Gain

Gain, also called brightness 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 (${\textstyle \mathrm {fL} /\mathrm {fc} }$).

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 ${\textstyle 2*10^{-6}\mathrm {fc} }$ of input light. A photometer is then used to measure the luminance (brightness) of the phosphor screen (in ${\textstyle \mathrm {fL} }$). The resulting value is divided by ${\textstyle 2*10^{-6}}$ to obtain the gain in ${\textstyle \mathrm {fL} /\mathrm {fc} }$.

To further test auto brightness control, the same procedure is usually repeated with ${\textstyle 2*10^{-4}\mathrm {fc} }$.

Metric

The metric units used to specify gain are candela per square-meter per lux (${\textstyle \mathrm {cd} /\mathrm {m} ^{2}/\mathrm {lx} }$).

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.

$$\mathrm {G} _{\mathrm {fL} /\mathrm {fc} }=\mathrm {G} _{\mathrm {cd} /\mathrm {m} ^{2}/\mathrm {lx} }*\pi$$

So, a gain of 10,000 ${\textstyle \mathrm {cd} /\mathrm {m} ^{2}/\mathrm {lx} }$ is equivalent to roughly 31,416 ${\textstyle \mathrm {fL} /\mathrm {fc} }$.

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), often referred to as manual gain, is a feature that allows the user to control the gain of an image intensifier with a potentiometer built into the housing. MX-11769 is the most popular tube format to feature manual gain, as well as a subtype of the MX-10160 format used in the AN/PVS-31.

Note that even tubes equipped with manual gain are also equipped with autogain, as the 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 (${\textstyle \mathrm {fL} /\mathrm {fL} }$), and in the metric system using lux per lux (${\textstyle \mathrm {lx} /\mathrm {lx} }$).

Both in imperial and metric, the system gain can be estimated using the following formula.

$$G_{\mathrm {System} }\approx {\frac {G_{\mathrm {Tube} }}{10}}$$