Phot to Foot-candle Conversion Table
| Phot (ph) | Foot-candle (fc) | Reference Benchmark | Formula Step |
|---|---|---|---|
| 1e-7 ph | 0.000093 fc | Overcast night sky with starlight only | 0.0000001 × 929.03 = 0.000093 fc |
| 0.000025 ph | 0.0232 fc | Full moon on a clear night at sea level | 0.000025 × 929.03 = 0.0232 fc |
| 0.0001 ph | 0.0929 fc | Emergency pathway and exit corridor egress lighting | 0.0001 × 929.03 = 0.0929 fc |
| 0.001 ph | 0.929 fc | Residential hallway and twilight ambient outdoor light | 0.001 × 929.03 = 0.929 fc |
| 0.005 ph | 4.645 fc | Residential living room or cinema auditorium ambient lighting | 0.005 × 929.03 = 4.645 fc |
| 0.01 ph | 9.29 fc | Warehouse bulk storage aisles and commercial stairwells | 0.01 × 929.03 = 9.29 fc |
| 0.02 ph | 18.581 fc | General dining room and school classroom background light | 0.02 × 929.03 = 18.581 fc |
| 0.03 ph | 27.871 fc | Library reading room and hotel conference workspace | 0.03 × 929.03 = 27.871 fc |
| 0.05 ph | 46.452 fc | Standard commercial open-plan computer office desktop | 0.05 × 929.03 = 46.452 fc |
| 0.075 ph | 69.677 fc | Technical CAD drafting and mechanical inspection bench | 0.075 × 929.03 = 69.677 fc |
| 0.1 ph | 92.903 fc | Precision electronic manufacturing and watchmaking bench | 0.1 × 929.03 = 92.903 fc |
| 0.2 ph | 185.806 fc | Hospital operating surgical theater task illumination | 0.2 × 929.03 = 185.806 fc |
| 0.5 ph | 464.515 fc | Television broadcasting studio lighting stage | 0.5 × 929.03 = 464.515 fc |
| 1 ph | 929.03 fc | Overcast overcast midday outdoor daylight | 1 × 929.03 = 929.03 fc |
| 2.5 ph | 2322.576 fc | Full indirect daylight under clear blue sky | 2.5 × 929.03 = 2,322.576 fc |
| 10 ph | 9290.304 fc | Direct overhead tropical noon sunlight | 10 × 929.03 = 9,290.304 fc |
Phot to Foot-candle Formula & Unit Definitions
The exact ratio derives from geometric conversions between metric centimeters and imperial feet: 1 foot is defined as exactly 30.48 centimeters, which means 1 square foot equals (30.48 cm)² = 929.0304 square centimeters. Since 1 phot equals 1 lumen per square centimeter, spreading that same luminous flux over 1 square foot yields 929.0304 lumens per square foot (foot-candles). Therefore, multiplying any illuminance value in phot by 929.0304 converts it directly to foot-candles.
Quick Estimation: To convert phot to foot-candles mentally, multiply by 930 (or multiply by 1,000 and subtract 7%). For example, 1 phot of daylight equals approximately 930 foot-candles (exact: 929.03 fc).
Phot (ph)
Source Unit- Centimetre-gram-second (CGS) illuminance unit defined as 1 lumen per square centimeter (lm/cm²).
- Historically standard in laboratory optics, photographic focal plane testing, and photometry.
- High-intensity unit where typical overcast daylight equals 1 ph and direct sunlight is ~10 ph.
Foot-Candle (fc)
Target Unit- Primary US customary illuminance unit defined as 1 lumen per square foot (lm/ft²).
- Mandated across North American architectural lighting codes, OSHA regulations, and IESNA standards.
- Represents human-scale room lighting, equaling roughly 1/929th of a phot per unit area.
Frequently Asked Questions
Q.How do you convert phot to foot-candles accurately?
Multiply the illuminance in phot by approximately 929.03 (or 929.0304). For instance, 1 phot multiplied by 929.03 yields approximately 929.03 foot-candles.
Q.What is the exact mathematical ratio between phot and foot-candles?
One phot equals exactly 929.0304 foot-candles because 1 square foot contains exactly 929.0304 square centimeters. Conversely, 1 foot-candle equals 0.00107639104 phot.
Q.What is the physical difference between Phot and Foot-candle?
Phot is a CGS metric unit measuring luminous flux per square centimeter (lm/cm²), while Foot-candle is an imperial unit measuring luminous flux per square foot (lm/ft²) used in American building codes.
Q.Why does 1 phot produce roughly 929 foot-candles?
Because one square foot contains 929.0304 square centimeters. Spreading 1 lumen over just 1 square centimeter creates 929.0304 times greater luminous intensity than spreading it over 1 square foot.