DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

A lumen is produced by about
1.5 mW of monochromatic power
at 555.5 nm. That wavelength,
a frequency of 540 THz, corresponds
to the peak luminous efficiency
of vision.
The quantity illuminance was
formerly called illumination.
However, use of illumination for
this specific quantity is deprecated,
owing to the more general
meaning of the word as the act of
illuminating, or the state of being
illuminated.
The old footcandle and metercandle
units for luminous exitance were
misleading, because the old candle
was a unit of intensity, not flux.
Radiometry and photometry are linked by the definition
of the candela: One candela (cd) is the luminous
intensity of a monochromatic 540 THz source having
a radiant intensity of 1 ⁄683 W·sr -1 . Once this definition
is established, the remaining photometric quantities
and units parallel those of radiometry. The
relationships are sketched in Figure B.2 opposite.
The photometric analog of radiant flux is luminous flux
(Φ v). To paraphrase James M. Palmer, luminous flux is
what you want when you buy a light bulb. The measure
of its brightness is lumens (lm), the photometric analog
of watts; its efficacy is measured in lumens per watt.
One lumen appears equally bright regardless of its spectral
composition.
Luminous flux per unit area – that is, luminous flux
density – arriving at a real or imaginary surface is
illuminance (E v), expressed in units of lux (lx), where
1 lux is defined as 1 lm·m -2. Illuminance is the photometric
analog of irradiance; it is the quantity measured
by a light meter. Luminous flux per unit area leaving
a surface is luminous exitance (M v ). Despite 1 lux being
equal to 1 lm·m -2 , luminous exitance is conventionally
given in units of lm·m -2: the lux unit implies
illuminance. Luminous exitance from a nonemissive
surface is its illuminance times its reflectance.
Luminous flux in a particular, specified direction is
luminous intensity (I v), expressed in units of lm·sr -1, or
candela (cd). The candela is the modern equivalent of
the old candle (colloquially, candlepower). Intensity is
independent of distance: A candle has a luminous
intensity of about 1 cd, at any viewing distance.
Luminous flux density in a particular direction is
luminance (L v , or Y). Formally, luminance is luminous
flux differentiated with respect to both solid angle and
projected area. Luminance is the photometric analog
of radiance; it is expressed in units of cd·m -2 , or nit.
Luminance is an important and useful measure, because
it is invariant under transformation by a lens.
604 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES