Luminous flux, measured in lumens (lm), is a core concept in lighting science that represents the total amount of visible light emitted by a source per unit of time. It is one of the most important metrics used to evaluate the performance of lighting products, such as LED lamps, luminaires, and other light-emitting devices.
All sources of light emit radiant energy, which is a general term for the flow of electromagnetic radiation, including infrared radiation, ultraviolet radiation, and visible radiation. The total radiant energy emitted in all directions per second is the radiant flux Φe, measured in watts (W).
Not all the emitted radiation, however, is useful in creating the “light” effect, for the human eye responds to a narrow band of the electromagnetic spectrum (visible light, approximately 380 to 780 nm) and is not equally sensitive to all wavelengths in this range. The human eye has maximum sensitivity to yellow-green light (at approximately 555 nm) and is much less sensitive to red and violet light (this variation is modeled by the spectral luminous efficiency function Vλ).
To account for this, the radiometric quantity, radiant flux, is weighted according to a function that models the response of the eye to the different wavelengths of light. The result of this weighting is the luminous flux, measured in lumens (lm).
Measured in lumens (lm): One lumen is defined as the luminous flux emitted in a solid angle of one steradian by a point source with a luminous intensity of one candela.
Human-eye weighted: Luminous flux accounts for how bright light appears to the human eye, using the luminosity function to weight different wavelengths.
Different from radiant flux: Radiant flux considers all electromagnetic radiation; luminous flux only considers visible light and how it is perceived.
Tied to perception, not appearance: Luminous flux reflects potential brightness, but not the actual visual brightness someone perceives from a specific angle or distance.
Luminous flux is typically measured using integrating spheres or goniophotometers in laboratories equipped with calibrated photometers or spectroradiometers. It can also be calculated with the formula:
Luminous flux (lm) = Illuminance (lx) × Area (m²)
This makes it a valuable metric for understanding how much usable, visible light a fixture provides in a space.
Luminous Intensity (cd): Measures how strong the light is in a specific direction.
Illuminance (lx): Measures how much luminous flux falls on a surface.
Luminous Efficacy (lm/W): Measures the efficiency of a light source in converting electrical energy into visible light.
Luminous flux is an essential unit of measurement in lighting design and product development. It provides a standardized, eye-related way for engineers, architects, and end-users to assess and compare lighting solutions. At ACE, we employ accurate photometric testing to ensure each product delivers optimal luminous flux. This helps us strike the perfect balance between brightness, efficiency, and visual comfort for industrial settings.
