Device, system and method for determining the concentration of a substance in the blood of a subject

1. A device for determining the concentration of a substance in the blood of a subject, comprising:

at least one processor programmed to:

receive detection signals reflected back or transmitted through a skin area of the subject in response to irradiation of the skin area by a radiation signal,

extract at least three photo-plethysmography, PPG, signals at different wavelengths from said detection signals,

direct current (DC) normalize said at least three PPG signals and to form a first difference signal between a first normalized PPG signal and a second normalized PPG signal and a second difference signal between a third normalized PPG signal and one other of the at least three normalized PPG signals and to form a ratio between estimated alternating current (AC) amplitudes of said first difference signal and said second difference signal, and

calculate the concentration of a substance in the blood of the subject based on said ratio.

2. The device as claimed in claim 1 , wherein the at least one processor is further programmed to:

extract two PPG signals at two wavelengths of infrared light the first and third PPG signals and one wavelength of visible or infrared light from said detection signals.

3. The device as claimed in claim 2 ,

wherein the two PPG signals at two wavelengths of infrared light represent the first and third PPG signals and the PPG signal at the wavelength of visible or infrared light represents the second PPG signal.

4. The device as claimed in claim 3 , wherein the at least one processor is further programmed to:

form the first difference signal by subtracting the second normalized PPG signal from the first normalized PPG signal,

form the second difference signal by subtracting the first normalized PPG signal from the third normalized PPG signal; and

form said ratio by dividing said first difference signal by said second difference signal.

5. The device as claimed in claim 4 , wherein the at least one processor is further programmed to:

calculate the concentration of the substance by multiplying said ratio with a second calibration parameter and subtracting the result of said multiplication from a first calibration parameter or to determine the concentration of the substance by use of a look-up table.

6. The device as claimed in claim 1 , wherein the at least one processor is further programmed to:

extract a first PPG signal at a first wavelength in the range from 780 nm to 850 nm, a second PPG signal at a second wavelength in the range from 550 to 780 nm and a third PPG signal at a third wavelength in the range from 840 nm to 1000 nm.

7. The device as claimed in claim 1 ,

wherein said detection signals are sensor signals of a contact PPG sensor contacting said skin area of the subject.

8. The device as claimed in claim 1 ,

wherein said detection signals are images of at least said skin area of the subject obtained by a white-balanced imaging device.

9. The device as claimed in claim 1 , wherein the at least one processor is further programmed to:

form the ratio between a standard deviation of said first difference signal and said second difference signal.

10. A system for determining the concentration of a substance in the blood of a subject, comprising:

a radiation detector configured to detect detection signals reflected back or transmitted through a skin area of the subject in response to irradiation of the skin area by a radiation signal, and

a device as claimed in claim 1 configured to determine the concentration of a substance in the blood of the subject from said detection signals.

11. The system as claimed in claim 10 ,

wherein said radiation detector comprises a contact PPG sensor configured to contact said skin area of the subject for detecting sensor signals as detection signals.

12. The system as claimed in claim 10 ,

wherein said radiation detector comprises an imaging unit for obtaining images of at least said skin area of the subject as detection signals.

13. The system as claimed in claim 10 ,

wherein said radiation detector comprises at least one processor programmed to obtain images of at least said skin area of the subject as detection signals.

14. A computer readable non-transitory medium having instructions stored thereon which, when carried out on a computer, cause the computer to perform a method for determining the concentration of a substance in the blood of a subject, the method comprising:

receiving detection signals reflected back or transmitted through a skin area of the subject in response to irradiation of the skin area by a radiation signal,

extracting at least three photo-plethysmography, PPG, signals at different wavelengths from said detection signals,

direct current (DC) normalizing said at least three PPG signals,

forming a first difference signal between a first PPG signal and a second PPG signal and a second difference signal between a third PPG signal and one other of the at least three PPG signals,

forming a ratio between estimated AC amplitudes of said first difference signal and said second difference signal, and

calculating the concentration of a substance in the blood of the subject based on said ratio.

15. A device for determining the oxygen saturation of a subject, comprising:

at least one processor programmed to:

receive detection signals reflected back or transmitted through a skin area of the subject in response to irradiation of the skin area by a radiation signal,

extract at least three photo-plethysmography, PPG, signals at different wavelengths from said detection signals,

direct current (DC) normalize said at least three PPG signals and to form a first difference signal between a first normalized PPG signal and a second normalized PPG signal and a second difference signal between a third normalized PPG signal and one other of the at least three normalized PPG signals and to form a ratio between estimated alternating current (AC) amplitudes of said first difference signal and said second difference signal, and

calculate the oxygen saturation of the subject based on said ratio.

16. The device as claimed in claim 15 , wherein the at least one processor is further programmed to:

extract two PPG signals at two wavelengths of infrared light the first and third PPG signals and one wavelength of visible or infrared light from said detection signals.

17. The device as claimed in claim 16 ,

wherein the two PPG signals at two wavelengths of infrared light represent the first and third PPG signals and the PPG signal at the wavelength of visible or infrared light represents the second PPG signal.

18. The device as claimed in claim 17 , wherein the at least one processor is further programmed to:

form the first difference signal by subtracting the second normalized PPG signal from the first normalized PPG signal,

form the second difference signal by subtracting the first normalized PPG signal from the third normalized PPG signal; and

form said ratio by dividing said first difference signal by said second difference signal.

19. The device as claimed in claim 18 , wherein the at least one processor is further programmed to:

calculate the concentration of the substance by multiplying said ratio with a second calibration parameter and subtracting the result of said multiplication from a first calibration parameter or to determine the concentration of the substance by use of a look-up table.

20. The device as claimed in claim 15 , wherein the at least one processor is further programmed to:

extract a first PPG signal at a first wavelength in the range from 780 nm to 850 nm, a second PPG signal at a second wavelength in the range from 550 to 780 nm and a third PPG signal at a third wavelength in the range from 840 nm to 1000 nm.