Define Lumen — What Is a Lumen?

流明是什么

The most common lighting-purchase mistake is buying by watts. Watts tell you how much energy a lamp consumes; they say nothing about how much light it produces. To define lumen correctly is to separate output (light) from input (power) — and that separation is exactly why LED lighting can deliver the same brightness as an incandescent bulb at a fraction of the wattage.

How the Lumen Works: Luminous Flux and the Eye

Light is electromagnetic radiation. A source emits a certain radiant flux (measured in watts) across many wavelengths. But the human eye is not equally sensitive to all wavelengths — it peaks in the green-yellow region near 555 nm and falls off sharply toward red and violet. To define lumen output, we weight the source's radiant power by the V(λ) photopic luminous efficiency function, standardized by the CIE.

The conversion constant is fixed: 683 lumens per watt of radiant power at 555 nm. This is the maximum possible luminous efficacy of radiation. A theoretical monochromatic 555 nm source radiating 1 W would produce exactly 683 lm. Real white sources spread energy across the spectrum (including invisible IR and UV that count for zero lumens), so their luminous efficacy of radiation is always lower — typically 250–350 lm/W of radiant flux for good white light.

This is why the lumen is a photometric quantity, not a purely physical (radiometric) one. To define lumen properly you must reference the observer: the same radiant spectrum produces different lumen totals under the photopic (daylight-adapted), scotopic (dark-adapted), or mesopic models. General lighting practice uses the photopic V(λ) function per CIE S 017:2020.

Luminous Efficacy: Lumens Per Watt

Divide a product's total lumen output by its electrical input power and you get luminous efficacy in lm/W — the single most important efficiency metric in lighting. To define lumen efficiency fairly, always use luminaire-level (system) efficacy from an IES LM-79-19 report, which measures total luminous flux and input power together. A bare LED chip might claim 200 lm/W, but after driver losses, optical losses, and thermal derating, the finished luminaire may deliver 130–160 lm/W.

Key Data: Lumen Output & Efficacy Benchmarks

How Lumens Relate to Lux and Candela

The lumen sits at the center of a family of three photometric quantities, all defined in CIE S 017:2020. Understanding the family is the fastest way to define lumen in context and to avoid confusing it with brightness at a surface:

1. Lumen (lm) — luminous flux: the total light emitted by a source in all directions. It answers "how much light does this lamp make?" This is a property of the source alone.
2. Candela (cd) — luminous intensity: the luminous flux emitted per unit solid angle in a specific direction (1 cd = 1 lm/sr). It answers "how concentrated is the light in this direction?" Optics redistribute a fixed lumen budget into candela.
3. Lux (lx) — illuminance: the luminous flux arriving on a surface per unit area (1 lx = 1 lm/m²). It answers "how much light lands on the desk?" This depends on distance and geometry, not just the source.

The relationships are precise:

• Lumens ↔ candela: luminous flux (lm) = luminous intensity (cd) × solid angle (sr). A source emitting uniformly over a full sphere (4π ≈ 12.57 sr) at 100 cd emits 1,257 lm. Focus those same lumens into a 15° beam and the on-axis candela shoots up dramatically.
• Lumens ↔ lux: illuminance (lx) = luminous flux (lm) ÷ area (m²). Spread 1,000 lm evenly over 1 m² and you get 1,000 lx; spread the same 1,000 lm over 10 m² and you get only 100 lx.

The critical insight: a lamp's lumen rating never changes, but the lux it delivers does. The same 1,000-lumen source produces high lux on a nearby desk and low lux on a distant floor. That is why lighting standards such as EN 12464-1:2021 specify lux at the task plane, while product datasheets quote lumens. A lighting designer's job is to convert one into the other using fixture count, mounting height, distribution, and room reflectances.

Practical Applications: Using Lumens Correctly

Buying Household and Office Lamps

Since incandescent phase-out, retail packaging lists lumens prominently. To define lumen needs for a room, work backwards from the target lux: a 12 m² home office at 300 lx needs roughly 3,600 lm of delivered light, then divide by a utilisation/loss factor (~0.6–0.7) to get ~5,000–6,000 installed lumens.

Specifying Commercial Fixtures

Lighting designers input each fixture's lumen output and photometric (IES/LDT) file into software such as DIALux or AGi32. The software computes lux distribution, uniformity, and UGR for the space. Accurate lumen data from an LM-79-19 report is the foundation — garbage lumens in, wrong lux out.

Comparing Products Fairly

Never compare a 40 W fixture to a 60 W fixture; compare their lumens and their lm/W. A 40 W fixture at 5,600 lm (140 lm/W) beats a 60 W fixture at 6,000 lm (100 lm/W) on both output and running cost. This is the essence of why the lumen — not the watt — is the currency of modern lighting.

Accounting for Lumen Depreciation

Initial lumens are not maintained lumens. LEDs lose output over time; the L70 or L90 figure (per IES TM-21-21) tells you when output drops to 70% or 90% of initial. Design to maintained lumens, not initial, so the space still meets its lux target years later.

Key Takeaways