according to an embodiment of the invention , instead of fixing the light modulation frequency bands upon manufacturing of the attenuation measurement device , the modulation scheme is adapted “ in - situ ” to the modulation spectrum of the ambient light . this modulation scheme is realized by actively monitoring the ambient light or the effect thereof on the detection performance , and changing the transmission and / or detection parameters such that interference with modulated ambient light is avoided or suppressed . fig1 shows a typical setup for transmission pulse oximetry : a red light source 1 and an infrared ( ir ) light source 2 are used for irradiating red light of 660 nm and ir light of 940 nm onto tissue of a patient , i . e . onto a finger 3 . the part of the light which is transmitted through the finger 3 is then collected with a common light detector 4 . fig2 depicts a general block diagram of a transmission pulse oximeter according to an embodiment of the invention . the system comprises a processing unit 5 that adjusts the parameters of a light modulator 6 which acts a pulse controller and modulates the light sources 1 , 2 . the configuration of the light modulator 6 depends on the specific multiplexing scheme applied , e . g . in case of tdm the light sources 1 , 2 are activated alternatingly , whereas for fdm the light sources 1 , 2 radiate light simultaneously but with different modulation frequencies . the reason for the multiplexing scheme is that in this way the same light detector 4 can be used to estimate the attenuation of the light from both light sources 1 , 2 . the light detector 4 detects the light that has propagated through the medium of the finger 3 and converts it into an electrical signal . this signal is then pre - processed by a signal - conditioning block 8 , which comprises analog amplifiers and band - pass filters , which make the signal suitable for conversion to the digital domain by an analog - to - digital converter ( adc ) 9 . correlators 10 , each comprising a demodulator 11 and a demultiplexer 12 , are used to simultaneously demodulate and demultiplex the detected light , and the results are presented to the processing unit 5 , which determines the parameters of interest by evaluating the transmitted and demodulated signals . for that , the processing unit comprises an interference analyzer 14 . the scheme according to the embodiment of the invention is independent of the specific multiplexing technique applied , since all attenuation measurement methods incorporate a certain modulation method . to simplify matters , the description of the following embodiments is restricted to a single light source , thereby disregarding the specific demultiplexing method . for a single light source , only one correlator 10 is necessary . this correlator 10 then simply equals a demodulator 11 , such as depicted in fig3 . here , the information on the light attenuation becomes present in the base - band by multiplying the received signal with a local reference of the same fundamental frequency ( fm = 1 / tm ). subsequently , only the base - band signal is preserved by passing the signal through a low - pass filter 13 , thereby disregarding out - of - band interference . it should be noted that the square wave in fig3 is only illustrative , as any periodic signal can be applied to both modulate the light sources 1 , 2 and demodulate the received signal as long as the fundamental frequencies and / or harmonics coincide . according to a first embodiment of the invention , the fact is applied that the system can operate at different frequencies or fdm modes . these fdm modes , as illustrated in fig4 , are approximately orthogonal in the frequency domain . when the experienced interference is spectral coloured , there exists a mode , which minimizes the experienced interference . further , it should be noted that a tdm system can be interpreted as also operating at a single or multiple fdm modes . the system operation frequency ( fm ) is adapted to a band where interference is low by evaluating the demodulator output ( y ). to that end , first the light source is turned off , then the demodulation frequency of the system is adapted , such that the demodulator output is minimized or becomes lower than a predefined threshold , and subsequently the modulation frequency is changed accordingly . the method can either alternate periodically between attenuation measurements and frequency adaptation , or an adaptation phase can be initiated based on the analysis of the demodulator output signal . the frequencies to be considered can be a discrete set or continuous within a certain range . the processing unit can either adapt the demodulation frequency until some selection criterion is fulfilled , e . g . a certain minimum sir is obtained ; or evaluate the entire range or set of frequencies and then select the optimum ; or evaluate the predefined frequency space by a search algorithm . a respective behaviour can also be realized according to another embodiment of the invention by directly evaluating the spectrum of the received signal , i . e . before demodulating the signal , and selecting a frequency or frequency set where interference is low . to that end , first the light is turned off , and then the power spectrum of the received signal ( x ) is determined using a discrete fourier transform . subsequently , a modulation and corresponding demodulation frequency ( fm ) is selected from the minimum of the power spectrum or by applying a threshold to the power spectrum . again the method can either alternate periodically between attenuation measurements and frequency adaptation , or an adaptation phase can be initiated based on the analysis of the received signal or the demodulator output signal , e . g . by variations in instantaneous power . alternatively , the modulation frequency ( fm ) can be changed continuously by cycling through a certain discrete set of at least two frequency modes or frequency bands , i . e . by frequency hopping . in this case , the set of frequencies is adapted in order to avoid interference by modulated ambient light . generally , the ambient light modulation spectrum does not cover the entire frequency set , and it can be assumed that at least one of the modulation frequencies is free of interference . to that end , the frequency set should be chosen initially such that sufficient spectral diversity is guaranteed . therefore the detected light for at least one of the frequencies in the set , i . e . the one that is free of disturbance , results in a maximum attenuation measurement . clearly , for a given colour of light , attenuation measurements are independent of the modulation frequencies . now , if another frequency in the set results in a lower attenuation measurement , this is a result of interference in that modulation frequency band , and the corresponding frequency should be replaced . in principle , a new frequency in the set can be chosen arbitrarily or based on some selection criterion . spectral sensing , as described in the previous embodiment , may be applied to select the initial set for frequency hopping and also to replace frequencies in the set . in order to achieve this functionality , the light source does not have to be turned off , and a new modulation frequency or a set can be selected while another modulation frequency is being used . it should be noted , that it is especially preferred that only two frequencies are used , wherein the second modulation frequency is chosen from the spectrum when the attenuation is measured using the first frequency and vice versa , thereby continuously seeking the optimal modulation frequency . the advantage of such adaptive modulation frequency hopping is that the method is based on the quasi - simultaneous evaluation of the received signal at multiple frequencies , thereby allowing continuous measurement without interruption by an adaptation phase . according to another embodiment of the invention , cdm is used , wherein the different light sources use unique , preferably orthogonal , codes to enable the light detector to distinguish their light contributions . suitable orthogonal codes are e . g . walsh - hadamard ( wh ) codes , where the number of light sources that can be accommodated is roughly equal to the length of the code . next to allowing for identification of the light contributions of the different light sources , these codes also shape the spectrum of the light signal . as an example all except one wh code achieve a dc - free spectrum and their spectra are mutually different . this is illustrated in fig5 for wh codes of length 256 . there , the spectra of 4 out of the 256 codes in the codebook are depicted . as can be concluded from fig5 , these codes have significantly different spectra . similarly , correlation at the receiving end with the spreading codes achieves suppression of frequencies not related to the transmitted code . it should be noted that , in contrast to the frequency multiplexing illustrated in fig3 , the spectra achieved by these codes will generally be overlapping . the modulation scheme is then implemented by adaptive code selection . the spreading or modulation code used by the system is adapted to the one , that is least affected by the interference , i . e . the code that is most orthogonal to the interference . this is done by evaluating the demodulator output . to that end , the technique of the first embodiment described above can be applied , where first the light is turned off , then the demodulation code of the system is adapted , such that the demodulator output is minimized or becomes lower than a certain threshold , and subsequently the transmitter spreading code is changed accordingly . it should be noted that also the accuracy / reliability of the attenuation measurement can be scaled in such a solution by applying codes with different codes lengths , i . e . when the level of interference is low , a short code is used and when it is high , a higher code length is used . the advantage of longer codes is that they achieve better spectral shaping and thus yield a better noise and interference suppression at the receiving end . the advantage of short codes is that they reduce the measurement time . above embodiments have been described for a single light source . in these solutions the system searches for the optimal modulation frequency / code . for a system with n light sources , however , the system finds a set of n codes / frequencies for which the impact of the interference and noise is minimized / below a certain threshold . here it is possible to choose to minimize the worst - case error for one of the light sources or the average error for all light sources , depending on the operation mode . while the invention has been illustrated and described in detail in the drawings and foregoing description , such illustration and description are to be considered illustrative or exemplary and not restrictive ; the invention is not limited to the disclosed embodiments . other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention , from a study of the drawings , the disclosure , and the appended claims . in the claims , the word “ comprising ” does not exclude other elements or steps , and the indefinite article “ a ” or “ an ” does not exclude a plurality . the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage . any reference signs in the claims should not be construed as limiting the scope .
the invention relates to a method of monitoring a vital parameter of a patient by measuringattenuation of light emitted onto tissue of the patient , comprising the following steps : modulating the light with a modulation frequency or / and a modulation code ; emitting the modulated light onto the tissue of the patient ; collecting lightwhich is transmitted through the tissue or / and which is reflected from the tissue ; demodulating the collected light ; analyzing the demodulated collected light with regard to interference with ambient light ; determining amodulation frequency or / and a modulation code for which interference with the ambient light is minimized or falls under a predefined threshold ; and setting the modulation frequency or / and the modulation code for modulating the light according to the determined modulation frequency or / and a modulation code for which interference with the ambient light is minimized or falls under a predefined threshold . in this way a versatile and reliable possibility of monitoring a vital parameter of a patient with a high signal - to - interference ratio is provided .