The terms lumen, candela and lux occur very frequently in light measurement. They are all photometric quantities. The following diagram shows the relationship between the three terms.
The unit lumen stands for the total light output of a luminaire emitted in all directions. However, since the light emitted by luminaires is not emitted evenly in all directions, the luminous intensity is given in candela. So candela is the unit of light intensity. This value indicates how much light is emitted in a particular direction. In contrast to these two emitted quantities, there is also the receiver quantity lux. The lux unit does not measure how much light is emitted, but how much of the emitted light reaches a certain surface.
To assess the brightness of a luminaire or illuminant, it is essential to consider all three values. The units lumen and candela are quoted by most manufacturers. Manufacturers cannot quote the lux value because this value depends on the ambient conditions of the area of application.
What is the solid angle?
The solid angle is the three-dimensional size of the light cone. If a luminaire emits light, the angle of the emitted light is three-dimensional. The unit of the solid angle is the steradian (sr). The light intensity indicates the amount of light that a light source emits per solid angle.
The solid angle is calculated by dividing the area (A) by the radius (r²)
What is the density of luminance (cd/m²)?
The density of luminance provides information about the impression of brightness of a luminaire. It is expressed in luminous intensity per area (cd/m²). The density of luminance describes how bright a surface appears to us. This factor is also influenced by other circumstances, such as the condition of the illuminated surface.
Post time: Aug-24-2022