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2 Basic Principles of LED
2.3.3 Basic Photometric Concepts in LED
Photometry is the study of the perception of light intensity by the human eye. LED
is a kind of light-emitting element, so photometric parameters are also critical for
evaluating the performance of LED.
Luminous Intensity: Also known as photometry or intensity, luminescence intensity refers to the intensity of a point light source in a certain direction, symbolized as
I, in candela (cd). It is defined as the quotient between the luminous flux emitted by
the light source in this direction and the solid angle. Light intensity is often used to
describe the distribution density of light flux emitted by light source in all directions
of space or in selected directions. 1 cd = 1 lm/sr (lumen per sphericity).
Luminous flux: The sum of the energy emitted by the light source and received
by the human eye is the luminous flux, denoted as Fv, in lumen (lm). Light flux, also
known as light beam, is the radiation flux commonly used in the international evaluation of human visual characteristics. When LED is used for illumination, luminous
flux is the basic quantity to evaluate the luminous ability of light source. Lumen is a
unit of luminous flux. The luminous flux emitted by a point light source with uniform
luminous intensity of 1 cd (candela) is 1 lm per unit solid angle, or all the luminous
flux received from the source is 1 lm per unit area per unit distance.
Luminance: It refers to the light source or reflected by the object entering the eyes
and imaging on the retina so that we can recognize its shape and shade. Brightness
is the density of light intensity per unit surface in a certain direction. It is equal to
the quotient of the intensity of light in this direction and the projection area of this
plane in this direction, which is expressed by the symbol L. The unit of brightness
is Candela per square meter (cd/m
2 ).
Illuminance: It refers to the surface density of the light flux received on the
irradiated plane. The unit of illumination is Lux, and the symbol is lx. 1 lx is equal
to the illumination produced by the uniform distribution of the luminous flux of 1 lm
on the surface of 1m
2 , i.e. 1 lx = 1 lm/m
2 . Illuminance is an important indicator of
illumination design, which indicates how bright the illuminated object is.
Luminous efficiency: It refers to ratio of the luminance of the light emitted by
the light source to its power consumption, that is, the luminous efficiency (lm/W)
= the luminance (lm)/the power consumption (W). Luminescent efficiency refers to
the amount of light converted by each watt of electricity. The larger the value, the
higher the efficiency of the light source is.
2.3.4 Electrical Characteristics of LED
The current–voltage characteristic of LED is similar to that of ordinary diode. Therefore, when the device is operated under forward bias, injection current and the luminance of the device increases with increasing voltage. When the forward voltage
increases to a certain value, the luminance decreases while the forward current still
2 Basic Principles of LED
2.3.3 Basic Photometric Concepts in LED
Photometry is the study of the perception of light intensity by the human eye. LED
is a kind of light-emitting element, so photometric parameters are also critical for
evaluating the performance of LED.
Luminous Intensity: Also known as photometry or intensity, luminescence intensity refers to the intensity of a point light source in a certain direction, symbolized as
I, in candela (cd). It is defined as the quotient between the luminous flux emitted by
the light source in this direction and the solid angle. Light intensity is often used to
describe the distribution density of light flux emitted by light source in all directions
of space or in selected directions. 1 cd = 1 lm/sr (lumen per sphericity).
Luminous flux: The sum of the energy emitted by the light source and received
by the human eye is the luminous flux, denoted as Fv, in lumen (lm). Light flux, also
known as light beam, is the radiation flux commonly used in the international evaluation of human visual characteristics. When LED is used for illumination, luminous
flux is the basic quantity to evaluate the luminous ability of light source. Lumen is a
unit of luminous flux. The luminous flux emitted by a point light source with uniform
luminous intensity of 1 cd (candela) is 1 lm per unit solid angle, or all the luminous
flux received from the source is 1 lm per unit area per unit distance.
Luminance: It refers to the light source or reflected by the object entering the eyes
and imaging on the retina so that we can recognize its shape and shade. Brightness
is the density of light intensity per unit surface in a certain direction. It is equal to
the quotient of the intensity of light in this direction and the projection area of this
plane in this direction, which is expressed by the symbol L. The unit of brightness
is Candela per square meter (cd/m
2 ).
Illuminance: It refers to the surface density of the light flux received on the
irradiated plane. The unit of illumination is Lux, and the symbol is lx. 1 lx is equal
to the illumination produced by the uniform distribution of the luminous flux of 1 lm
on the surface of 1m
2 , i.e. 1 lx = 1 lm/m
2 . Illuminance is an important indicator of
illumination design, which indicates how bright the illuminated object is.
Luminous efficiency: It refers to ratio of the luminance of the light emitted by
the light source to its power consumption, that is, the luminous efficiency (lm/W)
= the luminance (lm)/the power consumption (W). Luminescent efficiency refers to
the amount of light converted by each watt of electricity. The larger the value, the
higher the efficiency of the light source is.
2.3.4 Electrical Characteristics of LED
The current–voltage characteristic of LED is similar to that of ordinary diode. Therefore, when the device is operated under forward bias, injection current and the luminance of the device increases with increasing voltage. When the forward voltage
increases to a certain value, the luminance decreases while the forward current still