JPH0254522B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic focusing method for an optical instrument.

Conventionally, the following methods have been announced as automatic focus adjustment methods for microscopes.

(1) Analysis of image signals This method uses the mountain-climbing method to find the peak value of high-frequency components contained in image signals from imaging devices such as TV cameras and line-scan solid-state image sensors, and was published in Japanese Patent Publication No. 12106/1973. has been done.

This method is suitable for biological microscopes, but in order to detect changes in high-frequency components, it is necessary to remove low-frequency components, and in order to improve focus accuracy, it is necessary to increase the cutting frequency. The absolute value of the high frequency component becomes small and gets lost in the noise. On the other hand, there is a problem that the accuracy decreases when the cutting frequency is lowered.
Particularly, if the amount of light or contrast is even slightly insufficient, there is a drawback that the accuracy deteriorates considerably.
Even if an attempt is made to increase the amount of light by separately irradiating parallel light as in the above-mentioned Japanese Patent Publication No. 42-12106, it cannot be said that the above-mentioned drawbacks have been solved.

(2) Air Micro This is a method of installing an air nozzle on the objective lens and jetting air toward the subject, checking the back pressure to maintain a constant distance between the objective lens and the subject. From Japan Automatic Control Co., Ltd. It's on sale.

This method is used in the field of metallurgical microscopes, such as IC.LSI mask plates and wafers, but the thickness of the cover glass may be uneven for objects that are tightly packed inside, such as slides with cover glasses. In this case, the disadvantage is that the focus shifts by that amount.

The present invention solves the problems of the conventional method by improving the method of moving between the objective lens and the subject.

That is, in the present invention, when an object is imaged through a microscope with a line-scan solid-state image pickup device, the distance D between the object and the objective lens is set to an arbitrary value S in order to adjust the focus, and then the distance D is set to an arbitrary value S, and then the distance approaches the correct focal length. Calculate the total value of the difference values between adjacent pixels among the video signals captured and output at each position while changing the direction in fixed increments B, and calculate the total value after the change from the total value before the change in D. Repeat the above operation until the total value becomes small, then return the value of D to 1 to 1.5 times B, and then increase the value to 1/2 to 1/10 of B and 1 to 1/1 of the depth of focus of the microscope. Compare the total value before and after the change while changing D in the same direction as the change in B increments by a constant step C of 4, and first change when the value after the change becomes smaller than the value before the change. This is an automatic focus adjustment method in which the position indicating the previous value is set as the in-focus position.

The present invention will be explained in detail below with reference to the drawings.

FIG. 1 is a diagram for explaining an embodiment of the present invention,
FIG. 2 is a diagram for explaining the automatic focus adjustment method of the present invention.

A video signal from a line scan solid-state image sensor 2 attached to the lens barrel of the microscope 1 is input to a microcomputer 4 via an A/D converter 3.
The total value of the difference values between adjacent pixels of the video signals captured by the line scan type solid-state image sensor 2 is determined. This operation is performed while driving the pulse motor 5 attached to the fine movement dial of the microscope 1 to determine the focal point position.

Next, the automatic focus adjustment method of the present invention will be explained with reference to FIG.

In FIG. 2, the X-axis indicates the number of times the pulse motor is driven, and the Y-axis indicates the distance D between the objective lens and the subject. When the automatic focusing device is driven, the sum of the difference values is determined at an arbitrarily set position S. Next, the pulse motor 5 is driven to change D by a distance B in a direction that approaches the correct focal length, and the sum of the difference values is similarly determined. The values before and after driving the pulse motor 5 are compared, and if the latter is larger than the former, similar operations are repeated until the latter becomes smaller. Here, the direction in which the focal length approaches the correct focal length corresponds to the direction in which the total value of the difference values before and after the pulse motor 5 is driven is large.
Therefore, if the direction in which the focal length approaches the correct focal length is unknown in advance, the direction of movement can be determined by changing D by B in an arbitrary direction and comparing the sum of the difference values before and after the change.

If the latter becomes smaller than the former, repeat the same operation as above except that the step width B is set to a value C smaller than B from the position where D is returned by 1 to 1.5 times B, and the pulse motor 5 is driven. The position before driving when the later total value first becomes smaller than the total value before driving is set as the in-focus point.

Here, it is desirable that the distance S between the object and the objective lens initially set is sufficiently close to the correct focal position, but a distance roughly adjusted by the naked eye using a normal method is sufficient.

This distance may be larger or smaller than the distance at the correct focal position, but if it is small, the S focal position will be within the slide glass even if dust etc. adheres to the surface of the cover glass, and the dust will be focused. Since the in-focus position of the subject is reached before the object is in focus, a smaller size is preferable because there is no possibility of focusing on dust or the like by mistake.

The values of the increments B and C are arbitrary as long as B is larger than C, but the value of C is preferably equal to or 1/4 of the depth of focus of the optical equipment used, and more preferably not 1/4 of the depth of focus. 1/2 is preferable. B
The value of is preferably from twice to 10 times the value of C, more preferably from 3 to 6 times the value of C.

If B and C are larger than these values, there is a disadvantage that the position of the finally determined in-focus point will not be sufficiently close to the correct focal length, and the image will be blurred.
Even if B and C are closer than this, the image will be sufficiently clear, but there is a drawback that focusing will take time.

The present invention is characterized in that by driving the pulse motor in two stages as described above, automatic focus adjustment can be performed at high speed and accurately. Furthermore, since the image signal itself is used as a reference for adjustment, there is no room for blurring in the image after adjustment.

Further, in the method of the present invention, due to sensitivity unevenness and illumination unevenness between line scan type image pickup elements, even if a perfectly uniform object is imaged, the sum of the difference values is often not 0. Therefore, it is practically preferable to perform the magnitude comparison only when the total value of the difference values is equal to or greater than a certain value (threshold value). As this threshold value, for example, the sum of the difference values between adjacent pixels of the video signal outputted by imaging a glass slide without a subject, or a value several times that value can be used. This makes it possible to suppress the influence of noise.

In addition, when adjusting the focus when changing the position of the subject to be imaged within the same slide glass, it is expected that the focus will not shift by a wide range, and the distance D between the objective lens and the subject at that time will be adjusted by a minute amount, for example, about 20μ ( Automatic focusing can be performed using the method of the present invention using the retracted position (corresponding to A in FIG. 2) as the initial position. Normally, by using this method, the initial position will be closer to the subject than the correct focal position, and therefore the direction of movement with the increment width B will be in the direction away from the subject, and the subject, for example the slide glass and objective lens, will be moved in the direction away from the subject. This has the effect of eliminating the risk of damage due to collision. In addition, even if the slide glass is replaced, carry out the same procedure as above, and obtain the above-mentioned total value while moving it a certain distance, for example, about 40μ (maximum working distance) in increments of width B, and find the focal point position within this range. If this is not possible, restoring the appropriate D value S by visual inspection or by comparing the above total values before and after driving the step motor will only result in a delay of several seconds to more than 10 seconds. This is preferable because it saves money. If the focal point position cannot be determined in this way, it is practical to stop the operation after a certain distance, issue an alarm to the operator, and have the operator select an appropriate starting position, rather than searching for an unnecessarily long distance. It's a time saver. Furthermore, if the contrast of the subject is weak and the total value of the difference values exceeding the threshold value cannot be obtained, this maximum working distance setting allows an early check.

In the method of the present invention, a microscope with an objective lens of 20x magnification is used, the start position S is started from a position 20μ shorter than the correct focal length, B is 2μ, and C is 2μ.
By setting 0.5μ, it was possible to adjust the focal shift from -20μ to +10μ within the maximum working distance.

Furthermore, even if the focus was further out of focus, automatic focus adjustment was easily possible by once correcting the starting position. The focus adjustment time varied depending on the degree of defocus, but was several seconds at most.

If it is necessary to further increase the processing speed, it can be easily achieved by increasing the processing speed by employing a high-speed A/D converter or the like.

As explained above, the present invention is suitable as an automatic focusing method for optical equipment, and its effects are significant.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining an embodiment of the present invention, and FIG. 2 is a diagram for explaining an automatic focus adjustment method of the present invention. 1 is a microscope, 2 is a line scan type solid-state image sensor, 3 is an A/D converter, 4 is a microcomputer, and 5 is a pulse motor. S is the start position,
B and C are the drive units of the pulse motor, and D is the distance between the objective lens and the subject.

Claims (1)

[Scope of Claims] 1. A method for automatic focus adjustment of a microscope by capturing an image of a subject through a microscope with a line-scan type solid-state imaging device, which method comprises: (a) adjusting the distance between the subject and the objective lens in order to adjust the focus; D is set to an arbitrary value, and then the total value of the difference values between adjacent images among the video signals captured and output at each position while changing in a fixed step B in the direction approaching the correct focal length. Calculate and repeat the above operation until the total value after the change is smaller than the total value of the difference values before the change in D; (b) Next, return the value of D by 1 to 1.5 times B,
Compare the total value of the difference before and after the change while changing D in the opposite direction at a constant step C that is 1/2 to 1/10 of B and 1 to 1/4 of the depth of focus of the microscope. An automatic focus adjustment method in which the focal point position is the position where the value before the change first becomes smaller than the value before the change.