What is light is being detect in night vision camera?

The primary type of light being detected by most night vision security cameras in low-light conditions is infrared (IR) light.

Night vision security cameras typically use Infrared Light-Emitting Diodes (IR LEDs), which are the source of the light you're asking about:

Illumination: The camera has a ring of small IR LEDs (often visible as the "red dots") around the lens. When the ambient light drops low enough, the camera turns on these LEDs, which emit an invisible beam of infrared light to illuminate the scene.

Detection: The light reflects off objects in the area and is captured by the camera's image sensor (typically CMOS or CCD). These sensors are sensitive to the infrared spectrum, allowing them to "see" the illuminated scene even though you cannot.

Image Formation: The camera processes this captured IR data to create a visible image, which is usually displayed in black and white (or sometimes with a slight green tint) because it's only using the single-spectrum IR light, not the full spectrum of visible color.

The "Red Dot" on the Camera

The faint red glow you see on some security cameras at night is coming from the Infrared LEDs.

Near-Visible Spectrum: While most of the light emitted is in the infrared spectrum (which is invisible), some IR LEDs operate at a wavelength close to the edge of the visible light spectrum (which is red).

Accidental Visibility: This small amount of light bleed into the visible red range is what causes the faint, reddish dots or glow that a person can sometimes see around the camera lens. It's essentially an unavoidable byproduct of the technology used in those specific IR emitters.

Purpose: The red dots are not status lights (though some cameras may have a separate status light); they are the active infrared illuminators providing the light source for the camera to see in the dark.