Hemoglobin absorbs blue and green very strongly, but it absorbs red very weakly. The absorption of a chromophore is proportional to its concentration.
Spectrophotometers quantify optical absorber concentrations in optically clear media.
Differences in the absorption spectra of oxygenated and reduced hemoglobins allow quantitative distinction between these two states. NIR light is inherently sensitive to tissue blood-oxygenation state without exogenous contrast agents. Optical methods can be rapid, noninvasive, portable, and inexpensive. Figure 1 NIR radiation penetrates deeply and diffusively into tissues. Absorption and scattering contribute to optical losses, and a single attenuation measurement cannot distinguish between these two loss mechanisms.