Page "NES Zapper" Paragraph 6

When the trigger on the Zapper is pressed, the game causes the entire screen to become black for one frame.

Then, on the next frame, the target area is drawn in all white as the rest of the screen remains black.

The Zapper detects this change from low light to bright light, as well as at which screen position the change was detected.

This is how the game knows which target has been hit.

Because of the way the Zapper works, it can only be used on CRT displays.

It will not work on LCDs, plasma displays or other flat panel displays.

A known glitch about the zapper is you can get a perfect hit score every time in some poorly programmed games by simply pointing the gun right next to and into a light bulb.

After all target areas have been illuminated, the game returns to drawing graphics as usual.

The whole process is almost imperceptible to the human eye, although one can notice a slight " flashing " of the image.