lighting - Why is the Fluorescent filter for a flash green?

I just bought a Nikon SB-910 speedlight.

The filter that comes with it to use under fluorescent lighting it green, why is this? because certainly to my eye there is not a hint of green under normal fluorescent lighting.

Answer

Balancing for fluorescent lights is harder than say tungsten. The reason for this is that tungsten bulbs produce the same sort of spectrum (set of intensities at different wavelengths) as a daylight balanced flash, just shifted.

A fluorescent light doesn't have the same bell curve shaped spectrum, it produces a set of spikes at very particular frequencies. In particular there aren't many spikes in the red part of the spectrum. The reason you don't see a green tint is probably that the brain is filling in the missing information for you.

Using a green gel on the flash allows you apply a magenta colour tint to your image (to cancel out the green) which helps restore some red that will be missing from the fluorescent lit skintones.

You can never fully replace the missing frequencies with fluorescent lights, and some are much worse than others, for example sodium lights produce very few frequencies, no matter how you try and adjust the colours, there's no information there to recover.

Here's an example of an incandescent light source (a fire!):

Now because this source produces a similar spread of frequencies as the sun, albeit shifted toward orange, we can correct this to a achieve a daylight white balance:

Now let's take a shot under the worst kind of fluorescents:

It looks orange like the first shot. However, even if we shift the image by the same amount we don't get any colours, they simply aren't there in the first place:

So while the filter may help you take out the slight green tint that results from missing certain red frequencies, it wont replace certain colours that are lost.

Fluorescent lights are good for the environment, but terrible for photography. There is hope however, newer designs are improving the width of the spectrum, as defined by their CRI (colour rendering intent) number.