= FODIS and ILS =

The aircraft has a downwelling light sensor, with a cosine collector mounted on the roof bringing light via a fiber optic cable to the instruments. Both CASI and Eagle have systems for measuring this light
 * ILS. The NERC ARSF CASI System incorporates an Incident Light Sensor (ILS) measuring downwelling radiation at the aircraft; this is Lumogen coated for enhancement of the blue response below 450 nm. 
 * FODIS. The Eagle instrument incorporates a downwelling radiation sensor, similar to the CASI ILS, called the Fiber-Optic Downwelling Irradiance Sensor (FODIS). 

== Physical layout ==

DCALM's downwelling light collector (prior to cleaning!) mounted on the roof:

[[Image(wiki:Processing/FODIS:dcalm_fodis_collector.jpg)]]

Side view of DCALM, showing location of downwelling light collector:

[[Image(wiki:Processing/FODIS:dcalm_side_view.jpg)]]