PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
Daylight and the Occupant
Visual and physio-psychological well-being in built environments
DR SERGIO ALTOMONTE
Institute of Architecture, School of the Built Environment, University of Nottingham, United Kingdom
ABSTRACT: To substantiate an extensive use of daylight in commercial buildings it is necessary to demonstrate that,
other than just providing potential energy savings, natural lighting can foster further benefits (financial, environmental
and in terms of productivity) for both the owners and the occupants. Specifying daylighting solutions for energy efficiency
can be a very complex task, where many factors (e.g. illuminance, glare, solar gains, views) can diverge from each other
making design choices extremely difficult. Nonetheless, to foster the design of energy-conscious buildings which are also
conducive to human well-being, these variables have to be related with qualitative and behavioural factors such as
time/duration of exposure, directionality and spectral composition of visible radiation, psychological stimulation and user
preference. Correlating literature research with lighting standards and field measurements, this paper looks at the
relationship between quantitative physical factors of the luminous environment (e.g. horizontal/vertical illuminance,
luminance ratio, colour temperature), qualitative aspects of vision (e.g. uniformity, distribution, contrast) and physio-
psychological human response to daylighting. The aim of the study consists in defining a framework to implement existing
recommendations based not only on photopic requirements for visual tasks but also containing awareness of the demands
for photobiological stimulation that can influence the well-being of occupants, whilst also enhancing energy savings.
Keywords: daylight, energy, physiology, psychology, well-being, sustainability
The presence of natural light in commercial buildings -
with its fluctuations, the variations in its spectral
composition, and the provision for external views - is of
great importance for the comfort and well-being of
occupants, potentially resulting in enhanced productivity.
If carefully designed, a daylight strategy can also bring
tangible energy savings, as long as it minimises energy
use for artificial lighting, manages to balance heat gains
and losses and prevents visual discomfort (e.g. glare).
The specification of daylighting solutions can however
be a very complex task, whereas many factors and
variables can diverge from each other making design
choices extremely difficult. The main task for the
designer generally consists in selecting the most
appropriate daylighting systems and strategies that foster
trade-offs between conflicting requirements of
transmission and protection, and optimise quantitative
physical measures such as illuminance, luminance,
colour rendering and daylight factor to provide sufficient
luminous levels and always guarantee visual comfort.
Nevertheless, if energy-conscious buildings are designed
to be also conducive to human health and well-being,
these variables have inevitably to be related with
qualitative and behavioural factors such as time/duration
of exposure, directionality and spectral composition of
radiation, psychological stimulation and user preference.
In this context, photobiology is a new stream in
daylighting research revealing the complex interactions
between biological functions and external stimuli. Recent
research has indeed proved that daylight, other than
providing vision for tasks, has also an important non-
visual effect on biological processes, synchronizing the
circadian clock, stimulating circulation, controlling the
level of hormones, etc. In addition, further studies
suggest that visual performance and comfort can be
influenced by perceptive cues (such as an interesting
view) other than merely by physical parameters.
According to these findings, the routes by which daylight
can affect the ocular performance and the well-being of
occupants seem to involve not only visual but also
circadian and perceptual factors which take over once
the luminous image has been processed by the eye.
Lighting recommendations have thus to consider
awareness of many more factors than those currently
suggested in standards, involving, other than visual and
energy criteria, also non-visual attributes conducive for
human health and physio-psychological well-being .
DAYLIGHT AND WELL-BEING
Daylight is one of the basic immutable forces of Nature,
a primary element that can create significant and
suggestive spatial experiences . Architecture literally
depends on light, which reveals its forms and spaces, and
simultaneously discloses the meanings and intentions
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
that the architect anticipated through conceiving and
designing a building. Light can emphasize the experience
of architecture, telling about its structure, materials,
textures, the place it belongs to, the tasks to be performed
in it, marking the experience of time, uncovering the link
between inside and outside, providing orientation, focus,
hierarchy, and increasing the significance of a building
beyond functional use. And yet, light is often considered
either solely for aesthetic purposes or for providing
visibility for tasks. In fact, light should always render
both these aspects and, ultimately, acquire also a further,
biological, importance. In the path of human evolution,
daylight has actually represented the only way for
marking basic daily moments and one of the most
important means of maintaining biological rhythms in
connection to the rhythms of Nature. For millennia, all
human processes have been connected with the natural
daily (circadian) and seasonal (circannual) cycles of
daylight, which dictated the pace of activities other than
influencing metabolic functions. As a matter of fact,
scientific research has recently convincingly proven the
very close relationship existing between an appropriate
visual contact with the luminous cycle of day and night,
seasons, weather variations, etc. and healthy life
conditions and physio-psychological well-being .
In this context, a new stream in lighting research –
photobiology – reveals the existence of an alternative
pathway from the eye to the brain, regulating the various
interactions between biological functions and external
luminous stimuli . In fact, light, other than being
fundamental to visual tasks has also an important non-
visual effect on the body’s biological processes .
Adequate light received during the natural day period
synchronises the internal biological clock, stimulates
circulation, increases the production of vitamin D,
regulates protein metabolism, and controls the levels of a
number of hormones such as cortisol (the ‘stress
hormone’) and melatonin (the ‘sleep hormone’) .
Luminous stimuli thus involve the whole of the physical
(energetic exchanges), physiological (transformation of
energetic fluxes into nervous stimuli) and psychological
(neural interpretations of those stimuli) functions that
inform us about the surrounding environment and
contribute to the functioning of the human organism.
For almost two centuries of ophthalmic research, cones
and rods have been considered as the only two
photoreceptor cells in the human eye. Cones are active
mainly in bright light conditions (photopic vision), whilst
rods regulate vision in dim environments (scotopic
vision). When luminous signals reach these cells, a
chemical reaction occurs which determines electrical
impulses to be sent to the visual cortex located in the
brain, where these impulses are interpreted as ‘vision’.
However, recent studies have demonstrated that
biophysical processes regulating circadian rhythms are
very different from those that govern vision. Berson at al.
 have actually detected a third cell-type of
photoreceptor – an ‘intrinsically photosensitive retinal
ganglion cell’ (ipRGC) – which seems to be responsible
for the regulation of non-visual metabolic processes.
The new photoreceptor has its own neural connections to
the pineal gland, responsible for hormone regulation, and
to the Suprachiasmatic Nuclei (SCN) in the
hypothalamus, the brain’s biological clock (Fig. 1).
Figure 1: Visual and biological pathways from the eye to
the brain and daily rhythms (Source: Van Bommel, 2006)
Since the sensitivity of this new photoreceptor differs
radically from that of cones and rods, this discovery is
extremely significant for the specification of a ‘healthy’
luminous environment able to sustain both the visual and
the metabolic well-being of individuals, although most of
the international lighting standards are still specified
basing solely on the characters of photopic vision .
The light sensitivity of cones varies with the wavelength
(and thus the colour) of the luminous signal, and reaches
its maximum at 555 nm (green-yellow light). Conversely,
the sensitivity of the new photoreceptor shows a peak at
about 465 nm, in the green/blue part of the spectrum.
Also the temporal resolution of the new photoreceptor is
quite peculiar, since it is slow to react to luminous
changes but then gives a continuous response after
adaptation has taken place (around 20 minutes) .
In terms of the luminous stimulus, an important role in
the triggering of the photobiological processes seems to
be played by the vertical illuminance received by the eye
corrected for anatomic restrictions, i.e. the light received
in the retina . In fact, although the receptors for
metabolic regulation appear to be randomly dispersed in
the eye, the lower part of the retina shows greater
sensitivity for the entrainment of circadian processes, as
it is plausible considering that the sky tends to illuminate
this area rather than the upper part of the retina.
Threshold values for the retinal illuminance are assumed
to be around 1,500-2,000 lux, thus of an order of
magnitude significantly higher than the recommended
illuminance on the working plane for most tasks (300-
500 lux). This result implies that the vertical spatial
distribution of the luminous signal is a significant factor
for biological stimulation. Finally, also the dynamics of
lighting in terms of intensity, spectral composition and
direction during the day seem to play an influential role.
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
Obviously, sufficient retinal illumination to entrain
biological processes could eventually be provided by
artificial lighting, although this solution is less likely to
obtain the same results of daylight in terms of combined
visual comfort and bio-regulation, since natural light
usually produces a high illuminance at the eye with a
spectrum that matches its circadian sensitivity .
Daylight, with its variability in spectral content and the
fact that, at all times, it provides signals in all parts of the
visible range, thus represents the most appropriate
stimulus for vision and the concurrent regulation of the
biological clock. In particular, morning light plays an
essential significance in the synchronisation of metabolic
rhythms to the 24h rotational cycle of the Earth. Without
regular daylight entrainment, in fact, the human circadian
clock (circa meaning approximately, and dies meaning
day) would run, on average, on a 24h and 15-30m cycle,
ultimately leading to a shift of the circadian pacemaker
and a de-synchronisation of the biological clock .
In summary, due to new discoveries, it becomes clear
how natural light, other than just providing visibility for
tasks, orientation in space and time and environmental
stimulation, can also mediate and control a large number
of biochemical processes in the human body, which are
fundamental for health and well-being. Yet, current
practice for lighting design in buildings is still based on
outdated visual criteria related solely to horizontal task
illuminance and luminance in the field of view (e.g.
glare). Conversely, to foster the health of occupants and
truly enhance the sustainability of built environments,
these criteria have to be extended to non-visual demands.
‘HEALTHY’ DAYLIGHTING CRITERIA
Nowadays people spend more than 90% of their time
indoors – often in offices – and in all cases the lighting
strategy is uniquely based upon the notion that, whatever
the time of day or the season, the tasks have to be
performed efficiently and with an adequate level of
visual comfort. Yet, despite the options offered by
daylight, internal lighting is often designed to provide
fairly constant luminous conditions at all times,
irrespective of the occupants’ preferences, differences in
metabolic requirements and needs for individual tasks.
As a matter of fact, regardless of the widespread use of
computer-based activities in offices (which require a
vertical gaze), traditional paperwork to be performed on
desks is still considered as the prevailing visual design
parameter, with photopic vision remaining the key factor
in lighting regulations. Actually, most international
lighting standards – as the European Norm EN 12464-1 –
specify recommendations for a wide range of activities
according solely to visual comfort criteria, such as the
maintained illuminance on the work surface (E
) – i.e.
the value below which the horizontal illuminance is not
allowed to fall – the unified glare rating limit (UGR
i.e. the rate of discomfort glare (although this can be
calculated with a certain degree of certainty only for
luminaries) – and the colour rendering index (R
On the contrary, awareness of the requirements for
photobiological stimulation (e.g. vertical illuminance,
spectral composition of light, duration of exposure, etc.)
is generally ignored. The question now is to establish
how serious are the consequences of working and living
with much less light than outdoors and with a luminous
environment fundamentally detached from biological
needs, and how this can be compensated by a ‘healthy’
lighting design able to, at once, meet visual requirements,
reduce energy consumptions and guarantee a correct
metabolic stimulation for occupants.
Light can be described in terms of a number of factors
that govern visual and photobiological functions:
quantity, spectrum, spatial distribution, timing, duration.
In first instance, it must be considered that all human
processes are physiologically adapted to the availability
of large amounts of outdoor illuminance (up to 100,000
lux on a sunny day) and to significant daily and seasonal
variations in daylight levels. Conversely, according to the
standards, internal lighting is often set to fairly constant
levels at day and night and with an intensity which could
be 40 to 200 times smaller than outside. This implies
that, in most working places, the lighting levels required
for circadian processes can not be achieved if not in areas
close to the perimeter, where, on the other hand, daylight
ingress can be compromised by competing thermal
needs. Yet, if criteria for lighting were to be modified
basing on photobiological demands, it would follow that
illuminance levels required in indoor spaces would need
to rise significantly, at least for some phases of the day.
However, if this increased vertical illuminance to
stimulate biological functions was to be provided by
daylight alone, the risk of solar gains, thermal losses or
potential visual discomfort (glare) would obviously be
considerably higher . Rather, if provided solely by
artificial lighting, higher illuminance at the eye would
imply increased energy needs. As a consequence, a
thoroughly-designed daylight strategy has always to find
a balance between all the factors at play (sometimes
compensating its deficiencies with flexible social and
cultural habits, such as spending part of the day outside!).
Secondly, just as the spectral composition of daylight
shows large variations throughout the day and seasons, a
suitable photobiological entrainment would require
temporal variations in the correlated colour temperature
(CCT) of the lighting stimulus. As far as visual comfort
is concerned, according to the Kruithof Diagram the
higher the illuminance, the higher the CCT should be.
However, although artificial light sources are available
with a spectral content similar to that of daylight on some
occasions (e.g. xenon incandescent lamps), most of them
are adjustable only in output levels and not in CCT.
Moreover, it must be considered that circadian photo-
receptors seem to be mainly stimulated by short
wavelengths, while luminaries are generally designed to
maximise their energy output according to the photopic
visual sensitivity, which peaks in the yellow-green band.
Thirdly, natural light is highly dynamic in its intensity,
spatial distribution and direction, and it seems that people
would strongly prefer to be kept aware of these changes.
Another aspect that lighting standards should consider is
concerned with the timing and the duration of exposure.
From the point of view of vision, light is needed just as
long as a visual task is involved. Yet, metabolically, the
timing and duration of exposure should follow the
biological rhythms of the body and provide sufficient
stimulation during the course of the day to avoid phase
advances or delays. Duration of exposure and luminous
quantity are also strictly related. For example, abundant
light onset in the morning enables the biological clock to
maintain synchronicity with the changes in the light-dark
cycle (this could be a concern in winter, when people go
to work in the dark, and spend the day in interiors whose
illumination is based solely on photopic criteria).
A further important issue not to be neglected is the
influence of colour on biological processes, a matter that
may involve objective as well as subjective responses
(e.g. heart rate, respiration rate, oxiometry, eye blink
frequency, etc.). Whether the association between colour
and physiological indexes is direct (i.e. the response is
elicited without being mediated by a cognitive
intermediary reaction) or indirect (i.e. exposed to a
colour the observer makes certain associations) is yet to
be defined and involves also psychological issues .
Finally, in terms of the integration between natural and
artificial lighting, a good combination can generally
make it possible to dim the amount of electric light when
daylight is sufficient. If an automatic dimming system is
used, a personal over-ride is in general highly valued in
terms of perceptive comfort, although it can sometimes
jeopardise optimum energy performance. It must also be
noted that artificial lighting usually has a mainly
horizontal component, while daylight through windows,
with its vertical illuminance, is surely more beneficial to
biological processes. Suspended luminaries should thus
be favoured for non-visual stimulation, while also
providing copious light on the task and the ceiling.
In summary, photobiological research leads to the
hypothesis that healthy lighting is influenced by many
more factors than what is suggested in most standard and
regulations, involving, other than well-known visual
comfort criteria, additional non-visual factors which can
influence the ocular performance as much as the physio-
psychological well-being of building occupants.
PERCEPTION AND PSYCHOLOGY
To further support the notion that daylight is generally
superior than artificial lighting for stimulating, at once,
visual and photobiological responses, there is no doubt
that, given a choice, building occupants would prefer to
live by daylight and enjoy a view out. However, despite
this general preference for natural light, it is hard to
demonstrate that just the presence of windows would
improve users’ comfort, well-being and productivity,
even because people will swiftly give up daylight if it is
associated with visual or thermal discomfort .
Each visual task demands a specific relationship with the
environmental factors surrounding it and has to meet
complex needs that reflect people’s desire for orientation
in space and time (genius loci), expectations (functions,
aesthetics, ergonomics, privacy, concentration, details,
etc.), and also aspects related to society and culture.
During the day, the presence of daylight should render
the spaces lively, activating and motivating in
accordance with the human circadian rhythm. Daylight
associated with a view should tell about the time of the
day, the season, the weather, and improve the sense of
orientation and feeling of spaciousness. In addition,
views out are also extremely beneficial to reduce muscle
strain by allowing the eyes to shift from the near field
surrounding the task area towards distant objects.
A good view should preferably include three ‘layers’:
upper (distant, the sky – from natural to human-made
skyline); middle (natural or human-made objects, such as
trees, hills or buildings); and lower (the foreground,
including plants and paving) . Since these three
layers are stacked vertically, if the area of glazing is
limited (e.g. for thermal needs) it would be generally
better to have a tall, thin window rather than a short,
wide opening in order to get as much information content
from the view as possible. Conversely, horizontal
windows guarantee a better view of the landscape.
Nevertheless, daylight through windows can also imply
major drawbacks. As previously noted, in modern offices
the extensive use of computer displays has caused the
primary work gaze to shift from a horizontal desk plane
to a vertical display screen surface. Vertical windows can
thus frequently constitute glare sources (e.g. from the sky
vault, bright clouds, the sun, reflections off surrounding
buildings, etc.), although also internal reflective surfaces
or artificial lights can be at the origin of the distress.
Luminance ratios in the field of view should always be
contained within certain limits: too large, and it will be
difficult for the eyes to adapt; too small, and depths and
distances will be hard to estimate. The visual task should
normally be brighter than its immediate and general
surroundings. To this aim, the rule of thumb 1:3:10
generally applies, although, in case windows can be seen
within the VDT area, studies suggest that the tolerable
luminance ratio can be much higher, reaching up to 1:50
if the patches of bright natural luminance are small .
Glare is a serious source of visual strain that can prevent
the viewer from executing his task (disability glare) or
cause a decrease in performance (discomfort glare).
Disability glare is generally due to a saturation effect or
to a bright light source striking directly in the field of
view of the observer (e.g. after-images). Conversely,
discomfort glare can be associated with visual contrast
and is normally linked to the location and intensity of the
light source relative to the average luminance in the eyes
of the viewer, although the disturbance depends also on
the nature of the task and on personal tolerance .
The full comprehension of the process linked to
discomfort glare is however still incomplete, especially
when the visual strain is linked to daylight . Lighting
fluctuations coming from windows seem anyway to be
generally more easily accepted than glare from artificial
luminaries. This notion is consistent with the results of a
number of studies , which also suggest that an
increased tolerance to glare can be due to the natural
light source being accompanied by a view. In particular,
recent research has highlighted that there is less
discomfort glare from a window with an interesting view
than from a window of the same mean luminance but
with a view of less interest, specifically when natural
scenes and multiple distance layers are observed .
The implication of these studies could be that the
perceptive subjective effect of the interest in a view can
have a greater effect on perceived comfort than the
relative objective brightness range. It follows that if a
glare source contains some information regarded as
interesting, standard formulae and calculations are likely
to overestimate the degree of visual discomfort, even if it
is still unclear whether this increased tolerance should be
considered as a short-term effect or one that lasts .
In any case, these results provide further evidence that,
when examining visual comfort and well-being, a purely
physical approach can be insufficient. Consideration of
non-visual factors, on the other hand, can be particularly
important for the design of windows in situations where,
due to low-angle sun, discomfort glare is likely to occur
and would suggest the design and use of shading devices.
The use of obstructive blinds, in fact, by reducing the
luminance in the field of view of the observer, would
also decrease the amount of available illuminance, to the
detriment of metabolic processes and with increased
energy demands. In addition, by impairing visual contact
with the outside, shading devices would also deprive the
observer of the interest, information and variation given
by a view that could have increased his tolerance to
extreme lighting conditions and discomfort glare.
MEASUREMENTS AND OCCUPANTS’ SURVEYS
Field measurements linked to occupants’ surveys are a
fundamental mean to relate physical quantities to physio-
psychological human response to environmental stimuli.
A pilot daylight testing was performed to these aims at
the Council House 1 (CH
) offices in Melbourne,
Australia, as part of a comprehensive Post-Occupancy
Evaluation campaign set by the City Council to
benchmark the building’s energy performance and users’
comfort before moving to the newly built Council House
), the first building in Australia to receive a 6-
Green Star rating for excellence in sustainability .
comprises a typical open plan layout fitted with
conventional partitioning and furniture, overhead
recessed fluorescent luminaries and task lighting. Most
of the office desks have direct access to daylight and a
view out. The windows are mostly non-operable and are
equipped with adjustable internal blinds for light control.
The daylight testing consisted of measurements taken at
regular intervals at selected locations. Surface area
) was measured with a calibrated Nikon
Coolpix 5400 camera equipped with a fish-eye lens to
characterise the luminance homogeneity or non-
homogeneity in the field of vision of the occupants, and
then processed via the software Photolux 1.3.5 (ENTPE).
Concurrently, horizontal and vertical illuminance and
correlated colour temperature of the light sources were
measured to fully characterise the visual environment.
The figure below provides an observed example of a
workplace that exhibits potential glare problems (Fig. 2).
Figure 2: Luminance measurements (in cd/m
) taken at
10 am during winter (left) and summer (right)
The testing was performed in two seasons, winter and
summer, at the same time of the day (10am), in an
easterly-oriented work station. In the winter testing, it is
evident how the low-angle sun potentially creates risks of
glare in the field of vision of the occupant. The
luminance ratio between the window (7,436 cd/m
the Video Display Terminal (132 cd/m
) is actually 1:56,
a value which clearly flags probable discomfort glare.
Yet, the venetian blind is not drawn by the user to
counteract this visual annoyance. As a matter of fact,
upon interview the occupant revealed his awareness of
glare but reported his preference to mainly concentrate
on file processing and paper-work rather then screen-
based tasks in the early hours of the morning, in order to
benefit of a bright environment and a view out for
waking up and feeling better for the rest of the day.
Conversely, the measured conditions were well-suited to
paper-work tasks as per current standards. Actually, at
the time of testing the horizontal illuminance on the work
plane was 1,600 lux, while the CCT was slightly above
5,000 K (mainly coming from daylight), and thus in the
comfort area also according to the Kruithof Diagram.
During summer months a different situation applies.
Paradoxically, although the sun has a higher course and
path in the sky – and thus does not necessarily constitute
a source of direct glare – the blind here has been drawn
by the user to reduce the brightness of the window and its
contrast with his visual task. The resulting luminance
ratio between the visible part of the sky (2,461 cd/m
and the VDT area (206 cd/m
) is around 1:11, and thus
within the limits conventionally suggested by lighting
practices. Yet, the decrease in luminance ratio due to the
use of the blind also results in lower visual and luminous
stimulation for the occupant. This choice may be due to
the fact that the user had already benefited of sufficient
exposure to daylight – and thus metabolic and
psychological stimulation – on its way to work, and thus
did not require high level of illumination to feel better.
Because of the blind, also illuminance on the work plane
was much lower than the winter measurement (600 lux) –
yet sufficient for the task – while CCT was around 4,000
K (i.e. mainly due to the cool-white fluorescent lamps).
The measured conditions are still within the standard and
the comfort band of the Kruithof Diagram, although they
significantly rely on the use of artificial lighting sources.
This is a clear example of how physio-psychological
factors can influence and eventually compensate physical
luminous discomfort to the benefit of the user (well-
being) and the organisation (productivity and energy
savings), although, in the measured building, the absence
of a light sensor able to dim or turn off artificial lights in
presence of sufficient daylight prevents this saving.
Basing on the literature and on field measurements, this
paper has substantiated that, other than providing
potential energy savings, daylight can foster significant
advantages to the quality of architectural spaces, bringing
benefits to the health of people that live in buildings, and
to the finances of the organisations commissioning them.
As a source of electromagnetic radiation, natural light is
not intrinsically better than artificial light to ensure visual
performances. However, daylight is endowed with
unique features which are conducive to human health, as
it is generally delivered in large quantities and ensures a
continuous variation in spectral content, thus presenting
an effective stimulation for the human circadian system.
In addition, windows are favoured also for the view out
they provide, meeting the occupants’ psychological
needs of a continuous contact with the surrounding
context (biophilia) and offering environmental stimuli
that are beneficial for the well-being, attitude, mood,
concentration, and, possibly, productivity of occupants.
In the practice of design, hence, daylighting should not
be considered as an afterthought which is taken into
account only when the spatial characters of a building
have already been formulated. Rather, daylight should be
valued as a necessity that drives and directs the design
from its early stages, ultimately leading to better
architecture which is cheaper to run, less harmful for the
environment, and healthy and stimulating for its users.
ACKNOWLEDGEMENTS. Research for this paper
was initiated under the support of the Melbourne City
Council. Daylight testing was conducted in collaboration
with the MABEL (Mobile Architecture and Built
Environment) Laboratory, Deakin University, Australia.
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