The development of the phot paralleled the industrialization of light in the late 19th and early 20th centuries. Before standardized units existed, light was measured by comparing it to physical candles . As gas and electric lighting became competitive, industries needed more precise photometric standards to quantify output and efficiency. Modern Usage
The word (symbol: ph ) is a legacy unit of illuminance , representing the amount of luminous flux passing through a specific area. While largely replaced by the International System of Units (SI) today, it remains a notable part of scientific history in the study of optics and photometry . Understanding the Phot The development of the phot paralleled the industrialization
In modern practice, scientists and engineers primarily use lux or foot-candles (common in the U.S.) to measure how light hits a surface. To convert between these units, you can use the following standard equivalence factors : = 10,000 lux (lx) 1 phot (ph) ≈ 929.03 foot-candles (fc) Modern Usage The word (symbol: ph ) is
Derived from the Greek word phōs , meaning "light," the phot is defined as . It was a standard in the older CGS (centimetre–gram–second) system of units. The name was officially coined in 1921 by French physicist André Blondel. To convert between these units, you can use
= 10 lux (often used for measuring dim light sources) Historical Significance
Because it measures light density over a small area (a centimeter) compared to the standard meter, a single phot represents a very high level of brightness. For context, —the current SI unit for illuminance . Comparison and Conversion
Today, the phot is considered a "deprecated" unit. Most professional lighting design, architectural planning, and photography exposure calculations rely on the lux. However, the term "phot-" still thrives as a prefix in related fields like photonics , which explores the generation and harnessing of light for technology like fiber optics and medical imaging .