Hyperspectral Image Projector (HIP)

The Hyperspectral Image Projector (HIP) is intended for radiometric testing of instruments ranging from complex hyperspectral or multispectral imagers to simple filter radiometers. Based on the same digital mirror arrays used in commercial digital light processing (DLP) displays, HIP is capable of projecting any combination of many different arbitrarily programmable basis spectra into each pixel of the unit under test (UUT). The resulting spectral and spatial content of the image entering the UUT can simulate, at typical video frame rates and integration times, realistic scenes to which the UUT will be exposed during use, and its spectral radiance can be calibrated with a spectroradiometer. Use of such generated scenes in a controlled laboratory setting would alleviate expensive field testing, allow better separation of environmental effects from instrument effects, and enable system-level performance testing and validation of space-flight instruments prior to launch.

Spectral Engine

For the prototype spectral engine, a computer-interfaced mirror array, called a Digital Micromirror Device (DMD), having 1024 columns and 768 rows, has been used. It is illuminated with broadband spatially uniform light as shown in figure below. The tiny mirrors that make up the mirror array are on a 13.6 µm pitch. When powered, each aluminum mirror can be set to be either "on," reflecting light to the projection optics, or "off," reflecting light to a beam dump. Switching times are such that binary images can be updated at a frequency on the order of 5000 Hz.

A spectrally dispersive prism (or diffraction grating) maps each column of the mirror array to the exit slit, such that each column corresponds to a particular wavelength. That is, the exit slit of the traditional spectrograph is replaced by the mirror array, and each column of the mirror array corresponds to an exit slit that has a one-to-one correspondence with wavelength. At any instant in time, the number of elements turned on in a given column determines the relative spectral radiance at the wavelength corresponding to that column. Thus, the spectrum can be programmed simply by writing a binary image to the mirror array. The resulting spectral radiance is alternately projected into the instrument under test and a reference radiometer, enabling the calibration of the instrument under test to be tested with controlled arbitrary spectra. We have made visible and infrared prototypes of spectral engines.

Hyperspectral Image Projector

The concept for the complete HIP is shown in the above figure.

It uses two mirror arrays, optically in series. DMD1 is used in the spectral engine to generate arbitrary programmable basis spectra. DMD2 is illuminated by the spatially uniform light from the spectral engine, and the spatial image programmed into DMD2 is projected into the instrument under test. Alternatively, it is projected into the reference radiometer for spectral radiance calibration. For a UUT frame rate of 50 Hz, the 5000 Hz binary update frequency of the spectral light engine means that it can cycle through up to about one hundred basis spectra within the single-frame integration time of the UUT. The duty cycle that a given mirror (image pixel) of DMD2 spends in the "on" state during the "on" time of a particular basis spectrum determines, during that frame, the fractional component of that basis spectrum projected from that image pixel. Thus, arbitrary programmable spectra can be projected into each spatial pixel of the instrument under test. We have made a visible prototype HIP.

References

• "Development of Infrared Scene Projectors for Testing Fire-Fighter Cameras ,"
  J.E. Neira, J.P. Rice, F.K. Amon, Proc. SPIE 6942, 694207 (2008).
  
  
• "Characterization of Earth observing satellite instruments for response to spectrally and spatially variable scenes,"
  S.W. Brown, B. Meyers, R.A. Barnes, J.P. Rice, Proc. SPIE 6677, 667705 (2007).
  


• "Hyperspectral image projector for hyperspectral imagers,"
  J.P. Rice, S.W. Brown, J.E. Neira, and R. Bousquet, Proc. SPIE 6565, 65650C (2007).
  


• "Hyperspectral image projectors for radiometric applications,"
  J.P. Rice, S.W. Brown, B.C. Johnson, and J.E. Neira, Metrologia 43, S61-S65 (2006).
  


• "Hyperspectral image projector for advanced sensor characterization,"
  S.W. Brown, J.P. Rice, J.E. Neira, R. Bousquet, and B.C. Johnson, Proc. SPIE 6296, 629602 (2006).
  


• "Development of hyperspectral image projectors,"
  J.P. Rice, S.W. Brown, and J.E. Neira, Proc. SPIE 6297, 629701 (2006).
  


• "Spectrally tunable sources for advanced radiometric applications,"
  S.W. Brown, J.P. Rice, J.E. Neira, B.C. Johnson, and J.D. Jackson, J. Res. Natl. Inst. Stand. Technol. 111, 401-410 (2006).

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