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Description / Abstract:
INTRODUCTION
Specification of the colour of objects transmitting or
reflecting light requires specification of both the spectral
transmittance or reflectance of the object and the spectral
characteristics of the illumination. This illumination must be
reproducible, unlike ever-changing natural daylight. Practical
applications must therefore use artificial light sources. These may
be standard sources (when their relative spectral power
distribution [SPD] is identical to that of the standard illuminant
and has been accepted as such by the CIE), or simulators,
when the SPD of the source approximates that of a standard
illuminant, but has not been accepted as standard.
The SPD of the light source is important when making visual
assessments (whether by the naked eye or in a colour comparator)
and in tristimulus colorimeters. In spectrophotometers it is only
important when measuring the spectral transmittance or reflectance
of fluorescent specimens.
Why do we need simulators? The SPD of CIE standard illuminant
D65 for average daylight (CIE 2006) is based on experimental
measurements of natural daylight, as reported by Judd, MacAdam and
Wyszecki (1964). D65 has replaced Illuminant C as the primary CIE
standard for daylight, but even today, over 40 years after the
publication of the SPD, there is no corresponding CIE standard
source.