the direction of the maximum value can be detected correctly without being affected by local peaks or noise. However, if it is set to range 4, indicated by D■ in Figure 6, for example, the range will be too wide and will exceed the maximum value. Therefore, range D is set to an appropriate value.

When the direction in which the maximum value exists is determined, then the same calculation as in equation (3) above is performed for each predetermined range 4M from range 40 in that direction. This range IIJIM is M1, M
2, M3, etc., and when it reaches the value M0 (main size) which is set and stored in advance in the memory 9, from then on.
It is assumed to be constant (M0). When the polarity of the value S is opposite to that in range 40, the variance of the boundary with the immediately preceding range M is Vn+. is determined to be the appropriate focus position. However, since the range M0 has a certain width in order to quickly find the position of the maximum value Vm, it does not necessarily correspond to the exact position. Therefore, as described above, after a rough search is performed for the position of the maximum value Vm, a fine search is performed to find a more accurate position. In the memory 9, a second progress rate and range W for performing a fine search are set and stored in advance.

As shown in FIG. 7, the control circuit 6 moves the table 11 at a second speed within a range 8W centered on the appropriate focus position detected by the rough search. During this time, as in the case of coarse search, the display 5
ii at each focus position! j image is displayed, and the dispersion is calculated at predetermined time intervals (loo+++s), and the calculated value is stored in the memory 8 together with the corresponding focus position. The second speed of the fine search is set to a slower speed than the first speed of the coarse search. This makes it possible to detect more precise focus positions. The variance calculated by the fine search is sequentially compared with the variance of the adjacent focus position, the maximum value thereof is taken as the true maximum value Vm, and that focus position is finally taken as the optimum focus position.
In this manner, when the optimum focus position is determined, the control circuit 6 drives the driving means 10 to move the table 11 to that position. As a result, the clearest image of the subject is displayed on the display 5. When determining the direction of the coarse search, if an incorrect determination is made due to noise or the like, the proper focus position cannot be detected. As mentioned above, this can be prevented by setting range 40 somewhat wide. However, if it is made too wide, it will pass the proper focus position and detection will take time. Therefore, as shown in FIG. 8, a predetermined basic increase value R1 is set in advance and stored in the memory 9, and only variances larger than this basic value Rl are selected. It is also possible to have it set.

Thereby, it is possible to remove noise and the like in the image without making the range R so wide.

Also, a reference value R2 is set in advance for the maximum value obtained, and when the maximum value Vm of variance is less than this base value R2, the obtained value is considered not to be the correct maximum value, and the value is re-set again. It can also be configured to perform a detection operation. Thereby, as shown in FIG. 8, when a relatively large peak value exists in addition to the normal maximum value, it is possible to prevent that peak value from being detected as the maximum value. In addition, in the above, the variance of the brightness of the pixels of the entire screen was calculated, but as shown in No. 91'2I,
Coordinate Lx. It is also possible to perform calculations only on pixels within the range of the window set by Rx and y coordinates Uy and Dy. In this case, the coordinates are input to the memory 9 via the input means 7.
It is possible to set the focus to a predetermined part of the subject by changing the image to Hakusan and storing it.

Furthermore, depending on the object to be inspected, the dispersion may not necessarily be symmetrical in concept, and a relatively large peak value may occur near the maximum value. For such objects, the distribution state of the dispersion may be stored in the memory 9 in advance and compared with that data to find the maximum value without being influenced by relatively large peaks. can.

〔Effect of the invention〕

As described above, according to the autofocus method of the present invention, the variance of data related to image brightness is calculated for each focus position, and the maximum value of the variance is set as the appropriate focus position. Brightness etc! It becomes possible to accurately detect the proper focus position without being distorted.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment of an autofocus device to which the autofocus method of the present invention is applied; Fig. 2 is a front view of a screen explaining the principle of the conventional autofocus method; (c) is the conventional O1-
FIG. 4 is a brightness histogram explaining the principle of the focus method, and FIG. 4 is a timing chart [-. Figures 5 to 8 are 11? FIG. 9 is a front view of a screen explaining the operation of the embodiment shown in FIG. 1. 1... Video camera 2... A/D converter 3... Memory 4... RAM 5... Display storage 6... Control circuit 7... Input means 8, 9... Memory IO...driving means 11...table

Claims (9)

[Claims] (1) Shoot a subject by sequentially changing the focus position within a predetermined range, calculate the variance of data regarding the brightness of the subject for each focus position, find the maximum value of the variance, and set the value to the maximum value. Autofocus method that selects the corresponding focus position as the optimal focus position. (2) From the difference in the dispersion in the first range of focus positions, a direction determination is performed to find the focus direction in which the maximum value exists, and the difference in the dispersion in the second range of focus positions in the focus direction is sequentially determined. 2. The autofocus method according to claim 1, wherein said maximum value is determined by calculation. (3) After the maximum value is determined from the difference in variance in the second range, within a predetermined range including the determined maximum value, a third value shorter than the second range is further determined. 3. The autofocus method according to claim 2, wherein said maximum value is determined by sequentially calculating said variance difference for each range. (4) The autofocus method according to claim 2 or 3, wherein the second range is shorter than the first range. (5) The autofocus method according to claim 4, wherein the second range is sequentially shortened. (6) The autofocus method according to any one of claims 2 to 5, wherein the direction determination is performed only for the dispersion that is equal to or greater than a predetermined reference value. (7) The autofocus method according to any one of claims 2 to 6, wherein the maximum value is determined as the maximum value only when the value is equal to or greater than a predetermined reference value. (8) The calculation of the variance is performed at predetermined time intervals, and
8. The autofocus method according to claim 1, wherein the speed at which the focus position is sequentially changed is variable. (9) The autofocus method according to any one of claims 1 to 8, wherein only data related to the brightness of the subject within a set window is used for calculation.