A method comprising optically detecting optical output states of a plurality of light sources of an optical device over a test interval; for each light source, optically detecting that the output state of the light source has changed from a first optical condition to a second optical condition; for each light source, optically detecting that the output state of the light source has changed from the second optical condition to a third optical condition; for each light source, determining a first time interval representative of the first optical condition; for each light source, determining a second time interval representative of the second optical condition; for each light source, determining a third time interval representative of the third optical condition; determining a test result for the device based on a comparison of the first, second and third time intervals with pre-stored time intervals.