How a Pulse Oximeter Reads Your Blood Oxygen

Measuring blood without touching it

A pulse oximeter clips onto your fingertip and, a few seconds later, shows a few numbers, mainly your oxygen level and your pulse rate. As the U.S. Food and Drug Administration explains, it uses light beams to estimate a person’s blood oxygen level, called oxygen saturation or SpO2, and their pulse rate, without having to draw a blood sample.

That is a remarkable trick. The device never touches your blood. It only shines light into your finger and measures what comes out. This article looks at how the engineering works, and why the result is always an estimate.

Blood changes color with oxygen

Oxygen travels through your body attached to hemoglobin in red blood cells. Hemoglobin carrying oxygen and hemoglobin without it absorb light differently. A Texas Instruments design note puts it simply: oxygenated hemoglobin absorbs visible and infrared light differently from deoxygenated hemoglobin, and appears bright red rather than darker brown.

A pulse oximeter uses that difference. According to an FDA technical presentation, a photodetector measures how strongly two or more wavelengths of light are absorbed, typically red at 660 nanometers and infrared at 940 nanometers. SpO2 is then estimated as the percentage of hemoglobin that is carrying oxygen, out of all the hemoglobin that could.

Inside the clip: two lights and one sensor

Open the clip and there are two small LEDs on one side, one red and one infrared, and a photodiode, a light sensor, on the other. A spring holds the clip on your finger. In a fingertip clip the light enters one side of the finger, and the sensor on the other side collects what emerges. Other sensors put the LED and photodiode side by side and measure light reflected back from under the skin instead.

The two LEDs do not shine at the same time. Microchip’s application note describes how the oximeter flashes the red and infrared lights alternately through the finger onto the photodiode. That lets one sensor measure both colors, one after the other, many times a second.

Very little light survives the trip. TI notes that most of the light is absorbed or scattered before it reaches the detector on the other side of the finger. Inside the tissue it scatters in every direction, which, TI explains, is why the fingertip glows.

The trick: only arterial blood pulses

Here is the problem. Light passing through a finger is absorbed by skin, muscle, bone, veins and capillaries as well as by the arteries. How can the device tell which part of the absorption comes from arterial blood?

The answer is the heartbeat. With every beat, a little more arterial blood surges into the fingertip and the amount of light reaching the detector dips slightly. Everything else, the bone, the tissue, the blood in the veins, stays roughly constant. TI describes this as a small pulsing signal, the “AC” part, riding on top of a large steady signal, the “DC” part. The pulsing part is only about 1 percent of the steady level.

By looking only at the part that pulses, the oximeter isolates the arterial blood. That is the idea behind the name pulse oximetry, and it was the key insight of Takuo Aoyagi, an engineer at the Japanese company Nihon Kohden. According to Nonin, Aoyagi invented the principle in 1974, after realizing that a pulsing signal that interfered with his measurements could be used to measure arterial oxygen saturation.

From a ratio to a percentage

For each color, the oximeter divides the pulsing signal by the steady one. Then it divides the red result by the infrared result. TI calls this the ratio of ratios, R. Because each color is compared with its own steady level, the ratio is much less sensitive to how bright the LEDs are. TI’s note also describes tuning the LED current, including for battery-powered SpO2 devices.

There is no perfect formula that turns R into a percentage. TI notes that accurate SpO2 is computed from an empirical calibration of the ratio for the specific device, and the FDA describes the ratio being converted using an algorithm and a lookup table developed with calibration data. According to the FDA, that kind of testing uses healthy adult volunteers who undergo controlled desaturation, so the oximeter’s readings can be compared with arterial blood measurements, the gold standard for oxygen saturation.

In short:

1. Flash. The red and infrared LEDs take turns.
2. Detect. The photodiode measures each color.
3. Separate. The steady part and the pulsing part are split apart.
4. Compare. Each color’s pulsing part over its steady part, then red over infrared, gives R.
5. Look up. The calibration turns R into SpO2, and, as TI notes, the timing of the pulses gives the heart rate.

How accurate is the number?

An oximeter reading is an estimate, and the FDA is specific about how big the error can be. Under its 2013 guidance, the accuracy target for fingertip (transmittance) oximeters is an “Arms” of 3 percent, and 3.5 percent for reflectance sensors. Arms works like a standard deviation, so the true value falls within plus or minus 3 points of the reading about 68 percent of the time, and, when the reading has little overall bias, within roughly plus or minus 6 points about 95 percent of the time. The FDA has discussed tightening these requirements.

MedlinePlus, from the National Library of Medicine, says results may be 2% to 4% higher or lower than your actual blood oxygen level.

Many things can push a reading further off. In a 2021 safety communication, summarized on its pulse oximeters page, the FDA listed poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use and fingernail polish. Its technical presentation adds forms of hemoglobin that cannot carry oxygen, severe anemia, low blood flow to the sensor site, dyes, ambient light and movement. The oximeter’s calculation does not account for dysfunctional hemoglobin at all.

Why skin pigmentation matters

Light has to pass through the skin twice in a fingertip oximeter, on the way in and on the way out. The FDA’s presentation describes melanin in the outer layer of skin as a significant absorber of red and near-infrared light that can reduce the detected light by up to about 70 percent and may change the paths the light takes.

The FDA says current evidence suggests some accuracy differences between people with lighter and darker skin pigmentation. Its 2024 technical presentation adds that there have been reports of readings that overestimate oxygen levels in people with darker skin, especially at low saturation.

A 2020 study in the New England Journal of Medicine compared oximeter readings with arterial blood tests in hospitalized adults. Black patients had nearly three times the frequency of hidden low oxygen, meaning low oxygen on the blood test that the oximeter did not flag, compared with White patients. The authors urged combining oximetry with other clinical and patient-reported information.

In January 2025, the FDA issued draft guidance on testing and labeling pulse oximeters for medical purposes, to replace its 2013 guidance once final. It recommends a controlled desaturation study in 150 or more healthy participants spread across the Monk Skin Tone scale, and the FDA’s announcement describes a prominent label for devices shown to perform comparably across skin pigmentations. It is still a draft.

Medical device or wellness gadget?

Not every device that shows an SpO2 number is the same. The FDA separates pulse oximeters intended for medical purposes, which are reviewed by the agency and clinically tested for accuracy, from general wellness and sporting or aviation oximeters, which are often sold directly to consumers and are not reviewed by the FDA before they are sold. A small number of medical-purpose oximeters are available over the counter after FDA clearance.

Getting a steadier reading

Because the device depends on a small pulsing signal, anything that weakens the pulse or adds movement makes the job harder. The FDA suggests:

• Follow your health care provider’s advice about when and how often to check, and the manufacturer’s instructions.
• Make sure your hand is warm, relaxed and held below the level of your heart.
• Remove any fingernail polish from that finger.
• Sit still and keep that part of your body from moving.
• Wait until the reading stops changing and shows one steady number.
• Write down the reading, date and time, so you can track changes over time.

The FDA lists signs of low oxygen, such as bluish lips or nails, worsening shortness of breath, chest pain and a fast pulse, and warns that some people may not show them.

The FDA also stresses that only a health care provider can diagnose a condition such as low oxygen, and that a reading should be considered together with symptoms and how you feel. Contact a health care provider if you are concerned about a reading, or if your symptoms are serious or getting worse.