JPH0746202B2 - Micro film reader autofocus method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an autofocus method for a microfilm reader for performing in-focus determination using an image sensor such as a CCD line sensor.

(Technical background of the invention) Various types of autofocus devices have been proposed using an image sensor such as a CCD line sensor.
For example, as a phase difference detection method, there is a method in which projected light is incident on two positions on a line sensor by using two optical path splitting lenses or prisms and the deviation from the in-focus position is detected from the difference in each projected position. is there. However, this has a problem that the optical system is complicated and a sensor having a special shape is required. Also, as a kind of this phase difference detection method, the projection light before and after the in-focus position is guided to different image sensors, and whether the image sensor in which the front light is incident on the regular in-focus position is in focus, A method of detecting whether the image sensor on which the rear light is incident is in focus (so-called front focus or rear focus) is also considered. However, in this case, a plurality of image sensors have to be used, and signals are processed in parallel, resulting in a complicated circuit configuration.

Therefore, a method has been considered in which one image sensor is used, the contrast of the image is obtained from the output signal of each pixel, and the position where this contrast becomes maximum is the in-focus position. For example, the output signal is differentiated to obtain the sharpness of the image (see, for example, JP-A-56-132313). However, when obtaining the maximum value of the image contrast in this way, the contrast must be obtained at a large number of lens positions while moving the projection lens, and the number of contrast measurement points increases. Further, when obtaining the contrast, the lens must be stopped, but in this case, the lens vibrates due to movement and stop of the lens, and therefore the contrast must be obtained by waiting until the lens completely stops. Therefore, there is a problem that it takes time to detect the in-focus position.

(Object of the Invention) The present invention has been made in view of the above circumstances, and provides an autofocus method for a microfilm reader, which has a small number of contrast measurement points and can shorten the time required to detect a focus position. The purpose is to

According to the present invention, an object of the present invention is to provide an autofocus method for controlling a projection lens to a focus position by using an output signal of an image sensor obtained by scanning a projection image with an image sensor. The MTF characteristic at a predetermined spatial frequency of the optical system when the lens is moved is stored in advance, and the component amount of the predetermined spatial frequency is calculated at least at three positions sandwiching the focus position of the projection lens with respect to the projection image. Obtained, determine the corresponding position of the projection lens position on the MTF characteristic using these component amounts, determine the deviation between the corresponding position and the in-focus position on the MTF characteristic, the projection lens by the amount of this deviation It is achieved by an autofocus method of a microfilm reader characterized by moving.

(Principle) Generally, a periodic function can be expressed as a spatial frequency of periodic waves by performing Fourier transform. For example, if the output voltage υ obtained based on the output signal of the image sensor changes as shown in FIG. 4 during one scanning, the spatial frequency component of the output voltage υ becomes as shown in FIG. Here, a, b, c, d indicate the amount of each component when the position of the projection lens is changed, and the higher the degree of defocusing, in other words, the lower the contrast, the spatial frequency component of the projection image. Of these, the amount of high frequency components becomes smaller. I.e. a, b, c, d
In this order, the projection lens is far from the in-focus position.

In FIG. 5, the change in the component amount when the projection lens is moved with respect to the constant spatial frequency f ₀ is as shown in FIG. 6e, and the point where the component amount is maximum is the focus of the projection lens. It can be obtained as a position. Here, FIG. 6m shows the MTF (Modulation Trn) of the optical system until it is projected on the image sensor.
ansfer Function), if the aperture is constant depends on the MTF characteristics of the projection lens used. Here, the horizontal axis of FIG. 6 shows the lens position x for the characteristic e and the lens position X for the characteristic m. The difference between these lens positions x and X is the film thickness, the image. It has a property of changing depending on whether the film is on the front or back of the film. However, for the same image on the same film, this difference (X−x) can be considered to be constant. Generally, although slightly affected by the content of the original image, these characteristics e and m are almost the same as those of the projection lens. And the shape is the same, and it has been translated. Therefore, if the MTF characteristic (m) at a predetermined spatial frequency of the lens is known and Y = F (f), the MTF characteristic (e) including the projected image can be expressed by y = aF (f). Here, a means the MTF component of the original image on the film and can be considered as a constant. Further, F represents a spatial frequency. That is, these show the MTF characteristics m and e with respect to a certain spatial frequency f.

Here, if the component amounts of three consecutive points A, B, and C on the spatial frequency component (e) of the image at equal intervals Δχ are obtained as y ₁ , y ₂ , and y ₃ , respectively, y ₁ / y ₂ = F ( χ ₁ ) / F (χ ₂ ) ... (1) y ₂ / y ₃ = F (χ ₂ ) / F (χ ₃ ) ... (2) On the other hand, the MTF characteristics of the lens corresponding to these points A, B and C ( m) If the lens positions of points A ′, B ′, C ′ that are continuous at equal intervals Δχ on X are X ₁ , X ₂ , and X ₃ , then Y ₁ / Y ₂ = F (X ₁ ) / F (X ₂ ) = y ₁ / y ₂ (3) Y ₂ / Y ₃ = F (X ₂ ) / F (X ₃ ) = y ₂ / y ₃ (4) Therefore, on the known lens MTF characteristics (m) X ₁ , X ₂ , and X ₃ can be obtained such that the three points of the interval Δχ satisfy the values of the equations (3) and (4). If, for example, X ₃ is obtained on the characteristic (m), the deviation (X _n − from the lens position X _{n at} which the characteristic (m) becomes maximum is obtained.
X ₃ ) is obtained.

Therefore, the lens position χ ₃ on the spatial frequency component (e) of the image
The position X ₃ on the MTF characteristic (m) of the lens corresponding to
If the projection lens is moved from the position of χ _{3 by} the deviation (X _n −X ₃ ), the projection lens comes to the in-focus position. In other words, the difference (X−x) between the lens position x on the characteristic e and the lens position X on the characteristic m is obtained, and the lens at the position x is moved by this difference (X−x) to obtain the coordinate X. Then, it can be considered that the focus position Xn on the characteristic m is set.

However, as shown in FIG. 6, the spatial frequency component of the image with respect to the movement of the lens gradually decreases in accordance with the degree of out-of-focus. Therefore, when the lens is largely displaced from the in-focus position, y ₁ , y The difference between ₂ and y ₃ becomes extremely small, and correct operation cannot be expected.

If the lens positions χ ₁ , χ ₂ , and χ ₃ are close to each other, y ₁ , y
A slight error in ₂ and y ₃ causes a large error in the in-focus position, resulting in a problem that control accuracy is reduced.

The present invention improves accuracy by setting at least one of the lens positions for obtaining the spatial frequency component e to one side with respect to the lens focusing position and the other at least two to the other side.

In place of the above formulas (1) to (4), (y ₁ −y ₂ ) / (y ₂ −y ₃ ) = (F (χ ₁ ) −F (χ ₂ )) / (F (χ ₂ )) It is also possible to find the corresponding points by using the denominator and the numerator as the form of the difference, as in −F (χ ₃ )). In this case, the difference between three points may be used, or the difference between four points may be used.

(Embodiment) FIG. 1 is an overall schematic view of a reader printer which is an embodiment of the present invention, FIG. 2 is a block diagram of its autofocus control device, and FIG. 3 is a flow chart of its operation.

In FIGS. 1 and 2, reference numeral 10 is an original image of a micro photograph such as a micro fish or a micro roll film.
Reference numeral 12 denotes a light source, and the light from the light source 12 is guided to the lower surface of the original image 10 via a condenser lens 14, a heat insulating filter 16, and a reflecting mirror 18. In the reader mode, the transmitted light (projected image) of the original image 10 is guided to the transmissive screen 28 by the projection lens 20 and the reflecting mirrors 22, 24 and 26, and an enlarged projection image of the original image 10 is formed on this screen 28. . In the printer mode, the reflecting mirror 24 is rotated to the phantom line position in FIG.
2, 30, 32 depending on the PPC type slit exposure type printer 34
Be led to. The reflecting mirrors 22 and 30 move in synchronization with the rotation of the photosensitive drum 36 of the printer 34, and a latent image is formed on the photosensitive drum 36. This latent image is visualized with toner charged to a predetermined polarity, and this toner image is transferred to the transfer paper 38.

Reference numeral 40 denotes a zone setting means, which includes a mark 42 indicating a focus zone and a manual knob 44 for moving the mark 42 on the screen 28. The position a of the zone is detected by the position detector 46 and sent to the control means 48.

A focus control optical system 50 includes a semi-transparent mirror 52 arranged on the optical axis of the image projection light, a projection lens 54, a CCD line sensor 56 as an image sensor, and a servo motor 58.
With. A part of the projection light that has passed through the projection lens 20 is guided to the line sensor 56 through the projection lens 54 by the semitransparent mirror 52. The line sensor 56 is movable by a motor 58 in a direction orthogonal to the optical axis. The projection lens 54
When the projection lens 20 is placed at a position where the projection light is focused on the projection surface of the screen 28 or the photosensitive drum 36, its focal length is determined so that it is accurately focused on the light receiving surface of the line sensor 56. ing.

The autofocus mechanism includes a servomotor 60 that moves the projection lens 20 back and forth in the optical axis direction, and the projection light is projected on the screen 28.
Alternatively, the focus is controlled by the control means 48 so that an image is correctly formed on the projection surface of the photosensitive drum 36.

The control means 48 is constructed as shown in FIG. That is, the CCD driver 64 drives the line sensor 56 in synchronization with the clock pulse output from the clock 62. The line sensor 56 outputs a pulse signal that changes corresponding to the amount of incident light on each pixel for each scanning. This pulse signal varies from pixel to pixel even if the same amount of light is projected due to variations in the characteristics of each pixel. The signal processing circuit 66 corrects the variation in the characteristics of each pixel, and shapes and rectifies the waveform to obtain the output signal voltage υ in FIG.

The output signal voltage υ thus signal-processed is input to the bandpass filter 68, and a predetermined spatial frequency component of the output signal voltage υ is obtained as the analog signal g. That is, the filter 68 has a large gain at the spatial frequency f ₀ , and this frequency f ₀
The gain decreases sharply when is separated from, for example, at least −
Use one with an attenuation characteristic of 6 dB / OCT, preferably about -12 dB / OCT.

The analog signal g is converted into a digital signal by the A / D converter 70 and input to the CPU 74 via the input interface 72.

In FIG. 2, 76 is ROM, 78 is RAM, 80 is output interface, 82 and 84 are D / A converters, and 86 and 88 are motors 5, respectively.
It is a driver that drives 8, 60.

In addition to the control program of CPU74, ROM76
The MTF characteristics of the spatial frequencies 0 and 54 (corresponding to the characteristic (m) in FIG. 6) are stored. Generally, the MTF is expressed not only by the projection lens but also by the product of the MTFs of the reflecting mirror, the glass plate for supporting the original image, or the line sensor itself. When is fixed, the MTF can be approximated with high accuracy. Of course, it is possible to strictly seek all MTFs and use the product of these. In this embodiment, the MTFs of the entire projection lenses 20 and 54 are stored.

Next, the operation of this embodiment will be described. The control means 48 first reads the position a of the zone set by the zone setting means 40, and the projection light of the area corresponding to this zone is read by the line sensor.
The servo motor 58 is controlled so as to be incident on 56. The user selects the reader mode with the reflecting mirror 24 in the position shown by the solid line in FIG. 1 and projects the target original image on the screen 28 (step
100). A part of this projection light is guided to the line sensor 56 by the semi-transparent mirror 52.

The control means 48 measures the exposure amount based on the output of the next line sensor 56 (step 102). That is, the signal processing circuit 6
The output signal voltage υ of 6 is read into the CPU 74 via the filter 68, the A / D converter 70, and the interface 72, and the CPU 74 controls the exposure amount. If the exposure is not appropriate (step 10
4) Change the light intensity (step 106) and measure the exposure dose again. The adjustment of the exposure amount is performed, for example, by selecting the voltage of the pixel corresponding to the background area from the output voltage of each pixel of the line sensor 56 and setting the light source 12 so that the voltage becomes a predetermined voltage.
This is done by adjusting the amount of light.

Next, the control means 48 determines whether the projection light input to the line sensor 56 contains an image (step 108). This judgment is made based on, for example, whether or not the number of black and white inversions of the image is a predetermined value or more. If it is the predetermined value or more, it is judged that the image exists (step 110). When it is determined that there is no image, the control means 48 issues an alarm such as a buzzer or a lamp and requests the change of the focus zone (step 112). The user operates the knob 44 while looking at the screen 28 so that the mark 42 overlaps the position where the projected image is located.
To move.

Next, the control means 48 performs autofocus control based on the output of the line sensor 56.

The CPU 74 first stores the position of the projection lens 20 as χ ₁ in the RAM 78, and scans the line sensor 56 at the point A at this position χ ₁ . The CPU 74 similarly obtains the component amount y ₂ at the position χ _{2 at the} point B, which is on the same side as the point A with respect to the in-focus position χ _n of the lens and is moved Δχ from the point A. The CPU 74 further moves the lens to the position χ ₄ at the point D opposite to the points A and B with respect to the in-focus position χ _n , and similarly obtains the component amount y ₄ (step 11
Four).

The CPU 74 then performs the above (1), with respect to these points A, B and D.
Perform the same calculation as in equation (2) (step 116),
Corresponding points A ', B', D'on the lens MTF characteristic (m) satisfying the points (3) and (4) are obtained (step 118). In this case, there are various methods of obtaining the corresponding points. For example, when the data of the characteristic (m) is stored in the ROM 76 in the form of a table, (Y ₁ / Y ₂ ) or the like is a predetermined value (y ₁ / The positions X ₁ , X ₂ and X ₄ corresponding to y ₂ ), etc. are obtained. When there is no matching position, the closest position is obtained and correction is performed. In addition, the characteristic (m) is defined by the function F
When stored in the form of (X), this function is used to calculate the position X ₁ ,
X ₂ and X ₄ are required.

The CPU 74 then determines the maximum lens position X _n of the characteristic (m) as RO
The deviation from the position of the projection lens 20, for example, X ₄ , ΔX = (X _n −X ₄ ) is read from M78 (step 120). Then, the projection lens 20 is moved by the deviation ΔX from the position χ ₄ of the projection lens 20 at that time (step 122). As a result, the projection lens 20 can be adjusted to the in-focus position. In this embodiment, the lens position immediately before moving the lens position to the in-focus position is x
₄ , the amount of movement ΔX from this position x ₄ to the in-focus position x _n is calculated. However, this is equivalent to obtaining the difference (X−x) between the coordinates X and x of the characteristics m and e and matching the focusing position X _n on the characteristic m with the focusing position X _n on the characteristic e. .

If the printer mode is set in this focused state (step 12
4), the reflecting mirror 24 is rotated to the phantom line position in FIG.
The image is transferred to and a hard copy is obtained.

In the above embodiments, the MTF characteristics of the optical system are stored in the ROM 76, but since the MTF characteristics are different for each lens, for example, in consideration of the convenience of exchanging the lens, the lens barrel of the lens is considered. While incorporating a memory that stores the MTF characteristics of the lens, an electrical contact is provided that connects and disconnects with the main body of the device when the lens is attached and detached, and when the lens is attached, the contents of the memory are read out via this electrical contact. You may do it.

(Effect of the invention) As described above, the present invention obtains the component amount of the predetermined spatial frequency at at least three positions of the optical system based on the output of the image sensor, and finds the corresponding points on the MTF characteristics of the optical system stored in advance. Then, the deviation between the corresponding point and the maximum point on the MTF characteristic of this optical system is found, and the projection lens is moved by this deviation to move the projection lens to the in-focus position. There are fewer lens positions to calculate
The number of times the lens is stopped also decreases. Therefore, the waiting time until the vibration of the lens is stopped is small, and the time required to detect the in-focus position can be shortened. Further, one of the at least three positions is on the opposite side of the other position from the in-focus position, so that the control accuracy is improved.

[Brief description of drawings]

FIG. 1 is an overall schematic view of a reader printer which is an embodiment of the present invention, FIG. 2 is a block diagram of its autofocus control device, and FIG. 3 is a flow chart of its operation. Further, FIG. 4 is a diagram showing an image sensor output, FIG. 5 is a diagram showing a spatial frequency component, and FIG. 6 is a diagram explaining the principle of the present invention. 20,54 …… Projection lens, 56 …… Line sensor. 68: Spatial frequency filter, m: MTF characteristic, e: Spatial frequency component.

Claims (1)

[Claims] 1. An autofocus method of a microfilm reader for controlling a projection lens to a focus position by using an output signal of the image sensor obtained by scanning a projection image of a microfilm with an image sensor. While pre-storing the MTF characteristic at a predetermined spatial frequency of the optical system when moved, the component amount of the predetermined spatial frequency is obtained at at least three positions sandwiching the focus position of the projection lens with respect to the projection image, The corresponding position of the projection lens position on the MTF characteristic is determined using these component amounts, the deviation between the corresponding position and the in-focus position on the MTF characteristic is determined, and the projection lens is moved by this deviation. An autofocus method for a microfilm reader, which is characterized in that