when performing reflectance pulse oximetry , two main reasons justify the increase of the physical separation between optical parts ( leds and photo - diode ). 1 . in any pulse oximetry probe , the relative pulse amplitude is a good indicator of the quality of the probe placement . this quality factor , usually depicted as perfusion index ( pi ), is also interpreted as a quantification of the width of the vascular bed traversed by a light beam . it was demonstrated ( y . mendelson , noninvasive pulse oximetry utilizing skin reflectance photoplethysmography , ieee transactions on biomedical engineering , vol 35 , no 10 , 1988 ) that , given a reflectance probe , the pi is linearly increasing with the increase of the physical separation between the optical parts . 2 . the probability of direct - light being short - cut from the light sources to the light detector by scattering in the outer part of the skin or / and successive reflection in the probe - skin interface is reduced when both optical elements are dispersed . due to the reduced light short - cut , probe design can be simplified ( no glue fixing is required anymore ). however , by increasing the distance between the optical parts , the absolute intensity of received light at the light detector is exponentially decreased and , thus , the quantification of the pulsatile signal becomes problematic , compromising the feasibility of successfully identifying cardiac activity . in the state - of - the - art reflectance probes , the facts here exposed have imposed a trade - off between : increasing the physical separation of optical elements , thus reducing cross - talk and increasing the perfusion index ( pi ). assuring enough light intensity at the photo - detector . this trade - off has historically forced transmittance probes to include severe fixing mechanism such as glue or vacuum approaches , as described in the already mentioned j . g . webster publication or by v . konig ( reflectance pulse oximetry — principles and obstetric application in the zurich system , journal of clinical monitoring and computing 14 : 403 - 412 , 1998 ). the following table summarizes the advantages and disadvantages of near and far - field photo - plethysmography : the present invention merges the advantages of both near and far field photo - plethysmography in a single method . as shown in fig1 , the invention consists in combining far and near photo - plethysmographs so that : a near - field photo - plethysmograph allows the continuous tracking / detection of cardiac activity ; a far - field photo - plethysmograph performs pulse oximetry measurements on the basis of estimated cardiac activity information . according to the invention , a near - field reflectance photo - plethysmograph and a far - field reflectance photo - plethysmograph are merged in a unique device comprising : for the near - field function , a first light source 10 , which can be a led emitting in the infra - red range at 940 nm , a first light detector 11 , such as a photo - diode , located to receive light from the source , and a first analog - to - digital converter ( adc ) 12 connected at the output of the light detector ; for the far - field function , a second light source 13 , such as a led , emitting in the infra - red range at 940 nm , a second light detector 14 , such as a photo - diode , located to receive light from source 13 , a second analog - to - digital converter ( adc ) 15 connected at the output light detector 14 , a third light source 16 , such as a led , emitting in the red range at 660 nm , a third light detector 17 , such as a photo - diode , located to receive light from source 16 , and a third analog - to - digital converter ( adc ) 18 connected at the output of light detector 17 ; a microprocessor 19 connected at the outputs of analog - to - digital converters 12 , 15 and 18 ; and a display device 20 connected at the output of microprocessor 19 . the above - mentioned wavelength values of 660 and 940 nm are just given as examples . more generally , these wavelengths must be in the visible infra - red region , i . e comprised between 400 and 2000 nm , and be different from each other . as shown in fig2 , light sources 10 - 13 - 16 and light detectors 11 - 14 - 17 are positioned at the surface of the skin s of a body part . the light detectors are at the same location . near - field light source 10 is at a shorter distance from the detectors than far - field light sources 13 - 16 , located at the same place . typically , the separation between the near - field light source and the light detectors is between 4 and 10 mm , whereas the separation between the far - field light sources and the light detectors is between 10 and 50 mm . fig2 shows that the light collected by the detectors has travelled in tissue t longer and deeper for the far - field beam f than for the near - field beam n . the above described structure is a simplified presentation of the device of the invention . needless to mention that a single light detector and a single analog - to - digital converter can also be used in association with time - sharing control means adapted to apply to microprocessor 19 data corresponding respectively to the three light sources 10 , 13 and 16 . according to the present invention , the light sources and the light detectors can be arranged at the skin surface in many different configurations , the only rule to respect being : to collect a light beam having travelled over a short distance in the body , and to collect two light beams of different wavelengths in the visible infra - red region having travelled over a longer distance in the body . thus , for example , the three light sources can be located at the same place , with a near - field detector at short distance and far - field detectors at longer distance . another example is to have a plurality of light sources distributed around far - field detectors , with a near - field detector located at a shorter distance from one of the sources . fig3 shows , as a further example ( with the same reference letters as fig2 ), that the device of the invention can be arranged around the finger of a person . in that case , light sources 10 - 13 - 16 are located at the same place , near - field detector 11 is near the sources and far - field detectors 14 - 17 stand opposite to the light sources . due to the increased distance between light sources 13 - 16 and far light detectors 14 - 17 , the digital signals provided by far - field analog - to - digital converters 15 and 18 are noise polluted and render very difficult a reliable identification of the cardiac activity . but the reduced distance separating light source 10 and near light detector 11 assures enough received light intensity and provides a much better identification of the cardiac activity . in stage 21 , the near - field signals are used , therefore , to base the pulse oximetry measurements on an improved far - field information . the signals provided by stage 21 are finally used for conventional pulse oximetry calculations . as shown in fig1 , the digital output of near - field adc 12 is first applied to a band - pass filter 23 , such as a chebyshev filter type 1 , 3 rd order , having a band - pass of 0 . 5 to 3 . 5 hz . knowing that the useful portion of the signal corresponds to the normal , around 1 hz , cardiac frequency of a person , this filter eliminates the portions of the signal which are outside the 0 . 5 - 3 . 5 hz range . similarly , the digital outputs of infra - red far - field adc 15 and of red far - field adc 18 are first applied respectively to band - pass filters 24 and 25 , identical to band - pass filter 23 . in addition , the digital outputs of infra - red far - field adc 15 and of red far - field adc 18 are applied respectively to identical low - pass filters 26 and 27 , such as butterworth filters , 2 nd order , which have the function to eliminate the portion of the received signals above 0 . 2 hz . the remaining portion of the signals are taken respectively as the dc - infra - red ( dc ired ) and the dc - red ( dc red ) components of the far - field signals . the operation shown in 28 is the detection of the temporal position of every maximum of the signal delivered by band - pass filter 23 . the sequence of the maximum position is then used to perform , respectively in 29 and 30 , a triggered averaging of the infra - red and red far - field signals produced by band - pass filters 24 and 25 . the triggered averaging is performed in a similar way to that described in the already mentioned publication of j . g . webster . the triggered averaged signals resulting from operations 29 and 30 are taken respectively as the ac - infra - red ( ac ired ) and the ac - red ( ac red ) components of the far - field signals . finally , in stage 22 , the dc ired , dc red , ac ired and ac red signals are used to perform classical pulse oximetry calculations 31 , as described by j . g . webster . the results of the calculations are displayed by device 20 connected at the output of microprocessor 19 . reference is made , now , to fig4 which presents another method for obtaining pulse oximetry measurements from the signals delivered by band - pass filters 23 , 24 and 25 and by low - pass filters 26 and 27 . the elements common to the device of fig1 are designated by the same references as shown in fig4 , the near - field signals produced by band - pass filter 23 are used , in 32 , to estimate a a - priori representation of the spectral distribution of the cardiac activity , as disclosed , for example , in the publication of d . g . manolakis , statistical and adaptive signal processing , mcgraw - hill higher education , 2000 . then , the estimated representation of the spectral distribution of the cardiac activity is used , respectively in 33 and 34 , to denoise and / or restore the corrupted infra - red and red far - field signals produced by band - pass filters 24 and 25 . the technique used is described , for example , in the already mentioned publication of d . g . manolakis . the restored signals resulting from operations 33 and 34 are taken respectively as the ac - infra - red ( ac ired ) and the ac - red ( ac red ) components of the far - field signals . they are finally used to perform the classical pulse oximetry calculations 31 . the present invention can be used in many optical - based pulse oximetry applications . for example , a probe carrying the light sources and the light detectors can be placed : in a head band , the frontal bone acting as reflectance surface ; in a mask , the maxillary bone acting as reflectance surface ; in a chest - belt , the manubrium acting as reflectance surface ; around a finger ; around the leg or arm of a neonate ; as a ear - phone .
the present invention concerns an optical based pulse oximetry device comprising : first , second and third light emitting means , for placement on the skin surface of a body part to inject light in a tissue of said part , the wavelengths of the light emitted by said second and third means being different from each other light detecting means located at a relatively short distance from said first light emitting means and at relatively long distance from said second light emitting means and said third light emitting means , for collecting at the skin surface light of said emitting means having travelled through said tissue , first computing means for denoising the output signals of said long distance light detecting means from the output signals of said short distance light detecting means , and second computing means for deriving oximetry measurements from the denoised output signals of said long distance light detecting means .