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.


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


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


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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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.


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


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.

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