Abstract:
A device by an optical sight with a light tunnel ( 10, 11 ). A light emitting diode is received in the light tunnel providing an illuminated aimpoint in the light tunnel. An element sensitive to the ambient light is provided in the light tunnel for controlling the brightness of the aimpoint in dependence of the intensity of the ambient light.

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical sight with a light tunnel, a light emitting diode mounted in the light tunnel for producing an illuminated aimpoint visible in the light tunnel, and an element sensitive to the ambient light for controlling the brightness of the aimpoint in dependence of the intensity of the ambient light, the brightness being reduced in a dark environment so that the shooter does not get dazzled by the illuminated aimpoint, and increased in a bright environment so that the aimpoint will be seen better against the bright background. 
     A sight of this kind is disclosed in SE-B-449 262. The sight includes an oscillator producing a pulsed current to operate the light emitting diode, and a photoresistor sensitive to the ambient light for controlling the oscillator frequency and pulse length in dependence of the ambient light. It is not indicated where this photoresistor shall be located. 
     A sight of the kind referred to above is characterized in that the element sensitive to the ambient light, the light sensor, is placed in the light tunnel. This element will thereby detect the ambient light exactly in the area that the shooter aims at when he looks through the light tunnel and places the illuminated aimpoint visible therein, on the designated target. A more adequate control of the brightness of the aimpoint in relation to the ambient light is obtained when the field of view of the light sensitive element is limited to the target area and integrates the light therein. To place a photoresistor in the light tunnel is out of the question since the dimensions of a photoresistor are large, of the order of 5 mm, and the photoresistor hence would interfere with the view through the light tunnel, whereas a photo diode is well suited for this placement since the dimensions of the photo diode are of the order of 0.2 mm, and the photo diode therefore will not cause any such interference. In addition to control of the brightness of the aimpoint being improved by the sight according to the invention this sight is advantageous in that it consumes less power than the known sight with a pulsed light source. 
     A sight with a light emitting diode is disclosed in U.S. Pat. No. 4,665,622, and a power supply circuit for controlling emission of a light emitting diode is disclosed in U.S. Pat. No. 4,330,206. 
     Preferably, the light sensitive element is arranged in the close vicinity of the light emitting diode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An illustrative embodiment of the sight according to the invention will be described in more detail in the following with reference to the accompanying drawing, in which 
     FIG. 1 is a schematic cross sectional view of an optical sight, taken axially through the tube, which forms the light tunnel of the sight, and illustrates the functional principle of the sight, 
     FIG. 2 is an enlarged axial cross sectional view of the battery housing of the sight, and 
     FIG. 3 is a circuit diagram of the electrical circuit system of the sight. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The sight comprises a light tunnel which is formed by an outer tube  10  to be attached to the barrel of the shotgun on which the sight is to be used. Mounted in said tube is an inner tube  11  with one end fixed to the outer tube and the other end fixed to an adjustment device, not shown here, for adjustment of the longitudinal axis of the inner tube relative to the longitudinal axis of the outer tube to the extent required to adapt the sight to the shotgun on which it is to be used. At said one end of the inner tube a double lens  12  is mounted with a coating  13  between the lenses reflecting red light. Inside the inner tube a light source  14  is provided comprising a light emitting diode which directs a beam of red light towards the coating  13  reflecting the light beam through a surface ground glass plate  15  with anti reflective coating facing the right end of the light tube, as indicated by dot and dash lines in FIG.  1 . When the shooter looks at the target through the light tunnel from this end, he sees a red dot which he puts on the spot on the target, where he wants the impact to take place. 
     On the outside of the outer tube  10  a battery housing  16  is provided, FIG. 2, which can be made integral with the outer tube, or made as a separate part and attached thereto. The battery housing can have its longitudinal axis substantially parallel to the longitudinal axis of the outer tube or at an oblique angle thereto. In the battery housing, a cylindrical battery chamber  17  is provided, closed at one end thereof by a screw lid  18  and dimensioned to leave room exactly for a conventional penlight battery  19  of the type AA according ANSI with the voltage rating 1.5 V. This voltage is delivered by the battery through conventional connectors in the battery chamber. The battery housing also comprises a chamber  20  in which the electrical circuit system  21  of the sight for operating the light emitting diode is received. This circuit system will be described in more detail with reference to FIG.  3 . 
     The battery  19  is connected to a step-up transforming circuit  22  which is built in a way known per se, based on a DC/DC-converter LT® 1307 from Linear Technology Corporation, USA, indicated at  23 . In addition to the DC/DC-converter  23 , the transforming circuit  22  includes resistors R 1 -R 3 , capacitors C 1 -C 3 , inductor L 1 , and diode D 1 , arranged in a known way. With the step-up transforming circuit  22 , the voltage supplied to the light emitting diode  14  may thus be increased to at least 1.6 V. The output voltage from the circuit  22  in the case illustrated in FIG. 3 is 3.3 V, i.e. a potential higher than that needed for controlling the light emitting diode, and is tapped off at a terminal post  24  to which a circuit is connected for controlling the brightness of the light emitting diode  14  in dependence of the ambient light so that the brightness may be reduced in weak ambient light and increased in intense ambient light, whereby the illuminated aimpoint always is clearly visible in the sight without being blinding, independently of the current light conditions. 
     The control circuit comprises a light sensor  25  which in principle consists of two Darlington coupled phototransistors, and this light sensor is placed in the inner tube  11 , FIG. 1, in the area of the light emitting diode  14 . Its field of view is indicated by double dot and dash lines. The light sensor is positioned within the tube such that its field of view is larger than the field of view of the eye by which the shooter looks through the sight, in order that the light sensor shall integrate the ambient light. The proper position of the light sensor to obtain this feature may vary depending on the size and shape of the inner and outer tubes, and on the specifications of the optics. The light emitting diode  14  is connected to the light sensor  25  by a current mirror, comprising two transistors  26  and balanced by two resistors R 4  and R 5  for obtaining the correct transfer factor from the left to the right transistor  26 . The current mirror is a circuit built in a known way, with two current carrying lines wherein the currents in the two lines are maintained in a predetermined ratio. If one current is changed the other current changes so that the ratio between the two currents is maintained. Across the current mirror  10  resistors R 6 -R 15  are connected with a switch  27  by means of which the balance in the mirror may be varied for coarse adjustment of the brightness of the light emitting diode to a value (base value) adapted to current light conditions (i.e. day and night) and to the individual perception of the illuminated aimpoint in the sight. A switch  28  is provided for connection of the light emitting diode and associated control circuit, but this may be replaced by sensor automatics, e.g. an accelerometer which may be connected in such a way that battery voltage is applied to the circuit, when the accelerometer is activated, and the voltage is switched off if the accelerometer is not activated within a given period. A further possibility to switch off the voltage is to use the shut down function in the DC/DC converter. 
     One inherent characteristic of the described circuit system is that the light emitting diode will twinkle, when the battery voltage approaches a value which is so low that the battery cannot operate the step-up transforming circuit and the light emitting diode. The light emitting diode is turned off when the voltage has dropped to the said value, whereupon the battery voltage recovers to again keep the light emitting diode twinkling during a short interval. The user thereby receives an indication, that battery replacement must take place. 
     The circuit system in FIG. 3 may as a whole be arranged in the chamber  20  in the battery housing  16  where it is indicated by the block  21 , excluding the light emitting diode  14  and the light sensor  25 , which, as mentioned, are arranged in the inner tube  11 .