Video signal mixing device for infrared/visible integrated imaging

1. A video signal mixing device for superimposing a visible video signal output from a visible camera and an infrared video signal output from an infrared camera, both said cameras being focused on a common object, said video signal mixing device comprising:

a black level discriminating circuit for discriminating a first part of said infrared video signal lower than a predetermined black level from a second part of said infrared video signal higher than said predetermined black level;

a visible video selecting circuit for enabling output of said visible video signal during the presence of said first part of said infrared video signal lower than said predetermined black level, and selectively disabling output of said visible video signal during the presence of said second part of said infrared video signal higher than said predetermined black level; and

a mixing circuit for superimposing said visible video signal, output from said visible video selecting circuit, and said infrared video signal.

2. A video signal mixing device as recited in claim 1, wherein said visible video signal is output from said visible camera in synchronization with said infrared video signal output from said infrared camera.

3. A video signal mixing device as recited in claim 1, wherein said black level discriminating circuit issues a binary signal comprising a first signal indicating presence of said infrared video signal having a level higher than said predetermined black level, and a second signal indicating presence of said infrared video signal having a level lower than said predetermined black level.

4. A video signal mixing device as recited in claim 3, wherein said visible video signal selecting circuit disables output of said visible video signal during presence of said first signal of said binary signal.

5. A video signal mixing device as recited in claim 1, further comprising:

an infrared video signal level adjusting circuit for receiving said infrared video signal from said infrared camera and adjusting the level of said infrared video signal to a level within a first range between the predetermined black level and a threshold level where brightness of a first image resulting from said infrared video signal is at maximum; and

a visible video signal level adjusting circuit for receiving said visible video signal from said visible camera and adjusting the level of said visible video signal to a level within a second range between the predetermined black level and a level where brightness of a second image resulting from said visible video signal is at maximum, said infrared video signal level and said visible video signal level are capable of independent adjustment.

6. A video signal mixing device as recited in claim 5, further comprising:

a low-pass filter, operatively connected to said infrared camera and to said black level discriminating circuit, for filtering said infrared signal from said infrared camera to eliminate electrical noises contained in said infrared video signal.

7. A video signal mixing device as recited in claim 1, further comprising:

a low-pass filter, operatively connected to said infrared camera and to said black level discriminating circuit, for filtering said infrared signal from said infrared camera to eliminate electrical noises contained in said infrared video signal.

8. A video signal mixing method for superimposing a visible video signal output from a visible camera and an infrared video signal output from an infrared camera, both of the cameras viewing a common object, said video signal mixing method comprising the steps of:

(a) discriminating a first part of the infrared video signal lower than a predetermined black level from a second part of the infrared video signal higher than the predetermined black level;

(b) enabling output of the visible video signal during the existance of said first part of the infrared video signal lower than the predetermined black level;

(c) disabling output of the visible video signal during the presence of the second part of the infrared video signal higher than the predetermined black level; and

(d) superimposing the visible video signal when output enabled and the infrared video signal.

9. A method as recited in claim 8, further comprising the step of:

(e) outputting the visible video signal in synchronization with the infrared video signal.

10. A method as recited in claim 8, wherein step (a) further comprises the steps of:

(e) issuing a binary signal comprising a first signal indicating the infrared video signal having a level higher than the predetermined black level, and a second signal indicating the infrared video signal having a level lower than the predetermined black level.

11. A method as recited in claim 10, wherein step (c) disables output of the visible video signal when said first signal indicates the infrared video signal has a level higher than the predetermined black level.

12. A method as recited in claim 8, further comprising the steps of:

(e) receiving the infrared video signal from the infrared camera;

(f) adjusting the infrared video signal to a level within a first range between the predetermined black level and a level where brightness of a first image resulting from the infrared video signal is at maximum;

(g) receiving the visible video signal from the visible camera; and

(h) adjusting the level of the visible video signal to a level within a second range between the predetermined black level and a level where brightness of a second image resulting from said visible video signal is at maximum.

13. A method as recited in claim 12, further comprising the step of:

(i) filtering out electrical noise contained in the infrared video signal received from the infrared camera.