Probably the most common and informative equation currently used to fit
light response data is the non-quadratic hyperbola. This equation fits
the initial linear region as well as providing a description of the degree
of curvature as photosynthesis approaches its maximum. The most
comprehensive interpretations of this equation are provided in papers by
Jerry Leverenz and Ogren. SAS can be used to fit the curve to the data
(with difficulty). One problem with this function is that most fitting
algorithms do not vary each fitting parameter independently. For example,
small changes in your initial selection of quantum yield or curvature may
influence the final fit equation.
I have a paper in Plant Cell and Environment, about two issues back, that
fits light response data with an equation.