The term candela is the Latin translation of the word candle. But what does candela mean in connection with an LED lamp? This refers to the physical unit of luminous intensity. This describes the luminous flux emitted by the illuminant in a certain direction. Here you will find the definition and background of the luminous intensity in candela.
What is the luminous intensity in candela?
The luminous intensity of a light source describes the luminous flux in lumens emitted in a certain direction into the environment. It is a photometric quantity with the unit candela and the abbreviation cd. Sometimes the values are also given in milli-candela, i.e. mcd. Since the luminous intensity describes the light emitted by a light source, it is also referred to as the transmission size.
Most LED lamps and other light sources do not radiate their light evenly all around. The luminous intensity varies depending on the solid angle. This applies both to spots with narrow beam angles and to omnidirectional spotlights. In the horizontal plane, these have a uniform radiation, but in the vertical plane the angle is limited. The behavior can be illustrated graphically with a distribution curve of the luminous intensity.
Unit of luminous intensity
The luminous intensity has the unit candela (cd). Some of the values are also given in thousandths of a candela, i.e. milli-candela (mcd).
Is the luminous intensity indicated on the packaging?
The luminous intensity is not mandatory on the packaging of LED lamps. Due to the direct reference to the solid angle, the candela value differs depending on the beam direction. Only for LED spots with very small beam angles information on luminous intensity can be found on the packaging or in the data sheet. However, one should not be dazzled by high candela values.
The selection of an LED light source based on the light intensity in candela is difficult in practice. Orient yourself better on the luminous flux in lumens in conjunction with the beam angle.
How to calculate luminous intensity?
The luminous intensity is a direction-dependent transmission quantity. The luminous intensity results from the luminous flux in relation to the solid angle. With two illuminants with the same brightness and different beam angles, the luminous flux is distributed over a different solid angle. As a result the illuminant with the smaller beam angle achieves a higher candela value.
Formula to calculate luminous intensity in candela:
luminous intensity (in cd) = luminous flux (in lm) / solid angle (sr)
I = Φ : Ω
Correlations to other quantities
Below you will find the correlation between luminous intensity and other photometric quantities.
Candela vs Lumens
As seen in the candela formula, the luminous intensity results from the quotient of the lumen value and solid angle. In order to get the luminous flux from the candela value, the formula can be changed accordingly:
luminous flux (in lm) = luminous intensity (in cd) ⋅ solid angle (sr)
Φ = I ⋅ Ω
Candela vs Lux
Lux is the unit of illuminance. In contrast to candela, this is a reception quantity. The lux value indicates the luminous flux used to illuminate a defined surface. The surface area is determined by the beam angle and the distance to the illuminant. The lumens lux calculator can help you with the conversion.
Now you have an overview of the light intensity in candela. It is a transmission quantity that describes the radiated luminous flux in a defined direction. This is often given for LED spots with small beam angles. To compare different light sources, you should better use the lumens value in conjunction with the beam angle.