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Robust Pulse-Rate from Chrominance-Based Rppg Gerard De Haan and Vincent Jeanne

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Robust Pulse-Rate from Chrominance-Based Rppg Gerard De Haan and Vincent Jeanne IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. ?, NO. ?, MONTH 2013 1 Robust pulse-rate from chrominance-based rPPG Gerard de Haan and Vincent Jeanne Abstract—Remote photoplethysmography (rPPG) enables con- PPG signal into two independent signals built as a linear tactless monitoring of the blood volume pulse using a regular combination of two color channels [8]. One combination camera. Recent research focused on improved motion robustness, approximated the clean pulse-signal, the other the motion but the proposed blind source separation techniques (BSS) in RGB color space show limited success. We present an analysis of artifact, and the energy in the pulse-signal was minimized the motion problem, from which far superior chrominance-based to optimize the combination. Poh et al. extended this work methods emerge. For a population of 117 stationary subjects, we proposing a linear combination of all three color channels show our methods to perform in 92% good agreement (±1:96σ) defining three independent signals with Independent Compo- with contact PPG, with RMSE and standard deviation both a nent Analysis (ICA) using non-Gaussianity as the criterion factor of two better than BSS-based methods. In a fitness setting using a simple spectral peak detector, the obtained pulse-rate for independence [5]. Lewandowska et al. varied this con- for modest motion (bike) improves from 79% to 98% correct, cept defining three independent linear combinations of the and for vigorous motion (stepping) from less than 11% to more color channels with Principal Component Analysis (PCA) [6]. than 48% correct. We expect the greatly improved robustness to With both Blind Source Separation (BSS) techniques, the considerably widen the application scope of the technology. component that carries the pulse-signal is a priori unknown. Commonly, the selection assumes the pulse-signal shows the strongest periodicity. Consequently, periodic motion as in a I. INTRODUCTION fitness setting will render the selection useless. Moreover, a HOTOPLETHYSMOGRAPHY (PPG) is an optical tech- fairly long observation interval is required to have sufficient P nique to monitor various vital signs, like pulse-rate, resolution in the frequency domain, which prohibits adaptation respiratory rate and blood oxygenation, first described in the to quickly changing statistics. 1930s [1] and highly popular because of its non-invasiveness. In Section II of this paper, we shall analyze how motion Essentially, PPG detects the optical absorption variations of enters the pulse-signal. From this analysis new techniques the human skin due to blood volume variations during the for rPPG emerge that shall be shown superior to all earlier cardiac cycle. methods both in SNR and motion robustness. In Section III, Earlier work has shown that these variations can also be we provide the assement details, the results of which are shown measured at a distance leading to remote-PPG (rPPG) [2], [3]. for 117 stationary subjects over a broad range of skin-types Publications have even shown successful rPPG using a regular in Section IV, and for some subjects exercising in a gym to color video camera in ambient light conditions [4], [5], [6], test motion robustness in Section V. Finally, we draw our [7]. rPPG is a highly relevant development for cases where conclusions in Section VI. contact has to be prevented because of extreme sensitivity, e.g. neonates, skin-damage, or when an increased unobtrusiveness is required/desired (surveillance, fitness). II. ANALYSIS The main concern with rPPG is robustness to subject For the following analysis, we assume an area of skin is motion. Recent research has brought some improvement, but illuminated by a light source and registered with an RGB video significant motion renders current algorithms useless. At this camera. We assume blood volume changes, due to heartbeat, point we aim to contribute by analyzing how motion enters in human skin lead to color changes in the reflected light. The the pulse-signal and deriving far superior motion robust rPPG intensity of a given pixel in image number i in color channel algorithms from the analysis. C 2 fR; G; Bg registered by the camera, can be modeled as: Essentially all motion robust rPPG techniques profit from the fact that the variations in optical absorption of the human Ci = ICi(ρCdc + ρCi); (1) skin depend on the wavelength used, as shown in Figure 1. Motion of the skin relative to the sensor, on the other hand, mostly affects the light reflected or transmitted by the skin regardless the wavelength. Huelsbusch and Verkruijsse, in 2008, found the PPG signal had different relative strength in the three color channels of a video camera pointed at human skin [8], [4]. Huelsbusch first exploited this difference to achieve motion robustness by separating the noise and the Gerard de Haan and Vincent Jeanne are with Philips Innovation Group, Research, Eindhoven, The Netherlands, e-mail: ([email protected], Fig. 1. As reported in [9], the amplitude of the ppg-signal in light reflected [email protected]). from the skin varies as a function of the wavelength, showing a strong peak Manuscript received January 15th, 2013; revised May 6th, 2013. around 550nm and a dip around 650nm. IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. ?, NO. ?, MONTH 2013 2 where ICi is the intensity of the light source integrated over the exposure time of the camera in image i for color channel C, ρCdc is the stationary part of the reflection coefficient of the skin in color channel C, while ρCi is used to indicate the zero-mean time-varying fraction caused by the pulsation of the blood volume. Consequently, the amplitude of the heartbeat induced color variation is proportional to the intensity ICi of the light source in each of the color channels. To produce a pulse-signal that is independent of the presumed stationary color of the light Fig. 2. Illustration of specular and diffuse reflection. Through scattering of source and its brighness level, we can normalize each color the light inside the skin, the diffuse reflection changes color with the blood channel C by dividing its samples by their mean over a volume of the skin, whereas the specular reflection exhibits the color of the light source and is not affected by blood volume changes. temporal interval: Ci Cni = ; (2) µ(Ci) where s is the additive specular reflection contribution. The where µ(C ) can be a running average centered around image i i specular reflection component s is identical for all color i, or an average of an overlap-add processing interval that i channels, whereas the stationary part of the skin reflection, includes image i. In either case, the average is preferably taken ρ , is different for the individual color channels C, with over a number of images such that the interval contains at least Cdc ρ > ρ > ρ [12]. Consequently, the registered a pulse-period. Rdc Gdc Bdc color depends on the specular reflection fraction in the total Normalization, however, cannot prevent influence of in- reflected light. The relative contribution of specular and diffuse during tensity variations the normalization period. A typical reflections, which together make the observed color, depend on situation where this occurs is when the skin surface moves 1 the angles between camera, skin and light source. Therefore with respect to the light source . If we assume that such they vary over time with motion of the person in front of intensity modulations are equal for all channels, a ratio of two the camera, and create a weakness in the proposal using normalized color channels would not be affected by motion. normalized red over green, as the additive specular component The pulsatility of the blood would still be available in this ratio is not eliminated in the ratio. Figure 2 illustrates the situation. provided that the pulsatility due to blood volume changes is Recognizing this weakness, we see a possible improvement different in the individual color channels. Given the pulsatility by adding the third color channel. If we, initially, assume white as a function of wavelength exhibits a strong peak in green light, we note that the specular reflection affects all channels and dips in red [9], [10], as illustrated in Figure 1, the ratio of by adding an identical (white light) specular fraction to their normalized green and red would make a motion robust pulse respective diffuse reflection component. This implies that we signal S : i can eliminate the specular reflection component by using color Gni Gi µ(Ri) difference, i.e. chrominance, signals. From three color chan- Si = − 1 = : − 1: (3) 2 Rni Ri µ(Gi) nels, e.g. RGB , we can build two orthogonal chrominance signals, e.g. X = R − G and Y = 0:5R + 0:5G − B3. We shall refer to this method as RoverG. We note that this Again the variations due to the blood volume changes in method is related to a method suggested by Huelsbusch [8], the skin will likely be different, while motion affects both where a weighted difference of red and green is suggested. chrominance signals identically. A ratio of the two would be The relation between ratios and differences shall be elaborated an interesting candidate rPPG algorithm that we shall reference 4 later in this section. to as XoverY : Xn In practice, the RoverG method works less perfect, since S = − 1 (5) Y the light reflected from the skin consists of 2 components as n described by the dichromatic reflection model [11]: Although this algorithm uses normalized color difference signals, X and Y , the color channels themselves, R; G; B, • A diffuse-, or body-, reflection component, which has n n traveled through the skin and shows its color including are not normalized and the algorithm will be imperfect for non- variations thereof due to the cardiac cycle; and white illumination.
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