Radiometry ↔ Photometry Converter (W ↔ lm)
Convert radiant power in watts to luminous flux in lumens (and back) for light of a chosen wavelength, using the CIE photopic V(λ) curve.
Luminous flux
683 lm
- Luminous efficacy at this wavelength
- 683 lm/W
- V(λ)
- 1
- Radiant power
- 1 W
Monochromatic light at 555 nm (standard air): V(λ) = 1, so 1 W = 683 lm. Values from the CIE 018:2019 photopic table (daylight vision).
Show the work
Eye sensitivity V(λ) at 555 nm
CIE 018:2019 photopic table
1
Luminous efficacy at this wavelength
K = 683 lm/W × V(λ)
683 × 1 = 683 lm/W
Luminous flux: efficacy × radiant power
Φv = K × Φe
683 × 1 = 683 lm
How it works
Φv (lm) = 683 lm/W × V(λ) × Φe (W), with V(λ) the CIE photopic luminous efficiency at the wavelength (1 nm table, linear in between).
For light of one wavelength (a laser or narrow LED). White or broad-spectrum light needs its whole spectrum, so a bulb's lumens per watt can't be found this way. V(λ): CIE 018:2019 Table 1, wavelengths in standard air, 360–830 nm (CIE dataset doi:10.25039/CIE.DS.dktna2s3, CC BY-SA 4.0). Km = 683 lm/W from the SI definition of the candela.
Questions
- How does the radiometry ↔ photometry converter work?
- Φv (lm) = 683 lm/W × V(λ) × Φe (W), with V(λ) the CIE photopic luminous efficiency at the wavelength (1 nm table, linear in between).
- What does it assume?
- For light of one wavelength (a laser or narrow LED). White or broad-spectrum light needs its whole spectrum, so a bulb's lumens per watt can't be found this way. V(λ): CIE 018:2019 Table 1, wavelengths in standard air, 360–830 nm (CIE dataset doi:10.25039/CIE.DS.dktna2s3, CC BY-SA 4.0). Km = 683 lm/W from the SI definition of the candela.
- Why is 555 nm special?
- The eye is most sensitive there in daylight (V = 1), so 1 W of 555 nm light is 683 lm — the most any light can give per watt.
- Does this tell me how many lumens my bulb makes?
- No — a bulb's watts are electrical input and its light covers many wavelengths. This tool is for a known amount of optical power at one wavelength.
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