the optical window

LITShine a torch through your fingers and the light comes out red. Haemoglobin absorbs blue and green tens to hundreds of times more strongly than red (measured Prahl spectra, below). Water absorbs strongly past about 950 nm. Between them lies the "therapeutic" or "optical window", roughly 650–950 nm, where light penetrates millimetres to centimetres into tissue.

COMPUTEDAbsorption by blood alone (2% blood volume, 150 g/L Hb, SO₂ slider) uses the measured extinction; transmitted light falls as exp(−μ_a·d). MODELScattering, which dominates real tissue, is ignored, and water (above 700 nm) is not in this table, so the page stops at 700 nm. Nesting: an orange torch, purple-green-yellow-blue absorbed away in the first millimetre, red emerging, then void.

tissue mm SO₂

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