The human eye is capable of seeing in the 500-700 nm spectrum of light with a peak at 550 nm. The night sky’s Peak irradiation can be found in the 600-900 nm range. The 100 nm overlap between what the human eye can see and the peak irradiation of the night sky is evidence of the rods in the human eye at work. This is how the human eye can see at night. However, the human eye only sees approximately 20/400 focal vision at night. Night vision is predominately peripheral vision and focal vision at night is limited due to the anatomy of the eye (zero rods in the fovea).
In order to minimize the human factor and enable him to see through a greater range of the electromagnetic spectrum, Night Vision Goggles (NVG) are utilized. NVG’s present day are either monocular or binocular. The difference being that the binocular style is two monoculars on a common platform. Each monocular or half of the binocular set is called an Image Intensifier Tube.
These Image Intensifier Tubes (I2 tubes) amplify light that is already present. Generation III NVG’s are most sensitive to this spectrum of 600-900 nm. This sensitivity is due to the photocathode in the I2 tube that consists of Gallium Arsenide (GaAs).
At the front of the I2 tube, we see the Objective Lens. This focuses the light onto the photocathode. The photocathode is responsible for transforming the image in the form of photons into electrons for amplification.
Once the image in the form of photons has passed through the photocathode, they are changed into electrons and proceed to the front of the micro-channel plate (MCP). The electrons pass through the MCP which is a nickel size disk approximately the width of two sheets of paper and consists of approximately six million tubes that are tilted at 5.1 degrees. This tilt causes each electron that enters the MCP to hit the walls of each tube 10-11 times. The tubes are made of aluminum oxide.
As the electrons bounce down each tube in the MCP, the electron releases the extra electron on an aluminum oxide atom provided in the tubes lining, thus providing more electrons that exit the MCP. For every electron that enters the MCP, 1025 exit. This is the primary light amplification process.
In order to optimize the output of the I2 tubes, the MCP has a variable voltage power supply (200 volts on the front and 1000 volts on the back of the MCP) that automatically adjusts to provide the optimal amount of electrons passing through in order to provide a consistent picture. The electrons then proceed from the MCP to the phosphor screen which converts the electrons into photons and back into a visible image.
In Gen III NVG’s, the phosphor screen utilizes P43 (type of phosphorus) in order to provide an image that most closely matches the peak sensitivity of the eye (545 nm) and falls in the green region of the spectrum. The green allows for reduced eye fatigue, greater definition of image due to the wide variety of shades of green, and allows the eyes to see the clearest picture. The final step is the new image is focused onto the fovea in the user’s eye by the eye piece.
NVG’s do not turn night into day. Once the process is reviewed, it can be inferred that the NVG’s must have a source of light to amplify and cannot make light of their own. Even though this tools can seem magical in their operation, understanding their operation can help the user optimize their functionality.