A Field Study of Illuminance Reduction-2006
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Transcript of A Field Study of Illuminance Reduction-2006
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A field study of illuminance reductionY. Akashi *, P.R. Boyce
Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
Received 11 August 2005; received in revised form 8 September 2005; accepted 11 September 2005
Abstract
Many offices are illuminated at levels much higher than necessary for office tasks. Lowering ambient illuminance, while maintaining task
illuminance, i.e., task-ambient lighting, can save energy without impairing visual performance. A downside of task-ambient lighting is rooms
appear dark and gloomy. Maintaining brightness perception is the key to success in task-ambient lighting. To examine the practicality of
brightness-enhanced task-ambient lighting for illuminance reduction a field study was conducted in a modern office setting. This study examined
office workersresponses to approximately one-third lower ambient illuminance than the initial illuminance and twomeasures designed to enhancebrightness perception: higher correlated color temperature lamps (6500 K) and sparkle elements. Surveys showed that after an initial adaptation
period, office workers were generally satisfied with the lower level of ambient lighting. They did increase their use of task lighting at their desks but
this had little impact on overall energy consumption. The 6500 K lamp was effective at increasing perceptions of brightness in offices with the
lower ambient light level. Over a longer period, the sparkle elements used did not significantly change workers perceptions of gloom.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Illuminance; Brightness perception; Correlated color temperature; Task-ambient lighting; Parabolic louvered luminaire; Office
1. Background
Lighting consumes approximately 20% of the electrical
energy supplied to commercial buildings. Some of this energyis wasted in lighting areas that are not in use or by providing
more light than needed in many areas and for many tasks. The
waste associated with the former can be dealt with by installing
occupancy sensors, but what can a lighting specifier or facility
manager do to reduce lighting energy consumption in areas,
such as open-plan offices that are occupied continuously
throughout the day and where many different tasks are done?
One answer is to use one of the task-ambient lightingsystems proposed in the 1970s [13]. The concept behind task-
ambient lighting is that as long as enough light is provided to
fulfill the visual requirements of the work, the ambient light
level in the office can be reduced significantly, thereby saving
energy. However, task-ambient lighting was never widelyadopted, for two reasons, one technical, and the other
perceptual. The technical reason was the difficulty of providing
electrical power to every desk. This is no longer a problem, the
widespread use of personal computers having necessitated a
solution. The perceptual reason was the concern that many
occupants would object to the gloomy appearance of the office.
To overcome this problem, a mechanism to enhance the
brightness of the office is needed. There are two approaches thatmight be used to increase the perception of brightness at a lower
illuminance. One involves the use of lamps with a high
correlated color temperature [46]. The other is to add
sparkle elements to the ambient lighting system [7,8].
While such brightness-enhancing approaches might be used
to alleviate any impression of gloom produced by a reduced
ambient illuminance in an office, it is also worth noting that
lower task illuminances may be more acceptable now than they
were in the1970s. The reason for this is that the nature of officework has changed dramatically since the 1970s. Today, many
people work mainly from a virtual desktop on a computer
monitor, the self-luminous nature of which means that reducing
the ambient illuminance will enhance the visibility of thedisplay rather than reduce it. Further, what printed material is
used is of better quality. The day of the fifth carbon copy is over.
Taken together, these changes in thevisual stimuli used in office
work imply that a lower illuminance on the desk may now be
acceptable.
This paper describes a study designed to examine the
practicality of brightness-enhanced task-ambient lighting for
illuminance reduction in a modern office setting.
www.elsevier.com/locate/enbuild
Energy and Buildings 38 (2006) 588599
* Corresponding author.
E-mail address: [email protected] (Y. Akashi).
0378-7788/$ see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.enbuild.2005.09.005
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2. Site
The study was undertaken in the Public Safety Department
building in Middletown, CT. It was built in 1985 on an isolatedgreen-field site. The offices are primarily large open-plan areas.
There is a continuous band of windows around the outside
walls. All the windows are fitted with beige vertical blinds,
many of which are drawn across. The decor of the building,which is a mixture of white ceiling and walls, grey carpet, and
light grayish-blue furnishing, is typical of many other office
buildings in the US. The building is occupied by the State
Police, by departments dealing with building inspection and fireinspection, and by the Office of the State Commissioner of
Public Safety. Examination of the building showed that the
open-plan areas of the first floor-South (IF-South), second floor-
North (2F-North), and third floor-Central (3F-Central) offices
were suitable for the study. The 1F-South office could easily be
divided as there was a long block of private offices in the center
that effectively separated the room into two parts (West and
East).
The ambient lighting of all these open-plan offices isprovided by a regular array of 2 ft 4 ft ceiling-recessed
parabolic luminaires on 8 ft 10 ft centers. Every ambient
lighting luminaire contains three 32 W fluorescent lamps with
a correlated color temperature (CCT) of 3500 K and a colorrendering index (CRI) of 75. The luminaire is fitted with an 18
cell, parabolic, semi-specular, pre-anodized louver (Fig. 1).
Each cubicle in the offices has at least two under-shelf units
to provide task lighting, the length of the unit depending on the
length of the shelf (4, 3, and 2 ft). Each under-shelf unit
contains a single fluorescent tube with a CCT of 3000 K and a
CRI of 75, the aperture of the luminaire being covered by a
linear prismatic panel.
3. Study design
Table 1 summarizes the study design.
4. Modifications to lighting installations
4.1. Stage 1
Before starting the first stage of the demonstration, the
existing 3500 K lamps in the ambient lighting luminaires and
the 3000 K lamps in the under-shelf luminaires in allthe offices
were replaced with new lamps and the luminaires werecleaned.
4.2. Stage 2
For the second stage, the central lamp was taken from each
ambient lighting luminaire in the 2F-North and 3F-Central
offices with the exception of the occasional luminaires used to
provide escape route lighting in an emergency. A shield plate
was installed to mask the empty central row of the luminaire
louver so that the luminaire looked like two single-lamp
luminaires close together (Fig. 2). In the 2F-North office, all
luminaires were re-lamped with 6500 K lamps. In the 3F-
Central office, the 3500 K lamps were retained. In the 1F-South-West office, the 3500 K lamps were replaced with
6500 K lamps (see Table 1).
4.3. Stage 3
For this third stage, sparkle elements were installed in
the luminaires on the 2F-North and 3F-Central offices. These
elements are essentially pyramid-shaped with openings top
and bottom, and a flat plate with an opening through whichthe pyramidal element can be seen. The sparkle elements
were made of white powder-coated aluminum. One sparkle
element was fitted into a single cell of the louvers of allambient lighting luminaires in the two offices (Fig. 3). In the
1F-South-West office, the 6500 K lamps in the ambient
lighting luminaires were replaced with 5000 K lamps (see
Table 1).
Y. Akashi, P.R. Boyce / Energy and Buildings 38 (2006) 588599 589
Fig. 1. Parabolic louvered luminaires (stage 1).
Fig. 2. Parabolic louvered luminaires with central panels (stage 2). Central
lamps were removed from all luminaires except emergency luminaires.
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5. Illuminance measurements
Illuminancemeasurements were recordedat three locations in
each cubicle equipped for occupationon the horizontal task
area of the desk, on the computer screen, and at the eye of the
worker, seated in the cubicle and looking at the computer screen.The 1F-South-West, 1F-South-East, 2F-North, and 3F-Central
offices had 24, 24, 42, and 46 occupied cubicles, respectively.
Table 2 shows the mean illuminances (with standard deviations)
provided by theambientlighting alone,in each office, at thethreelocations, in the three stages of this study.
Table 3 shows the mean illuminances (and standard
deviations) on the work area of the desk, the computer screen,
and the workers eyes, from the ambient lighting and task
lighting combined, for the three stages of the study.
Examination ofTable 2 shows that during the first stage of the
study, similar mean illuminances are produced in all the offices
by the ambient lighting alone. This is as it should be as at this
stage, all the offices have the same ambient lighting. In the
second stage, the mean illuminances provided by the ambientlighting alone in the 2F-North and 3F-Central locations were
reduced by about one-third because one lamp had been removed
from most of the ambient lighting luminaires. However, there
was also a noticeable reduction in the mean illuminance in the
two first floor locations where the number of lamps/luminaire
was unchanged. To check their validity, illuminance measure-
ments were repeated. Similar results were found. There are three
possible contributors to the reduction in illuminances that
occurred in the 1F-South offices; the smaller lumen output of the
higher CCT lamps in 1F-South-West, the lumen depreciation of
all thelamps in both offices, andthe appearancein both offices of
new computers with a black finish rather than the putty-coloredmodels present in the first stage of the study.
6. Surveys
6.1. Procedure
6.1.1. Short-form questionnaire survey
To obtain occupants opinions on the office lighting, two
types of questionnaire were administered. The short-form
questionnaire was anonymous and was administered using a
questionnaire card completed several times during each stage of
the study. The card contained twelve statements with which the
occupants had to indicate their agreement or disagreement,
except for statement 12, where the occupants were asked to
select from three possible answers, better, about the same orworse. The statements were:
(1) I like the lighting in this office.
(2) Overall, the lighting is comfortable.
(3) With this lighting, I can see my work clearly.
(4) The office looks bright.
(5) The office looks gloomy.
(6) The light fixtures in the ceiling are too bright.
(7) The lighting in my cubicle is too bright for the tasks I
perform.(8) The lighting in my cubicle is too dim for the tasks Iperform.
(9) The lighting in my cubicle is not in the right place.
(10) The lighting is too warm in color for an office.
(11) The lighting is too cool in color for an office.
(12) How does the lighting of your office compare with the
lighting of other offices you have worked in?
In total, 11 short-form questionnaire surveys were adminis-
tered; 5 during the 9 months of stage 1, 4 during the 9 months of
stage 2, and 2 during the 3 months of stage 3. The numbers of
Y. Akashi, P.R. Boyce/ Energy and Buildings 38 (2006) 588599590
Table 1
Study design
Month 1F-South 3F-Central 2F-North
First stage As isa As isa As isa
Second stage 1F-South-West 1F-South-East Task-ambient
lighting with
two 3500 K lamps
Task-ambient
lighting with two
6500 K lamps
Ambient lighting
with three
6500 K lamps
Ambient lighting
with three
3500 K lamps
Third stage 1F-South-West 1F-South-East Task-ambient lighting
with two 3500 K
lamps and sparkle
Task-ambient lighting
with two 6500 K lamps
and sparkle
Ambient lighting
with three
5000 K lamps
Ambient lighting
with three
3500 K lamps
a Under the as is condition, each ambient lighting luminaire had three 3500 K lamps.
Fig. 3. Parabolic louvered luminaires with central panels and sparkle elements
(stage 3). Central lamps were removed from all luminaires except emergency
luminaires.
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short-form questionnaires completed were 83, 90, and 71 for
the 1F-South, 2F-North, and 3F-Central offices, respectively, in
the first stage. The corresponding numbers for the three offices
were 52, 60, and 48 for the second stage and 30, 26, and 26 for
the third stage.
6.1.2. Detailed questionnaire survey
In addition to the short-form questionnaire survey, a more
detailed survey was administered in stages 2 and 3. The
objective of the detailed survey was to confirm and clarify theresults obtained from the short-form survey. Questions
corresponding to the statements in the short-form questionnaire
were used. Responses were made using 5-point scales instead
of simple agree/disagree choices. The questions in the detailed
questionnaire survey are listed below:
1. How do you like the lighting in this office?
2: dislike very much, 1: dislike, 0: neutral, 1: like, 2:
like very much.
2. How comfortable is the lighting in this office?
2: very uncomfortable, 1: uncomfortable, 0: neutral,
1: comfortable, 2: very comfortable.
3. How well can you see to do your work in your cubicle?
2: very poorly, 1: poorly, 0: neutral, 1: well, 2: very
well.4. How bright does the office look?
2: very gloomy, 1: gloomy, 0: neutral, 1: bright, 2:
very bright.
5. Are the light fixtures in the ceiling?
2: too dark, 1: dark, 0: neutral, 1: bright, 2: too
bright.
6. How bright is the lighting in your cubicle for the tasks you
perform?
2: too dim, 1: dim, 0: about right, 1: bright, 2: too
bright.
7. Is the under-shelf lighting in your cubicle located
appropriately for your work?
2: very inappropriately, 1: inappropriately, 0:
neutrally, 1: appropriately, 2: very appropriately?
8. Is the color of the lighting in the office?
2: too cool,1: cool, 0: neutral, 1: warm, 2: too warm.
9. How does the lighting in your office compare with other
offices you have worked in?
2: much worse,1: worse, 0: about the same, 1: better,
2: much better.10. If you chose (2) like very much in question 1, why do
you think so?
(1) The color of the lighting is appropriate for an office.
(2) The colors of objects look natural.
(3) The office looks bright.(4) My cubicle is bright enough for the tasks I perform.
(5) Other 11. If you chose (2) dislike very much in question 1, why
do you think so?
(1) The color of the lighting is inappropriate for an
office.
(2) The colors of objects look unnatural.
(3) The office looks gloomy.
(4) My cubicle is too dark for the tasks I perform.(5) Other
12. Please make any other comments you wish about your
office lighting below.
The first detailed questionnaire survey administered in
stage 2 of the evaluation obtained 16, 20, and 18 responses
from the 1F-South, 2F-North, and 3F-Central offices,
respectively. The second detailed questionnaire survey
administered in stage 3 of the evaluation collected 24, 16,
and 19 responses from the 1F-South, 2F-North, and 3F-
Central offices, respectively.
Y. Akashi, P.R. Boyce / Energy and Buildings 38 (2006) 588599 591
Table 3
Mean illuminances (and standard deviations) from ambient lighting and task lighting combined
Stage
Desk Screen Eyes
1 2 3 1 2 3 1 2 3
1F-South-West 761 (132) 490 (181) 506 (135) 430 (92) 385 (65) 356 (54) 214 (55) 199 (46) 177 (42)
1F-South-East 806 (147) 592 (140) 810 (197) 404 (83) 374 (87) 338 (84) 223 (41) 263 (66) 239 (63)
2F-North 767 (241) 496 (167) 514 (247) 357 (95) 262 (78) 221 (71) 217 (53) 180 (56) 151 (50)
3F-Center 847 (165) 472 (103) 464 (79) 364 (87) 237 (53) 246 (66) 223 (54) 190 (43) 166 (42)
Table 2
Mean illuminances (and standard deviations) from ambient lighting alone (lx)
Stage
Desk Screen Eye
1 2 3 1 2 3 1 2 3
1F-South-West 586 (189) 517 (128) 448 (102) 392 (85) 322 (85) 309 (86) 227 (49) 235 (58) 235 (119)
1F-South-East 571 (114) 462 (64) 421 (79) 383 (82) 339 (46) 339 (55) 223 (60) 196 (51) 164 (48)2F-North 558 (141) 346 (99) 324 (81) 338 (96) 226 (70) 193 (57) 224 (72) 152 (51) 130 (47)
3F-Center 544 (141) 370 (79) 362 (73) 332 (75) 237 (51) 201 (52) 189 (45) 153 (34) 138 (45)
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6.2. Survey results
6.2.1. Overall findings
6.2.1.1. Short-form questionnaire survey. For statements 1
11 in the short-form questionnaire, the evaluations of lighting
conditions were expressed as the probability of respondents
agreeing with each statement. For the question 12, evaluationswere quantified by calculating a mean rating on a 3-point scale:
(1) worse; (2) about the same; (3) better.
Fig. 4 shows the probabilities of agreement with each
statement based on all the short-form questionnaires collected
in the four offices during each stage of the evaluation. This
shows the long-term effects of the lighting modifications on the
occupant evaluations. The first thing to note is that in the first
stage, when all the offices were as is, the probability of
agreement with the statement, overall, the lighting is
comfortable is about 0.70 in all offices. This result suggeststhat the lighting at the demonstration site can be considered as
typical of office lighting in the USa survey of 13 large offices
in the North-East, using the same statement in a similar survey,
suggested that about 69% of office workers would agree that the
lighting of their offices was comfortable [9].
The next thing to note is that the overall pattern of
probabilities in Fig. 4 indicates that the response to the lightingin all the offices, in all three stages of the demonstration, was
positive in the sense that there is a high level of agreement with
positive statements and a low level of agreement with negative
statements.Of course, the above are gross observations made simply
from looking at Fig. 4. To examine the differences
quantitatively, statistical comparisons among the three stages
were conducted for all four offices. To determine whether or notany statistically significant changes occurred over the three
stages of the study, x2-tests were applied to questions 111 and
one-way analyses of variance (ANOVA) were applied to
question 12. Table 4 shows the results of these statistical
analyses in terms of the probability that any difference could
have occurred by chance (p-values). Following on from this, a
series of paired comparisons were conducted for the three
possible stage combinations: stage 1 versus stage 2; stage 2
versus stage 3; stage 3 versus stage 1. For each pair, x2-tests
Y. Akashi, P.R. Boyce/ Energy and Buildings 38 (2006) 588599592
Fig. 4. Probability of agreement with each statement in the short-form questionnaire for the four offices during each stage of the study.
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were applied to questions 111 and a t-test was applied to
question 12. Table 5 shows the results of these pairedcomparisons. It is important to note that these paired
comparisons should only be considered for statements for
which there is an overall statistically significant difference in
Table 4. Further, given the large number of statistical tests done
and the consequent increased possibility of a type 1 error (falsepositives) it is better when examining these tables to place
greater value on higher levels of statistical significance
(i.e., p < 0.001 and
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p < 0.05) are, overall, the lighting is comfortable and the
lighting in my cubicle is too dim for the tasks I perform. The
difference in the mean ratings of, how does the lighting of
your office compare with the lighting of other offices you haveworked in was also statistically significant at the p < 0.01
level. Examination of Fig. 4 and Table 5 shows that for the
statement, overall, the lighting is comfortable, the statisti-
cally significant effect is due to a smaller probability ofagreement with the statementoccurring in stages 2 and 3 than in
stage 1, a difference that is coincident with the change from
3500 to 6500 K lamps, and 5000 K lamps without a change in
ambient illuminance. For the statement, the lighting in mycubicle is too dim for the tasks I perform the statistically
significant effect is due to a greater probability of agreement
with the statement occurring in stage 2 than in stage 1, a
difference that is coincident with the use of 6500 K lamps rather
than 3500 K, both at the same ambient illuminance. As for the
mean rating in response to the question, how does the lighting
of your office compare with the lighting of other offices you
have worked in, examination ofFig. 4 and Table 5 shows that
when this office is lit with 3500 K lamps (stage 1) and 5000 Klamps (stage 3) the lighting is considered slightly better than
usual, but when it is lit by 6500 K lamps, the lighting is
considered somewhat worse than usual. These findings suggest
that the use of 6500 K lamps can cause difficulties when theilluminance is unchanged.
All the above analysis has been based on data from the short-
form questionnaire, summed over each complete stage. It is also
interesting to consider how opinions changed during each stage
of the study. Fig. 5 shows the probability of agreement with the
statements in the short-form questionnaire and the mean rating
for the final question, every time the questionnaire was
administered. Fig. 5 shows considerable variation in responses
to many statements but the most obvious and consistent featureis that for two statements and the final question there are clear
short-term changes during stage 2 in all four offices.
Specifically, for the statements, I like the lighting in this
office and overall, the lighting is comfortable and for thequestion, how does the lighting of your office compare with
the lighting of other offices you have worked in, there is a
strongly negative response at the start of stage 2, for all offices,
regardless of the nature of changes made to the lighting, if any.Further, there is a tendency for the responses to these statements
and question to become more positive over time until by the end
of stage 2, there is little difference from what they were in stage
1. These observations suggest two conclusions. The first is that
changes to the lighting in one part of building can influence
opinions in another part where no changes have occurred. The
second is it can take occupants some time to adapt to lighting
changes.
6.2.1.2. Detailed questionnaire survey. Table 6 shows the
mean ratings on questions 19 of the detailed questionnaire, for
the two occasions, it was administered which were towards the
ends of stages 2 and 3. The mean ratings in Table 6 do not
indicate any strong opinions, i.e., all mean responses on a 5-
point scale (from 2 to +2) are around neutral, ranging from
0.75 to +0.86. This suggests that at these times, the lighting in
all the offices was fairly acceptable. The greatest number of
negative ratings appears in the 1F-South-West office in the
Y. Akashi, P.R. Boyce/ Energy and Buildings 38 (2006) 588599594
Table 6
Mean ratings given on the detailed questionnaire administered in stages 2 and 3 in all four offices
Question 1F-South-West 1F-South-East 2F-North 3F-Central
Stage 2 Stage 3 Stage 2 Stage 3 Stage 2 Stage 3 Stage 2 Stage 3
1 How do you like the lighting in this office? 0.38 0.38 0.17 0.30 0.29 0.08 0.25 0.23
p-Value 0.277 0.827 0.840 0.654
2 How comfortable is the lighting in this office? 0.13 0.21 0.33 0.20 0.43 0.27 0.25 0.31
p-Value 0.553 0.623 0.850 0.842
3 How well can you see to do your work? 0.13 0.64 0.17 0.60 0.86 0.75 0.67 0.77
p-Value 0.087 0.619 0.800 0.764
4 How bright does the office look? 0.29 0.14 0.50 0.40 0.07 0.33 0.08 0.46
p-Value 0.038* 0.841 0.268 0.475
5 Are the light fixtures in the ceiling? 0.13 0.14 0.33 0.30 0.00 0.33 0.08 0.15
p-Value 0.662 0.651 0.326 0.928
6 How bright is the lighting in your cubicle? 0.25 0.14 0.50 0.22 0.00 0.08 0.33 0.15
p-Value 0.275 0.578 0.131 0.245
7 Is the under-shelf lighting located appropriately? 0.38 0.25 0.67 0.30 0.07 0.00 0.58 0.75
p-Value 0.857 0.344 0.239 0.915
8 Is the color of the lighting in the office: cool to warm? 0.13 0.07 0.50 0.10 0.64 0.08 0.08 0.15
p-Value 0.588 0.023* 0.144 0.451
9 How does the lighting in your office compare with others? 0.75 0.14 0.17 0.60 0.15 0.17 0.33 0.15
p-Value 0.008** 0.452 0.857 0.329
Also shown are the statistical significances (p-values) for the mean ratings given in the two stages, for each office.* p < 0.05.**
p < 0.01.
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Y. Akashi, P.R. Boyce / Energy and Buildings 38 (2006) 588599 595
Fig. 5. The probabilities of agreement and the mean ratings for the short-form questionnaire for the four offices, every time it was administered. The boundaries
between stages 1 and 2 and between stages 2 and 3 are identified by dashed vertical lines.
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second stage in which a lamp with a CCT of 6500 K was used
without a reduction in illuminance. To identify how similar (or
different) the evaluations of the lighting between the two stages
are, t-tests were applied to all responses. Table 6 also shows the
results (p-values) of these t-tests. Examination of Table 6
reveals that there are no statistically significant differences
between the mean ratings for the 2F-North and 3F-Centraloffices, suggesting that the addition of the sparkle elements to
the ambient luminaires in these two offices had little impact.
Where there are statistically significant differences is in the two
1F-South offices, specifically in response to the questions,
how bright does the office look? and how does the lighting
in your office compare with other offices you have worked in?
in the 1F-South-West office and is the color of the lighting in
the office: warm to cool? in the 1F-Souh-East office. The
direction of the statistically significant effects in 1F-South-
West is such that the 5000 K lighting used in stage 3 is
considered slightly brighter and slightly better than the 6500 Klighting used in stage 2. The direction of the statisticallysignificant result in 1F-South-East is that stage 3 is considered
slightly warmer than stage 2. In the 1F-South-East office, there
is no difference in the lighting for these two stages. The most
reasonable explanation for this odd result is that the lighting of
the adjacent 1F-South-West office is influencing the percep-
tions of the occupants of the 1F-South-East office.
6.2.2. Visual performance
To measure visual performance, the short-form question-
naire had four high contrast sentences printed on the reverse in
different point sizes, ranging from 12- to 4-point, in the same
font. The evaluations were quantified by providing a score from
1 to 5 depending on the minimum point size that could be read
by each participant: (1) larger than 12-point; (2) 12-point; (3)
8-point; (4) 6-point; (5) 4-point. Mean scores over all responses
from each short-form survey administration were obtained.
Fig. 6 shows the mean of the print sizes that could just be read.From Fig. 6, it can be seen that the overall mean level of visual
performance is somewhere between reading 6- and 4-point
print. This is consistent with previous work[9] that showed that
in a large sample of offices in the North-East, 95% of occupants
could read 6-point print.
Y. Akashi, P.R. Boyce/ Energy and Buildings 38 (2006) 588599596
Fig. 6. Results of visual performance tests. The vertical axis represents the
mean of the smallest print size that can just be read: (1) larger than 12-point; (2)
12-point; (3) 8-point; (4) 6-point; (5) 4-point.
Fig. 5. (Continued).
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It is also apparent from Fig. 6 that the lighting modifications
have little effect on visual performance apart from in the 3F-
Central office. In this office, the reduced illuminance in stage 2
initially produced a statistically significant (p < 0.05) changeof approximately one unit on the scale. However, over time, the
visual performance gradually increased so much so that there
was no statistically significant difference in visual performance
between stages 1 and 3. Contrarily, the change to stage 2 did notshow any statistically significant change in visual performance
in the 2F-North office, despite the reduced illuminance. These
trends do not follow the changes in actual illuminance
(Table 2), which suggests that they have more to do withmotivation than visual capabilities.
7. Observation of behavior for energy consumption
comparisons
Given that the underlying purpose of this study is to examine
the practicality of energy saving by reducing the ambient
illuminance in offices, it is important to determine if the
reduced ambient illuminances used in stages 2 and 3 in the 2F-North and 3F-Central offices produce an increase in the amount
and duration of use of the task lighting. This was done by
observation of behavior.
7.1. Method
Cubicle observations were undertaken to identify how long
occupants stayed in the office and how much lighting energy
they used. In each observation, a researcher selected a day
around the middle of each month and walked around the four
offices, once every hour, from 8:30 a.m. to 3:30 p.m. Eight
hourly observations were noted for every cubicle on the day.The researcher counted the number of under-shelf task
luminaires switched on and the number of cubicles that were
occupied at each visit. Previous data collection had already
been undertaken to determine the total number of task
luminaires in each of the cubicles, and the wattage of each
task lamp type.
7.2. Observation results
As different cubicles have different combinations of lamp
wattages, the energy consumption by task luminaires was
calculated for each office per day (8 h). Fig. 7 shows this energy
consumption per day of task lighting, for all four offices, overall the stages of the study. Table 7 shows the mean energy
consumption per day for task lighting, for the three stages of the
study. Examination of Fig. 7 and Table 7 clearly demonstrate
that the task lighting was more widely used in the 3F-Center
office than in any of the other offices, during all three stages.
It is also apparent from Fig. 7 that the reduction in ambient
illuminance that occurs at stage 2 in the 2F-North and 3F-Central offices leads to an initial increase in use of the under-
shelf task lighting but that over time, this usage returns to the
level in found in stage 1.
Table 7 also includes the energy consumption per day of the
ambient lighting and the total (task-ambient) lighting system
for the four offices. It is evident from Table 7 that the energyconsumption per day of the task lighting is only a small
percentage of the total energy consumption per day, a total that
is dominated by the contribution of the ambient lighting. Also,
the ratios of total energy consumption per day in the second or
third stage relative to the first stage (S2/S1 or S3/S1 in Table 7)
prove that more than a 30% reduction in total energy
consumption per day was achieved in the 2F-North and 3F-
Central offices in stages 2 and 3.
7.3. Task light luminaires
Observations of the offices in stage 1 revealed that many
occupants of the offices did not use the existing under-shelf task
luminaires. It was expected that even when the ambientilluminance was reduced by one-third, no additional task light
luminaires would be requested by the occupants of the offices.
However, just in case, a type of free-standing task luminaire
was selected to be made available if one was requested. After
the lighting modifications needed to introduce stages 2 and 3lighting to the 2F-North and 3F-Central offices were made, and
Y. Akashi, P.R. Boyce / Energy and Buildings 38 (2006) 588599 597
Table 7
Energy consumption per day (Wh/day) by task and ambient lighting, separately and combined, for all four offices, over all three stages
Stage 1 (Wh/day) Stage 2 (Wh/day) Stage 3 (Wh/day)
Task Ambient Total (S1) Task Ambient Total (S2) S2/S1 (%) Task Ambient Total (S3) S3/S1 (%)
1F-South-West 3193 33024 36217 3065 33024 36089 99.6 2975 33024 35999 99.4
1F-South-East 2242 39168 41410 2088 39168 41256 99.6 1865 39168 41033 99.1
2F-North 1636 76032 77668 2203 50688 52891 68.1 1966 50688 52654 67.8
3F-Center 4957 79104 84061 5632 52736 58368 69.4 4635 52736 57371 68.2
Fig. 7. Energy consumption by task lights per day (Wh/day).
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hence after the reduction in ambient illuminance by one-third,
only one occupant, in the 3F-Central office, requested such a
luminaire. The illuminance on this individuals desk, even with
the available task lighting, was only 150 lx, so this request wascertainly justified. The significance of this happening was to
remind us that although the mean illuminance under a lighting
installation may be satisfactory, illuminance uniformity is not
perfect so there may be some locations, particularly at the edgeof the installation, where the illuminances are much lower than
the average. Reducing the illuminance in such locations
requires care.
8. Discussion
The results presented above demonstrate that it is possible to
reduce the ambient illuminance provided by typical office
lighting by about one-third without producing dramatic long-
term changes in occupant satisfaction with the lighting. The
mean ratings on the detailed questionnaire shown in Table 6 for
the three offices are all around the neutral point on a 5-point
scale, despite the fact that the ambient illuminance on the deskin the 1F-South office is around 500 lx, while those in the 2F-
North and 3F-Central offices are about 360 lx. Such a reduction
in illuminance is associated with a similar reduction in energy
consumption, unless the decrease in ambient illuminance isoffset by an increased use of task lighting. Observation of the
pattern of use of the under-shelf task luminaires in the offices
did reveal a short-term increase in use, but this had little impact
on the overall energy consumption of the complete installation.
Further, only one person in one office requested additional task
lighting. Therefore, removing one lamp from most of the three-
lamp ambient lighting luminaires resulted in a commensurate
energy saving.
While saving energy, and hence money, is of some interest to
business, it will be if little interest if the reduction in energy
consumption is associated with a decrease in visual perfor-mance. The results from the visual performance test in the
short-form questionnaire show that reducing the ambient
illuminance from about 500 to 360 lx has no long-term effect on
visual performance.
Although reducing the ambient illuminance from about 500
to 360 lx produces no adverse reactions long-term, the results
clearly demonstrate that such an action can produce adverse
reactions short-term. A similar tendency appears in risk
homeostasis theory as applied to road safety [10] in which, after
an introduction of a road safety measure, accident rate may besignificantly reduced but, after an adaptation period, the
accident rate returns to the initial level. This tendency suggests
that in the long run, a wide range of illuminance reductions will
be acceptable as long as the final illuminance meets the
minimum illuminance recommendations [11]. The lack of a
long-term effect can be ascribed to the facts that the human
visual system can perform equally well over a wide range of
lighting conditions [12,13], and that people tend to adapt to new
lighting conditions and slowly accept them as the norm. The
dramatic short-term effects can be ascribed to the dislike of
change in their working environment.
The results obtained in this study also suggest that the
likelihood of a negative short-term response can be reduced by
making the change in a way that will minimize the reduction in
room brightness caused by the reduction in ambientilluminance, and that will make the change less conspicuous.
What is required to achieve these aims is to take action to
increase the perception of brightness, and to make the changes
in a series of steps rather than all at once.Two means to increase brightness perception were tested in
this study, the use of high CCT lamps, and the use of sparkle
elements in the luminaires. The results for the 2F-North and 3F-
Central offices suggest that the use of high CCT light sources,specifically 6500 K lamps, can be effective in avoiding the
perception of gloom, but the results for 1F-South-West office
show that this advantage can be offset by the cold appearance of
the light where the illuminance is high enough to be considered
bright enough already. This observation in turn suggests that it
would be better to steer a middle course by using a light source
with a CCT of 5000 K to increase brightness perception at an
ambient illuminance of about 360 lx as this has less risk of
being considered unpleasant in light color. It also implies that itis important to avoid using higher CCT lamps at high
illuminances. Of course these suggestions may only apply to
North America where it is usual to use fluorescent lamps with a
CCT in the range 35004100 K for office lighting. It must be
doubted if the same conclusion would apply in Japan, where
light sources with a higher CCT range, from 5000 to 6500 K,are normally used for offices.
As for the use of sparkle elements, the failure to show any
statistically significant long-term changes for statements
related to brightness or gloom when the elements are
introduced into 2F-North and 3F-Central in stage 3 implies
that these elements are ineffective as brightness enhancers. It is
always possible that other forms of sparkle element would have
been more effective.
As for measures that can be used to make the reduction inambient illuminance less conspicuous, recent studies have
demonstrated that an illuminance reduction by 1520% from an
initial illuminance is unnoticeable provided it is made smoothly
and slowly [1416]. This can easily be done if the installation is
equipped with dimming ballasts, but if the simple expedient of
removing lamps is to be used, then some care is required. A
three-step procedure is suggested. First, all the lamps in the
installation should be changed to the CCT required to enhance
brightness, preferably 5000 K rather than 6500 K. Second, one
lamp in every third three-lamp ambient lighting luminaireshould be removed across the whole installation, excepting
from luminaires at the edges of the installation. This will
produce a nominal 10% reduction in illuminance across most ofthe working space but leave the edges little changed. This is
important because the illuminances are always lowest at the
edges of a regular array lighting installation, particularly in the
corners. However, the reduction in illuminance is nominal
because the first step calls for the installation of new lamps
throughout, and these are likely to have a higher light output
than the lamps they replace. Consequently, there may be very
little difference in ambient illuminance even though some of the
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lamps have been removed. Third, after 3 months have elapsed,
the third lamp in every second luminaire that has three lamps,
except those round the edges of the installation, is removed.
This process can be repeated until the required ambientilluminance is achieved.
9. Conclusions
The results of this study lead to the following conclusions:
It is possible to reduce the ambient illuminance produced by a
typical office lighting installation by about one-third withoutproducing a long-term change in occupant satisfaction with
the lighting.
Such a reduction in ambient illuminance will lead to a short-
term increase in task lighting but this will have little effect on
the energy consumption.
Such a reduction in ambient illuminance can produce
negative short-term reactions. These reactions will tend to
fade as time passes. The likelihood of negative short-term
reactions occurring will be reduced if action is taken toincrease the perception of brightness as the illuminance is
reduced and if the reduction is made in small steps.
Higher CCT lamps (5000 and 6500 K) can be used to enhance
the perception of brightness, but care is required. This isbecause the cold appearance of these lamps can be considered
unpleasant when the illuminance is high enough to provide an
adequate perception of brightness. The 5000 K lamp will
usually be a safer choice than the 6500 K lamp.
More research is needed before sparkle elements can be used
to enhance the perception of brightness with any confidence.
Acknowledgements
This paper was prepared from work undertaken with the
financial support from Connecticut Light and Power Company.The authors would like to acknowledge Sharon Goslee, the
Property Manager, for her help in administering the short-form
questionnaire, and staff of the State of Connecticut Department
of Public Safety for their participation. Martin Overington,
Michael Myer, David Cyr, and Anjan Sarkar are also
acknowledged for their work on this project.
References
[1] P.L. Shellko, H.G. Williams, The integration of task and ambient lighting
in office furniture, Lighting Design and Application 6 (9) (1976)1423.
[2] N. Florence, The energy effectiveness of task-orientated office lighting
systems, Lighting Design and Application 9 (1) (1978) 2839.
[3] C.L. Amick, Modern office lighting trends in America, International
Lighting Review 29 (2) (1978) 5461.
[4] S.M. Berman, Photopic luminance does not always predict perceived
room brightness, Lighting Research and Technology 22 (1990) 3741.
[5] S.A. Fotios, Chromatic effect of apparent brightness in interior spaces II:
SWS lumens model, Lighting Research and Technology 30 (1998) 103
106.
[6] S. Han, P.R. Boyce, Illuminance CCT, decor, and the Kruithof curve, in:
Proceedings of the 25th Session of the CIE, vol. 2, 2003, pp. D3178.
[7] Y. Akashi, Gloom is doomed, Lighting Design and Application 30 (2000)
8893.
[8] Y. Akashi, Y. Tanabe, I. Akashi, et al. Effect of sparkling luminous
elements on the overall brightness impression: a pilot study, LightingResearch and Technology 32 (2000) 1926.
[9] N.H. Eklund, P.R. Boyce, The development of a reliable, valid, and simple
office lighting survey, Journal of the Illuminating Engineering Society 25
(1996) 2540.
[10] G.J.S. Wilde, L.S. Robertson, I.B. Pless, Does risk homeostasis theory
have implications for road safety: for and against, British Medical Journal
324 (7346) (2002) 1149.
[11] Illuminating Engineering Society of North America (IESNA), ANSI/
IESNA Recommended Practice for Office Lighting, RP-1-04, IESNA,
New York, 2004.
[12] M.S. Rea, M.J. Ouellette, Relative visual performance: a basis for
application, Lighting Research and Technology 23 (1991) 135144.
[13] N.H. Eklund, P.R. Boyce, S.N. Simpson, Lighting and sustained perfor-
mance: modeling data-entry task performance, Journal of the Illuminating
Engineering Society 30 (2001) 126141.[14] K. Kryszczuk, P.R. Boyce, Detection of slow light level reduction, Journal
of the Illuminating Engineering Society 31 (2002) 310.
[15] Y. Akashi, J. Neches, Detectability and acceptability of illuminance
reduction for load-shedding, Journal of the Illuminating Engineering
Society 33 (2004) 313.
[16] Y. Akashi, J. Neches, Potential recommendations for illuminance reduc-
tions by load-shedding, Lighting Research and Technology 37 (2) (2005)
133153.
Y. Akashi, P.R. Boyce / Energy and Buildings 38 (2006) 588599 599