The device offers a means for automatic calibration of an optical sight for firearms, by firing one round only.

Technical standpoint

To zero-in, or calibrate, a firearm equipped with an optical sight, typically a riflescope, is usually a tedious and to some extent error prone procedure. Several rounds are usually needed for a successful calibration. Hunters then usually fire a couple of high priced soft point rounds to confirm the calibration.

The device described in this application offers shooters the possibility of getting the optical sight (riflescope) automatically calibrated, by firing one round only, at a defined target.

Description of the invention

To achieve an automatic calibration of an optical sight for firearms, a traditional optical sight needs the following additional items, se Figure 1. A beam-splitter (1) lets the shooter and the image processing system view the same scene. The image is fed into a camera (2) (e.g. a CCD camera) for registration. A micro-processor with sufficient memory and software (3) analyzes the image and determines the error (i.e. calculates the need for calibration). Servo motors (4) with gears (5) can be used to operate the adjustment screws and the traditional mechanical cross-hair element (9). A switch is used to initiate the automatic calibration sequence (6). Lamps (8) (e.g. diodes) indicate the calibration result. The device is powered by a built-in or external electrical power source (7).

By cross-hairs is meant all existing variants of optical sights.

The calibration sequence is described by the following instruction to the shooter:

- Arrange a target (typically: a white square surface against a dark background) and assume firing position at the range for which the firearm is to be calibrated.

- Initiate the calibrating function by pressing the switch.

- Aim & fire a round at the target.

- After firing the round, aim again at the target and identify the bullet hole visually.

- Hold the cross-hairs steady at the target, when the green lamp lights up - the calibration is successful. If the red lamp lights up, repeat the sequence.

- The calibrating function is shut off by pressing the switch again.

Function: When the calibration procedure is initiated by the switch, the microprocessor and camera start up. The camera immediately starts taking frames (with a typical interval between frames of 40 ms) and stores them in a FIFO queue (First hi , First Out).

The microprocessor detects that a round has been fired when the target suddenly disappears from the view (it happens very fast; i.e. from one frame to the other). Alternatively an embedded sensor can be used to register this event.

The last exposure before the shot is stored for later analysis; the edges of the white target area are identified by contrast with the darker background.

After firing the round, the shooter resumes the aiming position while the camera continues to take frames and to store them in the FIFO queue. The best frame after firing is identified and is then compared with the best frame before firing. The bullet hole position on the target is identified by variation in contrast (black dot on the white surface).

While the shooter holds the cross-hairs on the target, the microprocessor calculates the needed adjustment. This means both horizontal and vertical adjustments in "clicks", to align the bullet hole with the cross-hairs . Since the cross-hairs position is determined relative to the target in the entire field of view- both before and after firing the round - the needed adjustment can be calculated independent of range and riflescope magnification. (However, the target must not be moved and that the shooter must not change position during the calibration procedure).

The servo motors are then activated so that the adjustment screws are turned the needed number of "clicks" in both horizontal and vertical directions. A green lamp lights up as an indication of a successful calibration. If the identification of the bullet hole fails, a red lamp lights up, and the procedure needs to be repeated (a possible cause for failure might be that the bullet has missed the target).

When the shooter finally presses the switch again, the calibration system is shut down (power off).

Alternative arrangements: The implementation of the device can be adjusted to the desired level of technology and automization as follows:

1. Traditional optical sight using a semi transparent mirror with no moving parts (1). This can be implemented using an electrically controlled electro-chromic film (EC film) placed in the line of sight. When powered, the EC film works as an semi-transparent mirror.

2. Traditional optical sight, splitting the beam using a moving semi-transparent mirror (1).

This can be implemented by a mechanically operated semi-transparent mirror. The mirror is moved between bi-stable positions by a lever on the outside of the sight, locked by form and/or friction.

3. Fully digital optical sight with traditional fixed or zoom lens for riflescopes, and a display in the viewfinder. Mechanical cross-hairs are not needed.

This can be implemented by a camera (2) placed directly in the line of sight, a microprocessor with memory and software (3), and a display (10) (typically a LCD - Liquid Crystal Display) in the viewfinder instead of a conventional lens. In this case the need for indicator lamps is eliminated since the calibration result can be displayed directly on the display. The switch (6) and power source (7) are still needed. In this variant the camera view is presented together with the cross¬ hairs as an all digital picture on the display. This fully eliminates the need for a mechanical adjustment of the cross-hairs, see sketch in figure 2. The calibration procedure is carried out as described above.

4. A separate device attached to a traditional optical sight. The separate device contains a camera and microprocessor and includes servo motors to be mounted onto the existing adjustment screws. This variant can be used together with traditional sights without any modification, whenever a calibration is needed.

The variants can also be implemented as semi-automatic devices for adjustment of mechanical cross-hairs. The device then presents the shooter with information about the needed number of clicks, both in horizontal and vertical directions, to achieve alignment between the bullet hole and the cross-hairs. The shooter then carries out the adjustment manually according to the information. (The information can be projected via the semi- transparent mirror onto the viewfinder).