Lightness and Brightness and Other Confusions

Y
0.8
0.7
510 –
– 520
– 530
– 540
– 550
0.6
0.5
500 –
– 560
– 570
– 580
0.4
– 590
– 600
0.3
490 –
– 610
– 620
—– 700~780
0.2
0.1
480 –
Line of purples
470 –
0.1
46
0–
—– 380~410
0.2 0.3
0.4
0.5
0.6
0.7
0.8
X
Figure 14. The CIE 1931 colour space chromaticity diagram (with a triangle
inside it describing a typical RGB gamut – for more about RGB see pp
57, 76 and 91). The outer perimeter with numbers from 460 to 780 describes
the wavelengths of the spectrum. The coordinates have no absolute relation
to perceived colours, but the colour space is often presented in the above
kind of coloured version. For another kind of CIE colour space, the CIELAB,
see figure 20 on page 87.
Colorimetric measurements are made with spectrophotometers. To be accurate,
these measurements have to be made under strictly controlled conditions,
including the use of standard illuminants such as the standardised
daylight simulator of D65. Another method is to use a colorimeter, which
typically works with a small number of differently coloured illuminants.
There is rapid development in colorimetric theory and technology, including
the use of a large number of LEDs and new techniques that combine methods
from both spectrophotometers and colorimeters. This has among other things
resulted in small portable colour scanners, which use inbuilt controlled light
sources and make colorimetric comparisons between the spectral reflectance
curve of the measured surface and those of standardised colour samples.
The theoretical foundations of colorimetry are constantly developed through
research, which means that methods and algorithms should not be seen as
fixed entities. From the above it is clear that classical colorimetric theory,
units, methods and instruments could and should not be used for describing
the perceived colour. There are, however, more recent algorithms that attempt
to tell something about how colour is perceived (‘colour appearance’).
They will be discussed later in this text.
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