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.

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