Systems for characterizing ambient illumination

1. A method of using a camera system to characterize ambient illumination, the camera system having an image sensor to capture a view of a scene, a flash to provide flash illumination of the scene, and a multispectral sensor to capture a spectrum of light from the scene in a plurality of wavelength channels, the method comprising:

capturing a first spectrum of light (D TFA ) from the scene using the multispectral sensor whilst the flash is operating to illuminate the scene in addition to ambient illumination;

capturing a second spectrum of light (D TA ) from the scene using the multispectral sensor whilst the flash is not operating and the scene is illuminated by the ambient illumination;

determining a difference between the first and second spectra representing a scene flash spectrum (D TF ), wherein the scene flash spectrum represents a spectrum of the scene when illuminated by the flash without the ambient illumination;

compensating the scene flash spectrum using a spectrum of the flash illumination (E(λ)) to determine a colour-compensated scene flash spectrum (R T (λ)), wherein the colour-compensated scene flash spectrum represents an average reflectance spectrum of the scene when compensated for the spectrum of the flash illumination; and

processing the second spectrum of light from the scene using the colour-compensated scene flash spectrum to estimate a spectrum of the ambient illumination (R A (λ));

wherein processing the second spectrum of light from the scene using the colour-compensated scene flash spectrum to estimate a spectrum of the ambient illumination comprises dividing a representation of the second spectrum of light (R TA (λ)) by the colour-compensated scene flash spectrum (R T (λ)).

2. The method as claimed in claim 1 further comprising compensating each of the first and second spectra for a response of the multispectral sensor (M TF ; M TA ).

3. The method as claimed in claim 1 wherein the multispectral senor has n wavelength channels, wherein the first and second spectra are represented by respective first spectrum and second spectrum vectors of length n (D TFA ; D TA ), and wherein determining the difference between the first and second spectra comprises subtracting one of the first spectrum and second spectrum vectors from the other to determine a scene flash spectrum vector (D TF ).

4. The method as claimed in claim 3 wherein the spectrum of the flash illumination is represented by a flash illumination vector (E(λ)) of length m, where m represents a number of wavelength points defined by the spectrum; wherein a sensitivity of the multispectral sensor at the wavelength points for each wavelength channel is defined by a n×m sensitivity matrix (S(λ)); and wherein compensating the scene flash spectrum using the spectrum of the flash illumination comprises multiplying the scene flash spectrum vector (D TF ) by a matrix (M TF ) defined by a combination of the sensitivity matrix and the flash illumination vector to obtain a colour-compensated scene flash spectrum vector (R T (λ)) representing the colour-compensated scene flash spectrum.

5. The method as claimed in claim 4 further comprising multiplying the second spectrum vector (D TA ) by an inverse of the sensitivity matrix (M TA ) to obtain a sensor-compensated second spectrum vector (R TA (λ)), and dividing sensor-compensated second spectrum vector by the colour-compensated scene flash spectrum vector (R T (λ)).

6. The method as claimed in claim 1 wherein the multispectral sensor has at least four wavelength channels.

7. The method as claimed in claim 1 further comprising adapting an RGB to CIE XYZ transformation matrix of the camera using the estimated spectrum of the ambient illumination.

8. The method as claimed in claim 1 further comprising using the image sensor to capture an image, and i) using the estimated spectrum of the ambient illumination to colour-correct the image and/or ii) storing data representing the estimated spectrum of the ambient illumination with image data representing the captured image.

9. The method as claimed in claim 1 further comprising processing the estimated spectrum of the ambient illumination to classify the ambient illumination into one of a set of discrete categories, and controlling one or both of image capture and image processing by the camera dependent upon the category of the ambient illumination.

10. The method as claimed in claim 1 further comprising processing the estimated spectrum of the ambient illumination to determine illumination data characterizing a colour or colour temperature of the ambient illumination, and i) using the illumination data to colour-correct the image and/or ii) storing the illumination data with image data from the image sensor.

11. The method as claimed in claim 1 further comprising determining a colour transformation matrix, wherein the colour transformation matrix comprises a matrix to transform from an RGB to a CIE XYZ colour space adapted to compensate for the estimated spectrum of the ambient illumination.

12. Processor control code, or one or more computer readable media storing processor control code, to implement the method of claim 1 .

13. A camera system ( 100 ) comprising:

an image sensor ( 106 ) to capture a view of a scene;

a flash ( 110 ) to provide flash illumination of the scene;

a multispectral sensor ( 108 ) to capture a spectrum of light from the scene in a plurality of wavelength channels;

an image processing subsystem ( 120 ) configured to:

capture a first spectrum of light (D TFA ) from the scene using the multispectral sensor whilst the flash is operating to illuminate the scene in addition to ambient illumination;

capture a second spectrum of light (D TA ) from the scene using the multispectral sensor whilst the flash is not operating and the scene is illuminated by the ambient illumination;

determine a difference between the first and second spectra representing a scene flash spectrum (D TF ), wherein the scene flash spectrum represents a spectrum of the scene when illuminated by the flash without the ambient illumination;

compensate the scene flash spectrum using a spectrum of the flash illumination (E(λ)) to determine a colour-compensated scene flash spectrum (R T (λ)), wherein the colour-compensated scene flash spectrum represents an average reflectance spectrum of the scene when compensated for the spectrum of the flash illumination; and

process the second spectrum of light from the scene using the colour-compensated scene flash spectrum to estimate a spectrum of the ambient illumination (R A (λ));

wherein processing the second spectrum of light from the scene using the colour-compensated scene flash spectrum comprises dividing each of a set of values (R TA (λ)) representing the second spectrum of light at each of a respective set of wavelength points by a corresponding value for the colour-compensated scene flash spectrum (R T (λ)) at the respective wavelength point.

14. A method of using a camera system to characterize ambient illumination, the camera system having an image sensor to capture a view of a scene, a flash to provide flash illumination of the scene, and a multispectral sensor to capture a spectrum of light from the scene in a plurality of wavelength channels, the method comprising:

capturing a first spectrum of light (D TFA ) from the scene using the multispectral sensor whilst the flash is operating to illuminate the scene in addition to ambient illumination;

capturing a second spectrum of light (D TA ) from the scene using the multispectral sensor whilst the flash is not operating and the scene is illuminated by the ambient illumination;

determining a difference between the first and second spectra representing a scene flash spectrum (D TF ), wherein the scene flash spectrum represents a spectrum of the scene when illuminated by the flash without the ambient illumination;

compensating the scene flash spectrum using a spectrum of the flash illumination (E(λ)) to determine a colour-compensated scene flash spectrum (R T (λ)), wherein the colour-compensated scene flash spectrum represents an average reflectance spectrum of the scene when compensated for the spectrum of the flash illumination; and

processing the second spectrum of light from the scene using the colour-compensated scene flash spectrum to estimate a spectrum of the ambient illumination (R A (λ));

wherein processing the second spectrum of light from the scene using the colour-compensated scene flash spectrum comprises dividing each of a set of values (R TA (λ)) representing the second spectrum of light at each of a respective set of wavelength points by a corresponding value for the colour-compensated scene flash spectrum (R T (λ)) at the respective wavelength point.

15. The method as claimed in claim 14 wherein the multispectral senor has n wavelength channels, wherein the first and second spectra are represented by respective first spectrum and second spectrum vectors of length n (D TFA ; D TA ), and wherein determining the difference between the first and second spectra comprises subtracting one of the first spectrum and second spectrum vectors from the other to determine a scene flash spectrum vector (D TF ).

16. The method as claimed in claim 15 wherein the spectrum of the flash illumination is represented by a flash illumination vector (E(λ)) of length m, where m represents a number of wavelength points defined by the spectrum; wherein a sensitivity of the multispectral sensor at the wavelength points for each wavelength channel is defined by a n×m sensitivity matrix (S(λ)); and wherein compensating the scene flash spectrum using the spectrum of the flash illumination comprises multiplying the scene flash spectrum vector (D TF ) by a matrix (M TF ) defined by a combination of the sensitivity matrix and the flash illumination vector to obtain a colour-compensated scene flash spectrum vector (R T (λ)) representing the colour-compensated scene flash spectrum.

17. The method as claimed in claim 16 further comprising multiplying the second spectrum vector (D TA ) by an inverse of the sensitivity matrix (M TA ) to obtain a sensor-compensated second spectrum vector (R TA (λ)), and dividing sensor-compensated second spectrum vector by the colour-compensated scene flash spectrum vector (R T (λ)).