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Consider the depth of necrosis achieved by a broad beam of light incident on a tissue surface. The equation that predicts the production of Reactive Oxidizing Species (ROS) is shown in the figure. Also, the many factors involved in the ROS production are listed. Most the factors scale the magnitude of ROS production. The exponential term describes how the light penetrates the tissue and hence describes the ROS production as a function of depth.

There is a threshold value for ROS production, P_th, expressed in units of concentration, [moles/liter] or [M]. If the ROS production, P, exceeds this threshold, then there is necrosis (or apoptosis). At a critical depth, z_necrosis, the value of P drops below P_th and necrosis no longer occurs.

In the figure, the general equation for P(z) is specifically written as P_th(z_necrosis), which pertains to the specific point in the graph that is indicated by the long red arrow.

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