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 ${\textstyle \mathrm {fL} /\mathrm {fc} }$.

System Gain

System gain refers to the gain of the entire night vision device instead of only the image intensifier tube. As the objective lenses and oculars are practically not 100% efficient, some of the light, both incoming and emitted, is absorbed by the glass.

Due to the lens system involved, 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.

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