LIGHT-EMITTING DEVICE FOR AUTOMATIC FOCUS ADJUSTMENT APPARATUS

Abstract: In an automatic focus adjustment apparatus with a light-emitting device comprising a condenser lens (2) and a light-emitting element (1) which radiates light onto an object and adjusts the focal point according to light reflected back from the object, undesired flare is eliminated by placing a diaphragm (3a) sufficiently close to the light-emitting ele­ ment (1) so that only the effective light from the element passes to the condenser lens by using a cap (3), an opaque paint (11) or a spatial filter (13), and/or by applying a low reflectance paint to the inner wall of an outer casing (5) of the light-emitting device.

Description: Background of the Invention 1. Field of the invention The present invention relates to a light-emitting device used in the automatic focus adjustment apparatus for a video camera or still camera. 2. Description of the prior art Various systems have been proposed for the automatic focus adjustment apparatus which drives the image forming lens to be in a focusing position. Among them is a system which emits light to an object and detects a reflected light from the object for focus adjustment (hereinafter referred to as active system). U.S. Patent No. 4,441,810 and European Patent Publication EP-A-0 140 650 disclose this type of system; a light emitter and a photo sensor are positioned a specified distance apart from each other based on the principle of trigonometrical survey. Light from a light-emitting device is collected by a condenser lens and projected to an object. Reflected light from the object is condensed by a receiving lens and fed to the photo sensor, which photoelectrically detects the position of the centroid of the image formed by the reflected light to obtain distance imformation for focus adjustment. The focus adjustment apparatus of the active system described above is based on the assumption that, when the distance between the apparatus and the object is fixed, the centroid of the spot image formed on the photo sensor by the reflected light is also fixed. In the focus adjustment apparatus of the active system, the light-emitting device is normally an infrared ray-emitting diode (hereinafter abbreviated as IRED). Light from this device is collected by a condenser lens and then projected onto the object. At the same time, lights reflected by the inner walls of the outer casing and the surface of the base of the device are also projected through the condenser lens onto the object; direct light forms a high intensity infrared light image at the center, surrounded by a lower intensity infrared light image (hereinafter referred to as flare image) formed by lights reflected from the inner walls of the outer casing and the surface of the base (hereinafter called flare). If the object has constant reflectance over its entire surface, the centroid of the spot image on the photo sensor is virtually fixed irrespective of the flare due to the flare's negligibly low light intensity. If the object has different reflectances, however, problems arise. Suppose that higher intensity image is formed by direct light on a low reflectance area of the object while a lower intensity flare image is partly formed on a high reflectance area. In such a case, the light image formed on the sensor will have intensity distribution different from that for a constant reflectance subject. Since the flare image light intensity would be significantly high, the position of the centroid of the spot image would change. In other words, though the distance from the object is constant, the position of the centroid of the spot image changes on the photo sensor, resulting in an out-of-focus picture. In the light-emitting device disclosed by JP-A-5856482, the outer casing 1 is bent at the front end defining the light-emitting window 2 for the purpose of hermetically sealing the device by bonding the window member 4 to the front end bent portion of the outer casing with a hermetically sealing material 3. To improve the light transmitting characteristics, bonding strength and air-tight characteristics, a projection or groove 11 is formed at the front end bent position of the outer casing 1. Japanese patent application JP-A-58216478, also discloses a light-emitting device having a basically similar construction to that of the present invention but with means for blocking flare light disposed midway between the base of the device and a cover glass. An object of the present invention is to provide a light-emitting device for an automatic focus adjustment apparatus which reduces undesired flare, thus minimizing erroneous focus adjustment. The present invention provides a light-emitting device comprising: a light-emitting element for emitting light; a base for mounting thereon said light-emitting element; and outer casing for encasing therein said base and said light-emitting element and provided with a aperture; a cover glass mounted on said outer casing to cover the aperture of said outer casing; and means for blocking flare light reflected from an inner wall of said outer casing and a surface of said base, which is characterized either in that said flare light blocking means is provided separately from said outer casing to cover an outer surface of said cover glass and has an aperture which is smaller in size than the aperture of said outer casing so as to pass therethrough only the light emitted from said light-emitting element, or in that said flare light blocking means comprises a spacial filter placed in front of said cover glass and having a frustro cone-shaped surface defining frustro-cone-shaped inner space and a shielding means forming in said inner space a plurality of coaxial and substantially frustro-cone-shaped shield surfaces, the light-emitting element being placed at a position corresponding to the imaginary apex of each of said frustro-cone-shaped surfaces so as to pass through said spacial filter only the light emitted from said light-emitting element. In the present invention, the undesired flare is reduced by extremely simple construction without decreasing the quantity of effective light necessary for focus adjustment so that erroneous focus adjustment can be minimized. Fig. 1 is a partially-sectional side view showing a light-emitting device of a first embodiment according to the present invention; Figs. 2 (a) and (b) are a partially-sectional side view and top view showing a light-emitting device of a second embodiment according to the present invention; Figs. 3 (a) and (b) are a side sectional view and a top view showing a light-emitting device of a third embodiment according to the present invention; and Figs. 4 (a) and (b) are a side sectional view and a top view showing a light-emitting device of a fourth embodiment according to the present invention. Description of the Preferred Embodiments Fig. 1 shows a light-emitting device of a first embodiment according to the present invention. Light ray radiated from a light-emitting element 1 is collected by a condenser lens 2 and projected onto an object (not shown). An outer casing 5 to which a cover glass 4 is fused is mounted on a base 6. Reference numeral 7 shows a terminal, and 8 shows the solid angle of an imaginary cone extending from the light-emitting element 1 to the effective diameter of the condenser lens 2. A cap 3 is mounted on the outer casing 5 and includes a diaphragm 3a for limiting the light ray reflected from the inner walls of the outer casing 5 and the base 6. The diaphragm 3a is an aperture formed in the cap 3 at the position opposite to the light-emitting element 1. The cap 3 is of a material which has a low reflectance to infrared light. The diaphragm 3a has a knife edge. The height of the outer casing 5 is low to enable the diaphragm aperture in the cap 3 to be placed close to the light-emitting element 1. Here, the diaphragm aperture in the cap 3 is larger than the solid angle 8. That is, the selected aperture diameter value  may satisfy the following condition: where, a: Effective diameter of the condenser lens 2 b: Diameter of the light-emitting element 1 d: Distance between the base 6 and the diaphragm 3a ℓ: Distance between the base 6 and the condenser lens 2 If the aperture diameter satisfies the condition (1), the quantity of effective light contributing to focus adjustment will not be reduced. The closer the cap 3 is placed to the light-emitting element 1, the more the aperture diameter can be reduced maintaining the condition (1), thereby effectively reducing the light reflected from the inner walls of the outer casing 5 and the base 6 and the surface of the cover glass 4. If the cover glass 4 is in contact with the cap 3, the flare from the cover glass 4 is eliminated. Figs. 2 (a) and (b) show a light-emitting device of a second embodiment of the invention. The cover glass 4 is coated at an outer surface thereof with a low reflectance opaque paint 11 except an aperture area thereof. The aperture area is determined to satisfy the above condition (1). The height of the outer casing 10 is low to enable the aperture area formed by the opaque paint 11 to be placed closer to the light emitting element 1. In this embodiment, an effect identical to that achieved by the first embodiment is obtained by simply applying the opaque paint coating to the cover glass. The screen printing method may be used to apply the paint. In addition to good dimensional accuracy, the screen printing has high productivity; a large number of light-emitting devices can be simultaneously painted at the same time at low cost. Figs. 3 (a) and (b) show a third embodiment of the present invention. Fig. 3 (a) is a sectional view of a light-emitting device of the third embodiment and Fig. 3 (b) shows the top view of the same. Light from the light emitting element 1 passes through the cover glass 4 and a spatial filter 13 mounted on the aperture of outer casing 12 and is collected by the condenser lens 2 for projection of light onto an object (not shown). The spatial filter 13 defines by an inner surface 13a thereof a frustro-cone in the following: semi-cone with the center of light emitting element 1 as the imaginary top and with the effective diameter of the condenser lens 2 as the imaginary bottom. Inside the spatial filter 13 are mounted thin, semi-cone-shaped shields 13b, 13c each with the center of light emitting element 1 as its imaginary top. The semi-cone-shaped surfaces 13a and the semi-cone-shaped shields 13b, 13c are arranged so that their center axes are placed on the optical axis of the condenser lens 2 on which the light-emitting element 1 is also placed. Accordingly, if the light-emitting device is viewed along the optical axis from the condenser lens 2, only the light-emitting element 1 can be seen through the spatial filter 13, but the base 6 and the inner wall of the outer casing 12 cannot be seen. Light radiated from the light-emitting element 1 passes through the spatial filter 13 and is projected onto the object through the condenser lens 2. The flare is blocked by the shields 13b, 13c in the spatial filter 13. The amount of the flare reaching the object can be further reduced by painting the surfaces of the shields 13b, 13c and the surface 13a black for lower reflectance. Figs. 4 (a) and (b) show a fourth embodiment of the present invention. Fig. 4 (a) is a sectional view of a light-emitting device of the fourth embodiment and Fig. 4 (b) is a top view of the same. The light-emitting device in Fig. 4 (a) is the same as that in Fig. 3 (a) except for a spatial filter 14. While in Fig. 3 (a) the spatial filter 13 is composed of a plurality of co-axial semi-conical shields, in Fig. 4 (a) the space filter 14 is composed of a shielding sheet 14b circinated into hone shape. The fourth embodiment thus provides the same effect as the third embodiment, but it reduces the number of components. As described above, the present invention uses extremely simple construction to minimize undesired flare from the light-emitting device, thus substantially reducing erroneous focus adjustment when an object has reflectance distribution. Two or more of the above embodiments may be employed in combination to double flare reduction effect.

Claims: A light-emitting device comprising: a light-emitting element (1) for emitting light; a base (6) for mounting thereon said light-emitting element; an outer casing (5, 10) for encasing therein said base and said light-emitting element and provided with an aperture; a cover glass (4) mounted on said outer casing to cover the aperture of said outer casing; and means (3, 11) for blocking flare light reflected from an inner wall of said outer casing and a surface of said base, characterized in that said flare light blocking means (3, 11) is provided separately from said outer casing (5, 10) to cover an outer surface of said cover glass (4) and has an aperture which is smaller in size than the aperture of said outer casing so as to pass therethrough only the light emitted from said light-emitting element (1). A device according to claim 1, wherein said flare light blocking means (3, 11) comprises a cap (3) covering said outer casing (5) and the outer surface of said cover glass (4) and provided therein with an aperture (3a) as said aperture of said flare light blocking means. A device according to claim 1, wherein said flare light blocking means (3, 11) comprises a low reflectance opaque paint (11) coating the outer surface of said glass (4) except an area corresponding to said aperture of said flare light blocking means. A light-emitting device comprising: a light-emitting element (1) for emitting light; a base (6) for mounting thereon said light-emitting element; an outer casing (12) for encasing therein said base and said light-emitting element and provided with an aperture; a cover glass (4) mounted on said outer casing to cover the aperture of said outer casing; and means (13, 14) for blocking flare light reflected from an inner wall of said outer casing and a surface of said base, characterized in that said flare light blocking means comprises a spacial filter (13, 14) placed in front of said cover glass (4) and having a frustro-cone-shaped surface (13a, 14a) defining a frustro-cone-shaped inner space and a shielding means (13b, 13c, 14b) forming in said inner space a plurality of coaxial and substantially frustro-cone-shaped shield surfaces, the light-emitting element being placed at a position corresponding to the imaginary apex of each of said frustro-cone-shaped surfaces so as to pass through said spacial filter only the light emitted from said light-emitting element (1). A device according to claim 4, wherein said shielding means (13b, 13c, 14b) comprises a plurality of coaxial frustro-cone-shaped shield members (13b, 13c) provided in said inner space. A device according to claim 4, wherein said shielding means (13b, 13c, 14b) comprises a shielding sheet (14b) circinated to form the plurality of coaxial and substantially frustro-cone-shaped shield surfaces in said inner space. A combination of a light-emitting device according to any one of claims 1, 2 and 3, and a condenser lens (12) for collecting the emitted light. A combination according to claim 7, wherein said aperture of said flare light blocking means (3, 11) is a circular aperture having a diameter larger than a solid angle of an imaginary cone defined by the diameter of said condenser lens (2) as a bottom and the center of said light-emitting element (1) as an apex.

Inventors: ASAKURA, HIROYUKI, HIRAO, YOSHIAKI, KUGA, RYUICHIRO, NAGAOKA, YOSHITOMI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: H01L33
IPC codes: H01L33