As we all know night vision goggles need a light source in order to work, since they are in their core light intensifiers. In the past night vision was very affected by stronger light sources, so much so that you could cause temporary blindness in a person using night vision, just by pointing a flashlight at them.
With the new technology, this danger has been reduced, all modern night vision equipment includes light limiters that prevent strong light sources like moonlight or headlights and room lights from interfering with the image. Thus enabling the user to get the best optimum night vision imagery.
The main purpose of night vision is to sense radiation that is invisible to the human eye. The human vision is limited to a very small portion of the electromagnetic spectrum called visible light. What the night vision dose is enhance the spectral range by using sources of the spectrum that are otherwise not visible. These include near infra-red or ultra violet radiation.
Currently we have three types of night vision, image intensification, active illumination and thermal vision. For the purposes of answering the question proposed by the title of this article we need to focus only on the first two.
So image intensifiers or enhancers actually magnify the amount or photons received from various sources in nature, such as moon or starlight. The reason why the image is intensified is the fact that the output visible light is brighter than the incoming light. The image-enhancement systems are also called night-vision devices or NVDs and they rely on a special tube – image-intensifier tube, to collect and amplify infrared and visible light.
The second night vision option is active illumination. This type combines the above mentioned image intensification with an active source of light usually in the form of near infrared or shortwave infrared. It is important to remember that the amount of energy in a light wave is directly related to its wavelength, which in return means that shorter wavelengths have higher energy. Out of all visible light, violet has the most energy, and red has the least. Now, in addition to the visible light spectrum there is also the infrared spectrum.
The infrared spectrum can be divided into three categories:
- Near-infrared– it is the closest to visible light, with wavelengths that range from 0.7 to 1.3 microns, or 700 billionths to 1,300 billionths of a meter.
- Mid-infrared– with wavelengths ranging from 1.3 to 3 microns. Both near and mid-IR are used by a variety of electronic devices.
- Thermal-infrared– Occupies the largest part of the infrared spectrum, with wavelengths ranging from 3 microns to over 30 microns.
As we can see there are several types of light that affect night vision, some of it is natural light that is visible to the human eye and some is part of a IR spectrum and is not noticeable by human vision. So in order to use night vision goggles it is important to keep in mind the effects of all light sources that are available at that moment.