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Light is life – its energy makesour presence on earth possible inthe first place.

But light is also philosophy andart, rouses emotions andgenerates moods.

How can we capture thisphenomenon? Can it beexplained, defined, measured?

Light is difficult to explain evenby help of theoretical physics.Moving on to the practical side ofqualitative and quantitativecollection, things get even moredifficult. With every further stepof technology, these problemsget easier and better to solve.

When subjective impressions addon, one and the same actual factsor lighting situation, respectively,can be valued completelydifferently. That makes thingscomplicated but also veryinteresting.

Good light – good lighting – doesnot catch the eye, but whenevergood light is missing theatmosphere is kind of wrong, onedoes not feel good. This isintuitive knowledge.

The concept of good lighting incontradiction to that, needssound standing proficiency oflighting technology. The basicsthereof are quoted in this RadiumLichtbrief (light letter).

General Lighting Technology – Basics

What is Light?

Light is the wave lengths orfrequency range of electromagneticradiation which can be received bythe eye, i.e. can be seen.

Relationship between wavelength (ë) and frequency (f):

ë = cf

The propagation velocity c of thisradiation is constant within vacuumand constitutes to about 300,000km/s.

Light velocity in vacuum:c = 299 792 458 m/s

The radiation spectrum of visiblewhite light – like light of incandes-cent lamps or daylight at noon of asunny day – consists of singlecoloured (monochromatic) fractionsof radiation. These are allocated tocertain wave lengths which reachfrom violet at about 380 nm (1nm =10-9 m) to dark red at 780 nm.

The short-waved UV-radiation,visible light and long-waved infraredradiation can be called ‘opticalradiation’ altogether.

Electromagnetic radiationLight is only a small part of the natural electromagnetic radiation

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Perception of light

Biological sensor technology

The eye is the organ by whichhigher developed creatures canperceive light. In this process, thesensitivity of the sensor is not thesame always: identical radiationpowers in different wave lengths areobserved differently. So, a humaneye senses green-yellow lightespecially bright, on the other hand,short-waved blue light at the sameradiation power only quite weak.

To the contrary, insects see withinlong-wave UV and the adjacent bluerange very well, yellow, however, isquite bad. Furthermore, eyesensitivity differs individually, too,but there are mean values whichcan and have to be used asorientation by technology.Due to this biological componentabsolute measurements in thedomain of lighting are much moredifficult than cases in classicalphysics.

Whereas a length in meters ormillimetres can be given veryexactly, meaning with smalltolerances, lighting technology mustlive with bigger tolerance ranges:How bright is bright?

Eye and process of sight

Electromagnetic radiation of certainwave lengths is perceived by thehuman being as light, only if enoughradiation power gets onto the retinaof the eyes and there are readyreceptors available.As this eye sensitivity is different forthe various wave lengths theperception of brightness varies evenat the same radiation power.

On average, the eye is adapted tolight when lighting is good (day), i.e.at high illuminance. With this photo-topic vision the 3 kinds of cones onthe retina are active – they are thereceptors for red, green and bluewhich makes them responsible forcoloured vision. Then, the curve ofeye sensitivity follows the so-calledV(ë)-function which has got itsmaximum at about 555 nm.In actual fact, every kind of conehas got its own sensitivity curve:

red x (ë), green y (ë), blue z (ë)

For practical use, with sufficientaccuracy the curve of their meanvalues V(ë) equals the green curvey (ë).

When light gets low (at night) theeye is dark adapted.

With scotopic vision the light-darkperception dominates, which iscontrolled by the rods on the retina.

The eye sensitivity curve moves asV’(ë) into a more short-waved rangeand then has got its maximum atabout 507 nm. In the mesopic zone(twilight) between day and nightvision the eye cannot align clearly,therefore, many human beings havegot optic difficulties.

Light and Colour

The human eye evaluates incominglight not only according to theimpression of brightness but alsoaccording to colours. The cones dosomething like additive colourmixing with the light impressions, sothat finest differences can bedetected.

If you do not take brightness intoaccount, the x-, y- and z-curves arethe so-called standard spectralpower functions of a light sourceand always sum up to 1. Therefore,it is enough then to quote themeasured x- and y-values of a lightsource in order to describe itscolour. These values are shown inthe standard colour graph.

As indicated by the laws of additivecolour mixing, white light isgenerated if all colours are includedin the same proportions. Therefore,the white point on the standardcolour graph emerges at x = 0,333and y = 0,333.

Colour temperatures – another wayof describing the light colours – areto be found on Planck’s curve andJudd’s straight lines.

For light sources which do not havechromaticity coordinates on Planckbut quite close, a so-called mostsimilar colour temperature can bespecified. Its value corresponds withthe point of Planck’s curve wherethe chromaticity coordinates of thelight source are nearest (minimaldistance).Eye sensitivity curve

The eye perceives equal radiation powers differently at different wave lengths

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Light and Matter

Reflection – Absorption –Transmission

In order to see an object, light mustfall on it, this light must be reflectedby it and the get into the eye. Does

an object let pass the light all in allor in parts – by technical term calledtransmission – this object appearstranslucent or transparent. Does theobject, however, absorb the lightfully or parts of it – absorption – itappears bright or dark, dependingon how much light it has absorbed.

Standard colourgraph according toDIN 5033Every light colour hasgot chromaticitycoordinates and canbe described exactlyby those

Extract from thecolour trianglePositions of typicallamp light colours onthe standard colourgraph

The basic properties reflection,absorption and transmission can bedifferent with different parameters:

Material structure and texture:The more penetrable a material isfor radiation, the more probabletransmission becomes (e.g. clearglass vs. satin/ frosted glass)

Surface:The smoother, the more probablereflection (e.g. mirror)

Angle of radiation:For example, total reflection atwater surface when looking at a flatangle

Spectral power:Please, refer to colour rendering

Within the lighting business we liveand work with that every day, forexample when measuring light (inthe Ulbricht sphere) or lighttransportation (through glass fibres)and also in light planning (surfacesin rooms), of course.

Colour Rendering

See colours as they really are

Not only light itself has got colour,by help of light the colours of theobserved objects shall be visible.This can only be possible when inthe spectral power distribution of thelight source the corresponding wavelengths exist. Therefore, for perfectcolour rendering only light sourceswith continuous spectral powerdistribution are suitable (like thermalradiators such as incandescentlamps). In other cases, the eye canperceive colours only with a change.

Example: Clothes Shopping

A shop is lighted by triphosphorfluorescent lamps, exclusively. Alight grey pair of trousers and a lightgrey patterned shirt fit and matchperfectly within the fitting room. Athome surprise hits: when unpackingat daylight the trousers have got ahint of green and the shirt is in factbrownish with a hint of red.

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Measurement and ColourRendering Index

The colour rendering of a lightsource is assessed by acomparative measurement. In thisprocess the reflective spectralpower distributions (remission-spectrum) of 8 certain colours aremeasured twice: first, when lit by astandard light source and secondwhen lit by the lamp to be valued.Additionally, the light colours of thetwo light sources shall be as similaras possible, the same at best.

These remission spectra are thencompared in an elaboratecomputing process for every singlecolour. Are they identical, the colourrendering index Ra is 100. Is thecolour change substantial, thecolour rendering index can evenbecome negative.

From the Ra-values of the singlecolours an all-over value can beestablished for the lamp to bevalued. Additionally, colourrendering of another 6 colours(sometimes even 8) might beinteresting for certain applications.

Colour Rendering andColour Fastness

Colour rendering does not haveanything in common with colourfastness or yellowing. Colourrendering means to be able to seecolour as they really are. Colourfastness, to the contrary, meanscolours stay as bright as they havealways been and do not yellow inspite of all the different ambientinfluences (light, heat, UV-radiation,humidity, chemicals, etc.).

Colour Rendering R9-R14

Test colours 9-14:RedYellowGreenBlueSkin colourLeaf green

Colour Rendering R1-R8

Test colours 1-8:Dusky pinkMustard yellowYellow greenLight greenTurquoise blueSky blueAster violetLilac violet

Spectrum Incandescent LampSpectral power distribution of incandescentlamps

Planck Distribution CurvePlanck distribution curve for different ‘black body temperatures’

Light Generation

Properties of Different LightSources

Thermal Radiation

A thermal radiator becomes so hotthat it emits energy in the form oflight such as fire or incandescentlamps.

Thermal radiators have gotcontinuous spectrum, i.e. they emitat all wave lengths. Which wavelengths are more and which onesare less included depends on thetemperature of the radiator. So, withevery change of temperature thedistribution of radiation power withinthe spectrum changes.

Planck enunciated on the results ofsome predecessors (mainly Wien’sdisplacement law) the theoreticalphysics’ basics of light colours.That’s how one can explain colourtemperatures of light: a black bodyis heated up until it emits visibleradiation – light.

The respective temperature of thebody is taken for describing thecorresponding light colour.

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As sunlight changes all the time,however (activities on sun surface,filter earth’s atmosphere), it cannotbe used for measurement or stand-ard means. The sun is definitely nostandard light source.

Unfortunately, a standard lightsource comparable to the suncannot be made artificially, there aresimply no suitable materials orstable chemical elements available.

For manufacturing filament wires forincandescent lamps tungsten isused, already, the element with thehighest melting point.

By exhaust of all possibilities – alsowith halogen processes etc. – inpraxis incandescent lamps withcolour temperatures up to 3,400 Kare feasible, only.

Isotherms of the spectral radiance distribution for tungsten as emitterDiagram of the radiation power of tungsten material at certain temperatures (isotherms) fordifferent wave lengths

Therefore, the rule applies – even ifit seems illogic at first glance:because the body needs to behotter for them, the cold (white)light colours have got higher colourtemperature values than the warm(yellow-red) ones.

Hence:The hotter a thermal radiatorbecomes the whiter is its light.

The surface of thesun, for example, isabout 6,000 K hot onaverage, but offersvery pleasant light forthe human eye sinceits radiation maximumis at about 500 nm.

Spectrum of Sodium Vapour LowPressure LampSpectral radiation power distribution ofsodium vapour low pressure lamps

Spectrum of Mercury Vapour LampSpectral radiation power distribution ofmercury vapour lamps

Colour TemperaturesThe hotter a black body thewhiter the light emitted

Gas Discharge

All chemical elements can bestimulated by energy supply andrelease this energy again whenfalling back to the original state byemitting radiation. The configurationof this radiation is typical for eachelement, for example, mercury (Hg)emits mainly in the UV-range, othersubstances such as sodium (Na)yellow in the first place.

The spectra of these light sourcesare discontinuous, they can be evenmonochromatic, i.e. containing onlyone wave length, such as sodiumvapour low pressure lamps. Byexternal circumstances thespectrum can be influenced further:for example, with sodium highpressure lamps the sodium line iswidened by the pressure in theburner. Addition of mercury createsthe needed blue quantities.

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Luminescence Processes

Luminescence is a description forall processes in which a non-luminous substance or body emitsradiation due to chemical reactionor activation by light.

Best known here is fluorescence:Some materials can glow eventhough they themselves cannotgenerate light. They absorb short-waved radiation (mostly UV) andtransform it to more long-wavedradiation (visible light).

This kind of luminescence is used ingenerating light essentially influorescent lamps. An uncoatedfluorescent lamp would be a violet-blue-ish UV-radiator (due to themercury content), the phosphorcoating applied to the glass bulbfrom the inside changes thisradiation to visible light.

The type of phosphor and its qualitydecide which light colour and howmany spectral lines in the spectralpower distribution of the light of thelamp there are in the end.

LASER

= Light Amplification byStimulated Emission of Radiation

Whereas a gas discharge – forevery single gas atom – is aspontaneous emission of light, isthe light generating substance in aLaser triggered by an ‘opticalpumping process’ so that highenergy radiation emission emerges.The wave length of this radiationdepends on the employedsubstance combination. Today, anycolour is possible, even blue.

The generated radiation has got anextremely narrow radiation angleand can be focussed very well. Thisleads to very high power densitieswhich is chance and danger at thesame time: medical operations canbe performed by laser beam butmajor damages can also be caused,naturally.

LED / OLED

Light Emitting Diode

The light of LEDs is generated by asemiconductor chip: a pn-junction isstimulated and then emits light atgreat intensity, quite similar to laserradiation. Therefore, LEDs wereclassified under the radiationprotection standard EN 60–825–1like Lasers. Due to less power, i.e.radiation power, and enoughmaterial around the light generatingchip LEDs are always within theboundaries stated by the standard,hence they do not need to beincluded into the same dangerclassification.

Furthermore, as normally additionalscattering optics are in employment,there is no danger for the eye. Atleast no more than with otherlamps, either.

LEDs generate – like lasers –monochromatic light (so a certainwave length) according to theapplied substance combinationwithin the pn-junction.

In this process coloured light isgenerated directly and does notneed to be filtered elaborately, inthe first place.White light can be created by LEDsin two different methods: via RGBcolour composition of a so-calledmulti-LED with 3 coloured chips inone LED or by employment ofphosphors in the compound arounda blue chip.

In order to get reasonable values ofcolour rendering (Ra>70) for whiteLEDs with phosphors, usuallyyellow-orange and yellow-greenphosphor are applied together.

The advantages of LEDs are clearat hand: smallest design with verylittle connexion powers at longservice life make their employmenteven at difficult spots attractive. Nointerference by operational sounds,flickering, UV or IR radiation.

Only, unsuitable ambient conditionslike humidity or high temperatureslead to failure. Now, LEDs/ OLEDsare celebrated as ‘light source of thefuture’, but experts remain in doubt.Because expenses and costs forgeneral lighting by LED are stilldisproportionately high.

Functional Principle of thefluorescent Lamp

Spectrum of Sodium Vapour HighPressure LampSpectral radiation power distribution ofsodium vapour high pressure lamps

Structure of a LED

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4.

3.

WXn ➝➝➝➝➝ W + nX

2.

W + nX ➝➝➝➝➝ WXn

Organic LED

Research and development isworking very hard to produce LEDsfrom organic materials, too: giantmolecules and polymers canachieve similar qualities like thecomplex metal compounds intraditional LEDs.

Advantages are clear: everythingstays nice and environmentallyfriendly and some applications arerendered possible in the first place(e.g. luminous foils via layered thinfilm objects).

Little displays – unichrome or multi-colour – are not a problem anymore, with bigger areas there aremaybe difficulties with heatdissipation and, as a consequence,power losses.

Halogen Processes

Even though the wide spread ofhalogen lamps suggests thathalogens could be concerned withgeneration of light, this is not so.Halogens (group VII in the periodictable of elements – fluorine,

A tungsten atom disassociates from the filament

Halogen Circle

The tungsten atom is caught by the halogengas and bound in a complex

The tungsten atom is released again to thefilament by the halogen complex

Tungsten is on the filament again and thehalogen gas free in the bulb volume

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Tungsten (W)Halogen (X)

FunctionalPrinciple of MetalHalide Lamps

chlorine, bromine, iodide) aresupporting substances which havegot an influence on light generation.

They enable material transport, e.g.tungsten atoms in incandescentlamps and filament electrodes ofdischarge lamps in the so-calledhalogen circle. In this process atsufficient temperatures (filament >2,700°C, lamp bulb > 200°C)blackening is prevented andluminous efficiency enhanced.

That means in detail: the atomsevaporated from tungsten filamentare ‘caught’ by the halogenmolecules near the ‘cold’ bulb wallat about 300°C. This complexmoves around the lamp volume untilit gets into the vicinity of a ‘colder’part of the filament at about2,000°C. The bonding is undoneand the atom adsorbs itself to thefilament.

As this ‘cold’ spots are to be foundin the leg (ends) area of the filamentusually, a certain material transportis carried out. Therefore, flaws in thetungsten wire lead to ‘hot spots’where material is only carried awayand the lamp’s failure is pre-programmed.

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Standard Illuminants

Colour Temperatures of theStandard Illuminants

Since the colour temperature ofdaylight varies as well due to theposition on earth as due to thecourse of the day (3,000 – 9,000 K /blue) it seems sensible to definecertain benchmark values forstandards and measurements inDIN 5031 or 5033.

As mean daylight D65 wascharacterized, for reproductivepurposes (such as colour slides)D55 is applied, there is still D75 andalso sometimes C can be seen(old-fashioned). For incandescentlamps’ light the standard illuminantA is applicable which derives fromPlanck’s radiator.

The colour temperature values are:

D65 = 6,504 K

D55

= 5,500 K

D75 = 7,500 K

C = 6,774 K

A = 2,855.6 K

Furthermore, known standardilluminants are B (sun light),G (vacuum incandescent lamps),P (petroleum and candle light) andXE (xenon lamps).

Quantities and Units inLighting Technology

Description and Measurement ofLight

Luminous Flux Ö

The evaluated radiation power(impression of brightness) of a lightsource altogether, i.e. in alldirections, is called luminous flux.Its unit is the lumen lm.

So as to reproduce the impressionof brightness accurately within thehuman eye the pure spectral powerof a light source must be rated bythe eye sensitivity curve V(ë), i.e.multiplied with it. Therefore, thephotometric radiation equivalent K

m

is needed which is defined for themaximum of the V(ë)-curve.

A monochromatic emitter at a wavelength of 555 nm with 1W radiationpower has consequently got 683 lmby definition.

Put differently:Km = 683 lm/W

If this calculation quantity is used fornight vision rated by V’(ë) it amountsto 1699 lm/W.For all light sources which do notemit mainly into one direction theirradiation power is given in lm (seeluminous intensity).

Luminous Efficiency

The luminous efficiency is ameasure for the effectiveness oflight sources. It shows how muchlight is generated by the exploitedelectric energy. Its unit is lumen perwatt lm/W.

Quantity of Light Q

The quantity of light is a name forthe rated radiation power in lmh orlms, so the radiation power in acertain period of time.

Q = Ö · t

This quantity is important for flashlights, moreover it helps with theenergy assessing of plannedlighting installations.

Luminous Intensity I

The radiation power of a lightsource into one direction in acertain solid angle Ù is calledluminous intensity. Its unit iscandela cd.

I = dÖdÙ

or approximately

I = ÖÙ

Thus, the solid angle can beimagined: a circle is cut out of thespherical surface of a ball. The cutcone down to the centre of the ballcan be imagined as an ‘ice-creamcornet’. The side areas of this coneenclose the solid angle, therefore, itis the ‘tip of the ice-cream cornet’.Its value in Steradiant (sr) can becalculated by the quotient of the cutout surface area and the square ofthe ball radius:

Ù = Ar²

Definition Solid Angle

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For all light sources that emit mainlyinto one direction (e.g. reflectorlamps) the luminous intensity in cdis quoted.The measured radiation distributionof light sources – especially lampsin luminaires – are shown in polardiagrams, the so-called lightdistribution curves.

Illuminance E

Whereas luminous flux andluminous intensity indicate theradiation power which emanatesfrom a light source, the illuminancetells how much radiation powerarrives at a certain recipient. Its unitis the lux lx.

E = ÖA

Normally, this rule applies, becausewe can assume a sufficiently evendistribution of luminous flux.

If the contemplated area is inclinedtowards the direction of the viewer,the tilt angle å must be considered:

E = Ö · cos åA

Therefore, it is crucial for theilluminance which area isconsidered in which plane. Inparticular for planning lightinstallations must be kept in mind ifthe horizontal illuminance Eh (ontables like in offices), the verticalone Ev (e.g.on the walls of a gallery)or the cylindrical one Ez (for good,vertical sight all around) isespecially important. Within thecorresponding planning programs isa mean, a minimum and amaximum illuminance calculated,respectively, in order to get animpression of the uniformity oflighting.

Examples of values of illuminances:

Summer day 60,000 – 100,000 lxHazy winter’s day 3,000 lxDesk place in office 500 lxFull moon night 0.25 lxNew moon night 0.01 lx

Luminance L

The impression of brightness givenby a certain area is calledluminance. Its unit is cd/m² or cd/cm².

I L = A

If the contemplated area is notobserved vertically, the deviationangle á must be considered:

I L = · cos á

A

The higher the luminance of anobject, e.g. of a lamp, the brighter itappears and it can rather causeglare.

Photometric Law ofDistance

The further off an object is from thelight source the weaker itsilluminance becomes. Or put moreexact: Its illuminance decreaseswith the square distance.

E = I · cos ár²

Light Exposure H

In photography everything dependson the all-over amount of light whichreaches the light sensitive area –the film – within a certain period oftime.

H = E · t = QA

Explanation Illuminance

Explanation Luminance

Light Distribution CurveExample of a LDC of a reflector lamp

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Light and Emotion

We need light for eye vision. Butthere is also a strong connection toemotion and well-being: lightcolours and lighting situations havegot a lot of influence on our feelings,mostly more subjective andindividual than can be measured orproven as a matter of fact.

Therefore, accepted findings usuallyderive from very elaborate statisticalstudies.

Warm colours (warm white,incandescent lamps’ light) areconsidered un-comfortable by mostpeople when illuminance excedes2,000lx and not only due to the highheat load.

Cold colours (daylight, coolwhite)are experienced quite unpleasant atilluminances under 1,000lx.Therefore, indoors the colourtemperature of the light should bechosen under 5,500 K or highilluminances greater than 1,000lxshould be planned right from thestart.

In fact, the sensation of brightnessalways depends on the luminance,but it cannot be measured exactlyby the human eye. That means, allthe time the eye compares betweenthe existing points and areas andtries to find a specific balance evenif there are strong contrasts.

Light Effects on aHuman Being

Biological Effects

Light controls our activity and hasgot its influence on our whole life.Especially the skin as an area organis exposed to the whole radiationwith UV and IR. As well, UVradiation is needed for triggeringcertain processes within the bodylike generation of vitamin D3. Toohigh doses, however, can causeconsiderable damages. Sun burn orconjuncitivitis are just a fewexamples of those injuries.

The human being as day activecreature is more energetic andmakes less mistakes with higherilluminance, even if the task itself isnot depending on the visual ability.This higher action is evenmeasurable, for example at thecomposition of the blood, there aremore white blood cells in it.

All in all, concentration is muchhigher, therefore, there are muchless accidents to be expected.Consequently, especially inoccupational safety and road trafficgood lighting must be observed.

Light Needs

Every human person has gotindividual needs as regardsillumination. So, it is statisticallyproven that at good lighting (about1,000lx) older people workapproximately as well as youngones. When lighting is bad the olderones make definitely more mistakes.Moreover, it is sensible to plan lightcontrolling installations so that theuser can have the possibility ofexerting manual influence.

Disturbance due to Light

Light and light sources cannot justbe help for eye vision, but also canbother significantly, for example aslight immission or by glare.

Optic IllusionOptic illusion: at the white crossing points theeye ‘sees’ dark spots

Moreover, higher heat load canarise especially due to incandescentlamps which has to be carried awaywith elaborate air conditioningtechnology.

Light immissions – meaningbrightness unasked for in the darkhours, e.g. indoors due to streetlighting – are just annoying most ofthe times, sensitive people can bedisturbed by that to a great intent.Relief can be brought quite easilyby suitable methods:

– Darken highly illuminatedbuildings at night

– Keep lighting constant and even,no use of dynamic light likebanners or animated colouredpictures

– Change positions of luminaires

– Change kind of luminaire:height, inclination, light distribution

Glare, on the other hand, is a bigproblem, not just uncomfortable butalso preventing people fromachieving.Scattered light, shine, mirror effects– i.e. reflexion in one direction – canlead to a very strong light beam intothe eye locally so that it must seeagainst the light almost. In roadtraffic or at dangerous tasks at workglare can be a great safety risk,therefore. Normally, it is ‘just’ verystrenous and tiring. Then, quicktiring and eye problems are theconsequences.

The more diffuse light is reflected(for example, off a white wall), theless it is conceived as glare.Further disturbances can emergefrom hard shadows at highilluminances, for example, or fromfrequency problems with dischargelamps when there are stroboscopiceffects arising from them.

No human being has to accept badlighting, there is always a possibilityto change at least something.Professional and independent lightplanning, good lamps andluminaires guarantee a comfortingatmosphere.