Hemoglobin changes color depending on how much oxygen it carries, and every shade corresponds to a measurable shift in how the molecule absorbs light. The hemoglobin absorption spectrum is the precise map of which wavelengths of light hemoglobin soaks up and which it lets pass through, across the ultraviolet, visible, and near-infrared range. Because oxygenated and deoxygenated hemoglobin absorb light differently, that spectrum has become the foundation for an unexpectedly wide range of technologies, from the pulse oximeter clipped to your finger during a routine checkup to forensic tools that estimate how old a bloodstain is.
Why Hemoglobin Absorbs Light the Way It Does
Hemoglobin’s light-absorbing power comes from its heme group, a ring-shaped structure with an iron atom at its center. When oxygen binds to that iron, the electronic structure of the heme shifts, which changes how strongly it absorbs different wavelengths. The result is two distinct spectral profiles: oxygenated hemoglobin (often written as O₂Hb) and deoxygenated hemoglobin (HHb). In the red part of the visible spectrum, around 660 nanometers, deoxygenated hemoglobin absorbs considerably more light than oxygenated hemoglobin. In the near-infrared range, around 940 nanometers, the relationship flips: oxygenated hemoglobin absorbs more. This crossover is the single most exploited feature of the hemoglobin spectrum in clinical medicine.
Beyond just oxy and deoxy forms, hemoglobin also exists in other clinically relevant states. Carboxyhemoglobin forms when carbon monoxide binds to the iron instead of oxygen. Methemoglobin occurs when the iron atom is oxidized to a state that cannot carry oxygen at all. Each of these derivatives has its own absorption profile across the visible and near-infrared range. Researchers have mapped the absorptivities of all four major hemoglobin derivatives across wavelengths from 450 to 1,000 nanometers for both fetal and adult blood, providing the reference data that virtually every hemoglobin-sensing device relies on.1PubMed. Absorption spectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhemoglobin, and methemoglobin
Pulse Oximetry
The most familiar application of hemoglobin’s absorption spectrum is pulse oximetry, the small clip-on device used in hospitals, clinics, and even some smartwatches. A pulse oximeter works by shining two wavelengths of light, typically one red and one near-infrared, through a thin part of the body like a fingertip or earlobe. Because oxygenated hemoglobin absorbs more near-infrared light and deoxygenated hemoglobin absorbs more red light, the ratio of light detected at each wavelength reveals the proportion of hemoglobin carrying oxygen.2PubMed. “Seeing red” reflects hemoglobin’s saturation state: a discovery-based activity for understanding the science of pulse oximetry The device isolates the pulsating component of the signal, which corresponds to arterial blood being pushed through the tissue with each heartbeat, and uses it to calculate arterial oxygen saturation.3PubMed Central. Pulse oximetry: fundamentals and technology update
The elegance of this approach is that it requires no blood draw and provides a continuous reading. But the simplicity comes with a trade-off: the device assumes it is dealing with only two absorbers, oxygenated and deoxygenated hemoglobin. In someone with significant levels of carboxyhemoglobin from carbon monoxide poisoning, for instance, a standard two-wavelength pulse oximeter can give a falsely reassuring oxygen saturation reading. Measuring carboxyhemoglobin and methemoglobin requires additional wavelengths. Laboratory spectrophotometric methods that specifically target these derivatives use absorbance in the Soret region, around 420 and 432 nanometers, where hemoglobin absorbs most strongly, along with chemical preparation steps that isolate each variant for accurate measurement.4Clinical Chemistry. Spectrophotometric measurement of carboxyhemoglobin and methemoglobin in blood
The Skin Pigmentation Problem
Pulse oximeters were calibrated primarily on lighter-skinned volunteers, and this has led to a well-documented accuracy gap. Melanin in the skin absorbs light too, and its absorption profile overlaps with hemoglobin’s in ways that the standard algorithms do not fully account for. The result is a positive bias in darker-skinned patients: the device reads higher than the actual oxygen saturation, particularly when true saturation is low and the clinical stakes are highest.5PubMed Central. Skin Pigmentation Influence on Pulse Oximetry Accuracy: A Systematic Review and Bibliometric Analysis A broad review of the evidence found that accuracy differences across skin pigmentations are significant enough to require particular attention in clinical practice.6PubMed Central. A review of the effect of skin pigmentation on pulse oximeter accuracy
This is not a minor technicality. If a pulse oximeter tells a clinician that a patient’s saturation is 94 percent when it is actually 88 percent, the decision to escalate oxygen therapy may be delayed. The problem has prompted regulatory scrutiny and ongoing engineering efforts to design devices that incorporate additional wavelengths or software corrections to reduce bias across all skin tones.
Seeing the Brain at Work
The same spectral difference between oxygenated and deoxygenated hemoglobin that makes pulse oximetry work also underpins a brain imaging technique called functional near-infrared spectroscopy, or fNIRS. Small light sources and detectors placed on the scalp shine near-infrared light into the brain. When a region of the brain becomes active, local blood flow increases, changing the ratio of oxy- to deoxyhemoglobin in that area. Those changes alter how much near-infrared light is absorbed, and the fNIRS sensors pick up the difference.7PubMed Central. Functional Near-infrared Spectroscopy (fNIRS) of Brain Function During Active Balancing Using a Video Game System
fNIRS is not as spatially precise as an MRI scanner, and it can only reach the outer layers of the brain because near-infrared light scatters and gets absorbed as it travels through tissue. But it has a major practical advantage: the equipment is lightweight, portable, and tolerant of movement. This makes it useful in situations where MRI is impractical, like studying brain activity while someone is walking, exercising, or performing a task in a natural setting. It is also better tolerated by populations that find the confines of an MRI tube distressing, including young children.
Deep Tissue and Neonatal Monitoring
A related but distinct application uses near-infrared spectroscopy to monitor oxygen delivery deep in tissue rather than mapping brain activation patterns. In neonatal intensive care, for example, hybrid devices combining near-infrared spectroscopy with diffuse correlation spectroscopy have been used to monitor cerebral blood flow and oxygen metabolism in critically ill newborns with congenital heart disease. These optical tools can track changes in oxyhemoglobin, deoxyhemoglobin, and total hemoglobin concentrations at the bedside without moving a fragile infant to a scanner.8PubMed Central. Optical measurement of cerebral hemodynamics and oxygen metabolism in neonates with congenital heart defects
The reason near-infrared light is useful for deep-tissue work is that biological tissue is relatively transparent to wavelengths between roughly 650 and 950 nanometers. Water absorbs strongly at longer wavelengths, and hemoglobin and melanin absorb strongly at shorter visible wavelengths, but in this “optical window” the light can penetrate centimeters into tissue before being absorbed or scattered away entirely.9PubMed Central. Emergence of two near-infrared windows for in vivo and intraoperative SERS A second window at longer near-infrared wavelengths, roughly 1,000 to 1,700 nanometers, offers even better tissue penetration and less background interference, and is an active area of development for next-generation imaging.
Photoacoustic Imaging
Photoacoustic imaging takes a creative detour around the limitations of purely optical techniques. Instead of trying to detect dim light after it has scattered through tissue, a photoacoustic system fires short pulses of laser light into the body. Hemoglobin absorbs that light and heats up momentarily, generating a tiny pressure wave, essentially a sound. Ultrasound detectors then pick up these waves to build an image. Because the contrast comes from optical absorption but the detection is acoustic, photoacoustic imaging combines hemoglobin’s strong and wavelength-specific light absorption with the deep-reaching, high-resolution capabilities of ultrasound.10PubMed Central. Photoacoustic imaging for microcirculation
This makes hemoglobin a natural contrast agent, no injection needed. By pulsing light at different wavelengths, researchers can distinguish oxygenated from deoxygenated hemoglobin and map oxygen saturation throughout a volume of tissue.11Photoacoustics. Photoacoustic tomography of blood oxygenation: A mini review Recent work has applied photoacoustic imaging to peripheral arterial disease, using it to capture high-resolution images of peripheral blood vessels, quantify oxygen saturation, and assess microvascular health in ways that could inform treatment decisions.12PubMed Central. Emerging Photoacoustic Imaging Techniques for Peripheral Arterial Disease
Laser Treatment of Vascular Birthmarks
Hemoglobin’s absorption spectrum is not just useful for measurement. It is also the basis for a treatment. Port-wine stains, the reddish-purple birthmarks caused by malformed blood vessels in the skin, are treated with pulsed dye lasers tuned to a wavelength that hemoglobin absorbs strongly. The laser energy is selectively absorbed by the hemoglobin inside the abnormal vessels, generating localized heat that damages the vessel lining and eventually causes the vessel to collapse, while leaving surrounding tissue relatively unharmed.13PubMed Central. A numerical study on the effect of laser therapy against port-wine stain by administering artificial red blood cells This principle, called selective photothermolysis, works precisely because hemoglobin absorbs laser energy at the chosen wavelength far more strongly than the water and other molecules in surrounding tissue.14PubMed Central. A two-temperature model for selective photothermolysis laser treatment of port wine stains
The same principle applies to the laser treatment of spider veins, some rosacea symptoms, and other conditions involving visible superficial blood vessels. The success of the treatment depends on choosing a laser wavelength that aligns well with hemoglobin’s absorption peaks while penetrating deep enough into the skin to reach the target vessels. Too short a wavelength and the light gets absorbed in the outermost layers before reaching the vessel. Too long and hemoglobin does not absorb enough energy to generate the necessary heat.
Measuring Hemoglobin in the Lab
Before any of the optical imaging techniques existed, spectrophotometry was already the workhorse method for measuring how much hemoglobin is in a blood sample. The internationally recognized reference method, called the cyanmethemoglobin method, works by chemically converting all hemoglobin in a sample into a single stable derivative and then measuring its absorbance at 540 nanometers. The amount of light absorbed at that wavelength is directly proportional to the hemoglobin concentration in the sample.15PubMed Central. Methods and analyzers for hemoglobin measurement in clinical laboratories and field settings
Modern clinical analyzers, from full-scale hematology machines to portable point-of-care devices, are all calibrated against this spectrophotometric standard. Studies comparing various clinical devices have found that most stay within about five percent of the reference method across a wide range of hemoglobin concentrations.16PubMed Central. Current Status of Measurement Accuracy for Total Hemoglobin Concentration in the Clinical Context Newer devices have extended this principle beyond blood draws: optical sensors can now estimate hemoglobin concentration in surgical drainage fluid at the bedside, providing a fast, noninvasive way to monitor postoperative bleeding without waiting for a lab result.17PubMed Central. Evaluation of the Hemoglobin Concentration in Drainage Fluid After Surgery by Absorbance Spectrophotometry Using an Optical Sensor
A clever refinement involves isosbestic wavelengths, specific wavelengths where oxygenated and deoxygenated hemoglobin happen to absorb exactly the same amount of light. At these points, the absorption depends only on the total amount of hemoglobin present, regardless of how much is carrying oxygen. Using ratios of reflectance at pairs of isosbestic wavelengths allows researchers to estimate total hemoglobin concentration from optical measurements alone, independent of oxygen saturation and light scattering.18PubMed Central. Rapid ratiometric determination of hemoglobin concentration using UV-VIS diffuse reflectance at isosbestic wavelengths
Aging Bloodstains in Forensic Science
When blood leaves the body, its hemoglobin begins to change. Oxyhemoglobin gradually converts to methemoglobin and then to hemichrome, an irreversible degradation product. Each of these forms has a different absorption spectrum, so the spectral profile of a bloodstain shifts over time. Forensic scientists have exploited this to estimate how old a bloodstain is, a question that can be central to criminal investigations.
One approach uses reflectance spectroscopy to determine the fractions of the three hemoglobin derivatives present in a stain at any given time. Researchers found that these fractions follow a distinct time-dependent pattern: oxy-hemoglobin declines, methemoglobin rises and then falls, and hemichrome accumulates. By measuring these fractions and applying the known reaction kinetics in reverse, investigators estimated the age of bloodstains over a period of up to 60 days, with an uncertainty margin of about two weeks for older stains.19PubMed. Age estimation of blood stains by hemoglobin derivative determination using reflectance spectroscopy
A separate infrared spectroscopy approach has shown even tighter performance for distinguishing fresh stains from older ones. Using infrared absorbance measurements combined with statistical modeling, researchers built models that could estimate bloodstain age up to 107 days under both indoor and outdoor conditions, with strong predictive accuracy. The same method reliably distinguished stains less than a day old from stains older than a day.20Scientific Reports. Estimation of the age of human bloodstains under the simulated indoor and outdoor crime scene conditions by ATR-FTIR spectroscopy Ultraviolet-visible spectroscopy, which directly tracks the chromatic changes in hemoglobin as it degrades, has emerged as one of the most prominent methods for bloodstain dating.21PubMed. Chronology of forensic bloodstain age estimation using UV-Vis spectroscopy – A comprehensive review The underlying principle in every case is the same: hemoglobin’s absorption spectrum changes in predictable ways as the molecule breaks down.
Fetal Versus Adult Hemoglobin
Fetal hemoglobin has a slightly different protein structure than adult hemoglobin, which allows it to pull oxygen away from the mother’s blood across the placenta. That structural difference also produces small but real differences in absorption spectra. The spectral curves for fetal and adult oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin are similar in shape, but the millimolar absorptivities differ enough that using adult calibration data on fetal blood can introduce measurement errors in multi-component analysis.22Clinical Chemistry. Absorption spectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhemoglobin, and methemoglobin
This matters most in neonatal care, where newborns still have a large proportion of fetal hemoglobin circulating. Pulse oximeters and co-oximeters calibrated to adult hemoglobin can produce slightly off readings in newborns. Device manufacturers and researchers have published fetal-specific reference spectra to address this, but not all clinical devices incorporate the correction. If you have ever wondered why neonatal oxygen monitoring is considered trickier than adult monitoring, the subtle spectral mismatch is one of the reasons.
Hemoglobin Spectra in Food Science
Hemoglobin is not unique to humans, and its spectral behavior in other species has practical consequences outside medicine. In fish like Atlantic cod, hemoglobin is highly sensitive to pH changes. When pH drops, cod hemoglobin deoxygenates and undergoes rapid autoxidation, shifting its absorption spectrum in ways that are spectrophotometrically measurable and significantly more dramatic than what happens with mammalian hemoglobin under the same conditions. In human hemoglobin, researchers found a pH-independent isosbestic point at 523 nanometers in the spectra of oxyhemoglobin, deoxyhemoglobin, and methemoglobin. That same isosbestic point does not exist for cod hemoglobin.23PubMed Central / ACS Publications. pH-induced shift in hemoglobin spectra: a spectrophotometeric comparison of atlantic cod (Gadus morhua) and mammalian hemoglobin
These differences have implications for seafood quality and processing. The oxidation of hemoglobin in fish flesh is a major driver of color changes, off-flavors, and lipid oxidation during storage. Understanding how fish hemoglobin’s spectrum shifts with pH and oxygen exposure helps food scientists design better processing and packaging methods. It is a reminder that hemoglobin spectroscopy is not purely a medical tool; the same physics applies wherever the molecule shows up, and the absorption spectrum remains the most direct way to track what is happening to it.
Artificial Blood and Hemoglobin-Based Products
Researchers developing hemoglobin-based oxygen carriers, essentially synthetic alternatives to donated blood, face the same spectral questions that clinical labs do. These products contain hemoglobin packaged in various ways outside of red blood cells, and the hemoglobin can exist as a mix of oxy, deoxy, and met forms. Accurately determining the composition matters because methemoglobin cannot carry oxygen, so a product with too high a proportion of it would be functionally impaired. Spectral extinction measurements across a broad wavelength range, from 300 to 800 nanometers, allow researchers to distinguish between the three hemoglobin states and quantify their relative concentrations, even in these artificial formulations where the hemoglobin is embedded in microparticles rather than natural red blood cells.24PubMed Central. Function of Hemoglobin-Based Oxygen Carriers: Determination of Methemoglobin Content by Spectral Extinction Measurements Quality control for these products is, at its core, a spectroscopy problem.