How Lutron’s DaylightingControl System Works

Pekka Hakkarainen, Lutron Electronics Co., Inc.

Technical white paper7 January 2005

1. Daylight sensors

Daylighting control employs ceiling mounted sensors thatactively measure available daylight, and are used for maintaining light level to within a predetermined range. In these cases, we decide on thecritical task surface or surfaceson which the target light level is maintained.

Generally speaking, photosensorsrespond to light that is incidenton the sensor surface. Otherthan direct sun light, which inmost cases we want toeliminate from the task surfaceanyway, the other source ofdaylight is the reflected (andrelatively diffuse) sunlight fromthe sky and from clouds. Indaylighting applications, thechallenge is to orient the sensorin such a way that it measuresthis reflected daylight inproportion to how it varies onthe task surface. The idealplacement is such that thesensor has a high level ofillumination from daylight, but is shielded from any exteriorglare sources.

2. Electric Lighting Control:Proportional vs. Set point control

The choice of the controlmethod for electric lighting playsa significant role as far as overallsuccess is concerned. A setpoint control method aims tokeep the photosensor signalconstant during the operation ofthe system – somewhat like athermostat – while aproportional control methodallows the signal to vary duringthe operation. It has been foundthrough practice that theproportional control method isgreatly superior to the set pointmethod in interior applications.In a proportional control system,the sensor signal varies duringthe course of the day, and it isimplicitly assumed that thephotosensor signal is mostlycomposed of the daylightcontribution. In some cases thesignal is a mixture of daylightand electric light (this is calledclosed loop proportionalcontrol), while newer methodsof control eliminate thecontribution of electric lightingto the sensor signal completely(known as open loopproportional control). Thus, theplacement, directionality andarea of view of the photosensorare closely linked with thechoice of the control method.Lutron uses the proportionalcontrol method, which can beconfigured as either open loopor closed loop.

When the control systemincorporates both windowshades and electric lights, weput the above control methodstogether and operate thewindow shades in a “set point”or “dead band” fashion, whilethe electric lighting is operatedin an open loop proportionalfashion. The term “dead band”rather than “set point” ispreferred, since it moreaccurately describes thepreferred implementation, wherethe sensor illumination isallowed to vary within a presetband without moving theshades. When the illuminationmoves outside this band, theshades are moved to bring itback in the center of the band.This operation ensures that theshades do not movecontinuously, which would beannoying to the occupants. Ifthe control system is forskylights that are covered withmotorized solar fabric, thephotocell has to be mounted inthe skylight well, preferablypointing towards the north sothat the sun does not directlyilluminate the sensor.

3. Principle of operation –proportional control systems

The photosensor converts theamount of light it detects to aDC current signal that varies, forexample, between 0 and 3 mA.In the proportional controloperating mode, which is usedwith electric light control, thephotosensor signal level is

Lutron | 1

allowed to vary during thecourse of the day. The outputsignal from the control systemthat determines the levelsettings for the electric lights isthen proportional to thephotosensor signal, the higherthat signal the lower the electriclight level.

The “dead band” control modeis used with the shade control.In this mode, three thresholdlevels are defined thatcorrespond to three light levelsincident on the photosensor.These levels can be thought ofas “goal level”, “too bright” and“too dim”. There is a finite gapbetween all these levels thatshould be defined to be largeenough to provide a hysteresisfor the system that preventslights and shades fromoscillating. When a thresholdlevel is crossed, the controlsystem issues a command thatchanges the lighting preset orthe shade position to move thesensor signal back to theacceptable range.

4. Location of the photocell– optimize signal relevanceand strength

The correlation of ceilingillumination near the windowsand desk illumination isgenerally not very good, butimproves when moving awayfrom the window. Locating thesensor about two windowheights into the room works wellin this case.

When window shades are used,this correlation is improved nearthe window, and the sensor canbe located so that it has a directview of the window beingcontrolled. In a side lightedroom with the sensor mountedin the ceiling, a good location isabout one window height intothe room from the window. Inthese cases, the same sensorlocation provides a good signalfor controlling the electric lighting.

5. Calibration issues

The initial installation of adaylighting control systeminvolves a calibration procedure.It is necessary to ‘tell’ thesystem when the targetillumination level meets goal,and this defines the contributionof electric light (output tolighting zones is of courseknown) required at a particulardaylight level, as measured bythe photosensor. In addition, anight time set point is needed,which defines the electric lightcontribution when no daylight ispresent. (This can also be doneusing blackout window shades,if they are available.) During thenight time calibration procedure,a further step should be takenthat improves the performanceof the system by making itcompletely open loop: When theelectric lights are at full, thesensor reading is recorded inthe control system. Thisinformation can then be usedduring the day to subtract theelectric light contribution from

the sensor reading at all times(by scaling from the full lightoutput using the known electriclight dimming level), therebymaking the system a true openloop proportional controlsystem. The reason why thiscontrol method is preferred isthat it improves the signal tonoise ratio of the sensor reading.

In addition to calibrating theelectric light output, whenwindow shades are used, thecommissioning agent needs todefine the upper and lowerbounds for acceptable windowluminance. These limits willdetermine how often the shadesmove as well as how bright thewindow will appear.

6. Photosensor characteristics

The characteristics of the Lutronphotosensor are listed below:a) spectral response is close to

human eye responseb) uses spatial cosine correction,

which properly accounts forlight sources at variousangles of incidence

c) 60 degree angle of view vertically, 180 degrees horizontally gives a wideangle of view appropriate forproportional control systems

d) the view is oriented to oneside, providing directionalityto the sensor and making iteasily adaptable to a varietyof mounting locations

e) large dynamic range (0 to2000 fc) and linear responsewithin that range

Tel: 610.282.3800International: +1 610.282.3800

World HeadquartersLutron Electronics Co., Inc. 7200 Suter RoadCoopersburg, PA 18036-1299 USA

Barcelona +34 93 496 5742Beijing +86 10 8525 1867Berlin +49 30 9710 4590Hong Kong +852 2104 7733London +44 (0) 207 702 0657Madrid +34 91 567 8479Paris +33 1 44 70 71 86Shanghai +86 21 6288 1473Tokyo +81 3 5575 8411

www.lutron.com©2005 Lutron Electronics Co. Inc.