math test

pulseox.md

@@ -23,3 +23,10 @@ A quick teardown of a ~$20 500BL from Walgreens revealed no integrated photonics
 ![pulseox2](img/pulseox_2.jpg)
 
 ![pulseox3](img/pulseox_3.jpg)
+
+## Operational Theory
+Pulse oximetry is based on the [Beer-Lambert law](https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law), a principle that relates the concentration of a species to the attenuation of light through a sample:
+```math
+I=Iin*e^-(D*C*E)
+```
+where _I_ is the intensity of light transmitted through the sample; _Iin_ is the intensity of the light prior to absorption by the sample; D is the optical path length; C is the solute concentration; and E is the extinction coefficient, the sample's absorption at a given wavelength of light.