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N
 Nano. A one thousand millionth 10^-9 .
ND Filter.  A filter that attenuates all colours of light in the visible spectrum, equally.
ND Spot.  A graduated "spot" filter in a lens that restricts the amount of light passing
through the lens at small apertures.
Nit.  A measurement of luminance  = 1 candela/ m2.
Noise.  Any unwanted signal produced by all electrical circuits working above the absolute
zero.  Noise cannot be eliminated but only minimized.
Noninterlaced.  The process of scanning whereby every line in the picture is scanned during
the vertical sweep.
NTSC.  National Television Systems Committee.  American committee that set the standards
for colour television as used today in the US, Canada, Japan and parts of South America.
NTSC television uses a 3.57945 MHz sub-carrier whose phase varies with the instantaneous
hue of the televised colour and whose amplitude varies with the instantaneous saturation of
the colour.  NTSC employs 525 lines per frame and 59.94 fields per second (usually taken as
60 fields per second).
Numerical aperture.  A number that defines the light gathering ability of a specific optical fibre.
The numerical aperture is equal to the sine of the maximum acceptance angle.
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