A PRACTICAL METHOD FOR THE ASSESSMENT OFDAYLIGHT-RESPONSIVE LIGHTING CONTROL SYSTEMSREGARDING ENERGY SAVINGS AND LIGHTING QUALITY

Heiko Belendorf, Sirri Aydinli and Heinrich KaaseInstitute for Electronics and Lighting Technology, Technical University of Berlin, Einsteinufer 19, Sekr. E6, 10587

Berlin, Germany, Tel: +49 30 314 24208, Fax +49 30 314 22161, belendorf@ee.tu-berlin.de

Abstract – The increased utilization of daylight-responsive lighting-control systems in commercialbuildings requires the possibility to compare different systems regarding energy and lighting perform-ance. Measurements at the Technical University of Berlin (TUB) on various daylight-responsive lightingcontrols have been made analyzing the energy-saving potential and ability to fulfill general lighting re-quirements. The paper describes a basis for the calculation founded on these test-measurements in 1:1test-rooms in order to assess the energy and lighting performance of the system as well as calculating ageneral quantity (the system-triple) per room-zone to classify and compare different systems. The methodmeets practical requirements because it takes into account the official guidelines for lighting in offices(dependent on the specific country, e.g. in Germany the standard DIN5035 “Artificial Lighting of Interi-ors” or other equivalent European standards). The quantity called system-triple consists of three numbers,of which one number describes the energy saving performance (system-potential), the other two relate tothe behavior towards its maintaining the requirements to the illuminance level in the room (relative short-coming and exceeding light exposure). The system-potential tends to be independent from the sky-condition, although the situation in which the system is going to be applied has quite an influence on thesystem-potential. The method is an essential part of a comprehensive procedure, developed at the TUB,for predicting the energy saving potential regarding electric lighting in an office equipped with daylight-following lighting control systems.

1. INTRODUCTION

The prediction of energy savings and the performanceassessment of daylight-responsive lighting controls canbe considered a complex task. On one hand the systemshould save as much energy as possible, on the otherhand, the system should maintain the quantity of light(e.g. illuminance-levels) required by the standards (e.g.DIN5035, Germany). The performance is the result of thespecific system, the ambient in which it is applied, theused control strategy and the calibration method. Thebroad range of products on the lighting-control systemmarket makes it necessary to compare different systems.A lighting control system is expected to maintain thelighting requirements during the whole office day and tosave as much energy as possible regarding the require-ments to lighting conditions. Lacking daylight has to becompensated by artificial light.

2. ASSESSMENT AND INFLUENCE

The assessment of daylight-responsive lighting controlsmainly focuses on two parts: the energy-saving capabilityand the ability to maintain the lighting requirements ac-cording to the national or international standards for of-fice lighting. Daylight-responsive lighting controls cannotbe assessed isolated from its environment where they areto be applied. There are different factors influencing theperformance of the daylight-responsive lighting control,e.g. calibration method and the dimming characteristic ofthe artificial lighting system. They are interacting with

each other. Generally there are different approaches toassess the performance of a daylight-responsive lightingcontrol, from which mainly a practical and a theoreticalprocedure used at TUB can be mentioned (Figure 1),(Belendorf and Aydinli, 1999) and (Knoop, 1998).

Daylight-responsive lighting control

Test-room studies / measurements

Variation of parameters

Assessment of lighting performance Energetic assessment

Maintaining requirements

for lighting

Theoretical approach

Characterizing quantities

Practical assessment

Assessment of user acceptance

Assessment of system – performance / quality

Figure 1 Different ways to assess the performance ofdaylight-responsive lighting control systems.

3. ENERGY-SAVING

Daylight responsive lighting controls contribute to anoptimum utilization of daylight. Without daylight-

responsive artificial lighting, the energy saving-potentialusing daylight-systems (increased interior daylight-availability) cannot be realized. The characterization ofthe system’s energy saving potential is an important is-sue. In the energy-saving context one has first to definewhat kind of energy can be saved by installing a daylightresponsive lighting control. The energy generally con-sumed in a building can be classified into different types,of which the electric energy expended for artificial light-ing is the kind of energy that can directly be saved withintelligent daylight responsive lighting control systems(Figure 2). But there is some interaction between thesedifferent forms of energy, e.g. saved energy for lightinghas an impact on the necessary cooling or heating energyin the building; see energy flow in (Figure 3). This cansignify that the cooling loads can be reduced when thelighting is controlled or on the contrary that the necessaryheating energy is increased. Therefore the prediction ofenergy savings caused by daylight responsive controls forthe artificial lighting is a sophisticated matter and actuallyshould not be treated isolated from its thermal environ-ment. Thermal aspects of lighting can positively or nega-tively affect the potential energy savings.

other energy

cooling energy

electric energy for

lighting

ENERGY

heating energy

Figure 2 Rough classification of the energy used inbuildings

Although an overall energy reconsideration is necessarywhen making an exact prediction or calculation of the(overall) energy savings caused by control systems, some(simplified) methods to assess the saving potential can beemployed. A procedure to make an estimation of thepossible energy saving due to a certain daylight respon-sive lighting control will be presented. Although it doesnot take into account the overall energy aspect (e.g. ther-mal impacts) the estimation of the so-called direct energysaving potential is an important decision criterion for theselection of a system.

In order to compare systems with regard to their effec-tiveness, the control in combination with the building inwhich it should be applied has to be treated as a unit, inwhich besides the system’s property the amount of usabledaylight forms the major quantity of influence.

heat DEMAND ON

ENERGY

Energy saving potential

Electr. energy

SOLAR ENERGY

light, heat, ...

Figure 3 Energy flow into the building and energy savingpotential

The assessment of daylight-responsive controls regardingenergy-savings and lighting quality is a multidimensionalprocess. An accurate estimate requires a lot of informa-tion and a detailed calculation. The energy savings can bethe result of a lower consumption of electric energy forelectric lighting. This leads to a reduction of the internalheat gains in the building. In some cases this gives asubsequent reduction of cooling load and thus extra en-ergy savings. The direct energy savings may be calcu-lated straightforward, the estimate of the reduction ofcooling loads requires a detailed simulation (Belendorfand Zonneveldt, 1999).

4. COMPREHENSIVE PREDICTION AND AS-SESSMENT METHOD

For calculating the possible energy savings by the combi-nation of daylight responsive lighting controls and day-light in buildings at the Institute of Electronics andLighting (TUB), a method was developed, that enables todetermine and to quantify the expected energy saving-potential. Measurements on different daylight-responsivelighting controls also led to a defined way to assess theenergy and lighting performance that forms part of thecomprehensive method.

4.1 General idea of the entire approachThe motivation for the extended method is based on thefact that only a few measurements on a control system atsome test days are not sufficient to make an accuratelong-term prediction of the energy-savings as well as thelighting performance. The calculation (estimation) proce-dure can mainly be divided in two major parts:

• Determination of the room-zone system-potential

• Calculation of the relative annual usable lightexposure per room-zone (room-zone system po-tential)

4.2 Determination of the system-potentialFor the different sky-conditions test-room measurementsare to be performed on the daylight-responsive lightingcontrol in combination with the specific room, the artifi-cial lighting installation (incl. dimmable ballasts with its

dimming characteristic), the sensor (position, form, ori-entation) and calibration for the desired design illumi-nance. Using the assessment procedure, one can calculatethe specific quantity, i.e. the system-potential related tothe different room-zones.

4.3. Calculation of the room-zone potentialThe annual value of the relative usable light-exposure forthe local average sky type (average daylight-availability)has to be calculated for the specific room (with its prop-erties). The quantity also called room-zone potential canthen be used for the long-term prediction.

Possible influence factors of the artificial lighting system(dimming components, characteristics) also have to beconsidered doing a detailed calculation.

4.4. Energy-saving potential estimationThe estimation/calculation of the long-term energy sav-ings (potential) that can be achieved/expected with theinstalled system finally can be computed using Eq. (1).

SLSFRZSPRZPESPRZn

1xRZxRZx ⋅

⋅= ∑=

(1)

ESP Energy saving potentialRZP Room-zone-potentialnRZ Number of room-zones in the roomx Specific room-zone numberRZSP Room-zone system-potentialSLSF Specific lighting system factor

Normally, the lighting installation at nighttime dimmingstate (100%) defines the reference illuminance situationto which the control situation with daylight contributionwill be compared. In difference to former assessmentapproaches (Knoop, 1998) and (Knoop, Ehling, Aydinliand Kaase, 1997), the references are given through theaverage and the minimum illuminance in the definedroom zone.

4.5 Room-zone conceptGenerally a room can be divided into different zones, e.g.in the TUB test-rooms two room-zones (Figure 4). Forpractical reasons, each room-zone is assigned to thatregion of the room illuminated dominantly by the locallighting fixture. Each luminaire has to illuminate a de-fined region. Artificial lighting contribution of that part ofthe room is for major part caused by the correspondinglighting fixture.

Room-zones can characterize the different zones of day-light availability, e.g. daylight-zone, daylight- and artifi-cial light-zone (mixed zone) or artificial light-zone (nodaylight-responsive control necessary).

luminaire-row LB2

window-side

room-zone 1

room-zone 2

luminaire-row LB1

Figure 4 Dividing a room in different room-zones, viewof TUB test-room and sensor-locations

For the assessment procedure each room-zone can betreated separately, as it is also done referring to the day-light-availability with respect to the corresponding room-zone

4.6 Daylight situations in a day-lit roomIn a room with daylight-openings one can classify threedifferent daylight illumination situations of the interior(Aydinli, 1984), (Seidl, Aydinli, Van Bergem-Jansen andKeller, 1992).

A) The sun is not shiningB) The sun shines, but no direct sun hits the window

façadeC) The sun shines and direct sun is on the façade

(window) plane, but sun-protection is active.

Note that the characteristic of the daylight system has tobe taken into account.

4.7 TUB methodThe method for calculating the energy-saving potential bythe use of daylight in combination with control systemsfor artificial lighting introduces the advantage that it takesinto consideration the three different daylight illumina-tion options. This is in contrast to the daylight factormethod for predicting the energy saving potential (Hunt,1979).

The calculation can be divided into two aspects: theroom-zone potential and the system-potential per room-zone. The procedure uses the concept of the average skycondition model as a basis in order to characterize day-light availability in the room (Seidl, Aydinli, Van Ber-gem-Jansen and Keller, 1992), (German standard DIN5034, 1985).

4.8 Sky conditionThe sky condition is a relevant aspect for the assessmenton daylight in interiors. Obviously, the momentary skycondition is not very useful for a long-term estimation ofthe energy-saving potential. Therefore, average daylight-availability data is needed. Serving this purpose e.g. inGermany an average sky type is introduced in a Germanstandard (Aydinli, 1981), (Seidl, Aydinli, Van Bergem-Jansen and Keller, 1992) and (German standard DIN5034, 1985). For other countries, a kind of standard skymay be calculated with data on the local daylight avail-ability.

4.9 Relative usable light exposureThe relative usable light exposure shows how much ofthe required design illuminance level to be maintained atthe work plane can be achieved by the available daylight.If the lighting level can be adjusted by dimming in differ-ent steps, economical benefits referring to daylight utili-zation in buildings are to be referred to the quantity of theannual relative usable light exposure (Huse,A,rel ), that canbe calculated with Eq. (2), (Seidl, Aydinli, Van Bergem-Jansen and Keller, 1992) and (Aydinli and Seidl, 1986)

%

)(

,, 100NtE

dtEN

HAWd

T

T

P

12

1ii

relAuse

E

B ⋅⋅⋅

⋅⋅

=∫∑

= (2)

Ni Number of working days per monthNA Annual number of working daystwo = TE - TB Hours of daily working timeTB Begin of working time (e.g. 800 LTT)TE End of the working time (e.g. 1800 LTT)EP Daylight illuminance at a point in the

interiorEd Design illuminance

While calculating the relative usable light exposure withEq. (1) it is important to use daylight illuminance valuesEP from the range

sp EE ≤ only. Otherwise for values within

the range EP > ES one has to calculate with EP = ES. Theusable light exposure can be interpreted as the part of theartificial lighting that can be replaced by daylight.

4.10 Room-potentialUsing daylight responsive dimming, the room-potential isgiven as the mean value of the annual relative usable lightexposure over the room area A, Eq. (3).

A

dAH

H A

rel,A,use

rel,A,use

∫ ⋅= (3)

In this context, not all formulae will be presented; fordetails refer to (Seidl, Aydinli, Van Bergem-Jansen andKeller, 1992). Since dimmable electronic ballasts arecommon and many light sources are dimmable, the day-light responsive switching of artificial lighting can no

longer be seen as state of the art. Therefore it is not men-tioned here. Comparing Figures 5 and 6 one can observethe influence of the obstruction on the spatial distributionof the relative usable light exposure related to the tworoom-zones, calculated for an office test-room at TUB(vertical orientation, large window, Ed =500 Lux andworking time 9–1800 LTT) for the two cases: without andwith obstruction of 300), related to a dimming controlsystem for the artificial lighting (Aydinli, 1986)). Theobstruction’s influence on the room-zone potential at thewindow is distinctive to the zone in the room depth, asone can see in the Figures 5 and 6, for the room-zonedistant to the window, the obstruction leads to a moresignificant reduction of the room-zone potential(

2RZreAuseH ,,,) than for the window-near zone (

1RZreAuseH ,,,).

Figure 5 Relative usable light exposure without obstruc-tion

Figure 6 Relative usable light exposure with obstruction

Huse,A,rel [%]

Huse,A,rel [%]

The relative usable light-exposure in the room-zones isthe quantity that can be increased by the installation of adaylight-system.

The spatial distribution of the relative usable light expo-sure for one of the TUB test-rooms with large windowsize and a design illuminance of 500 Lux was calculatedfor the sensor locations in the working plane, Table (1).

Table 10,6 m 1,76 m 2,9 m

0,88 m 89,6 % 90,2 % 89,6 %2,04 m 83 % 84 % 82,9 %3,2 m 76,6 % 77,2 % 76,3 %4,36 m 72,8 % 73 % 72,5 %

The room-zone potential for the two room zones (RZ1and RZ2) than can be calculated as the average value ofthe usable relative light exposure per room-zone.

RZPRZ1 = Huse,A,rel,RZ1 = 86 %RZPRZ2 = Huse,A,rel,RZ 2= 75 %

4.11 Daylight Availability and Energy Saving PotentialDescribing the amount of daylight availability in a spe-cific room, one can use different methodologies. Theused method always stands in relation to the kind of sys-tem to be applied in the day lit room. For a dimmingsystem, the relative value of the usable light exposure isimportant, for switching systems the relative usable timeis relevant.

4.12 Room-zone potentialThe room-zone potential is used to describe the daylightavailability in the room-zone, i.e. the pre-given energy-saving potential to be used by control systems for thatregion. For more detailed calculations, the control anddimming characteristic of the artificial lighting controlhas to be taken into account.

Following it is assumed that the lighting design is doneproperly, because it can actually influence the real energysavings positively as well as negatively. The room-potential (related to the daylight availability) is influ-enced by the following parameters, which naturally influ-ence also daylight availability in the room:

• Local geographic latitude• Local daylight availability (referring to the mete-

orological data e.g. sunshine probability and theaverage sky condition)

• Installed daylight system• Building, room and window properties (e.g. win-

dow size and orientation, obstruction• Working time• Design illuminance

The calculation of the room potential considering allaspects and referring to the average sky condition is quitecomplex. Therefore, it is recommendable to calculate itwith a computer program.

4.13 System-potential per room-zoneThe room-zone system-potential characterizes the effec-tiveness of a certain system in a certain environment. Theformer approach of defining a term of a quantity, de-scribing the system, the system-potential (Knoop, 1998)was newly defined and extended to a room-zone relatedpotential. The room-zone system-potential quantifies,how well the installed system is able to realize the offeredenergy savings by the room potential. An ideal controlsystem has for every system-potential per room-zone avalue of 100%, but in practice most real control systemscannot maintain ideally the pre-set design illuminancewithout a certain illuminance deviation over its operatingtime. Therefore it is necessary to introduce quantitiesdescribing these deviations from optimal operation.

The system-potential per room-zone of a daylight de-pendent lighting control system can up to now only beassessed by measurements in special equipped test-rooms, which have to be able to measure the distributionof the daylight contribution on the illuminance on theworking plane and the power consumption per luminairerow. It is the resulting quantity of the entire system, in-cluding the sensor position, its environment and the cali-bration and strategy, and therefore cannot be pre-given bythe manufacturers. The room-zone system-potential forone part is decisively (mainly) influenced by the way ofthe daylight dependent control of the artificial lighting:

• Daylight dependent dimming or switching• Daylight dependent dimming with respect to the

room depth• Control strategy (open or proportional or non-

proportional / integral reset closed loop)• Comfort functions for higher user acceptance

(time-delays, smooth or slow dimming etc.)

Furthermore, the quality of the correlation between thesensor-signal and the illuminance to be controlled has astrong impact on the control system’s system-potentialper room-zone. The mentioned correlation, influenced bythe construction, the spectral responsitivity and the posi-tion of the sensor, has an impact on the room-zone spe-cific system-potential (Belendorf, Aydinli and Kaase,2000).

Measurements performed at the lighting department ofTUB have shown that the room-zone system potential ofa daylight-responsive control for artificial lighting, thatdims per luminaire band (room depth depending) andable to switch off, can achieve high system potentials perroom-zone for well-calibrated systems. The describedtesting can also be performed in combination with day-

light systems under various sky conditions. The per-formed measurements take place in various test rooms, inwhich different kinds of daylight responsive lightingcontrol systems are implemented (Knoop, 1998), (Belen-dorf, Sit, Aydinli and Kaase, 1998), (Belendorf and Ay-dinli, 1998).

5. PRACTICAL ASSESSMENT

5.1 Practical orientated requirementsBased on a method to assess lighting control and energy-performance that uses the exact nighttime illuminancedistribution, a practical method was worked out, usingdistinctive (practical oriented) reference values for light-ing. During its operating hours, the daylight-responsivelighting control has to maintain certain requirementstowards distribution and levels for the lighting corre-sponding the official guidelines (e.g. German standardDIN 5035).

In the test-rooms of the lighting division of TUB, theilluminance distribution in the working plane (height 0,85m) is measured in a 3x4 sensor grid. The reference illu-minance can be calculated from the nighttime illuminancedistribution that has to fulfill the local guidelines for theartificial lighting design. Referring to this, the relevantreferences are the average illuminance (calculated perroom-zone) and the minimal illuminance (of the room-zone), i.e. the maintaining of a defined uniformity. Fortest-rooms with two luminaire-rows a division in tworoom-zones is recommendable, but for the test of systemswithout room-depth dependent dimming, the whole roomcan be defined as one zone.

The following assessment calculations can be carried outfor each room-zone. The assessment procedure is dividedinto different assessment steps, calculating the describingquantities.

5.2 Reference-conditions / criteria for lighting qualityIt is assumed, that the artificial lighting installation ful-fills the requirements corresponding to the local guide-lines (nighttime light distribution). Therefore, the serviceilluminance Ed can be calculated from the spatial averageilluminance per room-zone for the nighttime dimming-state, Eq. (4). Within the test-procedure at TUB, a simpli-fied view is applied, which not considers planning fac-tors.

∑=

⋅==sN

1inighti

s

rzrzxnightavrzxd E

N

nEE ,,,,

(4)

Ns Number of sensors in the roomEi,night Nighttime illuminance caused by artificial light

at the sensor position irzx A certain room-zone xnrz Number of room-zones in the roomEi Illuminance at the sensor i

With a defined service illuminance, the German standardgives recommendations for the minimal admissible illu-minance in the room-zone, in order to keep certain uni-formity in the light levels. Referring to the average valuecalculation, the requirement in Eq. (5) is:

rzxdrzx E80E ,min, , ⋅≥ (5)

Actually, the minimal illuminance in the test-rooms ishigher than the standard requires. In this case the specificnighttime relationship between average and minimalilluminance is the reference.

The following figures for a control system (open loopcontrol, EIB bus based, date: 28.01.1999, overcast sky,small window size) show the course of the relative aver-age and the relative minimal illuminance of the differentroom-zones in relation to the power consumption (Figure7 and 9) and enlarged (Figure 8 and 10).

0%

20%

40%

60%

80%

100%

120%

140%

160%

8:00 10:00 12:00 14:00 16:00 18:00Time h:m (LTT)

Rel

ativ

e va

lue

E ave,rz0,relE min,rz0,relP LB 1 relP LB 2 rel

Figure 7 Open loop control, 1 room-zone division

90%

95%

100%

105%

110%

8:00 10:00 12:00 14:00 16:00 18:00Time h:m (LTT)

Rel

ativ

e va

lue

E ave,rz0,rel

E min,rz0,rel

Figure 8 Open loop control, 1 room-zone division (enlar-ged)

A more detailed analysis of the system’s behavior ispossible looking at the two room-zones seperately.

0%

25%

50%

75%

100%

125%

150%

8:00 10:00 12:00 14:00 16:00 18:00Time in h:m (LTT)

Rel

ativ

e va

lue

P LB 1 relP LB 2 relE ave,rz1,relE min,rz1,relE ave,rz2,relE min,rz2,rel

Figure 9 Open loop control, 2 room-zone division

In Figure 10 one can analyze the calibration for the twoluminaries. Obviously in the window-near room-zone, thedimming function needs to be recalibrated. The criteria tothe minimal illuminance is stricter than the average illu-minance criteria. A fulfilled average criterion not alwayssignifies that the criterion for the minimal illuminance isfulfilled. The view at the courses for the individual room-zones permits a detailed analysis and shows the quality ofthe calibration.

85%

90%

95%

100%

105%

110%

115%

8:00 10:00 12:00 14:00 16:00 18:00Time in h:m (LTT)

Rel

ativ

e va

lue

E ave,rz1,relE min,rz1,relE ave,rz2,relE min,rz2,rel

Figure 10 Open loop control, 2 room-zone division, (en-larged)

Normally a reference case defined through the nighttimestate with 100 % lamp light-output is assumed. The con-trol system for the whole operating-time has got to main-tain the following conditions for every room-zone.

Condition 1:

∑=

≥⋅=nrz

N

1irzxdi

s

RZrzxav

S

EEN

nE ,,

(6)

Condition 2:

nightrzxrzx EE ,min,min, ≥ (7)

These conditions differ from former approaches in whichthe exact nighttime reference illuminance distributionwas the reference. The presented criteria are more rele-vant towards practical considerations. It is obvious, thateven the best daylight-responsive control system cannotbe able to maintain a specific nighttime reference illumi-nance distribution during daylight-availability time. Most

of the working time in day-lit rooms illuminance is amixture of artificial and daylight.

Decisive for judging the lighting quality is the relation ofaverage and minimal illuminance in the work-plane (uni-formity criterion). Assuming that the room is divided intonRZ room-zones, the quantities can generally be calcu-lated using Eq. (8).

∑=

⋅=RZ

s

n

N

1ii

s

RZRZd E

N

nE ,

(8)

[ ] RZ

S

n

N

1ii1RZ EE == minmin,(9)

The requirements generally can be expressed as:

{ (Ei,RZ1≥Ed,rz1) ∧… ∧ (Ei,RZnrz≥Ed,rznrz) } (10)∧{ (Emin,RZ1≥Emin,RZ1,night) ∧ .. ∧ (Emin,RZ2 ≥ Emin,RZ2,night)}

Referring to the TUB measurements, two room-zoneswere defined, from which each one has its own referenceilluminance, e.g. the design illuminance for room-zone 0(rz0) one calculate with:

∑=

⋅=SN

1iinightart

S0RZd E

N

1E ,,,

(11)

In the tests, illuminance distribution is measured; thecourse of the average illuminance per room-zone is theinteresting quantity.

∑=

⋅=S

RZ

N

n

1ii

S

RZ1RZ E

N

nE (12)

∑⋅=S

S

RZ

N

N

ni

S

RZ2RZ E

N

nE

(13)

Considering the actual uniformity requirement of thelighting installations, the criteria are even stricter than theDIN 5035 prescribes for the uniformity. Assessmentcriteria has always to be based on the higher quality crite-ria, i.e. the actual minimal nighttime illuminance of therespective room-zone.

According to the room-zones different illuminances areconsidered. Based on former calibrations, daylight- andartificial light contribution on illuminance can be deter-mined. Distinguished by the contributions, average day-light- and artificial light per room-zone is measured, e.g.for artificial light:

∑=

⋅=6

1iiart1RZart E

6

1E ,,

(14)

RZx,dayRZx,artRZx EEE += (15)

In practical lighting design, the design-illuminance is sethigher than necessary, in order to compensate the lumendecrease of the lamps. In this inspection this effects willnot be considered, because it is assumed that the testmeasurements take place in a short period of time in

which the lumen depreciation can be disregarded. In thepractical design process the consideration is necessary.

6. ASSESSMENT QUANTITIES

Forming part of a comprehensive method to predict theenergy saving potential for a control system in an office,the assessment quantities are to be calculated. The room-zone system-potential is necessary for the calculation ofthe energy savings. In order to describe the accumulatedbehavior of the control system towards lighting controlquality towards the reference illuminance, the light-exposure quantities are introduced. Special care has to betaken referring the mathematic definition of the quanti-ties, because they form part of the comprehensive methodand have to be consistent with the criterion in Eq. (1).

5.3 Relative room-zone shortcoming light-exposureThis light exposure quantity is used to describe the cu-mulated amount of artificial light, which the system didnot provide sufficiently (lacking light-exposure). Valueshigher than zero mean that the reference illuminance wasnot always maintained properly. The relative short com-ing light-exposure has to be calculated separately for eachroom-zone. Doing so, a detailed analysis of the lightingcontrol system’s behavior in relation to the daylight con-tribution is possible. The calculation has to be done onlyfor defined time intervals, i.e. the operating time in whichdaylight is available but is not sufficient to maintain therequirements on lighting conditions (referring averageand minimal illuminance). That means to take only illu-minance at times in which artificial light is necessary anddaylight contribution is greater than zero.

For a room-zone the relative shortcoming light-exposurecan be computed referring to the average illuminance aswell as to the minimal illuminance referring to the refer-ences. The short coming light exposure for the room-zonereferring to the average illuminance can be calculated to:

∑

∫⋅

⋅

='iRZ,d

)t(

1RZ

1RZ,SLEA tE

dt)t(

H'

ψ(16)

with t’∈ {t with 0 < Eday,ave RZ1 < Ed,rzx,RZ}

−

=0

tEEt 1RZaveRZrzxd

1RZ

)()( ,,,ψ

1RZnenn1RZ

1RZnenn1RZ

EE

EE

,

,

≥<

∑=

=6

1ii1RZave tE

6

1tE )()(,

The relative shortcoming light exposure for the room-zone can also be calculated referring to the minimumilluminance (HSLEM,RZ1). The amount of this light-exposure can serve for itself as a sufficient criterion for agood calibration of the system. Maintaining the criteriatowards the minimal illuminance includes in the mostcases the maintaining of the condition for the average

illuminance. The requirement of the minimal illuminanceis the stronger criteria. Verification of the calibration canbe based on the minimal illuminance.

5.4 Relative exceeding light-exposure per room-zoneThis quantity describes the relative accumulated amountof artificial light, that the control system provides unnec-essarily referring to the guidelines (exceeding the refer-ence). Correspondingly, the relative exceeding light ex-posure has to be calculated for each room-zone. Thecalculation only has be performed for defined operatingtimes, i.e. time when daylight is available and one of thefollowing two situations in the considered time intervaloccurs:

Situation 1: The daylight quantity in the room-zone is notsufficient for maintaining the lighting requirements.Therefore, it is necessary for the concerned luminaire-row to supplement artificial light.

Situation 2: The daylight quantity in the room-zone issufficient for maintaining the presented lighting require-ments, but nevertheless the control system provides arti-ficial light unnecessarily.

As done with the relative shortcoming light exposure, therelative exceeding light-exposure can be calculated forthe minimal and the average illuminance per room-zone.The minimal admissible illuminance is a stricter criterionthan the requirement to the average illuminance; thereforethe relative exceeding light exposure related to the mini-mal illuminance permits a good analysis. The relativeexceeding light exposure referring to the minimal illumi-nance for room-zone 1 can be calculated with:

∑

∫⋅

⋅

='iRZmin,

)t(

1RZmin,

1RZ,ELEM tE

dt)t(

H'

ε(17)

−

=0

EtEt 1RZ1RZ

1RZmin,min,

min,

)()(ε

refRZ1RZ

refRZ1RZ

EE

EE

,min,min,

,min,min,

≤>

Note that only the defined time intervals t’ are to be con-sidered for the calculation process.

5.5 System-potential related to the room-zonesThe system-potential for this approach has to be calcu-lated for each room-zone. The calculation relates only tothe operating time defined above. The calculation can beperformed referring to the average illuminance referenceas well as to the minimum illuminance reference for theroom-zone. For the energy estimation using Eq. (1) thesystem-potential has to be calculated for the averageilluminance.

If the condition towards Emin,RZ is fulfilled, automaticallyusing a correct calibration method, the design illuminance

that corresponds to the average illuminance of the room-zone will be maintained.

The system-potential can be related to the minimal andthe average illuminance course of the room-zone. Thecalculation for the room-zone system-potential (Eq. (18))is based on a time integration of a term XRZx representingweighted deviations between the daylight illuminance,the reference artificial light illuminance and the actualilluminance caused by the artificial light control system.

System-potential per room-zone

∑

∫ ⋅

=Γ'

)( '

)(

i

t

RZx

RZx t

dttχ(18)

The whole control system’s behavior in its installed statein the room with a number of nRZ room-zones (i.e. thenumber of luminaire rows) can be described through asystem-potential vector.

System-potential vector { }RZn1 ,...,ΓΓ=Γ

r(19)

Correspondingly, a vector description for the results forthe relative exceeding and shortcoming light exposurescan be given:

Vector of the relative shortcoming light exposure{ }

1RZn,SLE1RZ,SLESLE H,...,HH =r

(20 )

Vector of the relative exceeding light{ }

1RZn,ELE1RZ,ELEELE H,...,HH =r

(21)

In summary, the behavior of the whole lighting controlfor all the relevant room-zones can be analyzed and de-scribed using a (3 x nRZ) – system-matrix:

ΓΓ=

Γ=

RZ

RZ

RZ

RZnELE1RZELE

RZnSLE1RZSLE

RZn1RZ

ELE

SLE

HH

HH

H

HSysM

,,

,,

...

...

...

r

r

r

(22)

The room-zone system potential values can be used todraw conclusions about how good the system can makeuse of the offered room-zone potential. e.g. 1RZx =Γ sig-nifies that the system made optimal use of the offeredroom-potential

Cumulative frequencies of the illumination deviationsTo assess the control performance of the system withrespect to the deviations from the reference illuminancein a qualitative manner, the presentation of cumulativefrequencies related to the relative short-coming or ex-ceeding light exposure per room-zone is used.

The course of the cumulative frequencies for an example(open loop control system, see also figures 7-10) for thedeviation from the reference illuminances with respect tothe average illuminance consideration (1 x room-zonedivision) is given in Figure 11 and 12 (Ed,rz0 = 730 Lux,Energy-savings: ESLB1 = 31,94%, ESLB2 = 2,96% andEStotal = 17,35%, test-day: 28.01.1999, overcast sky,small window size)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 5% 10% 15% 20% 25% 30%Short-coming of illuminance

Cu

mu

lati

ve f

req

uen

cy o

f sh

ort

-co

min

g

Figure 11 Short-coming light exposure for a open-loopcontrol system for the average-illuminance related toroom-zone 0 (Design illuminance 730 Lux).

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 5% 10% 15% 20% 25% 30%Exceeding of illuminance

Cu

mu

lati

ve f

req

uen

cy o

f ex

ceed

ing

Figure 12 Exceeding light-exposure for a open-loopcontrol system for the average-illuminance related toroom-zone 0 (Design illuminance 730 Lux).

5. CONCLUSIONS

The comprehensive method enables the prediction ofenergy-savings and lighting quality by a certain daylight-responsive lighting control system for a defined environ-ment. The different factors influencing the overall-energyperformance are discussed. Based on a few test-measurements, general quantities describing the entiresystem’s behavior can be computed. Furthermore, differ-ent systems, which are to be applied in a defined envi-ronment can be compared to each other and the assess-ment of the lighting performance meets practical re-quirements.

6. ACKNOWLEDGEMENT

A PhD study funding from the Siemens AG, Germanyand a project funded by the German BMBF/BMWi sup-ported this work.

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