Marc Steinmetz Photography | The seven SI base units

INTEGRATING SPHERE
Photometry at the PTB with an inte-
grating sphere 2.5 m in diameter.
A light source is completely enclosed
by the aluminum shell. The light is
scattered through diffuse reflection
off the matte white coating on the
interior and measured with a photo-
meter. Thus light sources can be
characterized by comparison with
standard transfer lamps of known
properties.

[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.’