Night Vision scopes are electro-optical devices that intensify (amplify) available light.
The main component of such a device is the Image Intensifier - basically a vacuum tube.
At the input end, the objective lens collects the particles of light (photons) arriving from the subject and focuses them on the image intensifier tube.
Inside the image intensifier tube a photocathode absorbs these photons and converts them into electrons which are amplified and projected on to a green phosphor screen at the rear.
When this highly intensified electron image strikes the phosphor screen, it causes the screen to emit light that you can see.
Since the phosphor screen emits this light in exactly the same pattern and degrees of intensity as the light that is collected by the objective lens, the bright night-time image you see in the eyepiece corresponds closely to the outside scene you are viewing.
The phosphor screen is coloured green because the human eye can differentiate more shades of green than any other phosphor colour.
The number of times the screen image is brighter than the one arriving at the photocathode is called the gain of the Intensifier.
However, gain is a function of two factors, the ability of the photocathode to convert the weaker photons into electrons, and the amount of amplification which accelerates these electrons onto the screen.
If the type of photocathode is fixed, more gain means more screen brightness, not the ability to see better in the dark.
There are two ways to measure gain - tube gain and system gain.
Tube gain is usually seen in values of tens of thousands and is more a laboratory figure that is not necessarily indicative of a devices performance.
If tube gain is too high, the tube will be “noisier” and the signal-to-noise ratio may go down.
System gain is based on the total system ie. tube, optics, power supply etc.
System gain is usually seen in the low hundreds for Gen 1, mid-hundreds for Gen 1+ and 1000-3000 for Gen 2/2+/SuperGen/HyperGen and Gen 3.
System gain is the figure that potential night vision purchasers should compare.