minimum transmitted light from the transmitted light in each wavelength, the effects of other tissues are corrected for, generating a continuous signal for pulsatile arterial blood. the ratio of the red light measurement to the infrared light measurement is then calculated by the processor (which represents the ratio of oxygenated hemoglobin to deoxygenated hemoglobin), and this ratio is then converted to spo2 by the processor via a lookup table based on the beer–lambert law. the signal separation also serves other purposes: a plethysmograph waveform ("pleth wave") representing the pulsatile signal is usually displayed for a visual indication of the pulses as well as signal quality, and a numeric ratio between the pulsatile and baseline absorbance ("perfusion index") can be used to evaluate perfusion. spo 2 = hbo 2 hbo 2 + hb =+hb}}}}} where hbo2 is oxygenated hemoglobin (oxyhemoglobin) and hb is deoxygenated hemoglobin. derived measurements