Photopic (black) and scotopic [1] (green) luminosity functions. The photopic includes the CIE 1931 standard [2] (solid), the Judd-Vos 1978 modified data [3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data [4] (dotted). The horizontal axis is wavelength in nm.

In photometry, luminous flux or luminous power is the measure of the perceived power of light. This article deals with the usage of this term in Optics and Lighting. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. In Radiometry, radiant flux or radiant power is the measure of the total power of Electromagnetic radiation (including Infrared, Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency.

Units

The SI unit of luminous flux is the lumen (lm). The lumen (symbol lm) is the SI unit of Luminous flux, a measure of the perceived power of Light. One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian. The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a Light source in a particular direction per unit Solid The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space In other systems of units, luminous flux may have units of power. In Physics, power (symbol P) is the rate at which work is performed or energy is transmitted or the amount of energy required or expended for

Weighting

The luminous flux accounts for the sensitivity of the eye by weighting the power at each wavelength with the luminosity function, which represents the eye's response to different wavelengths. The process of weighting involves emphasising some aspects of a phenomenon or of a set of data — giving them 'more weight' in the final effect or result The luminosity function or luminous efficiency function describes the average sensitivity of the human Eye to light of different wavelengths The luminous flux is a weighted sum of the power at all wavelengths in the visible band. A weight function is a mathematical device used when performing a sum integral or average in order to give some elements more of a "weight" than others Light outside the visible band does not contribute. The ratio of the total luminous flux to the radiant flux is called the luminous efficacy. Luminous efficacy is a property of Light sources which indicates what portion of the emitted Electromagnetic radiation is usable for human vision.

Contexts

Luminous flux is often used as an objective measure of the useful power emitted by a light source, and is typically reported on the packaging for light bulbs, although it is not always prominent. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 A lamp is a replaceable component such as an Incandescent light bulb, which is designed to produce Light from Electricity. Energy conscious consumers commonly compare the luminous flux of different light bulbs since it provides an estimate of the apparent amount of light the bulb will produce, and is useful when comparing the luminous efficacy of incandescent and compact fluorescent bulbs. Luminous efficacy is a property of Light sources which indicates what portion of the emitted Electromagnetic radiation is usable for human vision. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general

Luminous flux is not used to compare brightness, as this is a subjective perception which varies according to the distance from the light source. Brightness is an attribute of Visual perception in which a source appears to emit or reflect a given amount of Light.