Question

In pulse oximetry, the light sources used are typically:

• A. Blue and green
• B. Green and yellow
• C. Red and infrared
• D. Ultraviolet and visible

Hint:

Wavelength specifics in Pulse Oximeters: - Red Light ($\sim 660\text{ nm}$): Absorbed heavily by deoxygenated blood ($\text{Hb}$). This is why poorly oxygenated blood appears dark bluish-red. - Infrared Light ($\sim 940\text{ nm}$): Absorbed heavily by oxygenated blood ($\text{HbO}_2$). Highly oxygenated blood appears bright red to the eye because it reflects red light while absorbing infrared wavelengths.

Answer 1

The Correct Option is C

Concept: Pulse oximetry estimates functional arterial oxygen saturation ($\text{SpO}_2$) by measuring the differential light absorption of tissue at two distinct wavelengths. The two primary forms of hemoglobin present in blood exhibit markedly different absorption coefficients ($\mu_a$) based on whether they are carrying oxygen:
• Deoxygenated Hemoglobin (Hb): Has a substantially higher absorption capacity for visible Red light (near $660\text{ nm}$) than oxygenated hemoglobin.
• Oxygenated Hemoglobin ($\text{HbO}_2$): Has a significantly higher absorption capacity for Infrared light (near $940\text{ nm}$) than deoxygenated hemoglobin.

Step 1: Evaluating the optical absorption spectrum.
At the red wavelength ($660\text{ nm}$), reduced hemoglobin ($\text{Hb}$) absorbs significantly more light energy than oxygenated hemoglobin ($\text{HbO}_2$). At the infrared wavelength ($940\text{ nm}$), this relationship flips: $\text{HbO}_2$ absorbs more light energy than $\text{Hb}$. The point where their absorption lines intersect (near $805\text{ nm}$) is known as the Isosbestic point. By passing light at these two specific wavelengths through a vascular tissue bed, a pulse oximeter tracks the changing light levels over a cardiac pulse cycle. It computes a ratio of ratios ($R$): \[ R = \frac{\left( \frac{\text{AC}_{\text{red}}}{\text{DC}_{\text{red}}} \right)}{\left( \frac{\text{AC}_{\text{ir}}}{\text{DC}_{\text{ir}}} \right)} \] This empirical ratio $R$ maps directly to the absolute arterial oxygen saturation level ($\text{SpO}_2$) via a calibration curve.

Step 2: Evaluating alternative spectrum options.
Wavelengths such as ultraviolet, blue, green, and yellow are not chosen for pulse oximetry because:
• They do not offer the clear, contrasting absorption differences between $\text{Hb}$ and $\text{HbO}_2$ found in the red/infrared windows.
• Shorter wavelengths (UV, blue, green) suffer from high scattering and absorption by water, melanin, and other baseline tissue pigments, preventing light from deeply penetrating into pulsatile arterial beds. Consequently, pulse oximetry monitors universally rely on paired Red and infrared light sources. This matches Option (C).