The four
basic lighting quantities
1. Luminous
flux (Φ) is the rate at which light is emitted by a lamp. Measured in lumen
(lm), it defines the visible light radiating from a light source in all
directions.
2. Luminous
intensity (I) is the amount of luminous flux radiating in a particular
direction. It is measured in candela (cd).
The spatial
distribution of luminous intensity – normally depicted by an intensity distribution
curve (IDC) - defines the shape of the light beam emitted by a luminaire,
reflector lamp or LED.
3. Illuminance
(E) – measured in lux (lx) – is the luminous flux from a light source falling
on a given surface.
Where an
area of 1 square metre is uniformly illuminated by 1 lumen of luminous flux, illuminance
is 1 lux.
The flame of an ordinary candle, for example,
produces around 1 lx at a distance of 1 m.
4. Luminance
(L) is the brightness of a luminous or illuminated surface as perceived by the
human eye.
Measured in
cd/m2 or cd/cm2, it expresses the intensity of the light emitted or reflected
by a surface per unit area.
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