From high-altitude hiking and intense workdays to overnight sleep tracking, blood oxygen saturation (SpO2) has become a key indicator of overall vitality.

But can a smart watch really estimate oxygen levels in your blood using just a few tiny lights on the back?

Behind those subtle red glows lies a well-established optical technology borrowed from medical monitoring. This article breaks down how SpO2 measurement works in smartwatches—and why the data is grounded in real science.

The Science Behind Blood Oxygen Monitoring: Light Absorption and Reflection

1. The Color of Blood: Hemoglobin Explained

Blood appears red because of hemoglobin, the protein responsible for carrying oxygen.

What many people don't realize is that hemoglobin changes how it interacts with light depending on whether it is carrying oxygen:

• Oxygenated hemoglobin (HbO₂):

Absorbs more infrared light and reflects more red light, giving blood its bright red appearance.

• Deoxygenated hemoglobin (Hb):

Absorbs more red light and reflects more infrared light, appearing darker.

This optical difference is the foundation of SpO2 monitoring.

2. PPG Technology: Turning Light Into Data

Smartwatches use a method called photoplethysmography (PPG).

How it works:

• LEDs emit specific wavelengths of light into the skin.

• A photodiode sensor measures how much light is reflected back.

• As blood volume changes with each heartbeat, the reflected light signal fluctuates.

Algorithmic calculation:

By comparing the absorption ratios of red and infrared light, the system estimates the percentage of oxygenated hemoglobin—expressed as SpO2.

This is not guesswork, but a calculation based on optical physics and statistical modeling.

Why SpO2 Uses Red and Infrared Light Instead of Green

Many users notice green LEDs on the back of a smartwatch and assume they are used for everything. In reality, different wavelengths serve different purposes.

• Green light:

Highly sensitive to surface-level blood flow, making it ideal for heart rate and HRV monitoring.

However, it lacks sufficient penetration depth for reliable blood oxygen estimation.

• Red + infrared light:

Offer deeper tissue penetration and clearer differentiation between oxygenated and deoxygenated blood.

This red/infrared combination is considered the industry standard for SpO2 monitoring, both in medical-grade equipment and advanced wearables.

Key Factors That Affect SpO2 Accuracy in Smart Watches

Even with solid technology, measurement conditions matter. The most common factors influencing accuracy include:

1. Wearing Fit

A loose fit allows ambient light to leak into the sensor area, introducing noise and skewing results.

2. Skin Conditions

Tattoos, dense body hair, or thick callused skin can interfere with light reflection and absorption.

3. Ambient Temperature

In cold environments, reduced peripheral blood circulation may cause temporarily lower readings.

4. Body Position and Movement

For best results, measurements should be taken while the arm is relaxed, still, and positioned at heart level.

Where BWOO Fits Into the Equation

As a long-term participant in the smart wearable supply chain, BWOO designs its devices in alignment with international optical sensing standards and validated PPG measurement principles.

Rather than relying on raw sensor output alone, BWOO emphasizes:

• Multi-channel optical sensor integration

• Stable light emission control

• Signal filtering and algorithm calibration during mass production

This standards-driven approach ensures that SpO2 readings remain consistent and meaningful across different usage scenarios.

Conclusion

Smartwatch SpO2 data is not a diagnostic tool—but it is a valuable reference for daily health awareness and risk management.

Understanding how optical sensing works helps users interpret the numbers correctly and use them responsibly.

Behind every reading is a combination of physics, algorithms, and manufacturing precision—not just a flashing light.