We measured transient photoinduced birefringence ( $\delta n$ ) in various azobenzene dye films by pumping with a nanosecond pulse at $532\mathrm{nm}$ and probing at $633\mathrm{nm}$ . The switch-on times for the photoinduced birefringence range from nanoseconds to milliseconds and are systematically related with the lowest optical transition energies for those films. Moreover, our results suggest that the transient photoinduced birefringence measurement is a convenient way to determine the relative energies of $\pi –{\pi }^{*}$ and $n–{\pi }^{*}$ states in azo-based materials.