Light

Decided: the lam is defined by fixing the luminous efficacy of green light at K_cd = 3 × 10^4 lam·sr/vg (exact), for light of frequency 19,042,90X,764,540 per blink (exact; 540 THz). That makes 1 lam ≈ 0.980246 cd.

Why: every other base unit is defined by a constant stated in Paludal units; the lam used to be the SI candela carried over, so it couldn't be defined without SI. A round K_cd per vig is the Paludal equivalent of SI's 683 lm/W. Replaces the earlier decision to keep lam = 1 cd ("rarely used, nothing to gain").

Advantage: every base unit is now defined within Paludal, and still converts exactly to SI.

  • The frequency is SI's 540 THz exactly, expressed per blink. It isn't round (≈ 1;9043 × 10^11), for the same reason the caesium count isn't: a round frequency (eg 1;9 × 10^11 per blink, 556 nm instead of 555 nm) would mean converting to the candela through the eye's sensitivity curve, so the conversion would no longer be exact
  • 683 lm/W expressed per vig is 2E,357;1E (60,979 dec). 3 × 10^4 (62,208 dec) is the nearest one-digit round number; 2E,000 would be closer (lam = 1.008 cd) but isn't as round
  • Lamp ratings change by 2%: an 800 lumen bulb is about 580 paludal lumens (lam·sr; 816 dec)