The most widely used optogenetic tool, Channelrhodopsin2 (ChR2), is both light- 
and voltage-sensitive. A light-triggered action potential or light-driven 
perturbations of ongoing electrical activity provide instant voltage feedback, 
shaping ChR2 current. Therefore, depending on the cell type and the light pulse 
duration, the typically reported voltage-clamp-measured ChR2 current traces are 
often not a good surrogate for the ChR2 current during optically-triggered 
action potentials. We discuss two experimental methods to reveal ChR2 current 
during an action potential: an "optical AP clamp" and its approximation 
employing measured current-voltage curve for ChR2. The methods are applicable to 
voltage- and light-sensitive ion currents operating in excitable cells, e.g. 
cardiomyocytes or neurons.