Auto-focus night vision device

1. A night vision device comprising:

a housing having a first end and a second send;

an objective lens mounted on the housing nearer to a first end than a second end, and arranged for receiving light from ambient into the housing;

an image intensifier mounted on the housing and located toward the second end of the housing from the objective lens;

an eyepiece lens mounted on the housing and located toward the second end of the housing from the image intensifier;

an adjustable iris defining an iris opening, the adjustable iris being mounted on the housing and located toward the second end of the housing from the objective lens;

a sensor for detecting light intensity connected to the housing and located toward the second end of the housing, the sensor being configured to control a size of an area of the iris opening in response to the light intensity detected for keeping the area of the iris small so as to maintain a depth of field of the night vision device.

2. The night vision device as set forth in claim 1 wherein the sensor is configured to maintain the area of the iris opening so long as the light intensity detected remains in a range between an upper boundary and a lower boundary.

3. The night vision device as set forth in claim 2 wherein the sensor is configured to cause the area of the iris opening to increase if the light intensity detected is below the lower boundary, and to cause the area of the iris opening to decrease when the light intensity detected is above the upper boundary.

4. The night vision device as set forth in claim 3 wherein at least one of the upper and lower boundaries is adjustable.

5. The night vision device as set forth in claim 1 further comprising a power switch operable to turn the night vision device on and off so that the night vision device can operate automatically when turned on or switched off to allow manual operation of both the objective lens focus and the size of the iris opening.

6. A method of operating a night vision device comprising measuring at a location proximate the eye of the user the intensity of light reaching the user's eye, using the measured light intensity to maintain a depth of field of the night vision device large so long as the amount of light reaching the user's eye from the night vision device is sufficient to permit visual perception.

7. The method of operating a night vision device as set forth in claim 6 wherein said step of using the measured light intensity comprises maintaining an adjustable opening of an iris only as large as required to permit visual perception.

8. The method of operating a night vision device as set forth in claim 7 wherein said step of measuring the intensity of light reaching the user's eye comprises reading the level of light intensity using a sensor positioned in the night vision device proximate to the position at which the user positions the eye for using the night vision device.

9. The method of operating a night vision device as set forth in claim 8 further comprising adjusting the sensor according to the visual acuity of the user.

10. The method of operating a night vision device as set forth in claim 6 further comprising intensifying the light received from an objective lens at an input end of the night vision device.

11. The method of operating a night vision device as set forth in claim 6 wherein measured light intensity can be used to automatically close the iris opening if by opening the iris opening the light intensity measured becomes larger than a maximum amount.