When comparing two light fixtures that produce the same amount of light, the one that draws less electrical power to do so is the more efficient product. Luminous efficacy is the metric that makes this comparison possible: it expresses how much visible light a fixture produces for every watt of electricity it consumes, stated in lumens per watt (lm/W). The higher the number, the more efficiently the fixture converts electrical energy into light.

This single figure carries considerable weight in specification decisions. It determines whether a fixture meets the minimum thresholds set by energy regulations in the markets where it will be installed. It predicts the operating cost of the fixture over its lifetime. And it provides a standardised basis for comparing fixtures across different technologies, power ratings, and manufacturers — without which comparisons based on wattage or lumen output alone can be misleading.

The definition: what luminous efficacy actually measures

Luminous efficacy is defined as the ratio of luminous flux to input power: lm/W = total lumens output ÷ total watts consumed. A fixture that emits 1,200 lumens while drawing 10 watts has a luminous efficacy of 120 lm/W. A fixture that emits the same 1,200 lumens while drawing 15 watts has a luminous efficacy of 80 lm/W — it produces the same light, but at a higher energy cost.

The lumens figure used in the calculation must be measured at the fixture level, not at the LED chip or light source alone. A bare LED chip may carry a rated efficacy significantly higher than the fixture in which it is installed, because the fixture assembly introduces optical losses through its lens, diffuser, or reflector; thermal losses that affect LED output over time; and driver losses from converting mains voltage to the low-voltage DC required by the LEDs. The fixture-level efficacy — sometimes called system efficacy — is the number that matters for specification purposes, as it reflects the actual performance of the complete product as installed.

Four factors that determine a fixture's luminous efficacy

How luminous efficacy has evolved across light source technologies

The historical context of lm/W figures helps to place the current 120 lm/W threshold in perspective. The incandescent lamp — the dominant light source through most of the twentieth century — operated at approximately 10–15 lm/W. The halogen lamp, an evolution of the incandescent, reached roughly 15–25 lm/W. Both technologies generated light through thermal radiation, heating a filament until it glows, a process that converts most of the electrical input to heat rather than light.

Fluorescent lamps improved substantially on this baseline, reaching 60–100 lm/W in their more efficient forms — a step change made possible by the different mechanism of fluorescent excitation. Compact fluorescent lamps brought comparable efficacy to the form factor of the screw-base lamp, though with trade-offs in light quality and dimming behaviour.

LED technology has advanced further and faster than any previous lamp type. Early LED products entering the market around 2010 offered 50–70 lm/W at the fixture level. By the mid-2010s, mainstream commercial products were regularly exceeding 100 lm/W. Current production-standard products from quality manufacturers routinely fall in the 120–160 lm/W range at the fixture level, and laboratory results on advanced chips have demonstrated values considerably higher. The 120 lm/W threshold that now appears in regulatory frameworks worldwide represents current mainstream attainability, not the leading edge.

The 120 lm/W threshold in global energy standards

The figure of 120 lm/W appears across a range of regulatory and certification frameworks as the minimum efficacy expected of energy-compliant LED products. Understanding why this specific threshold has been adopted — and how it is applied in different markets — is useful for anyone purchasing or specifying fixtures for international projects.

Luminous efficacy vs. related metrics that are often confused with it

Several related figures appear on specification sheets and data tables alongside luminous efficacy, and it is worth understanding precisely how each differs — because conflating them leads to specification errors that only become apparent after installation.

LED chip efficacy or package efficacy is the lm/W figure measured at the bare LED package, typically under standard test conditions of 25°C junction temperature and a defined drive current. This figure is always higher than the fixture-level efficacy because it does not account for driver losses, optical losses, or the thermal rise that occurs during normal fixture operation. A chip rated at 180 lm/W may be the basis for a fixture that achieves 130 lm/W at the system level — both figures are accurate, but they measure different things.

Luminous intensity, measured in candela (cd), describes how much light is emitted in a particular direction rather than in total. It is relevant to beam angle and distribution characterisation but says nothing about efficiency. A fixture could have very high intensity in a narrow beam while being highly inefficient — or vice versa.

Luminous efficacy of radiation (LER) is a theoretical measure of how efficiently a given spectral power distribution converts to human-visible light, based on the eye's sensitivity curve. It is distinct from luminous efficacy as used in fixture specification. LER is of interest in photometric research but does not appear on product data sheets in the same way.

Reading efficacy data on product specification sheets

When reviewing a fixture specification sheet, the luminous efficacy figure should be stated as a luminaire or system efficacy — not as chip or driver efficacy alone. The measurement conditions should correspond to a recognised photometric testing standard: LM-79 in the United States, EN 13032 in Europe, or equivalent national standards. These standards define how fixtures should be conditioned before measurement, at what ambient temperature, and how the photometric sphere or goniophotometer is calibrated.

LM-79 testing, specified by the Illuminating Engineering Society, is particularly significant because it measures the complete luminaire at its actual operating temperature rather than at a controlled low-temperature baseline. This matters because LEDs produce less light when hot than when cold, and a test conducted at artificially low temperatures will report optimistic lm/W values that the fixture will not achieve in a real installation. Independent LM-79 test reports from accredited laboratories provide more reliable data than manufacturer-conducted tests or chip-level ratings presented as fixture-level claims.

How efficacy interacts with colour rendering index and colour temperature

Luminous efficacy and colour rendering index (CRI) exist in a trade-off relationship that is important to understand. The CRI of a white LED is determined by the phosphor blend used to convert the blue LED emission to a broad-spectrum white light. Phosphor blends that produce a higher CRI — more accurate colour rendering, closer to the Ra 90+ range — generally do so by adding red-wavelength components to the spectrum. Red wavelengths are relatively inefficient in terms of the human eye's sensitivity response (which peaks in the green region), so adding more red content for better CRI typically reduces the lm/W figure at a given drive current.

The practical implication is that a fixture rated at Ra 90 CRI will typically have a lower luminous efficacy than an otherwise equivalent fixture rated at Ra 80 CRI, all other factors equal. When specifying against a minimum efficacy threshold such as 120 lm/W, the CRI requirement must be stated alongside the efficacy requirement, not treated as an independent variable. A fixture that reaches 125 lm/W at Ra 80 may achieve only 108 lm/W at Ra 90 — satisfying the efficacy threshold with one CRI specification but not the other.

Colour temperature also interacts with efficacy. In general terms, cool white LEDs (5000–6500K) tend to offer slightly higher lm/W than warm white (2700–3000K) at comparable CRI levels, because the spectral distribution of cool white sources aligns more closely with the eye's peak sensitivity. This difference is typically modest — within the range of 5–10% — but is worth accounting for when specifying warm white sources against a tight efficacy threshold.

Practical efficacy ranges by fixture application type

Why wattage alone is not a reliable comparison basis

In the era of incandescent lamps, wattage was a reliable shorthand for brightness because all incandescent lamps of the same wattage produced roughly the same amount of light — efficacy was consistent across the technology. The widespread adoption of LED lighting has invalidated this shorthand entirely. Two LED fixtures rated at 20W may produce 1,600 lumens and 2,400 lumens respectively, depending on their efficacy. Comparing them by wattage alone tells a buyer nothing useful about the light they will receive.

The consequence of wattage-based comparisons for buyers is real and measurable. A fixture specified as a "20W LED downlight" without an efficacy figure or lumen output specification could be delivering 80 lm/W or 130 lm/W — a difference of 62% in light output for the same electricity consumption, and potentially a difference in whether the product meets regulatory requirements in its installation market.

Specifying on the basis of lumen output and luminous efficacy together — rather than wattage — resolves this ambiguity. A requirement stated as "minimum 1,500 lm at minimum 120 lm/W" defines both the quantity of light and the efficiency at which it must be produced, leaving no room for a fixture to meet one criterion by sacrificing the other.

Efficacy over the fixture's lifetime: lumen maintenance and its interaction with rated efficacy

Luminous efficacy as measured at the beginning of a fixture's life will not remain constant over its operating hours. LEDs depreciate — they produce less light as they age, a process driven primarily by phosphor degradation and changes in the LED chip itself. A fixture that measures 130 lm/W at hour zero may measure 104 lm/W at hour 50,000 if it maintains 80% of its initial lumen output (an L80 rating in the terminology of lumen maintenance standards).

The lumen maintenance rating — expressed as L70, L80, or L90 at a stated number of hours — describes the percentage of original output retained at that point in the fixture's life. When evaluating efficacy in the context of a project's lifetime operating cost, it is worth considering what the effective average efficacy will be across the fixture's expected service life, not only its initial value. A fixture with high initial efficacy but poor lumen maintenance may deliver less over time than one with slightly lower initial efficacy but superior long-term stability.

Thermal management is the primary determinant of lumen maintenance quality. Fixtures that run their LEDs at lower junction temperatures — through better heat sink design, lower drive currents, or higher-grade thermal interface materials — demonstrate superior lumen maintenance and therefore sustain their efficacy advantage over their operating lifetime.