Although we aren’t able to see without light, there are some animals that can do this. The retina in our eye is responsible for this, and it consists of cones and rods, which are special cells.
Cones detect colored lights and rods can spot movement and things in lower light conditions. Human eyes have 6 million cones and 120 million rods, but other animals including cats, have a different ration of cones and rods which makes their vision abilities different to those of humans. The retinas of a nocturnal creature have more to do. The eyes of a cat for example, have a particular type of mirror – a tapetum. This mirror reflects light twice, giving cats a better chance of seeing in the dark. Despite this, given a total lack of light cats are not able to see.
As humans we are only able to pick out seven colors – violet, indigo, blue, green, yellow, orange and red. Although we can see these parts of light we are not able to see several types of radiation, including Gamma rays, X-rays, Ultraviolet rays, infrared rays or radio waves.
Gamma rays come from radioactive elements, such as radium and cesium. Medical practitioners use X-rays to investigate our bodies. Germs can be destroyed by ultraviolet rays. Operating electrical equipment requires the use of infrared rays. These rays are all part of the electronic spectrum, and although human beings can’t see them, there are some animals that can, for example, the mantis shrimp.
How can humans improve their night vision with a lack of rods and no tapetum? Technology can assist us. To see at night we need the ability to use the lack of light by combining the range of light intensity and the spectrum. We need to use similar principles to those used for spectacles, and magnifiers that increase the focus and the light rays coming from an object. Electricity can be used. As a result of this, we now have night vision equipment that is able to increase the amount of light so we can ‘see in the dark’.
To make night vision goggles you need the following:
- Front lens,
- Photomultiplier, and
- Phosphur screen
This equipment combines to allow night vision through the goggles. The human eye is particularly sensitive to green so this color is used to produce the image the human eye can see.
Despite this technology night vision goggles will not work without a small amount of available light. Other technology uses infrared to produce an image. This is called a thermal image because of the use of heat. For example, if a night vision feature is installed in a digital camera it also uses infrared. The most important uses for night vision goggles is for night time searches, security and studying wildlife at night.