Multichromatic calibration method and device

Patent metadata

Field	Value
Publication number	US-11226232-B2
Application number	US-201816651060-A
Country	US
Kind code	B2
Filing date	Sep 26, 2018
Priority date	Sep 26, 2017
Publication date	Jan 18, 2022
Grant date	Jan 18, 2022

Abstract

A Multichromatic Calibration (MC) method of at least a spectral sensor which is one of a list comprising at least a spectrometer, a multispectral sensor, a hyperspectral sensor, a spectral camera, a color camera. The method comprises a. generating a plurality of different multichromatic spectra, wherein i. a spectrum from the plurality of different multichromatic spectra contains light intensity measurable by the at least one spectral sensor and by a reference spectral device, and ii. a spectrum from the plurality of different multichromatic spectra contains light centered around at least two different wavelengths and is configured to be integrated during an exposure time of a single measurement from any of the at least one spectral sensor or the reference spectral device; b. measuring each multichromatic spectrum of the plurality of different multichromatic spectra with the reference spectral device and the at least one spectral sensor; and from all data of the measured multichromatic spectra, compute a transfer function which relates a response of the at least one spectral sensor to a corresponding response of the reference spectral device, without measuring the spectral response of the at least one spectral sensor.

First claim

The invention claimed is:

1. A Multichromatic Calibration (MC) method of at least a spectral sensor which is at least one from a list comprising a spectrometer, a multispectral sensor, a hyperspectral sensor, a spectral camera, a color camera, the method comprising the steps of:
a. generating a plurality of different multichromatic spectra, wherein
i. a spectrum from the plurality of different multichromatic spectra includes light intensity measurable by the at least one spectral sensor and by a reference spectral device, and
ii. a spectrum from the plurality of different multichromatic spectra includes light centered around at least two different wavelengths and is configured to be integrated during an exposure time of a single measurement from any of the at least one spectral sensor or the reference spectral device;

b. measuring each multichromatic spectrum of the plurality of different multichromatic spectra with the reference spectral device and the at least one spectral sensor; and
c. from all data of the measured multichromatic spectra, compute a transfer function which relates a response of the at least one spectral sensor to a corresponding response of the reference spectral device, without measuring the spectral response of the at least one spectral sensor.

2. The method of claim 1 , wherein the plurality of generated different multichromatic spectra and the corresponding measurements from the reference spectral device are saved, and then loaded and used for multiple calibrations of the at least one spectral sensor to be calibrated.

3. The method of claim 1 , wherein the at least one spectral sensor is a camera for which the multichromatic calibration is performed spatially localized on a sensor surface of the camera thereby enabling to compensate for local camera deviations such as sensor imperfections, sensor manufacturing errors, local sensor spectral sensitivity variations, lens transmission variations, dust in the optical path and lens vignetting, wherein
a. the sensor surface of the spectral sensor to calibrate is segmented into a plurality of areas; and
b. a calibration transfer function is estimated for each of the areas of the plurality of areas.

4. The method of claim 1 wherein measurements of the reference spectral device are transformed by a custom function before being used to compute the transfer function between the measurements of the at least one spectral sensor and measurements the reference device.

5. The method of claim 1 wherein the transfer function is computed via a linear regression method.

6. The method of claim 1 wherein the transfer function is computed using a nonlinear machine-learning-oriented method.

7. The method of claim 1 wherein the transfer function is computed using a nonlinear computational solver-based method.

8. The method of claim 1 , wherein the generating step generates random or pseudo-random multichromatic spectra.

9. The method of claim 8 , wherein the multichromatic spectra generated are piecewise smooth.

10. The method of claim 1 , wherein the multichromatic spectra generated are weighted by a predefined light source spectrum, as to simulate reflectances of real objects under a given light source.

11. The method of claim 1 , wherein the multichromatic spectra generated are square waves or present randomly distributed square transitions.

12. The method of claim 1 , wherein the generated multichromatic spectra are weighted by a defined or random spectral function.

13. An automated self-calibration method which enables mapping of a digital input and the spectral output of a multichromatic calibration device by comparing the digital commands of a digitally tunable spectral filter to the spectral measurement of the reference spectral device, thus estimating automatically and in real-time digital command wavelength and intensity correction functions necessary to generate accurate spectra during the multichromatic calibration of claim 1 , the method comprising the steps of:
a. mapping digital commands to spectral reference device wavelengths by emitting a multichromatic spectrum composed of multiple narrow-band peaks of light and measuring that spectrum with the spectral reference device, then matching emitted and detected peaks to obtain a digital command wavelength correction function;
b. mapping digital commands to spectral reference device intensity measurement by initializing a digital command intensity correction function to have no effect, then iteratively executing the following steps:
i. (i) generating a desired broadband spectrum with a set of digital commands and the digital command intensity correction function,
ii. (ii) measuring that spectrum with the spectral reference device,
iii. (iii) updating the digital command intensity correction function from the deviation between the digital commands and the measured spectrum, and
iv. (iv) repeating steps i-iii until the digital command intensity correction function does not change.

Assignees

• Ecole Polytechnique Fed Lausanne Epfl

Inventors

• Constantin Dragos
• De Visscher Simon

Classifications

• G01J2003/2826

  Multispectral imaging, e.g. filter imaging · CPC title

• G01J3/0229

  using masks, aperture plates, spatial light modulators or spatial filters, e.g. reflective filters · CPC title

• G01J3/36 Primary

  Investigating two or more bands of a spectrum by separate detectors · CPC title

• G01J3/2823

  Imaging spectrometer · CPC title

• G01J2003/1247

  Tuning · CPC title