Monitoring device and method for compensating non-linearity effects in vital signs monitoring

1. A monitoring device comprising:

a light source configured to emit light into a body part of a living being;

a light sensor configured to receive light including an ambient light component and a measurement light component resulting from interactions of said emitted light with said body part and configured to generate an output signal, wherein a transfer function describes the relation between the output signal and the received light; and

a microprocessor device comprising:

an ambient light cancellation unit configured to separate the output signal into a first signal portion corresponding to the ambient light component and a second signal portion corresponding to the measurement light component;

an ambient light modulation removal unit configured to compensate a variation of the ambient light component by demodulating the second signal portion based on the transfer function and the first signal portion to generate a measurement signal; and

a processing unit configured to derive and monitor information on a vital sign of the living being from the measurement signal.

2. The monitoring device of claim 1 , wherein the ambient light modulation removal unit is configured to determine a derivative of the transfer function.

3. The monitoring device of claim 2 , wherein the ambient light modulation removal unit is configured to demodulate the second signal portion by multiplying it with an inverse of the derivative of the transfer function.

4. The monitoring device of claim 1 , wherein the ambient light cancellation unit is configured to determine an estimate for the first signal portion component and subtract said estimate from the output signal to determine the second signal portion.

5. The monitoring device of claim 1 , further comprising a calibration unit configured to determine the transfer function, wherein the calibration unit is configured to determine a calibration value from the output signal of the light sensor when being exposed to calibration light of predefined intensity.

6. The monitoring device of claim 5 , wherein the calibration unit is configured to apply a curve fitting algorithm to a plurality of calibration values.

7. The monitoring device of claim 1 ,

wherein the light source is configured to emit light pulses; and

wherein the ambient light cancellation unit is configured to separate the output signal based on the output signal at a point in time when the light source is powered off and based on the output signal at a point in time when the light source emits light.

8. The monitoring device of claim 7 ,

further including a filtering unit configured to filter the output signal with a first low-pass filter when the light source emits light to generate a first filtered signal and with a second low-pass filter when the light source is powered off to generate a second filtered signal;

wherein the ambient light cancellation unit is configured to subtract the second filtered signal from the first filtered signal.

9. The monitoring device of claim 1 ,

further comprising an additional ambient light sensor configured to receive ambient light and for generating an ambient light signal;

wherein the ambient light cancellation unit is configured to separate the output signal based on an estimate for the ambient light component derived from the generated ambient light signal.

10. The monitoring device of claim 1 , wherein the light source is configured to emit light of a first wavelength, further comprising an additional light source configured to emit light of a second wavelength different from the first wavelength.

11. The monitoring device of claim 1 ,

wherein the light source and the light sensor are configured to be arranged on opposite sides of the body part of the living being; and

wherein the measurement light component of the light results from a transmission of the emitted light through said body part.

12. The monitoring device of claim 1 ,

wherein the light source and the light sensor are configured to be arranged to face a body part of the living being; and

wherein the measurement light component of the light results from a reflection of the emitted light at said body part.

13. A monitoring method comprising the steps of:

emitting light into a body part of a living being;

receiving light including an ambient light component and a measurement light component resulting from interactions of said emitted light with said body part and for generating an output signal, wherein a transfer function describes the relation between the output signal and the received light;

separating the output signal into a first signal portion corresponding to the ambient light component and a second signal portion corresponding to the measurement light component;

compensating a variation of the ambient light component by demodulating the second signal portion based on the transfer function and the first signal portion to generate a measurement signal; and

deriving and monitoring information on a vital sign of the living being from the measurement signal.

14. The monitoring method of claim 13 , wherein the transfer function is a non-linear function.

15. The monitoring method of claim 13 , further comprising the step of determining a calibration value from the output signal of the light sensor when being exposed to calibration light of predefined intensity.