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Luminous efficacy
Common symbols
K
SI unitlm⋅W−1
In SI base unitscd⋅s3⋅kg−1⋅m−2
Dimension

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.[1][2][3] 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,[4] and the latter luminous efficacy of a light source[5] or overall luminous efficacy.[6][7]

Not all wavelengths of light are equally visible, or equally effective at stimulating human vision, due to the spectral sensitivity of the human eye; radiation in the infrared and ultraviolet parts of the spectrum is useless for illumination. The luminous efficacy of a source is the product of how well it converts energy to electromagnetic radiation, and how well the emitted radiation is detected by the human eye.

Efficacy and efficiency

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Luminous efficacy can be normalized by the maximum possible luminous efficacy to a dimensionless quantity called luminous efficiency. The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.

Luminous efficacy of radiation

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By definition, light outside the visible spectrum cannot be seen by the standard human vision system, and therefore does not contribute to, and indeed can subtract from, luminous efficacy.

Explanation

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The typical response of human vision to light under daytime or bright conditions, as standardized by the CIE in 1924. The horizontal axis is wavelength in nanometers.[8]

Luminous efficacy of radiation measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the luminous flux by the radiant flux.[4] Light wavelengths outside the visible spectrum reduce luminous efficacy, because they contribute to the radiant flux, while the luminous flux of such light is zero. Wavelengths near the peak of the eye's response contribute more strongly than those near the edges.

Wavelengths of light outside of the visible spectrum are not useful for general illumination[note 1]. Furthermore, human vision responds more to some wavelengths of light than others. This response of the eye is represented by the luminous efficiency function. This is a standardized function representing photopic vision, which models the response of the eye's cone cells, that are active under typical daylight conditions. A separate curve can be defined for dark/night conditions, modeling the response of rod cells without cones, known as scotopic vision. (Mesopic vision describes the transition zone in dim conditions, between photopic and scotopic, where both cones and rods are active.)

Photopic luminous efficacy of radiation has a maximum possible value of 683.002 lm/W, for the case of monochromatic light at a wavelength of 555 nm  .[note 2] Scotopic luminous efficacy of radiation reaches a maximum of 1700 lm/W for monochromatic light at a wavelength of 507 nm.[note 3]

Mathematical definition

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Luminous efficacy (of radiation), denoted K, is defined as[4]

where

Examples

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Photopic vision

[edit]
Type Luminous efficacy
of radiation (lm/W) 		Luminous
efficiency[note 4]
Tungsten light bulb, typical, 2800 K 15[9] 2%
Class M star (Antares, Betelgeuse), 3300 K 30 4%
Black body, 4000 K, ideal 54.7[note 5] 8%
Class G star (Sun, Capella), 5800 K 93[9] 13.6%
Black-body, 7000 K, ideal 95[note 5] 14%
Black-body, 5800 K, truncated to 400–700 nm (ideal "white" source)[note 6] 251[9][note 7] 37%
Black-body, 5800 K, truncated to ≥ 2% photopic sensitivity range[note 8] 292[9] 43%
Black-body, 2800 K, truncated to ≥ 2% photopic sensitivity range[note 8] 299[9] 44%
Black-body, 2800 K, truncated to ≥ 5% photopic sensitivity range[note 9] 343[9] 50%
Black-body, 5800 K, truncated to ≥ 5% photopic sensitivity range[note 9] 348[9] 51%
Monochromatic source at 540 THz 683 (exact) 99.9997%
Ideal monochromatic source: 555 nm (in air) 683.002[10] 100%

Scotopic vision

[edit]
Type Luminous efficacy

of radiation (lm/W)

Luminous

efficiency[note 4]

Ideal monochromatic 507 nm source 1699[11] or 1700[12] 100%
Spectral radiance of a black body. Energy outside the visible wavelength range (~380–750 nm, shown by grey dotted lines) reduces the luminous efficiency.

Lighting efficiency

[edit]
Main article: Wall-plug efficiency

Artificial light sources are usually evaluated in terms of luminous efficacy of the source, also sometimes called wall-plug efficacy. This is the ratio between the total luminous flux emitted by a device and the total amount of input power (electrical, etc.) it consumes. The luminous efficacy of the source is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve (the luminosity function). When expressed in dimensionless form (for example, as a fraction of the maximum possible luminous efficacy), this value may be called luminous efficiency of a source, overall luminous efficiency or lighting efficiency.

The main difference between the luminous efficacy of radiation and the luminous efficacy of a source is that the latter accounts for input energy that is lost as heat or otherwise exits the source as something other than electromagnetic radiation. Luminous efficacy of radiation is a property of the radiation emitted by a source. Luminous efficacy of a source is a property of the source as a whole.

Examples

[edit]

The following table lists luminous efficacy of a source and efficiency for various light sources. Note that all lamps requiring electrical/electronic ballast are unless noted (see also voltage) listed without losses for that, reducing total efficiency.

Category Type Overall luminous
efficacy (lm/W) 		Overall luminous
efficiency[note 4]
Combustion Gas mantle 1–2[13] 0.15–0.3%
Incandescent 15, 40, 100 W tungsten incandescent (230 V) 8.0, 10.4, 13.8[14][15][16][17] 1.2, 1.5, 2.0%
5, 40, 100 W tungsten incandescent (120 V) 5.0, 12.6, 17.5[18] 0.7, 1.8, 2.6%
Halogen incandescent 100, 200, 500 W tungsten halogen (230 V) 16.7, 17.6, 19.8[19][17] 2.4, 2.6, 2.9%
2.6 W tungsten halogen (5.2 V) 19.2[20] 2.8%
Halogen-IR (120 V) 17.7–24.5[21] 2.6–3.5%
Tungsten quartz halogen (12–24 V) 24 3.5%
Photographic and projection lamps 35[22] 5.1%
Light-emitting diode LED screw base lamp (120 V) 102[23][24][25] 14.9%
5–16 W LED screw base lamp (230 V) 75–217[26][27][28][29] 11–32%
21.5 W LED retrofit for T8 fluorescent tube (230 V) 172[30] 25%
Theoretical limit for a white LED with phosphorescence color mixing 260–300[31] 38.1–43.9%
Arc lamp Carbon arc lamp 2–7[32] 0.29–1.0%
Xenon arc lamp 30–90[33][34][35] 4.4–13.5%
Mercury-xenon arc lamp 50–55[33] 7.3–8%
Ultra-high-pressure (UHP) mercury-vapor arc lamp, free mounted 58–78[36] 8.5–11.4%
Ultra-high-pressure (UHP) mercury-vapor arc lamp, with reflector for projectors 30–50[37] 4.4–7.3%
Fluorescent 32 W T12 tube with magnetic ballast 60[38] 9%
9–32 W compact fluorescent (with ballast) 46–75[17][39][40] 8–11.45%[41]
T8 tube with electronic ballast 80–100[38] 12–15%
PL-S 11 W U-tube, excluding ballast loss 82[42] 12%
T5 tube 70–104.2[43][44] 10–15.63%
70–150 W inductively-coupled electrodeless lighting system 71–84[45] 10–12%
Gas discharge 1400 W sulfur lamp 100[46] 15%
Metal-halide lamp 65–115[47] 9.5–17%
High-pressure sodium lamp 85–150[17] 12–22%
Low-pressure sodium lamp 100–200[17][48][49][50] 15–29%
Plasma display panel 2–10[51] 0.3–1.5%
Cathodoluminescence Electron-stimulated luminescence 30–110[52][53] 15%
Ideal sources Truncated 5800 K black-body[note 7] 251[9] 37%
Green light at 555 nm (maximum possible luminous efficacy by definition) 683.002[10][54] 100%

Sources that depend on thermal emission from a solid filament, such as incandescent light bulbs, tend to have low overall efficacy because, as explained by Donald L. Klipstein, "An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 °C (6600 K or 11,500 °F). Even at this high temperature, a lot of the radiation is 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."[22] At temperatures where the tungsten filament of an ordinary light bulb remains solid (below 3683 kelvin), most of its emission is in the infrared.[22]

SI photometry units

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SI photometry quantities
Quantity Unit Dimension
[nb 1] 		Notes
Name Symbol[nb 2] Name Symbol
Luminous energy Qv[nb 3] lumen second lm⋅s T⋅J The lumen second is sometimes called the talbot.
Luminous flux, luminous power Φv[nb 3] lumen (= candela steradian) lm (= cd⋅sr) J Luminous energy per unit time
Luminous intensity Iv candela (= lumen per steradian) cd (= lm/sr) J Luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 (= lm/(sr⋅m2)) L−2⋅J Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.
Illuminance Ev lux (= lumen per square metre) lx (= lm/m2) L−2⋅J Luminous flux incident on a surface
Luminous exitance, luminous emittance Mv lumen per square metre lm/m2 L−2⋅J Luminous flux emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s/m3 L−3⋅T⋅J
Luminous efficacy (of radiation) K lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to radiant flux
Luminous efficacy (of a source) η[nb 3] lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient V 1 Luminous efficacy normalized by the maximum possible efficacy
See also:
  1. ^ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
  2. ^ Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
  3. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

See also

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Notes

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Luminous efficacy

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from Grokipedia
Luminous efficacy is a measure of how efficiently a source produces visible , defined as the ratio of the (in lumens) to the electrical power consumed (in watts), with units of lumens per watt (lm/). This metric quantifies the ability of a source to generate that is perceptible to the , accounting for the eye's sensitivity to different wavelengths via the photopic luminosity function, which peaks at 683 lm/ for monochromatic at 555 nm. It is distinct from radiant efficacy, which ignores human visual perception and focuses solely on total radiated power. There are two primary forms of luminous efficacy: the luminous efficacy of radiation, which compares to the source's and depends on its spectral distribution, and the luminous efficacy of a source, which includes electrical power input and accounts for losses in conversion processes. For example, the theoretical maximum luminous efficacy of radiation for ideal white light spectra is around 250–350 lm/W, limited by the need to balance color rendering and efficiency across the . In practice, real-world sources fall well below this due to thermal, electrical, and optical inefficiencies. Typical luminous efficacies vary widely by technology: incandescent lamps achieve 12–18 , primarily converting most to rather than ; fluorescent lamps, including T8 types, reach 80–100 lm/W when including losses; and modern light-emitting diodes (LEDs) exceed 200 lm/W for high-performance white LEDs, with records up to 254 lm/W as of October 2025, meeting earlier U.S. Department of projections. These improvements have driven the shift toward LEDs in lighting applications, offering 80–90% savings over incandescents and enabling better in illumination systems.

Basic Concepts

Definition and Importance

Luminous efficacy is defined as the ratio of , which quantifies the perceived brightness of light as measured in lumens, to the electrical power consumed by the source, expressed in watts, yielding units of lumens per watt (lm/W). This metric evaluates how effectively a light source converts input energy into visible light that aligns with human perception. The concept and term "luminous efficacy" emerged in the early , coinciding with the widespread adoption of electric technologies such as incandescent bulbs, which necessitated standardized ways to assess their performance. Prior to this, efficiency was not systematically quantified, but the shift to electricity-driven illumination prompted the development of photometric standards to bridge —the measurement of physical radiant power—and photometry, which weights light output according to the human eye's sensitivity to different wavelengths. Luminous efficacy plays a pivotal role in , efforts, and international standards aimed at curbing use for illumination, which represents about 15% of global consumption. The (IEA) promotes higher efficacy through policies and minimum performance standards that require light sources to deliver more lumens per watt, facilitating substantial reductions in energy demand and associated . By prioritizing sources with superior efficacy, these initiatives support sustainable practices without compromising visual comfort or functionality.

Efficacy versus Efficiency

Luminous efficacy quantifies the effectiveness of a light source in producing visible light, expressed as the ratio of luminous flux (in lumens) to either electrical power input or radiant power output, typically in lumens per watt (lm/). In contrast, luminous efficiency is a dimensionless metric that normalizes the efficacy by dividing it by the theoretical maximum possible efficacy for the given , yielding a value between 0 and 1 that represents the fraction of the source's radiated power falling within the visible as perceived by the . This distinction highlights how efficacy emphasizes practical output in visible terms, while efficiency provides a relative measure of spectral utilization without units. A common point of confusion in the lighting industry arises from interchanging terms like "luminous of radiation" (LER), which measures lumens per watt of radiant power and focuses solely on optical output, with "luminous of the device" (LE), or wall-plug , which accounts for lumens per watt of electrical input and includes losses in conversion processes such as thermal dissipation. This misuse can lead to overstated performance claims, as LER values (often 200–300 lm/W for white spectra) ignore electrical inefficiencies, whereas wall-plug is typically lower (e.g., 100–200 lm/W for high-end LEDs) due to non-radiative losses. Such terminological overlap has been noted in technical literature as a barrier to accurate comparisons in product specifications and research evaluations. In regulatory contexts, luminous efficacy serves as the primary metric for assessing and labeling lighting products to promote energy savings. For instance, the European Union's energy labeling framework under Regulation (EU) 2019/2015 classifies light sources from A (most efficient) to G based directly on their luminous efficacy in lm/W, enabling consumers to compare devices like LEDs (often exceeding 100 lm/W) against incandescents (around 15 lm/W) and driving market shifts toward higher-efficacy technologies. This approach ensures that labels reflect real-world visible light output per unit of consumed, supporting broader energy efficiency goals without conflating it with broader radiant or thermal efficiencies.

Luminous Efficacy of Radiation

Explanation

Luminous efficacy of radiation (LER) quantifies the fraction of a light source's that contributes to visible as perceived by the , defined as the ratio of to for a specific . This measure is independent of the source's electrical-to-optical conversion efficiency, focusing instead on the inherent properties of the emitted spectrum within the visible range. By weighting the radiant power according to the eye's , LER provides a standardized way to assess how effectively stimulates , regardless of the technology used to generate it. The perceptual basis of LER stems from the human visual system's sensitivity, particularly under photopic conditions where cone cells dominate. The standard photopic , V(λ), peaks sharply at 555 nm in the green-yellow region, reflecting maximum sensitivity there, and declines rapidly outside the approximate 400–700 nm , rendering and radiation ineffective for vision. This function, established through psychophysical experiments, ensures that LER emphasizes wavelengths that align with daylight-adapted vision, prioritizing the portion of the that humans perceive as bright. Idealized spectra illustrate LER's extremes: at 555 nm achieves the theoretical maximum of 683 /, as it perfectly matches the eye's peak sensitivity without wasting energy in non-visible wavelengths. In contrast, blackbody radiators, which emit across a broad continuum, yield lower LER values that depend on temperature; for instance, those approximating solar or incandescent spectra distribute significant power into , reducing the visible fraction. Under low-light mesopic conditions, such as in street lighting where both and cones contribute, the effective luminous efficacy is intermediate between photopic and scotopic levels due to the shifted and broadened sensitivity curve. This adaptation enhances detection in dim environments, influencing applications like roadway illumination where spectral optimization can improve perceived brightness.

Mathematical Definition

The luminous efficacy of radiation (LER), denoted as KK, is mathematically defined as the ratio of the luminous flux Φv\Phi_v to the radiant flux Φe\Phi_e emitted by a radiation source, expressed in lumens per watt (lm/W). This quantity quantifies how effectively the spectral power distribution of the radiation contributes to visible light as perceived by the under photopic conditions. The foundational equation derives from the definition of luminous flux provided by the (CIE), where Φv=683∫0∞V(λ)Φe,λ(λ) dλ\Phi_v = 683 \int_0^\infty V(\lambda) \Phi_{e,\lambda}(\lambda) \, d\lambda with Φv\Phi_v in lumens, V(λ)V(\lambda) the photopic spectral luminous efficiency function (normalized to a maximum of 1 at 555 nm), Φe,λ(λ)\Phi_{e,\lambda}(\lambda) the spectral radiant flux in watts per nanometer, and the constant 683 lm/W representing the maximum luminous efficacy for monochromatic radiation at 555 nm, where V(λ)=1V(\lambda) = 1. The radiant flux is the integral of the spectral radiant flux over all wavelengths: Φe=∫0∞Φe,λ(λ) dλ\Phi_e = \int_0^\infty \Phi_{e,\lambda}(\lambda) \, d\lambda. Thus, the LER for a given S(λ)S(\lambda) (where S(λ)=Φe,λ(λ)S(\lambda) = \Phi_{e,\lambda}(\lambda) for unit total power) is K=683∫0∞V(λ)S(λ) dλ/∫0∞S(λ) dλ,K = 683 \int_0^\infty V(\lambda) S(\lambda) \, d\lambda \Bigg/ \int_0^\infty S(\lambda) \, d\lambda, which normalizes the weighted visible portion against the total radiated power. This derivation follows directly from substituting the luminous flux expression into the efficacy ratio, ensuring KK reaches its theoretical maximum of 683 lm/W only for pure 555 nm light; for broadband sources like blackbody radiators, values are typically lower, e.g., around 15 lm/W for a 2800 K filament. A related normalization is the luminous efficiency ηv=K/683\eta_v = K / 683 lm/W, which expresses the LER as a fraction of the photopic maximum, ranging from 0 to 1 and highlighting the spectral match to human vision. For scotopic vision (low-light conditions), an analogous framework uses the spectral luminous efficiency function V′(λ)V'(\lambda), peaking at 507 nm, with a maximum efficacy constant of 1699 lm/W to account for rod cell sensitivity, though effective values remain adjusted for overall visibility thresholds. Recent CIE standards, such as CIE S 018:2019, have refined the tabulated values of V(λ)V(\lambda) with higher-resolution data (1 nm steps) and physiological basis updates, improving accuracy for narrowband spectra like those from LEDs by better aligning with empirical visual response measurements.

Luminous Efficacy of Sources

Definition

Luminous efficacy of a source, often abbreviated as LES, quantifies the performance of a complete lighting system by measuring the total luminous flux produced, in lumens (lm), divided by the total electrical power input consumed, in watts (W), yielding units of lm/W. This metric serves as a practical indicator of how effectively electrical energy is converted into visible light output for real-world applications, encompassing the entire device from power supply to emission. In contrast to the luminous efficacy of radiation (LER), which evaluates only the theoretical efficiency based on the spectrum of emitted light relative to its radiant power, LES incorporates all practical conversion losses, including those from thermal dissipation, non-radiative recombination, and electrical drive circuitry. Mathematically, LES is given by the product of LER, radiant efficiency (the ratio of to the electrical power delivered to the emitter), and (the fraction of total input power that reaches the emitter without loss in drivers or ballasts): LES = LER × η_radiant × η_electrical. This formulation highlights how LES reflects the cumulative impact of device physics and on overall light production. Several factors influence LES, particularly in modern solid-state lighting. For light-emitting diodes (LEDs), driver efficiency—typically around 85% for converting to and managing control functions—plays a critical role, as inefficiencies here can reduce LES by over 30%. In fluorescent lamps, ballast losses from power regulation circuitry further diminish , though electronic ballasts mitigate this compared to older magnetic types, enabling system-level LES in the 80–100 lm/W range. Broader system design elements, such as thermal management to minimize junction heating (which can cut by up to 15%) and optical extraction to preserve flux, are essential for optimizing LES across lamp architectures. As of 2025, laboratory advancements in and LEDs have pushed LES beyond 200 lm/W, driven by improved spectral matching to the response and reduced non-radiative losses, signaling transformative potential for energy-efficient illumination.

Examples

Incandescent light sources, such as traditional tungsten-filament bulbs, typically achieve a luminous efficacy of around 15 lm/W, largely due to significant energy loss as infrared radiation rather than visible . Halogen lamps, an improved variant using halogen gas to extend filament life, offer slightly better performance at approximately 20 lm/W. Fluorescent lamps provide a substantial advancement, with compact fluorescent lamps (CFLs) reaching 50-75 lm/W and linear fluorescents up to 75-100 lm/W, thanks to their excitation of phosphors that better align with the eye's sensitivity . High-intensity discharge (HID) lamps, including and high-pressure sodium types, further improve to 80-120 lm/W, making them suitable for large-scale applications like street . Emerging organic (OLED) panels, valued for their diffuse light quality, currently attain about 100 lm/W in commercial products. Light-emitting diodes (LEDs) represent the current benchmark, with commercial white LEDs averaging 150 lm/W as of 2025 and high-end models exceeding 220 lm/W in specialized applications. The theoretical maximum for white under photopic vision conditions ranges from 250-350 lm/W, limited by the spectral distribution that optimizes both color rendering and eye sensitivity. Historically, Thomas Edison's early incandescent bulb in 1879 achieved only 1-2 lm/, marking a modest start to electric . Over the subsequent decades, efficiencies progressed incrementally to 15 lm/ for standard incandescents by the mid-20th century, followed by fluorescents in the 1930s at 30-50 lm/. The advent of LEDs in the accelerated gains, with average efficacies rising from under 20 lm/ to over 100 lm/ by 2010, driven by advancements in materials and conversion. According to the (IEA), this evolution has led to LEDs comprising 50% of global residential sales by 2022, with projections for 100% adoption by 2025, potentially reducing worldwide energy use by 50% compared to 2015 levels. The following table compares luminous efficacies across key source types, illustrating the progression toward higher efficiency:
Light Source TypeTypical Luminous Efficacy (lm/W)Notes
Early Incandescent (Edison-era)1-2Carbon filament, short lifespan.
Standard Incandescent~15High infrared output.
Halogen~20Gas-filled for longevity.
Compact Fluorescent50-75Phosphor-based spectrum matching.
HID (Metal Halide/Sodium)80-120For high-output applications.
Commercial White LED (2025 average)150Phosphor-converted blue LED.
High-End LED>220Optimized for specific uses.
OLED~100Diffuse, flexible panels.

Units and Measurement

SI Photometry Units

In photometry, the SI unit for is the lumen (lm), which quantifies the total amount of visible light emitted by a source, weighted by the human eye's sensitivity. The (cd) serves as the for , measuring the brightness of light emitted in a specific direction per unit . The (lx) is the SI unit for , defined as one lumen per square meter (lx = lm/m²), representing the amount of incident on a surface. Luminous efficacy is expressed in lumens per watt (lm/W), where the watt () is the SI unit for radiant or electrical power input to a light source. This unit ratio directly compares the visible output to the energy consumed. Key conversion factors include 1 lm = 1 ·sr, linking flux to intensity via the solid angle in steradians (sr). Additionally, the maximum luminous efficacy for at 555 nm (540 × 10¹² Hz) is defined as exactly 683 lm/W, tying photometric units to radiometric quantities like through the human visual response. The (CIE) has standardized these units since 1924, when it adopted the photopic V(λ) to weight spectral power distributions for human vision. Updates in the late 1970s and 1980s refined V(λ), incorporating Judd-Vos modifications in 1978 for improved accuracy and the 1988 CIE Publication 86 adopting the modified 2° V_M(λ) function, ensuring consistency in photometric measurements. To measure these quantities, integrating spheres are commonly used to determine total luminous flux by uniformly distributing light from a source across the sphere's inner surface, where a detector calibrated in lumens captures the averaged output. Spectrometers measure the spectral power distribution of the source, allowing computation of luminous flux or efficacy by integrating the spectrum against V(λ) and applying the 683 lm/W constant. These protocols, outlined in standards like IES LM-78 for sphere-based flux measurement, ensure traceability to SI units with uncertainties typically below 1%. Radiant flux, denoted as Φe\Phi_e, represents the total electromagnetic power emitted by a light source, measured in watts (W). In contrast, , Φv\Phi_v, quantifies the portion of that power perceived as visible by the , measured in lumens (lm) and weighted by the luminous efficiency function V(λ)V(\lambda). Luminous efficacy serves as the bridge between these quantities, defined for radiation as the ratio Φv/Φe\Phi_v / \Phi_e in lm/W, which converts the physical power output into its visual equivalent. Luminous efficiency, a , normalizes the luminous efficacy of radiation by the theoretical maximum of 683 lm/W at 555 nm, yielding values between 0 and 1 to indicate the fraction of radiant flux effectively contributing to visible . The color rendering index (CRI), which measures a light source's to accurately reproduce colors compared to a reference illuminant, often trades off against luminous efficacy; higher CRI values typically require a broader distribution, reducing the by shifting energy away from peak eye sensitivity wavelengths. Wall-plug efficacy, or the luminous efficacy of a source, accounts for the full energy conversion chain by dividing luminous flux by the total electrical input power in watts, incorporating losses from electricity to light emission. This differs from radiant efficacy, which focuses solely on the optical output stage as luminous flux divided by radiant flux, excluding electrical inefficiencies such as those in drivers or heat dissipation. In practice, wall-plug efficacy provides a comprehensive metric for overall system performance, often lower than radiant efficacy due to these additional losses. For AC-driven light sources like LEDs, (PF) and () influence measured luminous efficacy by affecting how input electrical power is accurately quantified. A low PF, caused by phase differences or harmonics, means that apparent power exceeds real power, potentially inflating efficacy calculations if not corrected, while high introduces waveform distortions that increase losses in drivers and reduce overall efficiency. Standards mitigate this by requiring PF above 0.9 and below 20% for compliant drivers, ensuring reliable efficacy measurements. Following the sunset of lighting certifications effective December 31, 2024, U.S. federal standards under 10 CFR Part 430 integrate these quantities into efficiency requirements; for example, general service lamps must achieve minimum efficacies of up to 120 lm/W for integrated LED types (as of 2023 rules, effective through 2028), while considering PF, THD, and CRI for overall performance in residential and commercial systems. This ensures that efficacy metrics align with real-world performance, including interactions with electrical infrastructure.

References

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  12. https://unfccc.int/climate-action/momentum-for-change/activity-database/momentum-for-change-enlighten-initiative
  13. https://www.iea.org/policies/1552-lighting-energy-efficiency-standards
  14. https://iea.blob.core.windows.net/assets/08a95c62-b6eb-4606-8130-92cece9a287d/Lightslabourslost.pdf
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  23. https://www.sciencedirect.com/topics/computer-science/luminous-efficacy
  24. https://cie.co.at/datatable/cie-spectral-luminous-efficiency-photopic-vision
  25. https://ies.org/definitions/luminous-efficacy-of-a-source/
  26. https://www.iea.org/energy-system/buildings/lighting
  27. https://www1.eere.energy.gov/buildings/publications/pdfs/corporate/hid_workshop-report.pdf
  28. https://www.mdpi.com/2079-9292/13/7/1299
  29. https://pubs.aip.org/aip/jap/article/111/10/104909/369697/Maximum-spectral-luminous-efficacy-of-white-light
  30. https://www.bipm.org/en/si-base-units/candela
  31. https://www.nist.gov/pml/sensor-science/optical-radiation/realization-candela
  32. https://www.mdpi.com/2075-5309/15/19/3510
  33. https://jov.arvojournals.org/article.aspx?articleid=2121738
  34. https://www.labsphere.com/wp-content/uploads/2021/09/Integrating-Sphere-Theory-and-Applications.pdf
  35. https://www.instrumentsystems.com/fileadmin/Downloads/Brochures_Datasheets/ITS_EN.pdf
  36. https://store.ies.org/product/lm-78-20-approved-method-total-luminous-flux-measurement-of-lamps-using-an-integrating-sphere-photometer/
  37. https://depts.washington.edu/mictech/optics/me557/Radiometry.pdf
  38. https://wp.optics.arizona.edu/jpalmer/wp-content/uploads/sites/65/2018/11/BKAPPNDX.pdf
  39. https://www.electrochem.org/dl/interface/wtr/wtr09/wtr09_p029-030.pdf
  40. https://opg.optica.org/abstract.cfm?uri=oe-18-1-340
  41. https://www.researchgate.net/publication/241340097_Color_rendering_ability_and_luminous_efficacy_enhancements_in_white_light-emitting_diodes
  42. https://www.sciencedirect.com/topics/engineering/luminous-efficacy
  43. https://www.nap.edu/read/24619/chapter/5
  44. https://pmc.ncbi.nlm.nih.gov/articles/PMC9920439/
  45. https://oam-rc.inoe.ro/articles/the-design-and-analysis-of-led-drivers-with-power-factor-correction-in-lighting-applications/fulltext
  46. https://www.federalregister.gov/documents/2024/04/19/2024-07831/energy-conservation-program-energy-conservation-standards-for-general-service-lamps
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