Night vision goggles are one of the simplest devices for adapting to changing light conditions or dark environments. They have been commonly used by law enforcement agencies and armed forces personnel for decades now. These goggles work by using an artificial light source in conjunction with a single vision lens (transparent or opaque) that is placed in front of the user’s eyes. The light source is used to illuminate scenes in front of the wearer, while the transparent lens helps them view things through it.
How Do Night Vision Goggles Work?
The mechanism of night vision goggles is quite simple. They feature a camera unit that detects visible light, infrared light and other electromagnetic energy in its surroundings. The camera unit then converts this light into electrical charges transferred to an image intensifier (or imager) device. The imager amplifies the electrical charges, turning them back into visible light, allowing them to see in dark or dim environments.
The difference between visible light and infrared light is slight. Only the density of infrared light in a given area makes the difference noticeable to the human eye. Indeed, infrared images can be received even though there is little to no visible light in a room. The camera unit of a night vision device picks up any infrared radiation with wavelengths of 700 nanometers or shorter. Those wavelengths are very close to the wavelengths of visible light, and the eye sees them as identical.
The human eye is susceptible to infrared wavelengths in the 400-700 nanometer range. However, any energy above 500 nanometers will not be seen. The imager in night vision goggles receives electrical signals from a photodiode. These signals are then converted into electric charges that are transferred to a tube containing phosphorescent material.
Opto-electronic image enhancement is used to enhance the image produced by the imager. The phosphorescent material is charged with an electric current. The current causes the phosphor to emit light when stimulated by the incoming images. The emitted light has wavelengths that are much longer than visible light but are shorter than infrared light. So adapted night-vision goggles amplify faint sources of infrared radiation into visible light, converting them into images that the normal eye can see.
An interesting point to note is that the intensity of the image is proportional to the amount of energy in a given area. In other words, a larger area will have more light and produce a more robust electrical signal. This, in turn, will produce a brighter image.
Image Intensifiers
Image intensifiers are composed of two main parts: an electron multiplier tube (EMT) and a phosphor screen. The EMT is a part of the imager that converts the electrical charges into light. The EMT amplifies electrical charges by changing them into ultraviolet (UV) light. The UV light is then passed on to phosphor, a substance that emits visible light when electric charges are applied. The phosphor screen amplifies this light and enables the user to see in darkness.
Thermal Imaging
Thermal night vision goggles operate similarly to conventional night vision goggles, but with one difference. Instead of amplifying visible light, they focus on infrared light. Thus, they boost the normally invisible energy to the human eye and produce a thermal image picked up by the camera unit and then transferred to the imager.
Infrared Lights
Some night vision goggles have an infrared illuminator. The purpose of the illuminator is to make sure that the camera can detect more energy in a given area. It also calibrates the amount of energy in a given area, suggesting that it adjusts how bright the image will be.
All infrared lights are invisible to the human eye, but some are easily seen with night vision. Infrared rays have longer wavelengths than visible light and are in the wavelength range of 700 nm to 1,000 nm.
Components of Night Vision Goggles
1 – Ocular lens
The ocular lens helps the user see more clearly when the night vision goggles are being used. When the user looks through the eyes, he sees a magnified image.
2 – Phosphor screen
The phosphor screen is an essential part of night vision. It amplifies the energy that passes through a tube, which then reaches the user’s eyes.
3 – Photomultiplier
The photomultiplier is responsible for receiving electronic charges produced by the camera. The electronic charges are then transformed into a light image, picked up and seen by the user.
4 – Photocathode
The photocathode is a part of the camera unit. It converts the visible light into electrical charges that are transferred to the imager.
5 – Objective lens
The objective lens is a part of the camera unit. It focuses the light energy coming from a given area onto the photocathode.
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Conclusion
Night vision goggles are used to track wildlife. Night hunters use thermal imaging to see their prey’s body heat instead of just relying on blindsight. Owl, deer, and turkey eyes are particularly effective at picking up thermal energy. Military operations use night vision goggles to see people and objects during the darkness of night. Also, pilots, boaters, and passengers on an airplane see clearly during nighttime flights.
Frequently Asked Questions
Q. Why do night vision goggles have an eye relief distance?
A. The eye relief distance is the distance between the ocular lens and the phosphor screen. For a better view, one must maintain a constant distance from the screen. Also, the ocular lens must be properly adjusted for both focal length and focus.
Q. How does thermal night vision work?
A. Thermal night vision works by detecting energy radiating from the object(s). Humans, animals, and vehicles all give off heat (infrared radiation). Thermal vision devices detect this radiation and display it as an image.
Q. Can night vision goggles be used for day vision?
A. Yes, night vision goggles can be used for day vision. However, one has to pick the right kind of goggles depending on the environment. For example, if one needs to see through smoke or fog, then he will need a smoke filter that is specially designed for certain situations.