No, people cannot see 850nm light because it falls outside the range of the human visible spectrum. This specific wavelength belongs to the infrared (IR) portion of the electromagnetic spectrum, which is imperceptible to the naked human eye.
Understanding the Human Visible Spectrum
The ability of humans to perceive light is limited to a very specific range of wavelengths within the vast electromagnetic spectrum. This range, commonly known as the visible spectrum, spans approximately from 400 nanometers (nm) to 700 nm.
- Wavelengths shorter than 400 nm are ultraviolet (UV) light, which is invisible.
- Wavelengths longer than 700 nm are infrared (IR) light, which is also invisible.
| Wavelength (nm) | Type of Light | Human Visibility | Common Color (if visible) |
|---|---|---|---|
| < 400 | Ultraviolet | No | N/A |
| 400 - 450 | Visible | Yes | Violet |
| 450 - 495 | Visible | Yes | Blue |
| 495 - 570 | Visible | Yes | Green |
| 570 - 590 | Visible | Yes | Yellow |
| 590 - 620 | Visible | Yes | Orange |
| 620 - 700 | Visible | Yes | Red |
| > 700 | Infrared | No | N/A |
| 850 | Infrared | No | N/A |
Why 850nm Light is Invisible to Humans
Light at 850nm is classified as near-infrared (NIR). Just like other infrared wavelengths such as 940nm, it is invisible to the human eye. This means that 850nm infrared light does not illuminate a scene in the dark for our eyes, unlike how a regular flashlight producing visible light would. Our eyes lack the specialized photoreceptors (rods and cones) capable of detecting these longer wavelengths.
While 850nm light is unseen, some individuals with particularly sensitive eyes, or in cases of extremely high intensity, might perceive a very faint, dull red glow. However, this is not true "seeing" of the light itself in the way we perceive visible colors, but rather a potential secondary effect or perception of the light's interaction with the eye. For practical purposes, 850nm light is considered invisible.
Practical Applications of 850nm Infrared Light
Despite its invisibility to humans, 850nm infrared light plays a crucial role in various technologies due to its specific properties. Its inability to be seen makes it ideal for covert illumination and communication.
Common applications include:
- Security and Surveillance Cameras: Many night vision cameras use 850nm IR illuminators to "see" in the dark. The cameras' sensors are sensitive to infrared light, allowing them to capture images even when there is no visible light.
- Night Vision Devices: Devices used by military and outdoor enthusiasts often incorporate 850nm IR to enhance visibility in low-light conditions.
- Remote Controls: The IR emitters in most TV remotes and other electronic device controllers operate within the infrared spectrum, often around 940nm, though similar to 850nm in principle of invisibility.
- Biometric Systems: Facial recognition and iris scanning technologies sometimes utilize IR light, including 850nm, for improved accuracy and consistency regardless of ambient lighting.
- Medical Imaging: Certain medical diagnostic tools use near-infrared light for non-invasive imaging due to its ability to penetrate tissues more effectively than visible light.
For instance, if you point a security camera's built-in IR illuminator at a wall in a dark room, you will not see the light with your eyes, but the camera will clearly show an illuminated scene. This demonstrates the "invisible" nature of 850nm light to human vision.
