The Spectral Irradiance Monitor: Scientific Requirements, Instrument Design, and Operation Modes

  • Jerald Harder
  • George Lawrence
  • Juan Fontenla
  • Gary Rottman
  • Thomas Woods


The Spectral Irradiance Monitor (SIM) is a dual Fèry prism spectrometer that employs 5 detectors per spectrometer channel to cover the wavelength range from 200 to 2700 nm. This instrument is used to monitor solar spectral variability throughout this wavelength region. Two identical, mirror-image, channels are used for redundancy and in-flight measurement of prism degradation. The primary detector for this instrument is an electrical substitution radiometer (ESR) designed to measure power levels ∼1000 times smaller than other radiometers used to measure TSI. The four complementary focal plane photodiodes are used in a fast-scan mode to acquire the solar spectrum, and the ESR calibrates their radiant sensitivity. Wavelength control is achieved by using a closed loop servo system that employs a linear charge coupled device (CCD) in the focal plane. This achieves 0.67 arcsec control of the prism rotation angle; this is equivalent to a wavelength positioning error of δλ/λ = 150 parts per million (ppm). This paper will describe the scientific measurement requirements used for instrument design and implementation, instrument performance, and the in-flight instrument operation modes.


Focal Plane Charge Couple Device Digital Signal Processor Total Solar Irradiance Entrance Slit 
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Copyright information

© Springer 2005

Authors and Affiliations

  • Jerald Harder
    • 1
  • George Lawrence
    • 1
  • Juan Fontenla
    • 1
  • Gary Rottman
    • 1
  • Thomas Woods
    • 1
  1. 1.Laboratory for Atmospheric and Space PhysicsUniversity of ColoradoBoulderUSA

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