Complex sentence: Infrared (IR) point sensors use radiation passing through a known volume of gas; energy from the sensor beam is absorbed at certain wavelengths, depending on the properties of the specific gas.
Split into simpler sentences: Infrared (IR) point sensors use radiation passing through a known volume of gas; energy from the radiation is absorbed at certain wavelengths as it passes through the gas. The range of wavelengths that is absorbed depends on the properties of the specific gas.