"bulb. The luminous intensity of a typical candle is approximately one candela. The SI unit, candela, was in fact based on an older unit called the ""candlepower"", which represented the luminous intensity emitted by a candle made to particular specifications (a ""standard candle""). The modern unit is defined in a more precise and repeatable way, but was chosen such that a candle's luminous intensity is still about one candela. The hottest part of a candle flame is just above the very dull blue part to one side of the flame, at the base. At this point, the flame is about"
"the SI unit of magnetic flux density. An older name for the lumen second was the lumberg. The photometric unit lumerg, also proposed by the Committee on Colorimetry in 1937, correlates with the old CGS unit erg in the same way that the lumen second correlates with the radiometric unit joule, so that 10 lumerg = 1 lm⋅s. Lumen second In photometry, the lumen second (lm s or lm⋅s) is the SI derived unit of luminous energy. It is based on the lumen, the SI unit of luminous flux, and the second, the SI base unit of time. The lumen"
"Lux The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, ""lux"" is used as both the singular and plural form."
"by focusing it tighter. The luminous intensity of a light depends on many factors, including the colour of the light and the eye's sensitivity to that colour, the optics involved, reflector and lens. Despite its complexity, it is a more useful measure than watts, because it defines how much usable light is shed in a given place: a dynamo headlight designed for road use and focused for seeing the road makes more efficient use of the power of the lamp than lights using rotationally symmetrical optics. The lumen (lm) is the SI unit for luminous flux, the total amount of"
"illuminance of 1 microlux—about the same as a magnitude 1 star. Unicode has a symbol for ""lx"": (㏓). It is a legacy code to accommodate old code pages in some Asian languages. Use of this code is not recommended. Lux The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the"
"Lumen second In photometry, the lumen second (lm s or lm⋅s) is the SI derived unit of luminous energy. It is based on the lumen, the SI unit of luminous flux, and the second, the SI base unit of time. The lumen second is sometimes called the talbot (symbol T). This name was coined in 1937 by the Committee on Colorimetry, Optical Society of America, in honor of the early photographer William Fox Talbot. The talbot is exactly equal to the lumen second: The use of the symbol T for talbots conflicts with T as the symbol for the tesla,"
"Specific radiative intensity Specific (radiative) intensity is a quantity used in physics that describes electromagnetic radiation. It is a term used in much of the older scientific literature. The present-day SI term is spectral radiance, which can be expressed in base SI units as . It gives a full radiometric description of the field of classical electromagnetic radiation of any kind, including thermal radiation and light. It is conceptually distinct from the descriptions in explicit terms of Maxwellian electromagnetic fields or of photon distribution. It refers to material physics as distinct from psychophysics. For the concept of specific intensity, the"
"authority of the Office of Weights and Measures of National Institute of Standards and Technology (NIST), enforced by the individual states. The International System of Units (SI) defines seven base units: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. By convention, each of these units are considered to be mutually independent of each other; however, in reality they are interdependent given some definitions contain other base SI Units. All other SI units are derived from the seven base units. Since the base units are the reference points for all measurements taken in SI units, if"
"if the wavelength is longer or shorter. Light whose wavelength is well outside the visible spectrum has a luminous energy of zero, regardless of the amount of radiant energy present. The SI unit of luminous energy is the lumen second, which is unofficially known as the talbot in honor of William Henry Fox Talbot. In other systems of units, luminous energy may be expressed in basic units of energy. Luminous energy In photometry, luminous energy is the perceived energy of light. This is sometimes called the quantity of light. Luminous energy is not the same as radiant energy, the corresponding"
"Luminous efficacy Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power (electric power, chemical energy, or others) consumed by the source. Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The former sense is sometimes called luminous efficacy of radiation, and the latter"
"conditions, such as those assaulting a building. The sensitivity of an image intensifier tube is measured in microamperes per lumen (µA/lm). It defines how many electrons are produced per quantity of light that falls on the photocathode. This measurement should be made at a specific color temperature, such as ""at a colour temperature of 2854 K"". The color temperature at which this test is made tends to vary slightly between manufacturers. Additional measurements at specific wavelengths are usually also specified, especially for Gen2 devices, such as at 800 nm and 850 nm (infrared). Typically, the higher the value, the more"
"most common (SI) unit is cd·m·lx, i.e. candelas per meter squared per lux. The older convention is Fl/Fc (foot-lamberts per foot-candle). This creates issues with comparative gain measurements since neither is a pure ratio, although both are measured as a value of output intensity over input intensity. This creates ambiguity in the marketing of night vision devices as the difference between the two measurements is effectively pi or approximately 3.142x. This means that a gain of 10,000 cd/m²/lx is the same as 31.42 Fl/Fc. This value, expressed in hours, gives an idea how long a tube typically should last. It's"
"Radiant intensity In radiometry, radiant intensity is the radiant flux emitted, reflected, transmitted or received, per unit solid angle, and spectral intensity is the radiant intensity per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. These are ""directional"" quantities. The SI unit of radiant intensity is the watt per steradian (), while that of spectral intensity in frequency is the watt per steradian per hertz () and that of spectral intensity in wavelength is the watt per steradian per metre ()—commonly the watt per steradian per nanometre (). Radiant"
"""ħ"" = ""c"" = 1), this simplifies to GeV = 0.3894 mb = 38 940 am. In SI, one can use units such as square femtometers (fm). The inverse femtobarn (fb) is the unit typically used to measure the number of particle collision events per femtobarn of target cross-section, and is the conventional unit for time-integrated luminosity. Thus if a detector has accumulated 100 fb of integrated luminosity, one expects to find 100 events per femtobarn of cross-section within these data. Consider a particle accelerator where two streams of particles, with cross-sectional areas measured in femtobarns, are directed to collide"
"the term ""luminosity"" means bolometric luminosity, which is measured either in the SI units, watts, or in terms of solar luminosities (). A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. A star also radiates neutrinos, which carry off some energy (about 2% in the case of our Sun), contributing to the star's total luminosity. The IAU has defined a nominal solar luminosity of to promote publication of consistent and comparable values in units of the solar luminosity. While bolometers do exist, they cannot be used to measure even"
"International System of Units The International System of Units (SI, abbreviated from the French "") is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities."
"Luminosity In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object. As a term for energy emitted per unit time, luminosity is synonymous with power. In SI units luminosity is measured in joules per second or watts. Values for luminosity are often given in the terms of the luminosity of the Sun, L. Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (M) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a"
"ratio of the visible light flux emitted (the ""luminous flux"") to the total power input to the source, such as a lamp. Visible light is measured in lumens, a unit which is defined in part by the differing sensitivity of the human eye to different wavelengths of light. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy (LER) is a measure of how well the distribution of energy matches the perception of the eye. The units of luminous efficacy are ""lumens per watt"" (lpw). The maximum LER possible"
"Luminosity (scattering theory) In scattering theory and accelerator physics, luminosity (""L"") is the ratio of the number of events detected (""N"") in a certain time (""t"") to the interaction cross-section (σ): It has the dimensions of events per time per area, and is usually expressed in the cgs units of cm·s or the non-SI units of b·s. In practice, ""L"" is dependent on the particle beam parameters, such as beam width and particle flow rate, as well as the target properties, such as target size and density. A related quantity is integrated luminosity (""L""), which is the integral of the"
"system. These also include the Gaussian units and the Heaviside–Lorentz units. In this table, ""c"" = 29,979,245,800 is the numeric value of the speed of light in vacuum when expressed in units of centimetres per second. The symbol ""↔"" is used instead of ""="" as a reminder that the SI and CGS units are ""corresponding"" but not ""equal"" because they have incompatible dimensions. For example, according to the next-to-last row of the table, if a capacitor has a capacitance of 1 F in SI, then it has a capacitance of (10 ""c"") cm in ESU; ""but"" it is usually incorrect"
"is most sensitive to greenish-yellow light at 555 nm. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. The curve which measures the response of the human eye to light is a defined standard, known as the luminosity function. This curve, denoted ""V""(λ) or formula_1, is based on an average of widely differing experimental data from scientists using different measurement techniques. For instance, the measured responses of the eye to violet light varied by a factor of ten . Luminous intensity should not be confused with another photometric unit, luminous flux, which is the total"
"(candelas), luminance (candelas/m) and illuminance (lumen/m). After the discovery of ipRGCs in 2002 additional units of light measurement have been researched in order to better estimate the impact of different inputs of the spectrum of light on various photoreceptors. However, due to the variability in sensitivity between rods, cones and ipRGCs and variability between the different ipRGC types a singular unit does not perfectly reflect the effects of light on the human body. The accepted current unit is equivalent melanopic lux which is a calculated ratio multiplied by the unit lux. The melanopic ratio is determined taking into account the"
"is measured just in this center area. Limitations with CRT video projectors result in them producing greater brightness when just a fraction of the image content is at peak brightness. For example, the Sony VPH-G70Q CRT video projector produces 1200 ""peak"" lumens but just 200 ANSI lumens. Brightness (white light output) measures the total amount of light projected in lumens. The color brightness specification Color Light Output measures red, green, and blue each on a nine-point grid, using the same approach as that used to measure brightness. Lumen (unit) The lumen (symbol: lm) is the SI derived unit of luminous"
"light emitted by a source, weighted according to the sensitivity of the human eye to various colours of light. Lumens per watt is a common measure of the efficacy of a light source. The luminous flux is of less value for bicycle lighting due to the importance of directionality. Luminous intensity is much more useful, but lumens per watt is a handy way to compare the output of otherwise similar lights. The lux (lx) is this SI unit for illuminance, that is the amount of light that illuminates a surface (the road, in the case of a bike light) per"
"indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye's pupil. Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between 50 and . The sun has a luminance of about at noon. Luminance is invariant in geometric optics. This means that for an ideal optical system, the luminance at the output"
"Illuminance In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. In SI derived units these are measured in lux (lx), or equivalently in lumens per square metre (cd·sr·m). In the CGS system, the unit of illuminance is the phot, which is equal to . The foot-candle is a"
"the wire (the position along its length, formula_7) Notice that these steps were the exact same ones we took before to find :formula_28 The SI unit of linear charge density is the coulomb per meter (C/m). In drawing or printing, the term linear density also refers to how densely or heavily a line is drawn. Common units include: Linear density Linear density is the measure of a quantity of any characteristic value per unit of length. Linear mass density (titer in textile engineering, the amount of mass per unit length) and linear charge density (the amount of electric charge per"
"Irradiance In radiometry, irradiance is the radiant flux (power) ""received"" by a ""surface"" per unit area. The SI unit of irradiance is the watt per square metre (). The CGS unit erg per square centimetre per second () is often used in astronomy. Irradiance is often called intensity because it has the same physical dimensions, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity. Spectral irradiance is the irradiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The"
"either infrared or ultraviolet, and the theoretical luminous [efficacy] is 95 lumens per watt. No substance is solid and usable as a light bulb filament at temperatures anywhere close to this. The surface of the sun is not quite that hot."" At temperatures where the tungsten filament of an ordinary light bulb remains solid (below 3683 kelvin), most of its emission is in the infrared. Luminous efficacy Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of"
"the light is separated into its different wavelengths (colours), the light intensity is measured with a photomultiplier tube or tubes physically positioned to ""view"" the specific wavelength(s) for each element line involved, or, in more modern units, the separated colors fall upon an array of semiconductor photodetectors such as charge coupled devices (CCDs). In units using these detector arrays, the intensities of all wavelengths (within the system's range) can be measured simultaneously, allowing the instrument to analyze for every element to which the unit is sensitive all at once. Thus, samples can be analyzed very quickly. The intensity of each"
"an absolute magnitude (although absolute magnitudes are often not measured relative to an absolute flux): Luminosity In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object. As a term for energy emitted per unit time, luminosity is synonymous with power. In SI units luminosity is measured in joules per second or watts. Values for luminosity are often given in the terms of the luminosity of the Sun, L. Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (M) of an object"
"irradiance, depending on the background of the person using the term. Radiance is also sometimes called ""intensity"", especially by astronomers and astrophysicists, and in heat transfer. Intensity (physics) In physics, intensity is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy. In the SI system, it has units watts per square metre (W/m). It is used most frequently with waves (e.g. sound or light), in which case the ""average"" power transfer over one period of the wave is used. ""Intensity"" can be applied to other circumstances"
"although this can be very uncomfortable and cause long-lasting afterimages. In astronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band. V-magnitudes can be converted to lux using the formula where ""E"" is the illuminance in lux, and ""M"" is the apparent magnitude. The reverse conversion is Illuminance In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate"