The phot (symbol ph) is the CGS (centimeter-gram-second) unit of illuminance. It equals one lumen per square centimeter. Because a square centimeter is 10,000 times smaller than a square meter, 1 ph = 10,000 lux. The phot was widely used before the SI system standardized on lux.

How many lux is 1 phot?

1 phot equals exactly 10,000 lux. This comes directly from the fact that 1 m² = 10,000 cm², so 1 lm/cm² = 10,000 lm/m² = 10,000 lux.

How many phot is 1 lux?

1 lux equals 0.0001 phot (1 × 10⁻⁴ ph). To convert, divide the lux value by 10,000.

Is the phot still in use today?

The phot is largely obsolete in modern practice, replaced by the SI unit lux. It still appears in older scientific literature, vintage optical engineering references, and some CGS-based physics textbooks. Converting between phot and lux is occasionally needed when reading historical data.

What is the difference between lux, phot, and foot-candle?

All three measure illuminance — lumens per unit area. Lux (lx) is SI: lumens per square meter. Phot (ph) is CGS: lumens per square centimeter (1 ph = 10,000 lx). Foot-candle (fc) is imperial: lumens per square foot (1 fc ≈ 10.764 lx). Lux is the modern standard worldwide.

Lux to Phot Converter

Name: Lux to Phot Converter
Availability: InStock
Author: Nham Vu

Enter a lux value and instantly get the equivalent in phot, plus the formula and common lighting reference points.

Lux to Phot

Illuminance in lux (lx)

Enter a lux value above to see the result.

Formula: ph = lx ÷ 10,000

Common Lighting Levels

Environment	Lux (lx)	Phot (ph)

Click any row to load that lux value into the converter.

Summary