We present the theoretical treatment which allows for the quantification of optical orientation processes in individualizable photoisomers, including the measurement of the relative orientation of molecular optical transitions and the photochemical quantum yields in ordered systems. The theory is confirmed experimentally for a spiropyran-type molecule introduced into a poly-methyl-methacrylate film. In particular we found that the transitions at 365 and 633 nm of the B isomer are oriented at an angle of 71.25 degrees to each other, and that the isomerization quantum yields are phi(AB)(365) = 0.053, phi(BA)(365) = 0.030 and phi(BA)(546) = 0.003.