A bi-directional driver device for opening and closing a light shield of a film cassette is alternately biased open and closed via an overcenter spring, to open and close the light shield. The final opening torque the spring exerts against the driver device when the light shield is completely opened is substantially less than the final closing torque the spring exerts against the driver device when the light shield is completely closed. Thus, the final opening torque will not cause any deformation of the light shield that might allow light leakage into the cassette interior when the light shield is closed.