I do not have a light meter however I have something that to me is just as useful and it only cost a pound.
I don't know how easy it is to get this time of year but come spring the poundshops used to have them.
they come as flowers and different shapes.
Basically the faster it swings - the more light there is. A vigorous fast swing is too much light - A gentle swing is just right.
Zygopetalums don't need that much light so as long as the arms are swinging slowly (ie there is some movement) then I have found there is enough light for them.
In my kitchen this time of year behind a net curtain feels bright enough for me - but my solar chicken doesn't move it's arms unless I remove the net curtain so it's useful