Convert lux [lx] to phot [ph] Online | Free lllumination Converter

The Standard Unit of Illuminance

Lux (lx) is the SI unit of illuminance, measuring the amount of luminous flux (light) falling on a surface per unit area. One lux is equal to one lumen per square meter (lm/m²). It quantifies how brightly a surface is illuminated, directly relating to human visual perception of light in an environment.

Lux is widely used in various fields such as architecture, photography, horticulture, and workplace safety to ensure proper lighting conditions. For example, typical office lighting ranges from 300 to 500 lux to promote productivity without causing eye strain. Outdoor daylight varies greatly but can reach up to 100,000 lux on a bright sunny day.

Because lux is based on the metric system, it is the preferred unit globally for lighting measurement and design. It helps engineers, designers, and scientists create environments with appropriate lighting levels, balancing visibility, energy efficiency, and comfort.

Understanding lux is essential for interpreting lighting standards, optimizing artificial illumination, and assessing natural light exposure, making it fundamental in both everyday life and technical applications.

A Unit of Illuminance in Photometry

The phot (ph) is a unit of illuminance formerly used in photometry, representing the amount of luminous flux falling on a surface per unit area. One phot is equal to 10,000 lux (lm/m²), meaning it measures a very high level of illumination.

The phot was commonly used before the widespread adoption of the lux in the International System of Units (SI). It is derived from the Greek word "photos," meaning light, and was particularly useful for expressing strong light intensities, such as direct sunlight, which typically measures about 100,000 lux or 10 ph.

Though largely replaced by lux, the phot is still occasionally referenced in older scientific literature and certain industrial contexts. Because 1 phot equals 10,000 lux, it provides a convenient scale for quantifying intense lighting conditions without resorting to large numbers.

Understanding the phot unit helps in interpreting historical data and in fields where very bright light levels are relevant, such as outdoor lighting design, solar energy studies, and some photographic applications.