--- title: Blood Oxygenation Sensing Principles for Optical Medical Device Development description: Discussing the technical principles of optical oxygenation sensing in human tissue and how tissue phantoms can help accelerate medical device development. image: https://blog.quelimaging.com/hubfs/undefined-Oct-22-2026-06-43-17-5850-PM.png --- [Skip to content](https://blog.quelimaging.com/blood-oxygenation-sensing-principles#main-content) [![QUEL\_web\_logo-1](https://blog.quelimaging.com/hs-fs/hubfs/QUEL_web_logo-1.png?width=320&height=120&name=QUEL_web_logo-1.png)](https://shop.quelimaging.com/) - [Home](https://www.quelimaging.com/) - [Products](https://shop.quelimaging.com/) - [Blog](https://blog.quelimaging.com/) - [News](https://www.quelimaging.com/news) - [About](https://www.quelimaging.com/about) Open main navigation Close main navigation - [Home](https://www.quelimaging.com/) - [Products](https://shop.quelimaging.com/) - [Blog](https://blog.quelimaging.com/) - [News](https://www.quelimaging.com/news) - [About](https://www.quelimaging.com/about) - [Contact us](https://share.hsforms.com/1qlwaYoKVQvGnvWZt0GNGagcxyzv) [Contact us](https://share.hsforms.com/1qlwaYoKVQvGnvWZt0GNGagcxyzv) ### View of Field Oct 29, 2026, 6:59:21 AM # Blood Oxygenation Sensing Principles for Optical Medical Device Development ![Picture of Alberto Ruiz](https://blog.quelimaging.com/hs-fs/hubfs/Alberto2.jpg?width=50&name=Alberto2.jpg) [Alberto Ruiz](https://blog.quelimaging.com/author/alberto-ruiz) Share: [facebook-f icon](http://www.facebook.com/share.php?u=https://blog.quelimaging.com/blood-oxygenation-sensing-principles) [linkedin-in icon](http://www.linkedin.com/shareArticle?mini=true&url=https://blog.quelimaging.com/blood-oxygenation-sensing-principles) [Twitter icon](https://twitter.com/intent/tweet?url=https://blog.quelimaging.com/blood-oxygenation-sensing-principles) [pinterest-p icon](http://pinterest.com/pin/create/link/?url=https://blog.quelimaging.com/blood-oxygenation-sensing-principles) [envelope icon](mailto:?body=https://blog.quelimaging.com/blood-oxygenation-sensing-principles) Optical blood oxygenation sensing supports medical applications ranging from routine patient monitoring to assessing tissue oxygenation during surgery. These technologies are possible because oxygenated and deoxygenated hemoglobin absorb light differently. These spectral differences can be seen in [the color of blood,](https://blog.quelimaging.com/if-blood-is-red-why-are-veins-blue) with oxygen-rich blood appearing bright red and less oxygenated blood appearing darker red. Additionally, the oxygen-dependent absorption properties of hemoglobin extend beyond the visible spectrum into the near-infrared, enabling technologies such as pulse oximetry and tissue oxygenation imaging. In this post, we explore the spectral properties behind these measurements, how two major sensing approaches use them, and why tissue makes quantitative measurement more challenging. ## The Spectral Fingerprint of Oxygenated and Deoxygenated Blood ![](https://blog.quelimaging.com/hs-fs/hubfs/undefined-Oct-22-2026-06-43-17-5850-PM.png?width=411&height=391&name=undefined-Oct-22-2026-06-43-17-5850-PM.png)Hemoglobin, the oxygen-carrying protein in red blood cells, has distinct optical absorption spectra in its oxygenated (O₂Hb) and deoxygenated (HHb) forms. When oxygen binds to hemoglobin, it alters the electronic environment, changing how strongly it absorbs light at different wavelengths. These spectral differences are evident across the visible and near-infrared regions. Both forms of hemoglobin absorb strongly in the violet-blue region (400 - 450 nm) through the **Soret band**, whose peak position changes with oxygenation. Between approximately 500 and 600 nm, oxyhemoglobin exhibits two prominent absorption peaks near 542 and 577 nm (the Q-band), while deoxyhemoglobin has a broader peak near 555 nm. At longer wavelengths, the relative absorption of the two forms also differs significantly. Deoxyhemoglobin absorbs more strongly than oxyhemoglobin throughout much of the 600–750 nm region, including near 660 nm, while oxyhemoglobin absorbs more strongly at selected near-infrared wavelengths, including near 940 nm. These opposing absorption characteristics provide the spectral contrast used in optical oxygenation measurements. The spectra also intersect at **isosbestic wavelengths**, where O₂Hb and HHb have equal absorption per unit concentration. One commonly cited crossing occurs near 805 nm. At a fixed total hemoglobin concentration, absorption at this wavelength is approximately independent of oxygen saturation, making it a useful reference for distinguishing changes in hemoglobin concentration from oxygenation levels. However, measured tissue reflectance at isosbestic wavelengths remains affected by scattering, other absorbers, and measurement geometry. > *Note on notation:* > > *Oxyhemoglobin is commonly abbreviated as O₂Hb or HbO₂ > Deoxyhemoglobin may be written as HHb or Hb.* > > *Since "Hb" can also refer to hemoglobin generally, we use O₂Hb and HHb throughout this article, consistent with* [*ISO/FDIS 80601-2-61*](https://www.iso.org/standard/84595.html) *terminology.* | **Abbreviation** | **Definition** | | --- | --- | | **O₂Hb / HHb** | Oxygenated / deoxygenated hemoglobin | | **cO₂Hb / cHHb** | Concentrations of the two hemoglobin species | | **SO₂** | Functional hemoglobin oxygen saturation: cO₂Hb / (cO₂Hb + cHHb) | | **SaO₂** | Functional oxygen saturation of arterial blood | | **SpO₂** | Pulse oximeter estimate of SaO₂ | | **StO₂** | Instrument-dependent estimate of hemoglobin oxygen saturation in sampled tissue | | **ctHb** | Total concentration of all hemoglobin species | | **R** | Pulse oximetry modulation ratio (ratio of ratios): ratio of normalized pulsatile changes in detected red and infrared light, approximately (ACred/DCred)/(ACIR/DCIR), used to estimate SpO₂ through empirical calibration | | **AC / DC** | Pulsatile (AC) and baseline (DC) components of the detected optical signal | ## From Spectral Absorption to Oxygenation Measurements These differences in hemoglobin absorption provide the physical basis for optical oxygenation sensing. **Functional hemoglobin oxygen saturation (SO₂)** describes the concentration of oxyhemoglobin relative to the combined concentrations of oxyhemoglobin and deoxyhemoglobin: `SO₂ = cO₂Hb / (cO₂Hb + cHHb) × 100%` Here, *cO₂Hb* and *cHHb* represent the concentrations of the respective hemoglobin species. While the relative concentrations determine oxygen saturation, the magnitude of optical absorption also depends on the total amount of hemoglobin present. In tissue, additional factors such as blood volume fraction, the spatial distribution of blood vessels, scattering, and the paths light takes through tissue further influence the detected signal. Importantly, changes in blood volume fraction do not translate linearly into measured tissue reflectance. Optical oxygenation techniques therefore require specific measurement approaches, models, or calibration methods to estimate oxygen saturation reliably. ## Pulse Oximetry: Estimating Arterial Oxygen Saturation Conventional pulse oximeters commonly use red light near 660 nm and infrared light near 940 nm, where the two hemoglobin species exhibit different absorption. Their defining feature is the use of **time-dependent optical changes produced by pulsatile arterial blood flow**. By comparing the relative pulsatile modulation at the two wavelengths, the device calculates a **modulation ratio**, also called the **ratio of ratios (R)**. An empirical calibration relationship then converts R into **SpO₂**, a noninvasive estimate of arterial functional oxygen saturation (**SaO₂**). Reference SaO₂ measurements used to evaluate pulse oximeter accuracy come from arterial blood samples analyzed by a CO-oximeter. A future post will explore the pulsatile signal, calibration, and effects of perfusion, motion, and skin pigmentation on measurement accuracy. ## Tissue Oxygenation: Looking Beyond the Arterial Pulse The same spectral differences support tissue oxygenation measurements using near-infrared spectroscopy (NIRS), multispectral imaging, and hyperspectral imaging. Rather than relying on the arterial pulse, many of these methods analyze wavelength-dependent attenuation or reflectance from tissue containing both oxygenated and deoxygenated hemoglobin. Because sampled tissue contains arterial, capillary, and venous blood, the resulting estimate, often reported as **tissue oxygen saturation (StO₂)**, reflects a mixture of vascular compartments. Its interpretation depends on the instrument, tissue sampled, and measurement model; StO₂ is not interchangeable with SpO₂ or tissue oxygen partial pressure. [Scattering](https://blog.quelimaging.com/understanding-fluorescence-depth-sensitivity), [background absorbers](https://blog.quelimaging.com/light-tissue-interaction-overview) such as melanin and water, and tissue geometry further complicate quantitative estimates. We'll examine these effects and approaches for separating them in a dedicated post on tissue oxygenation. ## Why Controlled Optical References Matter ![](https://blog.quelimaging.com/hs-fs/hubfs/undefined-Oct-22-2026-06-43-17-9093-PM.png?width=376&height=570&name=undefined-Oct-22-2026-06-43-17-9093-PM.png)Device developers need to understand how systems respond to oxygenation-dependent absorption as well as changes in blood-equivalent content, tissue scattering, and geometry. **Tissue-mimicking phantoms** provide stable, reproducible optical properties for testing spectral sensitivity, instrument repeatability, and algorithm robustness. For example, a 3D-printed colon-tissue phantom can reproduce selected absorption and scattering characteristics representative of different modeled oxygenation conditions, without reproducing living blood physiology. At QUEL Imaging, we develop such controlled optical references to help characterize device performance and isolate measurement variability. Our phantoms can help teams control for biological variability in skin tone, perfusion, and various physiological conditions that impact measurementy accuracy.   Phantom testing complements biological and clinical studies. In particular, static phantoms cannot establish clinical SpO₂ accuracy by itself, where appropriate arterial reference measurements. Rather, tissue phantoms provide a practical tool for oxygen sensing device development before trialing prototypes in live tissue models.  ## Conclusion Oxygen binding gives hemoglobin distinct absorption spectra, enabling optical oxygenation sensing. Pulse oximetry and tissue spectroscopy exploit these spectral differences in different ways, each requiring its own measurement and validation approach. In upcoming posts, we will explore how each technique turns optical signals into reliable oxygenation estimates. --- Developing an optical system to measure tissue physiology? QUEL Imaging provides reference-grade tissue phantoms, custom tools, and optical-characterization services to help teams accelerate translation from R&D through validation, regulatory evaluation, and adoption. [Talk with our team](https://quelimaging.com/#contact) about your project. [concepts](https://blog.quelimaging.com/tag/concepts), [clinical translation](https://blog.quelimaging.com/tag/clinical-translation), [optical properties](https://blog.quelimaging.com/tag/optical-properties), [tissue optics](https://blog.quelimaging.com/tag/tissue-optics), [optiblox](https://blog.quelimaging.com/tag/optiblox) ## Related posts [![Q-S Optiblox Product Demonstration illuminated with red and green lasers](https://blog.quelimaging.com/hs-fs/hubfs/Screenshot%202025-10-24%20at%2012.19.11%20PM.png?height=200&name=Screenshot%202025-10-24%20at%2012.19.11%20PM.png)](https://blog.quelimaging.com/optiblox-qs-overview) [optical properties](https://blog.quelimaging.com/tag/optical-properties), [tissue optics](https://blog.quelimaging.com/tag/tissue-optics), [optiblox](https://blog.quelimaging.com/tag/optiblox) ## [Soft Tissue Phantoms for Medical Device Development: Unlocking Tissue Compliance](https://blog.quelimaging.com/optiblox-qs-overview) ![Picture of The QUEL Team](https://blog.quelimaging.com/hs-fs/hubfs/QUEL_Q.png?width=50&name=QUEL_Q.png) [The QUEL Team](https://blog.quelimaging.com/author/quel) Oct 30, 2025, 6:45:00 AM When you press a pulse oximeter against your finger, the tissue deforms. 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Main St. Suite 142 White River Junction, VT, USA Copyright © 2026, QUEL Imaging ```json { "@context" : "https://schema.org", "@type" : "BlogPosting", "author" : { "@type" : "Person", "name" : "Alberto Ruiz", "url" : "https://blog.quelimaging.com/author/alberto-ruiz" }, "dateModified" : "2026-10-29T10:59:21.374Z", "datePublished" : "2026-10-29T10:59:21.000Z", "headline" : "Blood Oxygenation Sensing Principles for Optical Medical Device Development", "image" : [ "https://blog.quelimaging.com/hubfs/undefined-Oct-22-2026-06-43-17-5850-PM.png" ], "mainEntityOfPage" : { "@id" : "https://blog.quelimaging.com/blood-oxygenation-sensing-principles", "@type" : "WebPage" }, "publisher" : { "@type" : "Organization", "logo" : { "@type" : "ImageObject", "url" : "https://blog.quelimaging.com/hubfs/QUEL_Q.png" }, "name" : "QUEL Imaging" } } ```