Luminous flux

In photometry, luminous flux (or luminous power) is a photometric quantity that measures the perceived power of light. The sensitivity of the human eye varies according to the wavelength of the light emitted. The luminous flux differs from the radiant flux, which is instead the measure of the total power of electromagnetic radiation emitted.

The luminous flux is defined as the product between the power emitted by a point light source and the visibility coefficient \(\overline{y}(\lambda )\) where λ is the wavelength and the coefficient is dimensionless.

Luminous flux is often used as an objective measure of the useful power emitted by a light source, and is usually listed on the packaging of light bulbs. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

Luminous flux is not used to compare brightness, as this is a subjective perception that varies with distance from the light source.

The coefficient of visibility

Luminous flux takes into account the sensitivity of the human eye by weighting the power emitted for each wavelength by the spectral luminous efficacy function, which represents the response of the eye to different wavelengths. The luminous flux is a weighted sum of the power of all the various wavelengths in the visible radiation band. Light outside the visible band does not contribute. The ratio of total luminous flux to radiant flux is called luminous efficiency.

The coefficient of luminous visibility was obtained statistically as the expected or average value among a number of test subjects. It varies, as mentioned, with the wavelength between the violet and red areas and is maximum in correspondence of yellow-green light. The visibility curve has a narrow bell-shaped trend.

If a light source, considered point-like, emits one watt of power at the wavelength of maximum visibility (555 nanometers), the luminous flux is equal to 683 lumens. For the other wavelengths it will be necessary to multiply by the value of the visibility coefficient. From the luminous flux are derived the other photometric quantities such as illuminance, radiance and luminous intensity.

The amount of energy can be considered according to two factors: the monochromatic or heterochromatic light factor. The first one detects only one color while the second one detects a set of several radiations with different intensity value.

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