"Brian Gaff" wrote in message
...
I remember once shooting in a large branch of Boots and the fluorescent
lights on the ceiling seemed to strobe in waves it was very weird at the
time. As I gradually lost my sight I started to notice this effect with my
eyes as well so it probably has to do with exposure etc.

Fluorescents, if they are nearly but not quite in phase with the camera (eg
constant 25 fps camera, but mains at one extreme of the other - 49.5 or 50.5
Hz) produce weird effects because they produce two different lights: there
is a blueish light with very short persistence from the gas discharge and a
yellowish light with a longer persistence from the phosphor coating.
Together they produce a white light (for some value of white ranging from
warm white to daylight). But the strobing between the two allows you to see
the blue light fading in and out which means the flicker varies in colour as
well as brightness.

I saw the same effect with the blades of a desk fan, illuminated by
fluorescent desk lamp - at some fan speeds there was a sharp blue image and
a more blurred yellow ghost image alongside.


I'm always surprised that intermittent short-persistence lights are used for
filming, because of the problem with strobing. There is a scene in the film
The Krays where one of the twins is in a boxing ring at a fairground. The
sequence is shown in slow motion and the flicker of the mains-frequency
filming lights is very intrusive.