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.

Hardware

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.

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Channels

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.

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.