Spectral Irradiance Monitor (SIM)
The SIM instrument will take contiguous spectral readings of the near UV, visible, and near infrared portions of the solar spectrum, from 200 to 2000 nm, which includes the peak of the solar spectrum and together add up to more than 90% of the Total Solar Irradiance. The SIM instrument consists of two solar spectrometers set side by side within one casing. Only one of the spectrometers will be used to take measurements on a daily basis. Sunlight entering this instrument is directed into a prism which then directs different wavelengths of ultraviolet, visible, and near infrared into separate directions. The separate wavelengths of light will then illuminate an array of photodiodes. The photodiodes measure the specific wavelengths of light between low energy ultraviolet (200 nm) radiation and near infrared (2000 nm) radiation. (nm stands for nanometer, which is one one-billionth of a meter.) SIM will measure these wavelengths in intervals that vary in width from 0.25 nm in the ultraviolet to 34 nm in the near infrared. Scientists will be recording measured energy in separate bands of ultraviolet light, violet light, blue light, yellow light, green light, red light, and near infrared light coming from the Sun.

The Spectral Irradiance Monitor (SIM) will measure the solar spectrum in ultraviolet, visible, and near infrared wavelengths. (Image courtesy Solar Radiation and Climate Experiment Project)

SIM will be the first instrument in orbit to take readings of the full spectrum of visible and near-infrared solar radiation. By reviewing data from SIM, scientists may be able to tell how the solar cycles affect both visible and near-infrared wavelengths. Combined with improved measurements by ground and by aircraft, they may be able to discern just how much of this light goes into heating up the lower layers of the Earth’s atmosphere and how much goes into the land and oceans. SIM may also aid in efforts to discern exactly how much of the Sun’s energy is reflected by industrial aerosols and clouds.

A problem with prism spectrometers is that the glass in the prism can degrade over time. To account for this, the scientists will monitor how well the prism transmits light. Light exiting the first prism will be sent through a slit to create a monochrome (single wavelength) light source. The wavelength is selectable by adjusting the position of the slit. This light source is then directed through a periscope into the second spectrometer. The light is measured before and after it goes into the second prism. If the ratio between the “before” and “after” measurement changes, the scientists will know if the glass is degraded and how this degradation affects the transmission of the light at each selected wavelength. They can then take this information into account when they calibrate the data. To test the entire apparatus, the scientists will open both of the spectrometers simultaneously and compare the data between the two.

next: Solar Stellar Comparison Experiment (SOLSTICE)
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Solar Radiation and Climate Experiment (SORCE)
Introduction
Earth’s Energy Balance
Solar Variability
The Sun and Global Warming
Uncertainties in Solar Measurements

The SORCE Satellite
Total Irradiance Monitor (TIM)
Spectral Irradiance Monitor (SIM)
Solar Stellar Comparison Experiment (SOLSTICE)
Extreme Ultraviolet Photometer System (XPS)

Related Articles
SOLSTICE
Watching the Sun
ACRIMSAT
Sunspots and the Solar Max
Clouds and Radiation
Why isn’t Earth Hot as an Oven?

Related Datasets
Reflected Solar Radiation
Outgoing Heat Radiation