Gain
Gain, which stands for brightness gain, luminous gain or luminance gain, is the measurement of the increase in brightness between the input and the output of an image intensifier tube.
Specification
There are two different units used to specify gain, imperial and metric.
Imperial
The imperial units used to specify gain are foot-lambert per foot-candle (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{fL} / \mathrm{fc}} ).
They are used by all manufacturers and retailers as well as government organizations in the United States. However, the popularity extends beyond the United States due to the significant influence of the country in the night vision industry worldwide.
Measurement
To measure gain in accordance to the MX-10160/AVS-6 specification, the photocathode is exposed to Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle 2 * 10^{-6} \mathrm{fc}} of input light. A photometer is then used to measure the luminance (brightness) of the phosphor screen (in Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{fL}} ). The resulting value is divided by Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle 2 * 10^{-6}} to obtain the gain in Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{fL} / \mathrm{fc}} .[1]
To further test auto brightness control, the same procedure is usually repeated with Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle 2 * 10^{-4} \mathrm{fc}} .
Metric
The metric units used to specify gain are candela per square-meter per lux (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{cd} / \mathrm{m}^2 / \mathrm{lx}} ).
Most notable for using metric units in all specifications is the European image intensifier manufacturer Photonis Technologies SAS.
Conversion
To convert between metric and imperial units the following formula can be used.
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathrm{G}_{\mathrm{fL} / \mathrm{fc}} = \mathrm{G}_{\mathrm{cd} / \mathrm{m}^2 / \mathrm{lx}} * \pi}
So, a gain of 10,000 Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{cd} / \mathrm{m}^2 / \mathrm{lx}} is equivalent to roughly 31,416 Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{fL} / \mathrm{fc}} .
Autogain / Automatic Brightness Control (ABC)
All modern image intensifier tubes are equipped with automatic brightness control (ABC), also known as autogain. This feature automatically down-adjusts the gain of the image intensifier in brighter environments to ensure a consistent viewing experience for the user. It is unclear, however, whether ABC also helps to protect the image intensifier from damage, such as burn-in, in very bright environments.
Variable Gain / Manual Gain / External Gain Adjustment Circuit (EGAC)
The external gain adjustment circuit (EGAC), commonly known as manual gain, is a feature that enables users to adjust the gain of an image intensifier using a potentiometer built into the housing. MX-11769 is the most popular tube format to feature manual gain, as well as the gain contact subformat of the MX-10160 format.
It is important to note that even though tubes equipped with manual gain offer users some control, they also always come equipped with autogain, as these two features are not mutually exclusive.
System Gain
System gain refers to the gain of the entire night vision device instead of only the image intensifier tube, including all lens systems.
As such, system gain does not measure the luminance of the phosphor screen, but the brightness of the light projected by the ocular. As such, in the imperial system, it is specified using foot-lambert per foot-lambert (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{fL} / \mathrm{fL}} ), and in the metric system using lux per lux (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\textstyle \mathrm{lx} / \mathrm{lx}} ).
Both in imperial and metric, the system gain of a typical night vision device can be estimated using the following formula.[2]
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle G_{\mathrm{System}} \approx \frac{G_{\mathrm{Tube}}}{10}}
