Lux

The lux, represented by the symbol lx, is the standard unit of illuminance in the International System of Units (SI). Illuminance describes how much luminous flux is spread over a specific area. While luminous flux, measured in lumens, tells us the total amount of visible light present, the lux tells us the intensity of that light on a surface. This measurement is vital for designing everything from home kitchens to massive factory floors. The term itself is derived from the Latin word for light, which is "lux."

To understand how lux works, you must look at the relationship between light and area. One lux is defined as one lumen per square metre (1 lx = 1 lm/m²). Imagine you have a light source that produces 1,000 lumens. If you spread that light uniformly over one square metre, the illuminance is 1,000 lux. However, if you spread that same 1,000 lumens over ten square metres, the light becomes much dimmer, resulting in only 100 lux. Because illuminance is inversely proportional to area, a larger area requires more total lumens to maintain the same brightness level.

There is a major difference between lux and the radiometric unit known as watts per square metre (W/m²). The watt per square metre measures irradiance, which is the physical power of light across all wavelengths. In contrast, the lux is a photometric unit. This means it accounts for how the human eye actually perceives brightness. Scientists use a model called the luminosity function to weight different wavelengths of light. This function reflects the fact that our eyes do not respond to all colors with equal sensitivity.

The luminosity function is not flat; it has a distinct peak at a wavelength of 555 nanometers, which is green light. Our eyes are most sensitive to this specific color. For monochromatic light at exactly 555 nm, the illuminance is at its maximum relative to irradiance. At this wavelength, 1 W/m² produces approximately 683.002 lx. For other wavelengths, the number of lux produced per watt is much lower. If a light source consists only of green light, it is very efficient, but it would not look like natural white light.

To create "white" light, a source must include a mixture of red and blue wavelengths. However, because our eyes are less sensitive to red and blue than to green, these sources produce fewer lumens per watt. The ratio between the actual lumens per watt and the theoretical maximum is called luminous efficiency. For example, a standard incandescent light bulb has a luminous efficiency of only about 2%. This shows how much light energy is "lost" to our perception when we use colors outside the peak sensitivity of our eyes.

Light levels in our environment vary across a massive scale. On a moonless, overcast night, starlight provides only 0.0001 lux. A full moon on a clear night provides between 0.05 and 0.3 lux. In human environments, family living rooms might have 50 lux, while office lighting ranges from 320 to 500 lux. Even brighter settings, like a typical TV studio, use about 1,000 to 25,000 lux. The strongest natural source, the Sun, can provide up to 100,000 lux on the Earth's surface, depending on the time of year and the atmosphere.

Geometry also affects how much light a surface receives. If a light source hits a surface perpendicularly, the illuminance is at its strongest. If the surface is tilted at an angle, the illumination decreases. This happens because a tilted surface subtends a smaller solid angle from the source. For a point source, the reduction in light is equal to the cosine of the angle between the light ray and the normal to the surface. This is why a flashlight beam looks brightest when aimed directly at a wall.

We use these measurements in many specialized fields. Video camera specifications often list a minimum lux level required to record a satisfactory image. Cameras with good low-light capability have lower lux ratings. In astronomy, the brightness of stars is related to illuminance; for instance, a star with an apparent magnitude of 0 provides about 2.08 microlux at Earth's surface. While the lux is the SI standard, some regions use the foot-candle. One foot-candle is approximately equal to 10.764 lux.