Marc Steinmetz Photography | The seven SI base units

LIGHTING THE TORCH
Realization of the candela on the
PTB’s unique 40-meter photometric
bench with a Toshiba transfer stan-
dard lamp. Under specific conditions,
the lamp generates a luminous in-
tensity of exactly 1 cd. It is used for
realization, preservation and dissemi-
nation of the candela. A photometer
measures the illuminance at various
distances.

[cd] ‘The CANDELA is the
luminous intensity, in a given
direction, of a source that emits
monochromatic radiation of
frequency 540 · 10¹² hertz and
that has a radiant intensity in
that direction of 1/683 watt per
steradian.’