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NORDIC LIGHT & COLOUR
107
BACKGROUND
Humans get the major part of information about the world
through vision. Our visual perception is based on a continuous
stream of changing pictures created by light at the retina. We
can see objects and colours because of light. Colour and light
are inseparable. However, the established classical scientific
tradition prevailing to date separates these concepts. In this
essay we will show how strongly interdependent they are.
It is important to specify particular definitions of concepts used
in this paper. Thus,
equiluminant
(or isoluminant)
colours
are
the colours equal in luminance (Valberg 2005), which implies
they may be chromatically different.
Brightness
is the percep-
tual dimension that runs from dim to bright. The physical co-
unterpart of brightness is luminance, or put simply, brightness
is perceived luminance (Gilchrist 2007). Perceptual attribute
light level
represents the appearance of the room as a whole
as bright or dark. At the same time light level does not refer to
how well or bad a person can see in the room or in a special
place of the room (Liljefors 2003:13; 32-33). The appearance
of the observed room (or its light level) was registered with
the help of the
PERCIFAL
questionnaire (Matusiak et al. 2011),
(Arnkil et al. 2011), (Fridell Anter, Haggstrom et al. 2012).
Answers are marked on a scale ranging from very dark to very
bright. The PERCIFAL method was developed by scientists
from the SYN-TES group and is now recommended as a good
analytical tool for professional application (Fridell Anter, Arnkil
et al. 2012). This method is based on direct observations of the
space and recording of these observations by verbal-semantic
descriptions.
Our visual apparatus is an intricate and complex system with
amazing abilities such as luminance and colour adaptation
and colour constancy. Furthermore, the Helmholtz-Kohlrausch
phenomenon is a visual effect, showing that more saturated
colours appear brighter than less saturated colours at equilu-
minance. This may be explained by the fact that the human
brain adds the information from the chromatic channel to the
brightness channel. The Helmholtz-Kohlrausch phenomenon
appears both in selfluminous and surface colours (Valberg
2005: 178-178), (DeCusatis et al. 1997: 338).
Chromatic contrast is another strong effect that may influence
brightness of surfaces, e.g. in interiors. At low spatial frequen-
cies our sensitivity to chromatic contrasts is significantly higher
than for achromatic contrasts (see Figure 1). In this context it
is necessary to define the term
spatial frequency
. It represents
a measure of numbers of periods per degree of a repetitive pat-
tern (Valberg 2005: 432). Following the above, we can say that
equiluminant chromatic pattern of colours with distinctly con-
trasting hues is the best representation of chromatic contrast
effects. The perception of the contrast depends on the size and
form of the observed objects, its temporal variation. The visual
system deals with chromatic contrast in distinctly different
ways than with luminance contrast (Valberg 2005: 182).
Aims and hypothesis
In this study we pursued two main objectives. The first question
deals with the perception of the interior light level in relation
to the colours of the room surfaces. We have formulated the
hypothesis that the perceived light level is affected by not only
the luminance but also the chromatic properties and chroma-
tic contrasts of wall colours. This was tested in model studies
using low saturated equiluminant colours that are normally
used in interiors, and combinations of equiluminant colours of
contrasting hues. The way that subjects responded to these sti-
muli is discussed in relationship to other scientists` previously
published psychophysical experiments dealing with human
response to chromatic contrast stimuli (Valberg 2005).
The second objective was to examine the capabilities of high
dynamic range (HDR) images as a method for lighting studies
of interiors painted in low saturated colours. The High dynamic
range image is a merged image of several conventional low-
range images taken with different exposures. In photography,
the term dynamic range is a dimensionless quantity. It is a ratio
of the lightest and darkest pixel (Reinhard et al. 2010: 4). We
can also use the term “luminance map” instead of HDR ima-
ges, to accentuate that the picture has been used generally for
the luminance values measurements.
Luminance-based design
is a new approach to lighting design using such pictures. It is
currently being promoted by a number of scientists (Y. Na-
kamura, J. P. Skar, M. Fontoynont, and others – CIE TC 3-45) as
a perception-oriented method for lighting design (Nakamura et
al. 2011). The prime advantage of this method is that luminance
is a basis that forms perception of the visual environment and
its brightness.
It is known from previous studies (Anaokar & Moeck 2005)
that warm and low saturated colours has the highest accuracy
in luminance representation at HDR images while cool and
saturated colours have a higher level of errors. Therefore it is
supposed that in the current experiment, the luminance of the
surfaces painted in chosen low saturated colours will be repre-
sented with high precision.