This may be a Pic of the Day that only folks who have worked in the Grip/Electric or camera departments of productions will appreciate, but it made me laugh. To cut down the amount of output from a light or even a window – but not affect the color balance – a Neutral Density (ND) gel can be used. Because it can be difficult by eye to tell the difference between the various levels of ND – with are designated with numbers such as ND.3, ND.6, etc. – people will sometimes label the gels with a sharpie. If you look in the lower right corner of this frame grab from season 1 of MTV’s Scream, you’ll see that whoever wrapped the fluorescent tube in ND gel left the N6 label clearly visible.