Lux to Phot

How Many Phots in a Lux?

To convert lux to phot, divide the lux value by 10,000. The formula is ph = lx ÷ 10,000. For example, 50,000 lux equals 5 phot. The phot is a CGS (centimeter-gram-second) unit of illuminance equal to one lumen per square centimeter, making it 10,000 times larger than the lux (one lumen per square meter). While the phot is rarely used in modern lighting design, it appears in older scientific literature, particularly in European and Japanese optics research from the early to mid-20th century. Understanding this conversion helps researchers working with historical data or legacy instruments calibrated in phot. The phot also provides a convenient scale for very high illuminance levels — direct sunlight at noon, which registers as 100,000 lux, is a tidy 10 phot. That makes the conversion useful when translating archival papers into SI language for modern reports, databases, and instrument software. It also gives you a quick sanity check when unusually bright conditions are described in legacy units. That extra context reduces transcription mistakes. It improves consistency too.

How to Convert Lux to Phot

1. Start with the illuminance value in lux (lx).
2. Divide by 10,000 to get the value in phot.
3. The result is the illuminance in phot (ph).
4. Since the conversion is simply a factor of 10,000, you can also move the decimal point four places to the left.
5. For practical purposes, phot values are only meaningful for very bright environments (direct sunlight, industrial light sources).

Real-World Examples

History of Lux and Phot

The phot was part of the CGS system of units that dominated physics from the mid-19th century through the mid-20th century. Named after the Greek word for light (phos), the phot was defined as one lumen per square centimeter. The CGS system was particularly popular in European and Japanese scientific communities, and many foundational optics papers from this era use the phot rather than the lux. As the SI system gained universal adoption following the 11th General Conference on Weights and Measures in 1960, the lux replaced the phot as the standard unit of illuminance. The transition was natural because the relationship is a clean power of ten (1 phot = 10,000 lux), making conversion of historical data straightforward. Today, the phot survives mainly as a historical curiosity and in some specialized fields where CGS units remain in use for internal calculations. The clean 10,000:1 ratio between phot and lux reflects the relationship between the square centimeter and the square meter: there are 10,000 cm² in one m². Since both units measure lumens per unit area, the conversion factor is exactly the area ratio.

Common Mistakes to Avoid

• Multiplying instead of dividing. To convert lux to phot, divide by 10,000. Multiplying goes the other direction.
• Confusing the phot with the foot-candle. The phot (1 lm/cm²) is much larger than the foot-candle (1 lm/ft²). One phot equals 10,000 lux while one foot-candle equals only about 10.76 lux.
• Attempting to use phot for ordinary lighting measurements. The phot is simply too large a unit for indoor illuminance levels, which is why the lighting industry abandoned it in favor of lux.
• Mistaking the phot for a luminance unit such as the nit or stilb. The phot measures illuminance on a surface, not the apparent brightness of that surface to an observer.

Frequently Asked Questions