Visual Performance Analysis of Office
Occupants
Working Under Realistic Luminous Environment
Conditions
Rock Financial and
Quicken Loans Office Building
3252 University
Drive, Suite 130
Auburn Hills, MI
48326
By:
Mojtaba Navvab,
Ph.D., FIES
Changes in modern
offices and their interior environment have modified the interior design practices
of today’s designers. Interior design objectives for open-plan offices with
solid and or opaque materials with various fabric colored covers, glass, translucent
or transparent cubicles and everything in between are now often used in an
attempt to improve the environmental health, communication, and make better
working environments. The variations in size and the height of partitions
or ceiling materials are also used to provide flexibility in control of the
variables influencing the working environment.
These changes affect
the occupants and their physiological or mental effectiveness during the working
hours, which are a major concern of the office management at large office
organizations. The office exterior and interior environmental conditions would
affect the attitudes and behavior of the users in an office environment. Daylighting,
lighting, thermal comfort, indoor air quality acoustics, and space layouts
are the key variables in design for interior working space. This study focuses
on the specific research to provide guidance on designing the lighting system
in an office that will meet employees' needs and support organizational productivity.
As part of good lighting
design practices, a luminous environment should be optimally viewed when acuity
is best, under a healthy lighting system. There is a growing demand for data
related to spectral variability of light and its influence, not only on vision,
but also on all other architectural surfaces along with the user interaction
in a given space. Rapidly emerging areas of science require lighting system
characteristics to be identified for quality, spectral, special distribution,
timing and duration. Current designs for buildings and related systems do
not meet the demands for optimal performance, visual comfort, energy efficiency,
healthy lighting and high standards in aesthetics at the same time. There
are no clear criteria for the light that enters the eye or reaches the retina
under dynamic daylighting and or controlled lighting conditions. This is true
not only for people but also there are no criteria for plants and animals.
This study and thoroughly documented laboratory experiments shown in various
case studies bring into focus the need for relevant data to quantify and validate
or assess the performance of these effects, and to enhance our design capabilities
[1]. Current studies show an increased interest on spectral content
of light. Light with blue spectral content has been a major focus of their
research objectives. Results show this portion of the spectrum is more visually
efficient for perceived brightness and visual acuity, therefore a luminous
environment with scotopically or cirtopically rich content will provide efficient
lighting system.
The studies in health
science and vision show spectral response to circadian regulation with peak
wavelength in 480-485nm regions adding more evidence to support the role of
the photoreceptors in circadian regulation. The applied physiology studies
show that performing an exiting or active task on video display terminal with
bright display suppresses the nocturnal changes on melatonin concentration
and other physiological indicators of human biological clock [2]. The
bright lights of 2000 lx or even lower light of 300-200 lx from fluorescent
lamps can inhibit nocturnal melatonin concentration in adolescents [3-5].
Subjective drowsiness is higher under 3000K° lighting condition than 5900K°,
lower CCT illumination can be used effectively in an environment where lower
physiological activity is desired. Office workers spend over 90 percent of
their daytime indoors and as results do not receive enough exposure to this
portion of the spectrum. It is necessary to examine the visual performance
and related lighting design, and control strategies with consideration of
the circadian influence in architectural lighting design to optimize or maximize
the physiological influence and health [6-8].
Results are reported
here of 3 studies of near visual acuity (normal reading distance of about
18 inches) under realistic conditions of task and surround lighting. The surround
and or general fluorescent lighting is either low (3500 CCT) and high (5000
CCT) along with high (5900 CCT) task lights. The office workers participated
in an experiment where their primary tasks were a paper based task on a horizontal
desk top and the secondary tasks were on a vertical self louminous computer
screen. The office workers using the telephone obtain the information and
record the data on the horizontal task paper, after evaluation of the information
the results are input into the database and viewed on the computer screen.
The participants
in these studies were 33 young office workers. In order to provide different
task and surround lighting conditions, the visual task, either Snellen letters
or words of varying size, rested on a small size light table that were lit
by general lighting system. An office partition with fixed height and their
layout were positioned as such to not interfere with the task being lit by
other lights such a new task lighting if it was utilized. To prevent the task
lighting from illuminating the surround walls, they were aimed right at the
paper task. The surround lighting conditions were adjusted so that equal photopic
illuminance was provided at the eye for the two color temperature lightings
for before and after.
Task luminance varied between about 34 and 398 Nits (cd/m2)
[10 and 116 fl (cd/ft2)], illuminance on the task plane varied
between 184 and 1261 Lux (lm/m2) [17 and 117 fc (lm/ft2)]
and illuminance in the subjects’ eye varied between 129 and 961 Lux (lm/m2)
[ 12 and 89 fc (lm/ft2)]. The existing lighting system was used to
establish the base case for evaluating office workers acuity under realistic
working conditions.
Figure 1.
View of the office space (Before _Left and After_ Right)
Office test area: All testing took place in an enclosed individual cubical office area
within an open office plan layout while the participants were working as usual
on their daily tasks. The partition space and the working area dimension 5
sq-ft. A desk of dimension 2.5 ft (0.762m), height,
3 ft(1.524m), length and 2 ft(0.609m), width was used to support
the visual task. The acuity chart and word performance test charts were placed 1 ft (0.9144m), from the back partition
wall in the center of the desk and or the participant simply replaced their
immediate task surface in use with the chart under testing. Major effort was
made by the experimenter to maintain the equal distance of the eye from the
chart among all participants by measuring and indicating its importance for
this study. A view office interior workstation is shown in Figure 2.
Figure 2 - A view office interior workstation (Before and After)
Because
of the real time working hours and agreement with the office management to
minimize the interference with daily work routine under realistic condition and
its relative simplicity in reading writing and talking on the phone, all
studies used acuity as the measure of visual performance rather than a
measurement of full contrast sensitivity.
For study Before and After, there were two visual
tasks. One utilized several Snellen
letter charts of 24 black letters on white paper (size 8.5 in. by 11 in. or
21.59cm by 27.94cm) of decreasing size letters going from top to bottom in 5
horizontal lines with 4 letters on lines 1 & 2, 5 letters on lines 3 &
4 and 6 letters on line 5. Direct measurement of the letter size heights
indicated that they ranged from approximately 9 points to 1.6 points (1 point =
1/72 in. or .035278 cm). Mean letter and word sizes were obtained by averaging
over all letters and words in a given line For the conditions of the study, a
participant who read correctly all the letters in the last line would have at
least 20/15 near visual acuity and for the next to last line 20/25 near visual
acuity. The second visual task
utilized in the study, consisted of
several charts of the same overall size and material as the letter chart but
contained 81 common simple but
unrelated words of gradually
decreasing size (Bailey-Lovie word charts). The word sizes ranged from 1.4
points to 22 points and a person able to read all the words would have at least
20/13 near visual acuity. The letter and word sizes were obtained using a
highly calibrated microscope accurate to 0.0001 mm. The participant simply
replaced their immediate task surface in use with the chart under testing. For
the testing, subjects sat in a chair, leaned slightly forward and viewed the
charts at a fixed distance of 18 inches
(45.72cm) between their eyes and the charts (approximate typical reading
distance). They were asked to read aloud the letters or words and the
experimenter recorded the total number of correctly read letters or words. Figure 3 shows the task area of each workstation.
Figure 3 - A view office interior workstation (Before and After)
The scores were based on the
number of words (Max=81W) or letters (Max=24L) read by the subjects correctly.
To assure that the visual distance was maintained, the experimenter monitored
the subject’s posture during the visual test. Typically, the subject head
position was inclined forward about 25 degrees with respect to the vertical. The Figures 4
shows the utilization of the task lights by the subject during their working
hours and the subject at a typical posture for the study TA.
Figures 4 - The subject at a typical posture using the task
lights.
Task light was essentially prevented from striking the surround
walls by the use its shield and its location on the table with an arrangement
that is shown in Figure 5. This was
an attempt to minimize the surround room lighting from affecting the task
luminance in the direction of subject gaze. The maximum and minimum task luminance used with
the task lighting arrangement was about 64 to 250
(cd/m2) or 19 to 73 (cd/ft2) respectively. For
study B and A, the letter and word charts were used. For the TA study, the set up was the same as
study B and A except an additional condition was added where the surround
lighting was impacted by the addition task lighting except for three stations
with some amount of incoming daylight. See Figure
6 for the areas were daylight was reaching the task.
Figure 5 – Task light arrangement for the study TA
Figure 6 – Workstations with incoming daylight
For all test lighting
conditions before and after, acuity was highly significantly better for the
high color temperature surround lighting as compared to the low color temperature
surround. The results show that visual acuity depends on the color temperature
of the surround lighting. The data shows that to obtain equality of visual
acuity under the 2 different surround spectra requires at least 300% more
task luminance when the surround lighting is provided by the low color temperature
lighting as compared to the high color temperature lighting. Study shows that
word reading and letter acuity were significantly better when the room general
lighting at (5000K CCT) and or task lighting were provided by lamps of high
color temperature (5900K CCT).
Figure 7- shows a plot of the mean score data of the test conditions.
Figure 7- Mean score data of all studies
The results clearly
demonstrate that acuity is significantly better for the high color temperature surround lighting than the low color temperature lighting at equal photopic
illuminance at the eye. More elaborate statistical analyses could be undertaken
such as a MANOVA procedure but because of the very high level of significance
provided by the T-tests it is doubtful that whether any additional conclusions
would occur. [9-14].
More importantly, as is
readily seen by examination of Figure 7,
equality in acuity between the two different surround spectra and just the use
of task light requires at least 300 %
more task luminance under the low color temperature surround lighting than
under the lighting. As can also be readily seen from figure 7, there is no value of the task
luminance under the low color temperature surround lighting that will produce
the maximum acuity achieved with the high color temperature surround. The
implication of this result for lighting practice is that lighting spectra of
higher color temperatures will provide either better vision or equal vision
compared to low color temperature lighting but at lower levels of illumination.
Luminance choice: Subsequent to the acuity measurements, we asked the participants in
study A to adjust the task luminance to their preferred level. Each subject was
asked, "Would you please adjust the light level for the new task lights to
a level that you are most comfortable with for reading the letters". Given
that the participants had just completed the acuity testing, we surmised that
they would choose a lower luminance in the vicinity of what provided the best
acuity, but would that be different in absent of any general lighting?
Conclusions:
An important goal
of lighting practice is to provide environments that appear clear and crisp.
This means that the edges of viewed objects should be well enunciated. It
is not optimal just to see the objects, but to see them sharply enunciated.
It is these object edges that contain high spatial frequencies and where acuity
is essentially the measure of visual sensitivity. Thus, all lit environments
will be optimally viewed when acuity is best. In our studies we have shown
that at a given value of light level, acuity is best when the lighting spectrum
has high color temperature. Since acuity generally decays with age, it is
possible that the effects observed here could be more dramatic in older persons.
We note that lighting retrofits replacing 3500K lamps with 5000K+ lamps have
been carried out in many commercial and industrial environments where both
major energy savings and improved vision have been reported11, 13.
Lamps of varying color temperatures are readily available in the lamp marketplace
and lighting practice should take advantage of that opportunity to enhance
the efficacy and quality of the lit environment.
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