Night vision devices have become increasingly popular in recent years, with more affordable consumer models coming onto the market. One of the most recognizable features of night vision is the green tinted imagery it produces. This has led many people to wonder, if night vision works by detecting infrared light, why does it look green instead of red?
How Night Vision Works
Night vision devices work by detecting infrared radiation, which is light with longer wavelengths just beyond the visible red portion of the electromagnetic spectrum. Everything emits some level of infrared all the time, even objects that appear cool or cold to our eyes. Night vision takes advantage of this by using a photocathode tube that is sensitive to infrared wavelengths.
The photocathode tube converts the infrared photons it receives into electrons. These electrons are then amplified to boost the signal. Finally, the electrons hit a phosphorous screen, which emits visible light. It is this final visible light image that we see through night vision goggles.
Why Green Not Red?
When electrons hit the phosphor screen, it produces light across the visible color spectrum, not just in the infrared and red portion. So why do we just see green and not the full rainbow?
There are two main reasons:
- The phosphors used in most night vision goggles emit a broadband green glow when excited by electrons.
- The human eye is most sensitive to green light. We perceive green light far brighter than red light of the same intensity.
By tuning night vision phosphors to emit green, and our eyes heightened sensitivity to green, the images appear in a green monochrome. This also enables a clearer and brighter image compared to if it were red.
Comparison of Red vs Green Sensitivity
| Color | Wavelength (nm) | Relative Sensitivity |
|---|---|---|
| Red | 700 | 0.7 |
| Green | 550 | 1.0 |
As seen in the table, at equal power our eyes perceive green light of 550nm wavelength to be significantly brighter than 700nm red light. This further enhances the visibility of the green-emitting night vision compared to hypothetical red-emitting night vision.
Military vs Consumer Night Vision Colors
While standard night vision goggles usually have a greenish hue, there are some variations between military and consumer models:
- Military – Tend to be a purer green and better optimized for night vision use. This provides maximum clarity and detail.
- Consumer – More affordable models often have a yellow-green tint. This is from different phosphor materials that produce a wider, less optimized range of visible wavelengths.
However, the overall greenish tone remains the ideal for night vision regardless of variations. More advanced consumer models continue to emulate the military-grade greens.
Can Night Vision be Red?
While not standard, it is possible to produce night vision goggles that display a red image rather than green. This involves a few modifications:
- Use a custom phosphor screen optimized to emit longer red wavelengths when excited by electrons.
- Add an optical filter on the goggle output to block all non-red colors.
- Boost gain/brightness to compensate for the eye’s weaker red sensitivity.
However, such red night vision models are rare. The loss of detail and visibility compared to green makes this an inferior design for most purposes. But the red night vision effect remains useful for certain specialty applications, training simulations, or replicating older night vision technologies.
Color vs Black & White Night Vision
Night vision is most commonly single color green or monochrome green-scale imaging. However, technology advances are improving color night vision capabilities.
Color night vision has typically been limited to large military systems and vehicles. But newer designs are shrinking multi-band imaging down to goggle sizes. This enables seeing night scenes in actual color, rather than green monochrome.
Color night vision has many advantages:
- Easier to interpret surroundings from familiar color cues
- Spot camouflage and threats like a human eye does
- Distinguish light sources, objects, and terrain
However, single color green night vision remains popular for good reasons. It uses a simpler single gain image intensifier tube, providing:
- Lower costs compared to multi-band color night vision
- Lighter weight goggles
- Wider fields of view
- Clearer image quality
For most civilian uses, monochrome night vision is still adequate. But color models continue improving and coming down in price for scenarios that justify the benefits.
Night Vision Alternatives
While night vision goggles are the most well-known way to see in low light, there are other alternatives. These include:
- Thermal imaging – Detects infrared heat signatures rather than light. It does not require any ambient light to operate.
- Low-light cameras – Have larger light gathering sensors and lenses to boost signals in darkness.
- Ultrasonic imaging – Uses reflected ultrasonic echoes to construct visual images.
- Laser scanning – Uses scanning lasers to probe surroundings and reconstruct images.
Each technology has pros and cons depending on the application. But standard night vision remains highly effective and affordable for many low light uses. The iconic green glow makes night vision instantly recognizable as well.
Conclusion
The green color of night vision might seem counterintuitive at first given its detection of infrared light. However, it is the result of calibrated phosphor screens and the human eye’s peak sensitivity to green wavelengths. This synergistic effect produces the characteristic green tint that provides optimal nighttime viewing capability. While other colors and technologies exist, the green hue of night vision will likely remain its signature look for the foreseeable future.