Convert centimeter-candle [cm*c] to phot [ph] Online | Free lllumination Converter

A Historical Unit of Illuminance

The centimeter-candle (cm·c) is an older, non-SI unit of illuminance that measures the luminous flux falling on a surface area of one square centimeter. It is defined as the illuminance produced at a distance of one centimeter from a uniform point source of one candlepower (now replaced by the candela).

Since the centimeter-candle measures light intensity over a very small area, it represents a much finer scale compared to more common units like the foot-candle or lux. One centimeter-candle is equivalent to 10,000 lux because one square meter contains 10,000 square centimeters.

Historically, the centimeter-candle was used in photometry and lighting sciences before international standardization led to the widespread adoption of the lux as the preferred illuminance unit. Its usage today is rare and mostly of historical interest, but it remains useful for understanding older lighting data and certain precise scientific contexts where small-area measurements matter.

Understanding centimeter-candle helps bridge the gap between historical and modern units of light measurement and offers insight into the evolution of photometric standards.

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.