Extinctiometer Theory

The extinction process can be described by Beer's Law, 

where Io is the incident light intensity, I is the transmitted intensity, X is the path length, and bl is the volumetric extinction coefficient. In principle, the cloud extinctiometer is a fairly simple instrument consisting of a light source which produces a beam of light that falls on a detector. In this case, Io is the light intensity falling on the detector with no cloud particles in the beam and I is the light intensity falling on the detector with particles attenuating the beam. The path length is a system constant. Extinction is the sum of the scattered and absorbed radiation when cloud particles (spherical drops or ice crystals) interact with the light beam. The volumetric extinction coefficient is calculated from the ratio of the attenuated beam intensity to the incident beam intensity.

From Beer's law, optical path length determines the lowest values of extinction that can be measured. Cumulus clouds may have extinction coefficients on the order of 200 - 300 km-1, while thin cirrus clouds may have an extinction coefficient on the order of 0.5 - 5 km-1. The cloud extinctiometer achieves a long optical path in a small volume by using a 26 pass Herriott cell to achieve a total path of approximately 8 m in a very small volume. The instrument mounts in a standard PMS can and is very insensitive to vibration and g-loading. The cloud extinctiometer is a valuable research instrument for cloud radiation studies. 

Center wavelengths and bandwidths for each of the Extinctiometer channels.

Detector Channel Number Center Wavelength
( ± Tolerance) 		Bandwidth (FWHM)
 ( ± Tolerance) 		Notes
Detector #1
Silicon 		Channel 1 450 nm ( ± 14 nm) 70 nm ( ± 14 nm) -
Channel 2 550 nm ( ± 14 nm) 70 nm ( ± 14 nm) -
Channel 3 650 nm ( ± 14 nm) 70 nm ( ± 14 nm) -
Channel 4 750 nm ( ± 14 nm) 70 nm ( ± 14 nm) -
Channel 5 850 nm ( ± 14 nm) 70 nm ( ± 14 nm) -
Channel 6 950 nm ( ± 14 nm) 70 nm ( ± 14 nm) -
Detector #2
1.9mm Extended InGaAs 		Channel 7 1.375 mm ( ± 6 nm) 30 nm ( ± 6 nm) MODIS Channel 26 used for Cirrus Clouds and Water Vapor
Channel 8 1.640mm ( ± 5 nm) 25 nm ( ± 5 nm) MODIS Channel 6 used for Cloud Properties
Detector #3
2.6mm Extended InGaAs 		Channel 9 2.130mm ( ± 10 nm) 50 nm ( ± 10 nm) MODIS Channel 7 used for Cloud Properties
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