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.

The gain of an image intensifier is not constant, but increases as the light level decreases.

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

To further test auto brightness control, the same procedure is usually repeated with 2 x 10^-4 fc.

Metric

The metric units (also referred to as SI or CGS units) used to specify gain are candela per square-meter per lux (cd/m²/lx).

Most notable for using metric units in all specifications is the European image intensifier manufacturer Photonis Technologies SAS and NNVT.

Measurement

The only notable difference to measurement in the imperial system is the intensity of the input light at 20 μlx (micro lux) (2 x 10^-5 lx), an equivalent of 1.858 x 10^-6 fc, which is slightly less than the imperial standard of 2 x 10^-6 fc.

Conversion

As the gain is measured at different light intensities for metric and imperial units, an accurate conversion is not possible. However, for an approximate conversion the following formula can be used.

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

So, a gain of 10,000 cd/m²/lx is equivalent to roughly 31,416 fL/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 feature 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 (fL / fL), and in the metric system using lux per lux (lx / lx).

Both in imperial and metric, the system gain of a typical night vision device can be estimated using the following formula.

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