A pulse oximeter works by shining two wavelengths of light through your finger (or earlobe) and measuring how much of each wavelength is absorbed by your blood. The ratio of absorption between those two light wavelengths reveals how much of your hemoglobin is carrying oxygen. A healthy reading falls between 95% and 100%, while anything below 90% is considered low.
Two Colors of Light, Two Types of Hemoglobin
The core principle is surprisingly simple: oxygenated blood and deoxygenated blood are literally different colors, and a pulse oximeter exploits that difference using light your eyes can’t fully detect.
The device emits red light at 660 nanometers and infrared light at 940 nanometers. Hemoglobin that’s carrying oxygen absorbs more infrared light and reflects more red light, which is why oxygen-rich arterial blood looks bright red. Hemoglobin without oxygen does the opposite: it absorbs more red light and reflects more infrared light, giving deoxygenated blood its darker appearance. By comparing how much red versus infrared light makes it through your tissue, the device can calculate the proportion of your hemoglobin that’s loaded with oxygen.
How It Isolates Your Arterial Blood
Your finger contains arteries, veins, bone, skin, and other tissue. All of these absorb some light. So how does the device know it’s reading your arterial oxygen and not everything else mixed together?
The answer is your heartbeat. Every time your heart pumps, a small surge of arterial blood pulses through your fingertip, briefly increasing the volume of blood in the tissue. That pulse creates a tiny, rhythmic change in light absorption that the sensor can detect. This fluctuating signal is called the pulsatile (or AC) component, and it corresponds specifically to arterial blood flow synchronized with your cardiac cycle.
Everything else, including venous blood, bone, and skin, produces a steady baseline of light absorption called the non-pulsatile (or DC) component. Because this baseline doesn’t change with each heartbeat, the device’s software can mathematically separate it from the pulsatile signal. The oximeter essentially ignores the static tissue and focuses only on the rhythmic arterial component to calculate your oxygen saturation.
From Light to a Percentage
The device calculates a “ratio of ratios,” comparing the pulsatile absorption of red light to the pulsatile absorption of infrared light. When oxygen saturation is high, relatively little red light is absorbed (because oxygenated hemoglobin reflects red) and more infrared is absorbed. When saturation drops, red absorption increases and infrared absorption decreases.
This ratio is then matched against a calibration curve built into the device’s software. Manufacturers create these curves by collecting data from healthy volunteers breathing progressively lower oxygen concentrations while simultaneously measuring their blood oxygen with direct arterial blood draws. The result is a lookup table that converts the light ratio into the SpO2 percentage displayed on your screen. The formula the device is solving is straightforward: oxygenated hemoglobin divided by total hemoglobin, multiplied by 100.
How Accurate Is the Reading?
When your oxygen levels are in the normal or mildly low range (above 80%), pulse oximeters are quite reliable. Research comparing pulse oximeter readings (SpO2) to direct arterial blood gas measurements (SaO2, the gold standard) found that in patients with readings above 90%, the two methods produced nearly identical results with no statistically significant difference. In the 80% to 90% range, accuracy remained solid.
Below 80%, however, the story changes. In that range, pulse oximeters tend to underestimate true oxygen levels by a meaningful margin. At those severely low saturations, the calibration curves become less reliable because fewer volunteers can safely be tested at such extreme levels. For patients in critical condition with very low oxygen, clinicians rely on arterial blood gas analysis rather than the finger clip alone.
What Can Throw Off the Reading
Several factors can interfere with the light signals and produce inaccurate numbers.
Skin pigmentation. The FDA has acknowledged that pulse oximeters can show accuracy differences between individuals with lighter and darker skin tones. Because the device relies on light passing through tissue, higher melanin levels can affect how light is absorbed and scattered in ways that weren’t always well accounted for in calibration. The FDA has proposed updated performance standards to address this gap.
Nail polish. Colored nail polish sits directly in the light path on a finger sensor. Black, purple, and dark blue polish produce the largest effect, shifting readings by roughly 1 to 1.6 percentage points on average. Other colors, including clear polish, have a smaller impact (under 1 point), but the effect is still measurable. If you’re monitoring your levels regularly, bare nails give the cleanest reading.
Carbon monoxide exposure. This is one of the more dangerous blind spots. Carbon monoxide binds to hemoglobin in a way that looks almost identical to oxygen when measured at two wavelengths. A standard pulse oximeter can display a reassuringly normal SpO2 in someone with significant carbon monoxide poisoning. Specialized devices called CO-oximeters use additional wavelengths of light to distinguish carbon monoxide from oxygen, but most home and hospital pulse oximeters don’t have this capability.
Poor circulation and movement. Cold fingers, low blood pressure, or significant movement can weaken the pulsatile signal the device depends on. When there isn’t enough arterial pulse to detect clearly, the reading becomes unreliable or the device may fail to display a number at all. Warming your hands or sitting still for a moment usually helps.
What the Number on the Screen Means
The reading displayed is called SpO2, which stands for peripheral oxygen saturation. It represents the percentage of hemoglobin molecules in your arterial blood that are currently bound to oxygen. A reading of 97% means 97 out of every 100 hemoglobin molecules passing through your fingertip are carrying oxygen.
Normal values range from 95% to 100%. Some people with chronic lung conditions may have a lower baseline that their doctor considers acceptable for them. A reading below 90% generally indicates a level low enough to warrant medical attention, as tissues may not be receiving adequate oxygen at that point. The device also typically displays your pulse rate, derived from the same pulsatile signal it uses for the oxygen calculation.