Transmittance oximetry measures light that:

Transmittance oximetry measures light that:

• A. Light transmitted directly through tissue and arterial blood
• B. Light reflected back to the detector on the same side
• C. Light emitted as fluorescence by hemoglobin
• D. Light absorbed by surrounding tissue without passing through

Answer: (A)

In transmittance oximetry, the light source and detector are on opposite sides of the tissue. The detector measures light that has passed directly through the tissue and arterial blood. The arterial blood’s pulsatile absorption at two wavelengths (red and infrared) changes the amount of transmitted light, and from this the oxygen saturation is calculated using the ratio of absorbed light. This is different from reflectance oximetry, where light is detected after reflecting back from the same side, and from fluorescence, which relies on tissue emitting light after excitation. If light were simply absorbed by surrounding tissue without passing through, it wouldn’t reach the detector in this setup, so it wouldn’t provide the SpO2 measurement.

In transmittance oximetry, the light source and detector are on opposite sides of the tissue. The detector measures light that has passed directly through the tissue and arterial blood. The arterial blood’s pulsatile absorption at two wavelengths (red and infrared) changes the amount of transmitted light, and from this the oxygen saturation is calculated using the ratio of absorbed light. This is different from reflectance oximetry, where light is detected after reflecting back from the same side, and from fluorescence, which relies on tissue emitting light after excitation. If light were simply absorbed by surrounding tissue without passing through, it wouldn’t reach the detector in this setup, so it wouldn’t provide the SpO2 measurement.