Unlike nocturnal animals which can efficiently see in darkness human beings are not able to see objects in dark clearly. This is because of difference in number of rods in the eyes of them and also lacking a light reflecting layer tapetum lucidum behind the retina of eyes. Different types of night vision devices were developed to meet the night vision needs in the fields of military, wildlife, hidden objects identification and many more. The devices are scopes, cameras and goggles. Night vision goggles are becoming increasingly useful these days because of developmental growth in the particular fields and improving technology is providing with advanced models of night vision devices. Among all, widely used ones are night vision scope binocular which are hand held devices with two eye pieces and can be worn on head.
NVG – working
Night vision devices helps us to see objects in dark clearly by using the low intensity light and making it much brighter. But how is this done?
A night vision device comprises of a photo cathode, photo multiplier and a phosphor screen. Other parts of which it is made up are front lens, micro channel plate, high voltage power supply, eye piece and image intensifier. A less intensity light whether it is moon light or any small amount of light in surrounding environment enters the lens. Particles of light (photons) entered the object hits the photocathode surface and electrons are generated. The photomultiplier amplifies the number of electrons resulting in production of more electrons. These electrons are made to hit the phosphor screen where the original outside image is made brighter and thus one can clearly see the image in dark clearly through night vision scope binocular.
The technology administered in these devices is primarily of two kinds. One is image intensification which works on using little amount of light as described before. The other is thermal imaging which uses heat where there is no enough of moon light. During night time, objects releases heat and surface of the earth releases infrared radiation (IR) which has higher wavelength than visible range. The device captures the image using heat radiation and processes inside it converting it into visible light. The light particles are made to produce electrons, amplifies the electrons and finally giving out the image on screen.
Why images on night vision devices look green coloured? When the electrons of high intensity fall on the device screen the image obtained becomes black and white. As NVGs are employed in fields were they use for long time, the phosphor screen makes it green coloured image so that our eyes can look through the binoculars for longer periods without straining the eyes same as in the case of green plants we feel relaxed when we look at them.