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XII.8
Luminance
A measure of “surface brightness” when an
observer is looking in the direction of the
surface. It is measured in candelas per square
meter (or per square foot) and was formerly
referred to as “photometric brightness.”
Luminous Efficacy
The light output (lumens) of a light source
divided by the total power input (watts) to that
source. It is expressed in lumens per watt (see
LUMENS PER WATT).
Luminous Intensity
A measure of the visibility of a light source
generally expressed in candelas. It is defined as
luminous flux per unit solid angle (steradian) in a
given direction.
Lux (lx)
A unit of illuminance or light falling onto a
surface. One lux is equal to one lumen per
square meter. Ten lux approximately equals one
footcandle. (See FOOTCANDLE)
M
Magnetic Ballast
A ballast used with discharge lamps that
consists primarily of transformer-like copper or
aluminum windings on a steel or iron core. Also
called “Core & Coil” (see ELECTRONIC BALLASTS).
Maximum Overall Length (M.O.L.)
The end-to-end measurement of a lamp,
expressed in inches or millimeters.
Mean Lumens
The average light output of a lamp over its
rated life. Based on the shape of the lumen
depreciation curve, for fluorescent and metal
halide lamps, mean lumens are measured at
40% of rated lamp life. For mercury, high-
pressure sodium and incandescent lamps ,
mean lumen ratings refer to lumens at 50% of
rated lamp life (See LUMEN MAINTENANCE).
Medium Base
Usually refers to the screw base typically used
in household incandescent lamps. There is also
the medium bipin base commonly used in T12
and T8 fluorescent lamps.
Mercury Lamp
A high-intensity discharge light source
operating at a relatively high pressure (about 1
atmosphere) and temperature in which most of
the light is produced by radiation from excited
mercury vapor. Phosphor coatings on some
lamp types add additional light and improve
colour rendering.
Metal Cases
Case design used in both magnetic and
electronic ballasts. These ballasts are grounded
once they are mounted to the fixture. They
meet all safety codes, some of which do not
allow plastic in open plenum areas.
Metal Halide Lamp
A high-intensity discharge light source in which
the light is produced by the radiation from
mercury, plus halides of metals such as sodium,
scandium, indium and dysprosium. Some lamp
types may also utilize phosphor coatings.
Mesopic
Typically referring to nighttime outdoor lighting
conditions, the region between PHOTOPIC and
SCOTOPIC vision (See SCOTOPIC).
Mogul Base
A screw base used on larger lamps, e.g. many
HID lamps.
Monochromatic Light
Light with only one wavelength (i.e. colour)
present .
Mortality Curve
Lamps have a rated or expected life but individual
failures occur earlier and some lamps will last
longer. The mortality curve depicts the expected
percent surviving in a group of lamps at various
points between zero hours and rated life or
beyond. The curve starts with 100% at zero hours
and goes to 50% surviving at the rated life (e.g.
3000 hours or 20,000 hours, etc.) However, the
shape of the curve between these two end points
can vary depending on the lamp type.
Mounting Height
Distance from the bottom of the fixture to either
the floor or work plane, depending on usage.
Multi-Vapor™
A GE brand name for metal halide lamps.
N
Nanometer
A unit of wavelength equal to one billionth of
a meter.
National Energy Standards for Fluorescent
Ballasts
A federal law enacted in 1988 that sets energy
standards for ballasts consistent throughout
the United States.
National Electric Code (NEC)
A nationally accepted electrical installation
code to reduce the risk of fire, developed by the
National Fire Protection Association (US).
National Stock Number
The standardized part number used by the US
Government for procurement .
Non-PCB Capacitor
Capacitor used in ballasts to help provide power
factor correction. Contains no polychlorinated
biphenyls and meets EPA requirements.
Normal Power Factor
Ballasts with power factor less than .90 and do
not incorporate any means of Power Factor
Correction.
O
Open Circuit Voltage (OCV)
Open Circuit Voltage measured across the
socket the lamp screws into, with the ballast
powered on. It is dangerous to stick a voltmeter
into such a socket without precise knowledge of
the ballast because exceedingly high voltages
could be present .
Open Fixture Rated
Lamps that are approved for burning in open
fixtures (as opposed to enclosed fixtures which
have an acrylic lens or plate glass enclosure).
Operating Position or Burn Position
Mercury and High Pressure Sodium lamps
may be operated in any burn position and
will still maintain their rated performance
specifications. Metal Halide and Low Pressure
Sodium lamps, however, are optimized for
performance in specific burn positions, or may
be restricted to certain burn positions for safety
reasons.
U = Universal burning position
HBU = Horizontal -15º to Base Up
HBD = Horizontal +15º to Base Down
HOR = Horizontal ±15º
H45 = Horizontal to -45º only
VBU = Vertical Base Up ±15º
VBD = Vertical Base Down ±15º
If no special burn position is noted, the burn
position is universal.
Operating Voltage
For electrical discharge lamps, this is the
voltage measured across the discharge when
the lamp is operating. It is governed by the
contents of the chamber and is somewhat
independent of the ballast and other external
factors.