Important question, you always have to watch out for the light source. Best is to turn off the fluorescent lights and use daylight or light bulbs. I remember seeing some interesting effects for propeller planes starting up, in broad daylight, but now I can't remember exactly what I saw. Can anyone describe an effect of this type they've seen in daylight? Jens' point is interesting, you might not expect everyone to see the same thing at the same time if it was physiological.