How this instrument works
Illuminance measures how much visible light lands on a surface, and this instrument converts a source's raw radiant power in watts into that photometric quantity, in lux. The bridge between the two is the constant 683 lumens per watt, K_cd — the exact factor the SI itself uses to define the candela, fixed at the single wavelength of 555 nanometres, where the daytime, photopic, human eye is most sensitive to light.
The formula multiplies radiant power by 683 to get luminous flux in lumens, then divides by the illuminated area to spread that flux over a surface: E = 683 × P ⁄ A. That multiplication only holds at exactly 555 nm, where the CIE photopic luminosity function V(λ) equals 1, its peak. Move off that single wavelength, or broaden the spectrum even slightly, and the true conversion factor drops below 683, because V(λ) falls away on both sides of the peak.
That makes 683 lm/W a ceiling, not a typical figure. No commercial lamp is a single spectral line sitting exactly on the eye's peak sensitivity, so every real source — sodium-vapour lamps, phosphor-converted LEDs, fluorescent tubes — converts watts to lumens less efficiently than this. The number is most useful as a sanity check: if a datasheet or a calculation implies a luminous efficacy above 683 lm/W for a visible source, something upstream is wrong.
- Enter the source's output in the Radiant power (monochromatic, 555 nm) field, in watts — this must be power at exactly 555 nm, not a lamp's total rated wattage.
- Enter the Illuminated area in square metres: the surface the light actually falls on, not the size of the emitter itself.
- Read Illuminance, lux (theoretical maximum) — the highest lux value that radiant power could possibly produce spread over that area.
- To sanity-check a real lamp, compare its rated lm/W against 683; a figure above 683 for any visible source signals an error somewhere upstream.
Worked example — one watt of 555 nm light over one square metre
A photometry lab calibrating a monochromatic source sets its output to exactly 1 W of pure 555 nm green light and spreads it evenly over a 1 m² diffusing screen. With Radiant power at 1 W and Illuminated area at 1 m², the formula gives E = 683 × 1 ⁄ 1 = 683 lux — not an approximation rounded for convenience, but the literal, exact value the SI fixes for this wavelength.
Compare that with a 1 W green LED aimed at the same square metre: its datasheet might claim 150 lumens per watt, so the identical setup would read only 150 lux, not 683. The shortfall is not inefficiency in the ordinary sense — it is the unavoidable cost of emitting a spread of wavelengths instead of a single spectral line sitting exactly on the eye's peak.
Questions
Why is 683 lm/W the maximum possible value, not just a typical one?
Because it is how the candela itself is defined. The SI fixes K_cd, the luminous efficacy at 555 nm, at exactly 683 lm/W with zero uncertainty — a chosen constant, not a measured one. Every wavelength away from 555 nm scores lower on the eye's sensitivity curve, so no light, at any wavelength or mixture of wavelengths, can convert watts to lumens more efficiently than this.
Does this formula work for sunlight or a white LED?
Not directly. This formula assumes every watt sits at exactly 555 nm. Sunlight and white LEDs spread power across the visible spectrum, so the correct conversion multiplies each wavelength's power by the CIE luminosity function V(λ) and sums the result — always giving a factor below 683 lm/W, often well below it for broad, white-looking sources.
Why do the best LEDs only reach 150-220 lm/W, far short of 683?
Because a white LED's light is not a single 555 nm line — it mixes blue LED emission with a phosphor's broad yellow-red glow, and much of that spectrum sits away from the eye's peak sensitivity. Add real losses like heat and imperfect phosphor conversion, and even the best commercial LEDs land at 150-220 lm/W, under a third of the theoretical ceiling.
What area should I enter — the source's size or the lit surface?
The illuminated area: the surface the light lands on, such as a desk, a test screen, or a photodetector's sensing area. Spread the same power over a wider area and illuminance drops; concentrate it on a smaller spot and illuminance rises, even though the source's own output never changed.
How does 683 lm/W connect to the definition of the candela?
Directly. 683 lm/W is the constant K_cd that the SI uses to fix the candela's size, set at a frequency of 540 × 10¹² Hz, which works out to 555.017 nanometres, the greenish-yellow light where daytime vision peaks. This calculator applies that same fixed constant to convert radiant watts into photometric lux.
Can a real light source ever beat 683 lux per watt per square metre?
No, not for genuinely visible light. 683 lm/W is a hard ceiling fixed by the SI's own definition, so illuminance can never exceed 683 times radiant power divided by area unless the source truly is monochromatic at 555 nm. A reading claiming more than that points to a unit mix-up or a bad input, not a better lamp.