Candelor

Candelor, a lantern-warden monk of The SI Guard, keeps a lantern that burns one pure green, the colour the daylight-adapted eye sees best.

The Physics

The candela, the SI unit of luminous intensity, links watts of light to how bright that light looks to a standard human eye. It is defined by fixing the luminous efficacy of monochromatic radiation at 540 × 10¹² Hz (a green of about 555 nm) at exactly 683 lm/W. For other wavelengths the watts are weighted by the CIE photopic sensitivity curve V(λ).

Φ_v = K_m ∫ V(λ) Φ_e,λ dλ, K_cd = 683 lm/W

• Φ_v: luminous flux (lumen, lm)
• K_cd: luminous efficacy of 540 THz radiation, 683 lm/W exactly (defining constant of the 2019 SI)
• K_m: maximum photopic efficacy, ≈ 683 lm/W
• V(λ): CIE photopic luminous efficiency function, peak 1 near 555 nm
• Φ_e,λ: spectral radiant flux (W/nm)
• λ: wavelength (nm)

The value 683 has been part of the candela definition since 1979; the 2019 SI rewrote the definition in terms of fixing K_cd exactly, alongside exact h, c and Δν_Cs. The vacuum wavelength of 540 THz is about 555.17 nm (about 555.02 nm in standard air).

Honest caveat: 683 lm/W is not a law of nature. It is a convention chosen to keep the candela continuous with older standards, and V(λ) is a standardised average human eye. 540 THz sits very close to, but not exactly at, the peak of V(λ), so K_m is very nearly, but not exactly, 683 lm/W. Real lamps give far fewer lumens per watt, because not all their light is at the peak.

On the sticker

Candelor holds up his lantern of acid-green flame, which lights his face and hood from below. Printed text: CANDELOR with the line 683 lm/W. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.