Define Luminous Efficacy — What Is Luminous Efficacy?

光效是什么

Two floodlights both labelled "200 W" can produce 18,000 lumens and 32,000 lumens respectively. Wattage tells you what you pay; lumens tell you what you get; efficacy is the exchange rate between them. When you define luminous efficacy targets in a specification, you are directly fixing the energy cost of every hour the installation will ever run.

How Luminous Efficacy Works: From Watts to Lumens

To define luminous efficacy rigorously, you need both halves of the fraction:

Every real light source loses energy at several stages, which is why practical values sit far below the ceiling. For a typical LED luminaire the chain looks like this:

1. Driver losses (5–12%): AC–DC conversion at 88–95% efficiency.
2. LED conversion losses: electrical-to-optical conversion in the chip plus Stokes losses in the phosphor; the best white LED packages reach 220–240 lm/W at test current.
3. Thermal derating (3–10%): flux drops as junction temperature rises; a package rated at 25 °C delivers less in a fixture running at 85 °C junction.
4. Optical losses (5–15%): lenses, diffusers, and reflectors absorb light. A micro-prismatic office panel loses more than a bare-board high-bay.

Source Efficacy vs. System Efficacy — The Trap in Every Datasheet

The most common specification error is comparing a competitor's system (luminaire) efficacy against a vendor's source (chip) efficacy. A "200 lm/W LED" claim usually refers to the bare package at 25 °C and low drive current. After driver, thermal, and optical losses, the finished luminaire may deliver 140 lm/W. When you define luminous efficacy in tender documents, always specify luminaire efficacy, measured per IES LM-79-19, at stabilized operating temperature, including driver consumption. That single sentence eliminates the 20–40% inflation between chip marketing and delivered performance.

The Efficacy–Quality Trade-off

Efficacy competes with color quality. Raising CRI from 80 to 90 typically costs 10–15% efficacy, because high-CRI phosphor blends push more energy into the deep-red region where V(λ) sensitivity is low. Similarly, warm 2700 K sources test a few percent below cool 5000 K equivalents. This is why credible specifications define luminous efficacy together with CRI and CCT — "≥ 150 lm/W" alone invites bids with CRI 70 cool-white product that wins the efficacy race but fails the application.

📊 Key Data: Efficacy by Technology and Regulatory Thresholds

Practical Applications by Industry

Warehouse & Industrial High-Bay

Efficacy translates most directly into money where fixtures are numerous, powerful, and long-burning. Replacing 400 W metal halide high-bays (~75 lm/W maintained) with 150 lm/W LED units cuts connected load by roughly 60% for equal illuminance. At scale the arithmetic is decisive: 300 fixtures × 12 h/day, moving from 100 lm/W to 160 lm/W product at equal lumen output, saves ~37% of lighting energy — often six figures over a 10-year life. For these projects, define luminous efficacy minimums at 150–170 lm/W system level, CRI ≥ 70, verified by LM-79 reports.

Office & Commercial

Offices trade a little efficacy for visual comfort: UGR ≤ 19 optics (micro-prismatic diffusers) absorb 8–15% of flux, so realistic targets are 110–140 lm/W for compliant panels versus 160+ for bare-lens industrial product. Energy codes drive the numbers indirectly through Lighting Power Density (LPD, W/m²) limits — ASHRAE 90.1-2022 allows roughly 6.5 W/m² for open offices, only achievable with high-efficacy sources plus controls. Specify efficacy and UGR together; either alone produces a bad building.

Street & Area Lighting

Municipal tenders now routinely demand ≥ 130–150 lm/W plus photometric distribution requirements (EN 13201 / IES RP-8-22 classes). Efficacy matters doubly here because street lights burn 4,000+ hours a year and the optics (Type II/III distributions, full cutoff) consume part of the flux budget. Maintained efficacy is the honest metric: require TM-21 projections (L90 ≥ 100,000 h is common for quality product) so year-10 performance, not day-1 performance, meets the lux requirement.

Retail & Hospitality

Here the priority order reverses: CRI ≥ 90 (with R9 ≥ 50) and precise beam control outrank raw lm/W, and accepting 100–130 lm/W is a rational trade. The professional approach is still to define luminous efficacy floors — just calibrated to the color-quality class — so vendors cannot quietly substitute low-efficacy COBs while meeting the CRI line.

Horticulture (Special Case)

Plant lighting abandons lumens entirely, because plants do not share the human V(λ) curve. The equivalent metric is photon efficacy in µmol/J (PPE), measured to the same LM-79 electrical rigor. If a horticultural supplier quotes lm/W, that is a red flag — the industries define efficacy differently for good physical reasons.

How to Compare Efficacy Correctly: 5-Step Method

1. Normalize the level: compare luminaire (system) efficacy to luminaire efficacy — never chip to system.
2. Demand LM-79-19 reports from ISO 17025-accredited labs; check that test voltage, CCT, and CRI match the product you will actually receive.
3. Match color quality classes: only compare products at the same CRI and CCT bin, or apply a ~1.5%/CRI-point mental correction.
4. Use maintained values: multiply initial efficacy by the TM-21 lumen-maintenance factor at your design life to compare what each product delivers in year 8, not week 1.
5. Compute cost per megalumen-hour: (price + energy over life) ÷ total delivered lumen-hours. This single number ranks products across wattages and technologies and is the cleanest way to define luminous efficacy value in financial terms.

Key Standards

• CIE S 017:2020 — International Lighting Vocabulary. The normative definition of luminous efficacy, luminous flux, and the V(λ) weighting on which the lumen itself is built. Cite it whenever you define luminous efficacy in contracts.
• IES LM-79-19 — Optical and Electrical Measurements of Solid-State Lighting Products. The measurement method behind every legitimate efficacy claim: absolute photometry in an integrating sphere or goniophotometer with simultaneous electrical measurement.
• IES LM-80-15 / TM-21-19. LED package lumen-maintenance testing and lifetime extrapolation — the basis for maintained efficacy and L70/L80/L90 claims.
• EU Regulation 2019/2020 (Ecodesign) + 2019/2015 (Energy labelling). Legal efficacy floors for the EU market and the A–G energy label computed from total mains efficacy, registered in the EPREL database.
• EN 12464-1:2021. While primarily an illuminance/UGR standard, it frames the design context in which efficacy targets must still deliver required lux, uniformity, and comfort.
• GB 30255-2019 (China). Energy-efficiency grades and minimum allowable efficacy values for LED products in the Chinese market.

Key Takeaways