Lux

Lore & Background

The lux is defined as one lumen per square metre, where the lumen measures total visible light flux. Illuminance from a source depends on surface orientation: a surface tilted at an angle θ to the source receives illumination reduced by a factor of cos θ. For a point source, the illumination on a tilted surface is reduced because the surface subtends a smaller solid angle. Practical lighting calculations sum contributions from every point on each light source. The lux has a radiometric counterpart, the watt per square metre, but no single conversion factor exists because the human eye's sensitivity varies by wavelength. White light sources have far lower luminous efficiency—typical incandescent bulbs achieve only about 2% of the theoretical maximum.

Reader's Guide

The lux is fundamental to photometry, bridging physical light power and human visual perception. Its definition via the standardized luminosity function allows precise measurement of illuminance for applications ranging from architectural lighting to astronomy. In video cameras, minimal lux ratings indicate low-light performance, while still cameras rely on longer exposures instead. In astronomy, apparent magnitude relates to illuminance: a magnitude 0 star provides 2.08 μlx at Earth's surface, and a magnitude 6 star provides 8 nlx. The lux's legacy Unicode symbol (U+33D3) exists but is not recommended for new documents. As a standardized unit, the lux enables consistent measurement across disciplines, though its value depends on spectral composition and surface geometry.

Did You Know?

• One lux equals one lumen per square metre, or one candela-steradian per square metre.
• A star of apparent magnitude 0 provides 2.08 microlux at Earth's surface under clear skies.
• The word 'lux' is derived from the Latin word for 'light'.

Definition and Place in the Measurement System

The lux, symbolized as lx, stands as the SI unit for illuminance—essentially luminous flux distributed across a surface area. One lux equals one lumen per square metre, making it a direct measure of how intensely light strikes or passes through a given surface. Unlike its radiometric counterpart, the watt per square metre, the lux is weighted by the luminosity function, a standardized model (agreed upon by the CIE and ISO) that reflects how the human eye perceives brightness unevenly across wavelengths. This makes it a photometric rather than purely physical quantity. The term itself traces back to the Latin word for light. In English usage, lux serves as both singular and plural. SI prefixes apply as with other named units, so one kilolux (klx) represents a thousand lux, while microlux and nanolux capture the faintest glimmers of distant starlight.

Scale and Practical Illuminance in Everyday and Cosmic Settings

Understanding lux in practice means grasping how the same amount of light changes character depending on the area it covers. A thousand lumens spread across one square metre yields 1000 lux of brightness, yet those identical 1000 lumens diluted over ten square metres drop to just 100 lux. This inverse relationship between area and illuminance, holding flux constant, explains why a home kitchen might reach 500 lux from a single 12,000-lumen fluorescent fixture, while a factory floor spanning dozens of times that area demands dozens of such fixtures to maintain the same level. At the extreme low end, a magnitude-0 star delivers roughly 2.08 microlux at Earth's surface, and a barely visible magnitude-6 star contributes a mere 8 nanolux. At the high end, unobstructed sunlight can push illuminance to 100 kilolux, a figure tied to the solar illuminance constant of 128,000 lux and varying with season and atmospheric conditions.

The Human Eye and the Limits of Luminous Efficiency

The lux is fundamentally a measure shaped by human biology. The eye's image-forming photoreceptors respond most strongly to green light at 555 nanometres, where one watt per square metre of irradiance translates to a maximum of 683.002 lux. At every other visible wavelength the conversion yields fewer lux per watt, and outside the visible spectrum the luminosity function drops to zero entirely. This biological weighting creates a practical consequence: a light source that appears white must blend red and blue wavelengths to which the eye is far less sensitive, so its lumens-per-watt ratio falls well short of the 683 lm/W theoretical ceiling. A typical incandescent bulb achieves a luminous efficiency of only about two percent. While individual human eyes vary slightly in their spectral sensitivity, the lux remains precisely defined and measurable because it rests on an agreed-upon standard luminosity function derived from measurements across many human subjects.

Geometry, Angle, and the Design of Real-World Lighting

Illuminance on a surface is not fixed by the source alone; the angle at which light meets the surface matters profoundly. A pocket flashlight aimed straight at a wall produces a compact, bright spot, but tilting the beam so it strikes the wall at an increasing angle causes the illuminated patch to stretch and dim. The physical reason is that a tilted surface subtends a smaller solid angle from the source's perspective, receiving less total light. For a point source, the reduction follows a simple cosine law: illumination drops by a factor equal to the cosine of the angle between the incoming ray and the surface normal. In practical lighting design, engineers account for this by calculating the contribution of every point on every light source, factoring in emission patterns, distances, and the geometry of the target area, then summing those contributions to predict the final illuminance at any given location.