The word ‘Night vision’ itself means the ability to see in low light conditions. Humans have poor night vision compared to many other animals.So we all might have a question in our mind that is this really possible to see in the dark night? The answer is most definitely yes. With the proper night-vision equipment, you can see a person standing over 200 yards (183 m) away on a moonless, cloudy night!. Originally developed for military use, it has provided the United States with a strategic military advantage, the value of which can be measured in lives. Federal and state agencies now routinely utilize the technology for site security, surveillance as well as search and rescue. Night vision equipment has evolved from bulky optical instruments in light weight goggles through the advancement of image intensification technology.

Two technologies are used for night vision:-

(1)Thermal Imaging

This work by collection the tiny amounts of light including the lower portion of infrared light spectrum that are present but may be imperceptible to your eyes, and amplifying it to the point that we can easily observe the image.

(2)Image Enhancement

This technology operates by capturing the upper portion of the infrared light spectrum, which is emitted as heat by the objects instead of simply reflected as light. Hotter object, such as warm bodies, emit more of this light than cooler objects likes trees or buildings.

2. Types of Night Vision

There are two type night vision such as

2.1. Biological Night Vision

 In biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape as light is absorbed by them. The peak rhodopsin build-up time for optimal night vision in humans is 30 minutes, but most of the adaptation occurs within the first five or ten minutes in the dark. Rhodopsin in the human rods is insensitive to the longer red wavelengths of light, so many people use red light to preserve night vision as it will not deplete the eye's rhodopsin stores in the rods and instead is viewed by the cones.

.Some animals, such as cats, dogs, and deer, have a structure called tapetumlucidum in the back of the eye that reflects light back towards the retina, increasing the amount of light it captures. In humans, only 10% of the light that enters the eye falls on photosensitive parts of the retina. Their ability to see in low light levels may be similar to what humans see when using first or perhaps second generation image intensifiers

2.2. Technical Night Vision

A night vision device (NVD) is an optical instrument that allows images to be produced in levels of light approaching total darkness. They are most often used by military and law enforce agencies but are available to civilian users. In this technology various types of instruments are used.Details descriptions of this technology is described in next page.

3. Working of Technical Night Vision

Night vision can works in two very different ways depending on the technology used. Before discussing about the types of night vision technology first known basic things of light.

3.1. Light Basics

In order to understand thermal imaging, it is important to understand something about light. The amount of energy in a light wave is related to its wavelength: Shorter wavelengths have higher energy. Of visible light, violet has the most energy, and red has the least. Just next to the visible light spectrum is the infrared spectrum.

Infrared light can be split into three categories:-

3.1.1. Near-infrared (near-IR)

Closest to visible light, near-IR has wavelengths that range from 0.7 to 1.3 microns, or 700 billionths to 1,300 billionths of a meter.

3.1.2. Mid-infrared (mid-IR)

 Mid-IR has wavelengths ranging from 1.3 to 3 microns. Both near-IR and mid-IR are used by a variety of electronic devices, including remote controls.

3.1.3. Thermal-infrared (thermal-IR)

 Occupying the largest part of the infrared spectrum, thermal-IR has wavelengths ranging from 3 microns to over 30 microns.The key difference between thermal-IR and the other two is that thermal-IR is emitted by an object instead of reflected off it. Infrared light is emitted by an object because of what is happening at the atomic level.

Night vision can work in two very different ways, depending on the technology used.

3.2. Thermal Imaging

A special lens focuses the infrared light emitted by all of the objects in view. The focused light is scanned by a phased array of infrared-detector elements. The detector elements create a very detailed temperature pattern called a thermogram..