In the lighting industry, luminous flux is the core parameter to measure the total amount of visible light emitted by a light source. It is defined as the total amount of light energy radiated by a light source per unit time that can be perceived by the human eye. Its symbol is Φ and its unit is lumen (lm).
1. Core characteristics of luminous flux
Human eye perception weight
Luminous flux is not a simple calculation of the physical radiation energy of the light source, but a combination of the sensitivity of the human eye to light of different wavelengths (visual function). For example, the efficiency of converting the radiation power of green light with a wavelength of 555nm (the most sensitive band of the human eye) into luminous flux is the highest, while the conversion efficiency of red light or ultraviolet light is significantly reduced.
Formula expression: Luminous flux = radiant power × visual function × 683 (maximum spectral luminous efficacy).
Significance in practical applications
Light source brightness evaluation: The greater the luminous flux, the stronger the overall luminous ability of the light source. For example, the luminous flux of a 40W incandescent lamp is about 360lm, while the same power LED can reach more than 4000lm.
Energy-saving efficiency index: Luminous efficiency (luminous flux/power, unit lm/W) is a key parameter for measuring the energy efficiency of a light source. For example, the luminous efficiency of an incandescent lamp is about 15lm/W, while that of an LED can reach 80-200lm/W.
2. Comparison with related concepts
Luminous flux vs. light intensity (luminous intensity)
Luminous intensity (I, unit candela cd) describes the luminous flux of a light source within a unit solid angle in a specific direction, reflecting the directional luminous ability.
Example: The light intensity of the same light source in different directions may vary significantly (such as the high light intensity in the center of a spotlight).
Luminous flux vs. illuminance (E)
Illuminance (unit lux) is the luminous flux received per unit area of the illuminated surface, and the formula is E=Φ/S. For example, 1lux=1lm/m².
Practical meaning: When the luminous flux of the same light source is fixed, the farther the distance or the larger the illuminated area, the lower the illuminance.
Luminous flux vs. brightness (L)
Brightness (unit candela/square meter cd/m²) combines the luminous intensity and luminous area of the light source, reflecting the "brightness" subjectively felt by the human eye.
Example: For light sources with the same luminous flux, the smaller the luminous area, the higher the brightness.
3. Practical application scenarios of luminous flux
Product selection and design
The number of light sources required can be calculated by luminous flux. For example, if a room requires 500 lux illumination and an area of 20m², the total luminous flux requirement is 500×20=10,000lm. If a 100lm/W LED lamp is selected, the total power required is 100W.
Standards and specifications reference
The International Commission on Illumination (CIE) defines the standard spectral luminous efficiency function V(λ) as the basis for luminous flux calculation.
The luminous flux maintenance rate (luminous flux after use/initial luminous flux) is used to evaluate the life of the light source. For example, the maintenance rate of LED lamps must be ≥70% (economic life standard).