Focus Detecting Device

The present invention relates generally to improvements in focus detecting devices and it relates more particularly to an improved photoresponsive member for sensing the condition of focus of a lens on a focal plane in an optical instrument such as a camera or the like.

An automatic focusing mechanism has heretofore been proposed in which a scanner having slits or pin holes therein is rotated or oscillated in a focusing plane in a direction at right angle to the optical axis of a lens to thereby scan an image, and the light which traverses the aforesaid slits or pin holes is received by stationary photodiodes to produce a oscillating or video signal, the high frequency component contained in the video signal being uses as a sensing signal for use in the automatic focusing. However, in the mechanisms of the above type, the device, including the scanner and the motor for driving the scanner occupies considerable space, thus resulting in incompatibility with a camera which is desired to be compact in size and light in weight. Furthermore, the aforesaid mechanism possesses the important drawback of an excessive amount of energy required for the scanner drive as well as very high power consumption. There has also been heretofore proposed an automatic focusing system in which the light receiving element itself is oscillated as a scanner, more particularly a photoconductor having slits is reciprocated in the focusing plane, as described in U.S. Pat. No. 3,830,571 filed on Dec. 19, 1972 and issued on Aug. 20, 1974. However, with the increase in the number of slits or apertures, the number of the output terminals of the element must be accordingly increased, with the resulting increase in the number of circuits associated therewith. In addition, there results a photoconductor presenting a poor response to low light, thus failing to scan at a higher oscillation cycle.

It is a principal object of the present invention to provide an improved focus detecting device.

Another object of the present invention is to provide an improved focus detecting device which overcomes the disadvantages and drawbacks of those heretofore proposed.

Still another object of the present invention is to provide an improved focus detecting device which is highly compact and suitable for use in cameras and highly responsive to high and low light conditions.

A further object of the present invention is to provide a focus detecting device having a minimum of electrical terminals, which is compact and of light weight, requires a minimum length reciprocation stroke and a minimum of power and current consumption.

Still a further object of the present invention is to provide a device of the above nature characterized by its high reliability and great versatility and adaptability.

The above and other objects of the present invention will become apparent from a reading of the following description taken in conjunction with the accompanying drawings which illustrate preferred embodiments thereof.

In a sense, the present invention contemplates the provision of a focus detecting device comprising first and second side-by-side longitudinally extending sets or groups of photoresponsive elements, advantageously photodiodes, the photosensitive element in each set being longitudinally spaced from each other and being longitudinally offset relative to the photosensitive elements in the other set to define a checkered pattern. In its preferred form, the improved device includes a semiconductor substrate of a first type or polarity having superimposed thereon at least two side-by-side strips of semiconductor of an opposite type or polarity. Masking overlies the strips to divide them into successive masked and unmasked areas, the respective areas of successive strips being longitudinally offset to provide a checkered pattern. A terminal is provided for the semiconductor substrate and each semiconductor strip. Alternatively, the semiconductor strips may be replaced by longitudinally spaced semiconductor laminae, the longitudinally spaced laminae being interconnected by conductors.

Although the detecting device is advantageously oscillated in the focusing plane for maximum resolution, stability and consistency, it may be satisfactorily employed in a stationary condition, and in either case, maximum response is provided at optimum focus and suitable circuits may be employed to achieve automatic focusing. The device is light and compact and requires a minimum of power and current consumption.

FIG. 1 is a perspective view of a first embodiment of the present invention;

FIG. 2 is an exploded perspective view thereof;

FIG. 3 is a schematic view illustrating the operation of the first embodiment;

FIG. 4 is a circuit diagram showing one example of a circuit in which the first embodiment is connected;

FIG. 5 is a graph showing an example of an output of the circuit of FIG. 4;

FIG. 6 is a perspective view of a second embodiment of the present invention;

FIG. 7 is an exploded, perspective view, illustrating a modification of the first and second embodiments in comparison with the first embodiment.

Referring now to FIGS. 1 to 5 of the drawings which illustrate a first embodiment of the present invention, a focus detecting element having a pair of photodiodes PD1, PD2 is disposed in the focus reference plane of a lens l, the photodiodes being arranged in parallel on an insulating base plate 1, with the light receiving surface divided into plurality of areas which are positioned in mutually staggered relation.

As best seen in FIG. 2, a semiconductor substrate or base plate 2 is superimposed on the insulating base plate 1, and the semiconductor base plate is made, for instance, of n type semiconductor. Semiconductors 3 and 4 are superimposed on substrate 2 and present light receiving surfaces of the photodiodes, and the semiconductors 3, 4 are made of a p type, in contrast to the n type of the semiconductor 2. Alternatively, the n and p types may be interchanged. The semiconductors 3, 4 with the substrate 2 define the aforesaid pair of photodiodes PD1, PD2, independently, due to the contact with semiconductor base plate 2, and connected to output terminals Out1 and Out2. Out3 is an output terminal connected to the semiconductor base plate 2. A mask 5 having a checkered pattern consists of transparent portions ai, bi (where i is a given number) and opaque portions C, with the upper half A which consists of ai and C covering the light receiving surface of semiconductor 3 and with the lower half B which consists of bi and C covering the light receiving surface of semiconductor 4, respectively, thus dividing the light receiving surfaces of phtodiodes PD1, PD2 into light receiving photodiode element defining areas appearing in a staggered relation.

The aforesaid photodiodes and mask are advantageously prepared according to the known I.C. circuit technique. The semiconductors 3, 4 should preferably be as close to each other as possible whicle being mutually electrically insulated. The width t of the semiconductors 3, 4 is advantageously of the order of several hundreds microns, and the width d of the checkered pattern is advantageously of the order of several tens microns. The width d may be derived from a consideration of a spatial frequency contained in an image formed by the objective lens. However, limitations arising from the aspects of the I.C. circuit technique or SN ratio are imposed thereon.

FIG. 3 illustrates the sensing operation of the above described focus detecting element according to the present invention, in which the variation in brightness is shown in relation to the position on the photodiodes PD1, PD2. As shown in FIG. 3 a dark light is incident through a transparent portion a2 of the mask on the semiconductor 3, while a bright light is incident through the transparent portion b2 of the mask on the semiconductor 4 as shown by the line 6 of FIG. 3. The light having uniform brightness is incident through the remaining transparent portions. Assuming that load resistors R1, R2 having equal resistance are connected to PD1 and PD2, as shown in FIG. 4, the semiconductor 4 receives more light than semiconductor 3, so that the photodiode PD2 produces more photocurrent to thereby produce a larger potential across the opposite ends of the resistor R2 than across the opposite ends of R1. In brief, a voltage corresponding to the difference in potential will appear across the terminals P, Q. On the other hand, if the distribution of brightness on the light receiving surface of the element is such shown by the broken line 7 due to off-focusing, the output between the terminals P, Q is reduced. Accordingly, in the network employing the focus detecting element of the present invention, the focus detecting may be achieved by obtaining the maximum value of the difference in voltages appearing between Out1 and Out3 and between Out2 and Out3. The detecting element described above may be used in a stationary condition, but in order to achieve a more stable or consistent output, the detecting element is advantageously oscillated in a direction parallel to the length of the semiconductors 3 and 4 as shown by the arrow A in FIG. 1. When the detecting element is shifted by a distance d in parallel with the light receiving surface, then the light incident condition on the semiconductors 3 and 4 are reversed to that shown in FIG. 3, so that a voltage having a polarity reverse to that in the aforesaid case appears across the terminals P, Q. In addition, when the detecting element is oscillated at an amplitude of d/2, then an A.C. output appears across the terminals P, Q as shown in FIG. 5. As a result, by determining the maximum value of the amplitude of the A.C. output, there may be achieved a highly stabilized or consistent focus detection. In the aforesaid description, the distribution of the brightness of an image over the light receiving surface is so provided as to suit or enhance the detection. However, such a condition is commonly present in the case of an ordinary photographic object. In other words, there exists a contrast of light and shadow in an ordinary photographic object, and hence there exists a spatial frequency component on the boundary of the contrast, as shown in the uppermost portion of FIG. 3. Accordingly, it is possible in general, that the focusing may be achieved by detecting the position of an A.C. output across the terminals of P, Q which provides the maximum A.C. output, by oscillating the element.

A piezo-electric element for example, may be used as a means for oscillating the detecting element. In addition, a known circuit may be used for processing an output signal appearing across the terminals P,Q, it being noted that various methods may be utilized for the aforesaid purpose.

Referring now to FIG. 6 which illustrates a second embodiment of the present invention, an arrangement is used for achieving a pair of photodiode sets whose light receiving surfaces are divided into areas or sections disposed in a staggered relation. Specifically, a plurality of semiconductors 8a, 9a, 8a', 9a', 8a" and 9a" of the same size and of heights t and widths d, are located on the semiconductor substrate or base plate 2 in a staggered relation to each other. In this instance, one group of semiconductors 8a, 8a' and 8a" which are spaced in a line are connected by means of connecting wires 8b and 8b', while the other group of 9a, 9a' and 9a" are connected by 9b and 9b', thereby presenting a focus detecting element, without using a mask 5 such as given in the first embodiment. As shown in FIG. 6, the two groups of semiconductors are connected to Out1 and Out2 respectively.

In FIG. 7 of the drawing there is illustrated another embodiment of the present invention of the nature of the first embodiment. There is provided in the modified detector element another pair of semiconductors 10 and 11 similar to the pair of semiconductors 3 and 4 and formed on the common semiconductor substrate or base plate 2 carrying the pair of semiconductors 3, 4. Shown at 12 is a mask of checkered pattern, which consists of C, D in addition to A, B of the mask as used in the first embodiment for the other pair of semiconductors 10, 11. In case more than two pairs of photodiodes are placed in the focus reference plane of a lens, more than two pairs of semiconductors may be formed on the common semiconductor base plate 2, thus presenting a unitary focus detecting element. In FIG. 7, the output terminals of a pair of photodiodes including semiconductors 10, 11 are shown at Out3, Out4 and Out5. According to the present embodiment, a wider range of focus reference plane may be covered, thus promoting the positiveness of the focus detection, as well as the accurate positioning of a plurality of pairs of light receiving surfaces on the same plane, because of the use of the common semiconductor base plate.

Further, it should be noted that the focus detectng element according to the present invention may be applied not only to a focus adjusting device of a camera, but also to the measurement of OTF of a lens.

While there have been described and illustrated preferred embodiments of the present invention, it is apparent that numerous alterations, omissions and additions may be made without departing from the spirit thereof.