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

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

This converter turns any lux value into phot in real time using the exact factor: 1 phot = 10,000 lux. The phot (ph) is the CGS system unit of illuminance — one lumen per square centimeter — and appears in older scientific papers and photometric standards. All math runs in your browser with no server call needed. A reference table of common lighting levels lets you put any result in practical context.

How it works

Enter the illuminance value in lux in the input field.
The converter divides your value by 10,000 to get phot.
The result updates instantly as you type.
Copy the result with one click.
Use the reference table to compare your result against common lighting environments.

Use cases

Interpret illuminance values in older scientific papers that use CGS units.
Convert modern lux measurements for comparison with historical photometric data.
Translate SI illuminance readings into phot for legacy engineering documents.
Verify photometric calculations when working with CGS-based optical formulas.
Cross-check illuminance units in academic references or textbooks that predate SI adoption.
Convert light meter readings for research involving CGS photometry standards.

Frequently Asked Questions

Last updated: 2026-07-14 · Reviewed by Nham Vu