Convert Watts/Square Centimeter (at 555 nm) to Phots
Watt/Square Centimeter (at 555 nm) (W/cm²) to Phot (ph) illuminance conversion — enter any value below to get an instant result, or use the table for common values.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Result
1 W/cm² = 683 ph
1 Watt/Square Centimeter (at 555 nm) = 683 Phots
1 Phot = 0.0014641288 Watts/Square Centimeter (at 555 nm)
1 W/cm² in every supported unit
Conversion chart: Watt/Square Centimeter (at 555 nm) to Phots
Conversion table
| Watt/Square Centimeter (at 555 nm) (W/cm²) | Phot (ph) |
|---|---|
| 0.01 W/cm² | 6.83 ph |
| 0.1 W/cm² | 68.3 ph |
| 1 W/cm² | 683 ph |
| 2 W/cm² | 1366 ph |
| 3 W/cm² | 2049 ph |
| 5 W/cm² | 3415 ph |
| 10 W/cm² | 6830 ph |
| 20 W/cm² | 13660 ph |
| 50 W/cm² | 34150 ph |
| 100 W/cm² | 68300 ph |
| 1000 W/cm² | 683000 ph |
Watt/Square Centimeter (at 555 nm) (W/cm²)
Definition: A radiometric-to-photometric conversion unit expressing illuminance as raw optical power per unit area at 555 nanometers — the wavelength of peak human eye sensitivity — where the luminous efficacy constant (683 lm/W at 555 nm) ties radiometric watts to photometric lumens.
History: Defined from the CIE photopic luminosity function, which establishes 555 nm green light as the wavelength the human eye perceives most efficiently, making it the reference point for converting between radiometric power and photometric illuminance.
Current use: Used in optics, laser safety, and photobiology research where a light source's raw power output needs to be related to how bright it would appear to the human eye.
Phot (ph)
Definition: The CGS (centimeter-gram-second) system's unit of illuminance, equal to one lumen per square centimeter — because a square centimeter is far smaller than a square meter, one phot equals 10,000 lux, an enormous illuminance in everyday terms.
History: Defined in the early 20th century as the CGS-system counterpart to the lux once the CGS system of units was formalized for scientific work, following the same lumen-per-area logic as lux but scaled to centimeters.
Current use: Rarely used at full scale today since CGS units have largely given way to SI, but it still appears in older European scientific literature and some photographic and spectroscopy contexts, often as a millphot or microphot submultiple.
Supported Units
| Unit | Symbol | In Lux |
|---|---|---|
| Lux | lx | 1 lx |
| Meter-candle | m·c | 1 lx |
| Centimeter-candle | cm·c | 10000 lx |
| Foot-candle | fc | 10.76391 lx |
| Flame Phot | ph (flame) | 43.055642 lx |
| Phot | ph | 10000 lx |
| Nox | — | 0.001 lx |
| Candela Steradian/Square Meter | cd·sr/m² | 1 lx |
| Lumen/Square Meter | lm/m² | 1 lx |
| Lumen/Square Centimeter | lm/cm² | 10000 lx |
| Lumen/Square Foot | lm/ft² | 10.76391 lx |
| Watt/Square Centimeter (at 555 nm) | W/cm² | 6830000 lx |
About These Parameters
- Value
- The illuminance value you want to convert, expressed in the "From" unit. Accepts decimals, and can represent anything from faint starlight to bright studio lighting.
- From Unit
- The unit your input value is currently measured in — a light meter's lux reading, a lighting code's foot-candle requirement, or a scientific instrument's phot or nox figure.
- To Unit
- The unit you want the result converted into. Use the swap button to flip From and To instantly.
How Illumination Conversion Works
The Formula
Every unit here is defined by a fixed multiplier relative to lux. To convert a value from one unit to another:
result = value × (factor of "From" unit ÷ factor of "To" unit)
For Watt/Square Centimeter (at 555 nm) → Phot: multiply by 683. For example, 1 W/cm² × 683 = 683 ph.
Lux vs. Foot-candles
Lux and foot-candles are the metric and US customary measures of the same underlying idea — luminous flux landing on a surface — and the divide between them tracks the broader metric/imperial split. One foot-candle equals about 10.76 lux, because a square foot is much smaller than a square meter. Most of the world specifies lighting requirements in lux, but US architectural and lighting codes, published by bodies like the Illuminating Engineering Society (IES), still specify recommended illuminance in foot-candles, so American lighting designers routinely need to convert between the two.
Typical Illuminance Levels
For context, here's roughly where common lighting conditions fall on the lux scale:
- Direct summer sunlight: ~100,000+ lux
- Full daylight (not direct sun): ~10,000-25,000 lux
- Overcast daylight: ~1,000 lux
- Typical office lighting: ~300-500 lux
- Living room lighting: ~50-150 lux
- Twilight: ~10 lux
- Full moonlight: ~1 lux
- Starlight (clear night sky): ~0.001 lux
Example
An illuminance of 1 W/cm² equals 683 ph. For scale, a well-lit office sits around 300-500 lux (roughly 28-46 foot-candles), while overcast daylight reaches about 1,000 lux and full moonlight is only about 1 lux.
Frequently Asked Questions
How many Phots are in 1 Watt/Square Centimeter (at 555 nm)?
1 Watt/Square Centimeter (at 555 nm) (W/cm²) equals exactly 683 Phots (ph).
What's the difference between illuminance and luminance?
Illuminance (measured in lux or foot-candles) is the amount of light falling on a surface. Luminance is how bright that surface actually appears to an observer, which depends on both the illuminance and how much light the surface reflects. A white wall and a black wall under the same illuminance have very different luminance.
How many lux is one foot-candle?
One foot-candle equals approximately 10.7639 lux. This is why US lighting specifications in foot-candles produce noticeably larger numbers than the equivalent metric lux specification for the same physical brightness.
What is a phot and why is it so large?
A phot is the CGS-system unit of illuminance, defined as one lumen per square centimeter. Because a square centimeter is 10,000 times smaller than a square meter, one phot equals 10,000 lux — an enormous illuminance rarely encountered at full scale, which is why phot values are often expressed in millphots or microphots instead.
When would I use the nox unit?
Nox (1 nox = 0.001 lux) is used for very low illuminance levels, such as those encountered in night-vision equipment specifications or low-light astronomical and research work, where ordinary lux figures would require awkward, hard-to-read decimals.
How accurate are these conversions?
Every conversion factor used here is the standard, internationally recognized relationship between the underlying illuminance units — results are limited only by floating-point display precision, not by rounded conversion constants.