AIRS has a set of visible and near infrared (Vis/NIR) photometers in order to observe low-level clouds. Low-level clouds are about the same temperature as the ground. The AIRS spectrometer and AMSU-A depend on temperature differences to differentiate thermal infrared emission from various altitudes. Without these photometers, the AIRS instrument suite would not detect these low-level clouds. The photometers detect the clouds from reflected sunlight.


The Vis/NIR photometer is physically inside the scan head assembly. The Vis/NIR photometer is bore sighted to the IR spectrometer to allow simultaneous visible and infrared scene measurements. The Vis/NIR light goes through four sets of lens/filter pairs to the Vis/NIR detectors. The spectral filters are deposited directly on the detector arrays for Channels 1-3, while Channel 4 has a bandpass filter deposited onto a lens surface. Calibration software algorithms on the ground use comparison with on-board lamps to monitor the stability of the Vis/NIR photometer.

Unlike the field of view of the IR spectrometer channels (defined by a single field stop), the fields of view of the visible channels are defined by the locations of the four arrays in the image planes of the entrance apertures.

schematic drawing of the AIRS Vis/NIR optical system
  • AIRS Vis/NIR photometer system
  • AIRS Vis/NIR photometer system


The four spectral bands observed are

  • Channel 1: 0.40–0.44 μm
  • Channel 2: 0.58–0.68 μm
  • Channel 3: 0.71–0.92 μm
  • Channel 4: 0.49–0.94 μm

Channel 1 was designed to be most sensitive to aerosols. Channels 2 and 3 approximate the response of the Advanced Very High Resolution Radiometer (AVHRR) channels 1 and 2, and are particularly useful for surface studies. Channel 4 has a broadband response for surface energy balance studies.

bandpasses of the four AIRS UV/Vis photometer channels

Spatial Sampling

Each channel has nine pixels along track, each with a 0.185° IFOV. The Vis/NIR spatial resolution is approximately 2.3 km. See Footprints for more information.