History of Photoplethysmography: From 1930s Invention to Modern Wearables

Photoplethysmography (PPG) has undergone one of the most remarkable transformations in biomedical sensing history -- from a laboratory curiosity built with vacuum tubes and galvanometers in 1937 to a technology embedded in over two billion consumer devices today. This article traces the complete arc of PPG development across nine decades, examining the key scientific breakthroughs, engineering innovations, and clinical milestones that shaped the field. Understanding this history is essential for researchers and engineers working on the next generation of PPG-based health monitoring. For a technical introduction to how modern PPG works, see our PPG technology overview.

Precursors: Light and Blood Before PPG (1870s-1930s)

The idea that light interacting with living tissue could reveal information about blood flow predates PPG by decades. In 1876, Karl von Vierordt published his studies on the spectrophotometric properties of blood, demonstrating that hemoglobin absorbed specific wavelengths of light and that this absorption varied with oxygenation state. This work laid the physical foundation for all optical blood sensing methods.

In the early 1930s, Kurt Kramer developed the first in-vivo oximeter by passing light through the ear of an animal and measuring changes in transmitted intensity at different wavelengths (Kramer, 1935). His approach required heating the ear to "arterialize" capillary blood and could not distinguish arterial from venous contributions -- a limitation that would persist for four decades until Aoyagi's breakthrough.

Simultaneously, researchers were exploring photoelectric methods for measuring peripheral blood flow. Molitor and Kniazuk (1936) described a photoelectric device for recording blood volume changes in the rabbit ear, using a light source and photocell arrangement that bore strong similarities to what Hertzman would formalize the following year. These converging lines of investigation created the intellectual environment for PPG's emergence.

Hertzman and the Birth of PPG (1937-1950s)

Alrick B. Hertzman at the University of Saint Louis is credited with establishing photoplethysmography as a defined technique. In his 1937 publication, "The blood supply of various skin areas as estimated by the photoelectric plethysmograph," Hertzman described an instrument consisting of a light bulb directed at tissue and a photocell positioned to detect transmitted or reflected light (Hertzman, 1937). The photocell output was connected to a galvanometer that traced the pulsatile blood volume signal on a moving paper strip.

Hertzman's key insight was that the pulsatile variations in the optical signal directly corresponded to arterial blood volume changes driven by the cardiac cycle. He systematically characterized signals from the finger, toe, and ear, establishing that the waveform morphology reflected local vascular tone and systemic hemodynamics. His apparatus was large -- the light source alone required substantial power -- but the principle was elegant and generalizable.

Throughout the 1940s, Hertzman and his collaborators refined the technique. They demonstrated that the PPG waveform amplitude was modulated by sympathetic nervous system activity, temperature, and vasoactive drugs, establishing PPG as a window into peripheral vascular physiology. Hertzman's 1938 paper introduced the term "photoelectric plethysmograph," though the abbreviation "PPG" did not become standard until much later (Hertzman, 1938).

During this period, the instrumentation remained cumbersome. Vacuum tube amplifiers, analog galvanometers, and incandescent light sources limited PPG to laboratory settings. The signals were noisy, and no standardized analysis methods existed. Nevertheless, clinicians began using PPG to assess peripheral vascular disease, Raynaud's phenomenon, and vasomotor responses.

The Pulse Oximetry Revolution (1960s-1980s)

The 1960s brought a transformative shift. The development of semiconductor light-emitting diodes (LEDs) and silicon photodiodes eliminated the need for incandescent bulbs and vacuum tubes. Robert Shaw at Hewlett-Packard developed an eight-wavelength ear oximeter in 1964 that represented a significant engineering advance, though it still required ear heating and was too complex for routine clinical use (Severinghaus & Honda, 2007).

The pivotal breakthrough came from Takuo Aoyagi, a bioengineer at Nihon Kohden Corporation in Tokyo. In 1974, Aoyagi realized that the pulsatile component of the optical signal -- the very feature Hertzman had characterized -- could be isolated and used to compute arterial oxygen saturation without the need for tissue heating or calibration against venous blood (Aoyagi et al., 1974). His insight was deceptively simple: since the pulsatile signal originates exclusively from arterial blood (venous blood does not pulsate at cardiac frequency), the ratio of pulsatile absorbances at two wavelengths (typically red at 660 nm and infrared at 940 nm) could be mapped to arterial oxygen saturation (SpO2).

This ratio-of-ratios approach -- R = (AC_red/DC_red) / (AC_IR/DC_IR) -- eliminated the confounding effects of tissue thickness, skin pigmentation, venous blood, and non-blood absorbers. The relationship between R and SpO2 was empirically calibrated by comparing pulse oximeter readings against co-oximetry of drawn arterial blood samples, yielding the characteristic calibration curve used in all modern pulse oximeters.

The commercial impact was enormous. Nellcor, founded by William New and Jack Lloyd in 1981, produced the N-100 finger pulse oximeter that became the first widely adopted clinical device. By the late 1980s, pulse oximetry was adopted as a standard of care in anesthesia by the American Society of Anesthesiologists. Studies showed that intraoperative pulse oximetry reduced the incidence of undetected hypoxemia by more than 90% (Moller et al., 1993). The technology spread rapidly to intensive care units, emergency departments, and neonatal units.

The pulse oximetry era fundamentally changed PPG's status from a research tool to a life-saving clinical technology. For the first time, PPG hardware needed to meet stringent regulatory standards for accuracy (typically +/- 2% SpO2 over the 70-100% range), reliability, and manufacturing quality. This drove major investments in LED consistency, photodetector sensitivity, and signal processing algorithms -- advances that would later benefit the entire PPG ecosystem.

Reflectance Mode and Signal Processing Advances (1980s-2000s)

While pulse oximetry dominated the clinical landscape, academic research expanded PPG's measurement capabilities and form factors. A critical development was the maturation of reflectance-mode PPG. Unlike Hertzman's transmission approach, which required placing tissue between the light source and detector (limiting measurements to fingertips, earlobes, and toes), reflectance PPG placed both components on the same tissue surface. This enabled measurements from virtually any body site -- wrist, forehead, chest, and even the esophagus.

Mendelson and Ochs (1988) published influential work characterizing reflectance PPG sensor design, demonstrating that source-detector spacing of 3-5 mm optimized the pulsatile signal amplitude for green light at the wrist. Their work established design principles still used in modern wearable sensors.

The 1990s saw rapid advances in digital signal processing applied to PPG. As microcontrollers became powerful enough for real-time computation, researchers developed algorithms for heart rate extraction, respiratory rate estimation from PPG amplitude modulation, and automated beat detection. The seminal work of Allen (2007) provided a comprehensive review of PPG physiology and applications that remains one of the most cited references in the field (Allen, 2007; DOI: 10.1088/0967-3334/28/3/R01).

Continuous blood pressure estimation from PPG emerged as an active research area during this period. The relationship between pulse transit time (PTT) and blood pressure, grounded in the Moens-Korteweg equation, motivated numerous studies combining ECG and PPG to estimate beat-to-beat pressure changes. However, the need for individual calibration and poor long-term stability limited clinical adoption. For a detailed technical treatment of these methods, see our PPG blood pressure estimation guide.

Heart rate variability (HRV) analysis from PPG also gained traction. Researchers demonstrated that inter-beat intervals derived from PPG pulse peaks (pulse rate variability, or PRV) correlated well with ECG-derived HRV under resting conditions, with correlation coefficients exceeding r = 0.99 for time-domain metrics like SDNN and RMSSD (Gil et al., 2010; DOI: 10.1109/TBME.2009.2025659). This finding would later become central to the wellness features of consumer wearables. For more on HRV measurement, visit our HRV analysis page.

Multi-Wavelength PPG and Advanced Morphology Analysis (2000s-2010s)

The 2000s introduced multi-wavelength PPG as a research tool extending beyond the two-wavelength approach of pulse oximetry. Researchers explored green (525 nm), blue (470 nm), and near-infrared (850-950 nm) wavelengths for different applications. Green light emerged as the preferred wavelength for heart rate monitoring at the wrist because of its shallower tissue penetration depth (~1-2 mm) and higher pulsatile signal strength from the superficial microvasculature (Maeda et al., 2011; DOI: 10.1007/s10877-011-9279-y).

PPG waveform morphology analysis became increasingly sophisticated. The second derivative of the PPG waveform (SDPPG or acceleration plethysmogram) was shown to contain features correlated with arterial stiffness and vascular aging. Takazawa et al. (1998) demonstrated that the SDPPG aging index correlated with age (r = 0.80) and with the augmentation index measured by arterial tonometry (r = 0.71), suggesting PPG could provide a non-invasive window into vascular health (Takazawa et al., 1998; DOI: 10.1038/hr.1998.8). Modern approaches to vascular assessment are covered in our PPG vascular age assessment guide.

This period also saw the first attempts at PPG-based glucose monitoring. Near-infrared wavelengths (1000-1700 nm) were explored for detecting glucose-related absorption changes, though the signal-to-noise ratio remained far too low for clinically useful measurements. This challenge persists today, as discussed in our PPG glucose monitoring progress article.

The Consumer Wearable Revolution (2011-2020)

The consumer wearable revolution began in earnest with the introduction of optical heart rate monitors in fitness devices. The Mio Alpha, launched in 2012, was among the first wrist-worn devices to use reflectance PPG for continuous heart rate monitoring during exercise. It employed green LEDs and a silicon photodiode with proprietary motion artifact rejection algorithms.

Apple's entry into the market with the Apple Watch in 2015 was a watershed moment. The device included a custom PPG sensor with four green LEDs and four photodiodes, sampling at rates configurable from 1 to approximately 50 Hz. By 2020, over 100 million Apple Watches had been sold, making PPG one of the most widely deployed biosensing technologies in history.

The Apple Heart Study (2019), conducted in collaboration with Stanford Medicine and enrolling 419,297 participants, demonstrated that a smartwatch PPG-based irregular rhythm notification had a positive predictive value of 84% for atrial fibrillation when followed up with ECG patch monitoring (Perez et al., 2019; DOI: 10.1056/NEJMoa1901183). The study's scale was unprecedented in digital health research and contributed to the FDA's clearance of Apple's irregular rhythm notification feature.

Samsung, Garmin, Fitbit (now Google), Oura, and WHOOP followed with their own PPG-based health monitoring platforms. The Oura Ring demonstrated that a ring-based PPG sensor on the palmar digital artery could achieve heart rate accuracy within 1.5 BPM mean absolute error compared to ECG during sleep (de Zambotti et al., 2018; DOI: 10.3390/s18010125). Consumer demand for SpO2 monitoring surged during the COVID-19 pandemic, accelerating the integration of red and infrared LEDs alongside green in consumer devices.

Signal processing in consumer devices advanced rapidly during this era. Multi-channel sensor arrays with 2-8 photodetectors became common, enabling spatial diversity techniques for motion artifact rejection. Accelerometer-referenced adaptive filtering, spectral peak tracking, and increasingly, deep learning-based algorithms powered real-time heart rate estimation during exercise with mean absolute errors below 2 BPM in many scenarios. For detailed coverage of these algorithms, see our motion artifact removal guide and our algorithms overview.

Regulatory Milestones and Clinical Validation (2018-Present)

The transition of PPG from wellness monitoring to regulated medical applications accelerated after 2018. The FDA's De Novo classification of Apple Watch's ECG and irregular rhythm notification features (2018) established a regulatory pathway for software-based cardiac monitoring on consumer hardware. Subsequent clearances for PPG-based SpO2 measurement, atrial fibrillation screening, and sleep apnea detection followed from multiple manufacturers.

The Samsung Galaxy Watch received FDA clearance for a PPG-based blood pressure monitoring feature that requires periodic cuff calibration (2020, initially in South Korea). While not a fully cuffless solution, it represented the first regulatory acceptance of PPG for blood pressure trending in a consumer device.

In the European Union, the Medical Device Regulation (MDR 2017/745) imposed stricter requirements for clinical evidence, pushing manufacturers toward larger validation studies. The Fitbit Irregular Heart Rhythm Notification (2022) was validated in a study of 455,699 participants, demonstrating a positive predictive value of 98% among those who completed ECG follow-up (Lubitz et al., 2022; DOI: 10.1016/j.jacc.2022.08.801).

These regulatory milestones have created a new class of PPG applications occupying a middle ground between consumer wellness and traditional medical devices. Understanding the accuracy differences between clinical and consumer PPG is essential context for this ongoing evolution -- see our coverage of medical vs. consumer PPG accuracy.

Current State and Technical Frontiers (2024-2026)

Modern PPG technology bears little resemblance to Hertzman's original apparatus. A contemporary multi-wavelength PPG sensor module measures approximately 3 x 3 x 1.5 mm, integrates 4-8 LEDs across 3-5 wavelengths (green, red, infrared, and sometimes yellow and blue), multiple photodiodes, an ambient light sensor, and a dedicated analog front-end with 18-22 bit ADC resolution. Total power consumption for continuous monitoring is 0.5-2.0 mW, enabling multi-day battery life in wrist-worn devices.

Research frontiers in 2025-2026 include calibration-free cuffless blood pressure estimation, continuous glucose monitoring using advanced optical configurations, mental health biomarker extraction from PPG-derived autonomic nervous system metrics, and multi-modal sensor fusion combining PPG with bioimpedance, electrodermal activity, and inertial measurement. For a comprehensive view of where the field is headed, see our future directions article.

The computational infrastructure supporting PPG has also transformed. Cloud-based deep learning models process billions of heartbeats per day from consumer devices, enabling population-scale epidemiological research that Hertzman could not have imagined. Edge AI accelerators in modern wearable chipsets run neural network inference locally, enabling real-time complex analysis without cloud connectivity.

Timeline Summary

• 1876: Vierordt demonstrates spectrophotometric properties of blood
• 1935: Kramer develops first in-vivo ear oximeter
• 1937: Hertzman publishes foundational PPG work
• 1974: Aoyagi invents pulse oximetry at Nihon Kohden
• 1981: Nellcor N-100 becomes first commercial finger pulse oximeter
• 1988: Mendelson and Ochs characterize reflectance PPG design
• 1998: Takazawa demonstrates SDPPG for vascular assessment
• 2007: Allen publishes comprehensive PPG review (cited >3,500 times)
• 2012: Mio Alpha launches first wrist-based optical HR monitor
• 2015: Apple Watch brings PPG to mass consumer market
• 2018: FDA clears Apple Watch irregular rhythm notification
• 2019: Apple Heart Study enrolls 419,297 participants
• 2022: Fitbit AFib study validates PPG screening in 455,699 participants
• 2024-2026: Active pursuit of cuffless BP, glucose, and mental health applications

Conclusion

The history of photoplethysmography is a story of compounding innovation. Each decade built upon the last: Hertzman's basic science enabled Aoyagi's pulse oximetry breakthrough, which drove the semiconductor sensor development that made consumer wearables possible, which in turn generated the massive datasets fueling today's machine learning approaches. The technology has evolved from a room-sized apparatus measuring one signal at one body site to a millimeter-scale sensor capable of extracting dozens of physiological parameters from nearly any location on the body.

For researchers and engineers entering the PPG field today, this historical context provides essential perspective. Many "new" ideas -- multi-wavelength analysis, morphology-based vascular assessment, continuous blood pressure estimation -- have roots stretching back decades. The current wave of innovation is distinguished not by wholly novel physics but by unprecedented scale, computational power, and clinical ambition. The next chapter of PPG's history is being written by those who understand both its proven capabilities and its persistent limitations.

Explore the technical foundations further in our PPG technology learning center and algorithms reference.