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Laboratory for Atmospheric and Space Physics


The TSIS Spectral Irradiance Monitor (SIM) measures spectral solar irradiance (SSI) incident on a plane surface at the top of the atmosphere that is normal to the Sun two times per day from 200 to 2400 nm at variable (~1- 35 nm) spectral resolution. The integrated irradiance contribution measured by SIM  is approximately 97% of the total solar irradiance measured by the TIM instrument. The SIM is a follow-on to the SORCE SIM instrument with improvements in absolute accuracy (0.2% over the full spectrum), long-term stability (to better than 0.05% per year), and relative precision (0.01%) (see Section 2.4.3). These improvements are key requirements to understanding the contributions from varying spectral irradiance on climate. Long-term relative accuracy is maintained by duty cycling three independent spectrometers and directly measuring the prism transmission for each.

Figure: SIM integrated power versus wavelength


Table below lists the TSIS SIM instrument properties and the next figure provides an expanded view of the TSIS SIM instrument. The TSIS SIM measurement requirements derived from analysis of the variability in spectral irradiance in the visible and infrared measured by the SORCE SIM instrument and incorporating the needs of the atmospheric and solar physics modeling communities are listed in Table 6. The SIM instrument error budget allocation is provided in Table 28.

SIM instrument properties

      Parameter               Value

  Wavelength Range            200-2400 nm
  Resolution                  0.25-41 nm
  Sun Viewing f/#             f/115
  Focal length                400 mm
  Prism Vertex Angle          34.3°
  Front Surface Radius        421.5 mm
  Back Surface Radius         441.3 mm (aluminized)
  Prism Transmission          65-85% over wavelength range
  Exit Slit Sizes             7.5 x 0.3 mm (7.5 x 0.34 mm for UV detector)
  Entrance Slit               6.5 x 0.3 mm
  Scan Range in Focal Plane   78 mm (78 mm/60,000 subpixels=1.3μm/subpixel)
  Shutter Frequency           0.01 – 0.05 Hz
  Diffraction Correction      0.51-8.2%


Like the TIM, the primary detector for the SIM is an electrical substitution radiometer (ESR), which is essentially a bolometer where optical power is measured through heating instead of a photon to electron conversion. In traditional ESR designs, like TIM, the light collection is performed by a blackened cavity where light that is not absorbed on the first bounce is very likely to hit the cavity wall again resulting in around 99.99% light collection. However, for SIM, a diamond strip with NiP black (Section 3.2.5) on one side is used instead because the light levels are so low (dispersion from prism) and a cavity would have entailed too much thermal mass. To increase collection efficiency, a reflective re-imaging hemisphere of polished aluminum with a Magnesium Fluoride (Mg F2) coating sits on top of the diamond strip. Light not absorbed on the first bounce then reflects off the hemisphere and is for the most part reimaged back onto the diamond strip.

When the shutter is open and the cavity illuminated with sunlight, reduced electrical heater power is needed to maintain the active ESR’s temperature. This measured reduction in electrical heater power combined with calibrations of the absorptance of the diamond strip bolometer provides a measure of the entering radiant solar power. By accounting for the area of the exit slit (which defines the entrance aperture for the bolometer) and the prism rotation angle, the area and wavelength over which sunlight is collected is determined and when combined with the incident radiant power yields SSI (W m-2 nm-1) in ground processing. Phase sensitive detection is used to reduce noise in postprocessing.



For normal operation, the opening and closing of a channel’s shutter with a 100 second period modulates the sunlight entering the entrance slit as an f/115 beam directed to the Féry prism located 400 mm from the slit. Since the solar spectral irradiance varies dramatically over the SIM spectral range (see Figure 11), precise stability and knowledge of the prism position is required to resolve the solar structure at the SIM resolution. With precise knowledge of the prism rotation angle (obtained through a closed-loop controller that uses a CCD as a focal plane position encoder), the SIM provides an accurate measure of the index of refraction. There is a known and highly precise relationship between the index of refraction and the wavelength of incident light, including accounting for nonlinear temperature dependencies, which ultimately provides the knowledge of spectral solar irradiance to a relative wavelength error of < 150 ppm.