Hyperspectral sensing system and processing methods for hyperspectral data
A hyperspectral sensing device may include an optical collector configured to collect light and to transfer the collected light to a sensor having spectral resolution sufficient for sensing hyperspectral data. In some examples, the sensor comprises a compact spectrometer. The device further comprises a power supply, an electronics module, and an input/output hub enabling the device to transmit acquired data (e.g., to a remote server). In some examples, a plurality of hyperspectral sensing devices are deployed as a network to acquire data over a relatively large area. Methods are disclosed for performing dark-current calibration and/or radiometric calibration on data obtained by the hyperspectral sensing device, and/or another suitable device. Data obtained by the device may be represented in a functional basis space, enabling computations that utilize all of the hyperspectral data without loss of information.
1. A method of measuring a radiometrically calibrated spectrum using a spectral sensor deployed at an outdoor location without relocating the spectral sensor, the method comprising:
measuring a spectrum of light using a spectral sensor deployed at an outdoor location;
normalizing the measured spectrum using a selected normalizing factor;
computing a simulated spectrum based on a simulation of at least a portion of the atmosphere at the outdoors location, the simulation including one or more adjustable parameters;
adjusting at least one of the adjustable parameters to produce an adjusted simulated spectrum matching the measured spectrum according to one or more predefined criteria;
determining a mathematical transformation capable of transforming the measured spectrum, such that at least a portion of the transformed measured spectrum approximates at least a portion of the adjusted simulated spectrum; and
performing the mathematical transformation on the measured spectrum to produce a radiometrically calibrated spectrum.
2. The method of claim 1 , further comprising measuring a second spectrum using the spectral sensor deployed at the outdoor location and performing the mathematical transformation on the second spectrum to radiometrically calibrate the second spectrum.
3. The method of claim 1 , wherein the one or more predefined criteria for matching the adjusted simulated spectrum to the measured spectrum include agreement within a predefined first tolerance of a first absorption depth of a first measured absorption line of the measured spectrum and a second absorption depth of a corresponding first simulated absorption line of the adjusted simulated spectrum.
4. The method of claim 3 , wherein the one or more predefined criteria further include agreement within a predefined second tolerance of a third absorption depth of a second measured absorption line of the measured spectrum and a fourth absorption depth of a corresponding second measured absorption line of the adjusted simulated spectrum, and wherein the first and second measured absorption lines have respective central wavelengths differing from each other by at least 40 nanometers.
5. The method of claim 1 , wherein computing the simulated spectrum includes calculating an optical thickness of an atmospheric component based on an absorption line of the measured spectrum, and using the calculated optical thickness as an initial value of one of the adjustable parameters of the simulation.
6. The method of claim 1 , further comprising correcting the measured spectrum for near-infrared absorption by identifying a lowest value of a near-infrared portion of the measured spectrum and subtracting the lowest value from all values within the near-infrared portion.