natural light illumination system

Natural light illumination system

Allen Jong-Woei Whang,1,2 Yi-Yung Chen,1,* Shu-Hua Yang,1 Po-Hsuan Pan,1

Kao-Hsu Chou,1 Yu-Chi Lee,2 Zong-Yi Lee,1 Chi-An Chen,2

and Cheng-Nan Chen2

1Department of Electronic Engineering, National Taiwan University of Science and Technology,#43, Section 4, Keelung Road, Taipei, 106, Taiwan

2Department of Electro-Optical Engineering, National Taiwan University of Science and Technology,#43, Section 4, Keelung Road, Taipei, 106, Taiwan

*Corresponding author: [email protected]

Received 18 June 2010; revised 12 October 2010; accepted 12 October 2010;posted 18 October 2010 (Doc. ID 130198); published 9 December 2010

In recent years, green energy has undergone a lot of development and has been the subject of many ap-plications. Many research studies have focused on illumination with sunlight as ameans of saving energyand creating healthy lighting. Natural light illumination systems have collecting, transmitting, andlighting elements. Today, most daylight collectors use dynamic concentrators; these include Sun trackingsystems. However, this design is too expensive to be cost effective. To create a low-cost collector that canbe easily installed on a large building, we have designed a static concentrator, which is prismatic andcascadable, to collect sunlight for indoor illumination. The transmission component uses a large numberof optical fibers. Because optical fibers are expensive, this means that most of the cost for the system willbe related to transmission. In this paper, we also use a prismatic structure to design an optical coupler forcoupling n to 1. With the n-to-1 coupler, the number of optical fibers necessary can be greatly reduced.Although this new natural light illumination system can effectively guide collected sunlight and send itto the basement or to other indoor places for healthy lighting, previously there has been no way to man-age the collected sunlight when lighting was not desired. To solve this problem, we have designed anoptical switch and a beam splitter to control and separate the transmitted light. When replacing tradi-tional sources, the lighting should have similar characteristics, such as intensity distribution and geo-metric parameters, to those of traditional artificial sources. We have designed, simulated, and optimizedan illumination lightpipe with a dot pattern to redistribute the collected sunlight from the natural lightillumination system such that it equals the qualities of a traditional lighting system. We also provide anactive lightingmodule that provides lighting from the natural light illumination system or LED auxiliarysources, depending on circumstances. The system is controlled by a light detector. We used optical simu-lation tools to design and simulate the efficiency of the active module. Finally, we used the natural lightillumination system to provide natural illumination for a traffic tunnel. This system will provide a greatnumber of benefits for the people who use it. © 2010 Optical Society of AmericaOCIS codes: 220.2945, 220.2740, 220.4830, 080.3685, 230.1360.

1. Introduction

In response to the energy crisis, green energy hasbeen gaining popularity. This has led to increasinginterest in renewable energy, such as solar energy,wind energy, hydroelectric power, ocean wave energy,

geothermal energy, and energy from biomass. In re-search regarding the production of energy and itsuse, one of the most important topics is how to useenergy with the highest efficiency.

Among renewable energies, solar energy has manyadvantages. It is abundant, clean, and globally avail-able, so we can utilize it anywhere in the world. Inthis paper, our ultimate target is to determine howto collect sunlight for indoor illumination. Natural

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light illumination systems can be separated into col-lecting, transmission, and lighting components [1].In the collecting stage, we have designed a static con-centrator to collect sunlight for indoor illumination,which is prismatic and cascadable. For transmission,we have designed a geometrical fiber coupler unit. Byreducing multifiber cables to one fiber, we can satisfyenergy needs for indoor lighting. Although this newnatural light illumination system can effectivelyguide collected sunlight and send it to the basementor to other places indoors, there has previously beenno way to manage the collected sunlight when light-ing was not desired. We designed a light switch tocontrol and change the direction of the transmittedlight. To make up for the lack of artificial lightsources, we designed a beam splitter with high-passand low-pass dichroic coatings. It forces beams of dif-ferent wavelengths to travel in different directions.To maintain stable lighting throughout the day, wemust be able to compensate for when sunlight is notavailable; hence, electricity, LED lights, and sensorsare needed.

Currently, there are two kinds of natural light il-lumination systems, active systems and passive sys-tems. The difference between the active and passivesystems lies in the ability to track the Sun. Followingthe Sun’s position, the collecting part of an active sys-temwill track the Sun. This will include an electroniccircuit; in contrast, the passive system does not in-clude a circuit to actively track the sun. Active sys-tems include the following: monolens HIMAWARIsystems, which were developed in Japan in the late1970s by Kei Mori; hybrid lighting systems, from aU.S. partnership research project; SOLUX systems,a Fresnel lens-based day-lighting system developedby the German company, Bomin Solar Research(BSR); and heliostats. In a passive system, the mainapplications are made parts of the building, such aswindows [2]. This research paper discusses a naturallight illumination system that is a passive system.

Previous works on day-lighting system technologyinclude [3], which discussed two collecting methods:top lighting and side lighting. In [4], the experimen-ter’s experience proved that the installation of solarlight pipes in residences can greatly reduce the mor-bidity of the “seasonal syndrome.”

2. Indoor Lighting System

According to engineering concepts, any device can bedivided into three parts: input, output, and—between input and output—a black box, which doessomething.

Input includes natural light, which is collected by astatic solar concentrator, and electricity, which drivesthe LED auxiliary sources. The natural light illumi-nation system can be divided into three main struc-tures: a light collection unit, an optical transmissionunit, and a lighting unit. Sunlight is collected by alight collection unit and passes through the opticaltransmission unit to be guided into a room. Finally,it is emitted uniformly by a lighting unit to achievenatural light illumination (as shown in Figs. 1 and 2).

The collecting unit, called a static light concentra-tor, has a prism structure. It consists of two differentarrangements of prisms to change the direction of thecollected sunlight. Incident sunlight arrives from thetop of a plane. After being reflected and refracted, therays are finally directed from a square surface (asshown in Fig. 3). The output ray is guided to anindoor room for illumination via an optical fiber.

Fig. 1. Structure of an indoor lighting system.

Fig. 2. Illustration of the natural light illumination system.

Fig. 3. (Color online) Sunlight path in a static light concentrator.

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The optical transmission unit includes a couplerand an optical fiber. The coupler is used to enhancethe efficiency of light guided into the optical fiber.Collected sunlight is transmitted by the optical fiber.At the end of the fiber, we can use an optical switch tochange the path of the collected sunlight or a beamsplitter to divide the sunlight into red, green, andblue, or more colors.

The lighting unit, called a dynamic lighting mod-ule, includes a lightpipe, an optical sensor, and anLED array. The lightpipe will redistribute the col-lected sunlight; the optical sensor will detect the in-tensity of collected sunlight and control the auxiliarysources; and the LED array will support steady illu-mination. Our goal is to produce a steady light out-put, indoors.

In Section 3, we discuss the subsystems of theindoor lighting system.

3. Static Solar Concentrator

To date, most studies on collecting methods havebeen dedicated to dynamic concentrators, whichinclude Sun tracking systems [1,5–8]. They are pow-ered by small electric motors and require some typeof control module to follow the Sun’s path. Theyrequire maintenance and add complexity to the sys-tem. Therefore, the application area of a dynamicconcentrator is limited. To create a concentrator thatis inexpensive, easy to install, and able to be appliedto a large area in a structure, we needed to design aprismatic and cascadable static concentrator unit tocollect the sunlight for indoor illumination.

Our static solar concentrator consists of a pris-matic structure that includes two parts, as shown

in Fig. 4. The first part, three right-angled prisms,is used to change the plane source, such as sunlight,into a line source. Its prismatic structure can reducethe cross-section area of transmitted light. The sec-ond part can change the light line pattern from thefirst part into a point source for light transmission. Apoint source is easier to couple with a light pipe, ascompared to the plane and line light sources.

For indoor illumination and energy saving, wecombined the prism array and different right-angledprisms to create a static solar concentrator. The pris-matic structure using prisms at right angles to eachother can improve the total sunlight energy collected,and the prism array can create a uniform distribu-tion of efficiency, as shown in Fig. 5. It suits indoorillumination well.

To calculate the efficiency of the unit, the path ofthe Sun was simulated. To simulate the efficiency ofthe unit with different incident angles, we adoptedthe sunlight data for July 2006 in Taipei. The staticconcentrator was analyzed using information on sun-light from the Central Weather Bureau, as shown inTable 1 (θ is the elevation angle of sunlight, Φ is theazimuth angle of sunlight, and θi is the angle of sun-light incident to the prism, transformed from θ andΦ). The efficiency of the static prismatic concentratorwe designed with different incident angles each houris shown in Fig. 6. We used this static concentrator tocollect sunlight; it can use solar energy withoutoptical-electronic transformation [9].

Fig. 4. (Color online) Illustration of the cascadable unit, which isa prismatic structure and can be separated into two parts.

Fig. 5. (Color online) Static prism concentrator with prism array.

Table 1. Incident Angle of Sunlight at Different Timesa

Time 7 8 9 10 11 12 13 14 15 16 17 18

θ 24:10° 37:30° 50:70° 64:20° 77:80° 87:80° 24:10° 24:11° 24:12° 24:13° 24:14° 24:15°Φ 74:10° 78:60° 82:90° 87:40° 94:40° 226:40° 24:11° 24:12° 24:13° 24:14° 24:15° 24:16°θi −65:06° −52:15° −39:08° −25:78° −12:17° 1:59° 15:29° 28:85° 42:11° 55:17° 68:03° 80:94°

aθ is the elevation angle of sunlight,Φ is the azimuth angle of sunlight, and θi is the angle of sunlight incident to the prism, transformedfrom θ and Φ.


4. Coupler

A. Background of Coupler

This invention involves light extraction from a pris-matic structure, for example, to be directed into onelight fiber; and more specifically, it concerns the pro-vision of a nonimaging optical element that receiveslight from a prismatic structure or array of prismaticstructures and efficiently redirects the light into alight fiber without allowing any air gaps between theprismatic structure or optical-fiber entrance and thelight fiber.

B. Structure

An optical coupler is used to collect the energy frommultiple fibers into one fiber. Traditionally, an opti-cal coupler is a tapered structure, as shown in Fig. 7.According to the simulation result, the light outputefficiency is 7.65%. This means that energy is lost;therefore, we redesigned a new element that is madeup of a stepped structure, the surface of which willnot change the direction of the rays. On the left of thestructure are the two incident fibers, and on the rightare the exit fibers. The middle stepped structure is a2 × 1 coupler, as shown in Fig. 8. Based on the pur-pose of the coupler, the total energy of the exit beamshould be larger than the energy of one incident fiber.This means that the energy density in a fiber will beincreased when the collected sunlight passesthrough the coupler and then the stepped structurecan reduce the number of fibers.

C. Theory Analysis

As a numerical example, suppose the input angle is10° and the taper expands the diameter by a factor of1=2. The sine of the output angle θ2 would be sin θ2 ¼d1=d2 sin θ1 ¼ 2 (ðsin 10°Þ ¼ 0:347.

Thus, θ2 would be about 20:32° and light exitingthe narrow end of a taper would diverge at a biggerangle at the fiber axis than when it had entered.Conversely, light going from the broad end to the nar-row end would diverge at a broader angle. For thisreason, we can clearly see that the stepped couplercreates an advantage by maintaining the angle, asshown in Fig. 9.

D. Simulation

To improve the coupling efficiency, different lengthand width ratios were selected from 1∶1 to 10∶1,as shown in Fig. 10. According to the simulation,the stepped coupler can effectively enhance the cou-pler efficiency. The energy rate of the exit fiber to oneincident fiber is bigger than 1 when the ratio is 1∶1 or1∶2, as shown in Table 2. This means that the den-sity of the energy in the fiber can be increased by thecoupler. Even if the efficiency is insufficient, we canuse the coupler to reduce the number of fibers in thenatural light illumination system.

The summary of the simulations in each conditionis shown in Table 2. According to the simulation, ourstepped structure coupler enhances the efficiency to50.28%. The traditional coupling efficiency is just7.65% [10].

5. Optical Switch

Although this new natural light illumination systemcan effectively guide the collected sunlight and sendit to the basement or other indoor locations forhealthy lighting, previously there has been no way tomanage the collected sunlight when lighting is notdesired. To solve this problem, we designed a lightswitch to control and change the direction of thetransmitted light. With the switch, the natural lightillumination system can fit the needs of a variety ofstates and use sunlight efficiently. For example, the

Fig. 6. (Color online) Efficiency of the concentrator with differentincident angles θi.

Fig. 7. (Color online) Traditional coupler 2 × 1.

Fig. 8. (Color online) Redesigned stepped coupler.

Fig. 9. Schematic diagram of coupler ray tracing.


lighting could be turned off and the sunlight thenguided to a solar cell to save power when the inha-bitants are not at home. As shown in Fig. 11, the re-flecting surface is 45° for changing the path of thesunlight from the collecting part. When the sunlightfrom Fiber 1 enters the cubic structure at Surface 1,the sunlight is reflected by the 45° mirror and thenexits at Surface 2, as shown on the left of Fig. 11. Tocollect the most sunlight from Fiber 1 and offer themost energy to Fiber 2, Surface 1 and Surface 2 aredesigned and optimized according to the sameconditions.

According to the characteristics of the collectingpart of the natural light illumination system, the ra-dius of Fiber 1 is 9 mm, and the viewing angle of thecollected sunlight is �40°. The input source of theswitch is a telecentric system placed 20 mm in frontof the optical switch on Surface 1 or Surface 3. Thefunction of the subsystem is only for redirection, sothe output distribution of the switch is similar to theinput. Therefore, the radii of Fibers 2 and 3 are 9 and20 mm behind Surfaces 2 and 4, respectively. Finally,the size of the cubic structure is 52 mm and the ma-terial is BK7. The layout of the turn on function andthe rays are shown in Fig. 12. The left line is Fiber 1,and the bottom line is Fiber 2.

With these beginning conditions, we use OSLO, op-tical design software, to optimize the optical switchwith 9 operands. The operands define the y coordinateof the ray that is on the entrance surface of Fiber 2. Inthis type of operand, we need to define the y coordi-nate and exit angle of the ray on the starting surface.The rays of the operands have different y coordinates,0, 6.3, and9:0 mm,and exit angles,0°,20°, and40°, onthe exit surface of Fiber 1 in Fig. 12. We use the cur-vatures and conic constants of Surfaces 1 and 2 of theoptical switch as variables. After optimization, the er-ror function is 0.573668, which is 1.63%of the originalerror function. For these results, the curvature of Sur-face 1 is 0.086883842 and Surface 2 is 0.088274294,and the conic constant of Surface 1 is −1:553343253and Surface 2 is −2:842749601. The ray tracing isshown in Fig. 13.

Finally, we simulate themodel of the optical switchwith a real source condition, a plane source, as shownin Fig. 14. The variables in the optimization are thecurvature and conic constant, and the efficiency ofthe optical switch is 32.23% in the real condition.In the future, we will use other aspheric coefficientsas variables in optimization, consider the real condi-tion of our source to define the operands, and designSurfaces 3 and 4 [11].

6. Beam Splitter

Dichroic coating can be placed on amirror or prism tomake a dichroic mirror or dichroic prism to separatewavelengths, as shown in Fig. 15. In the natural lightillumination system, the exit beam from the opticalfiber has an expanding angle and the exit beam fromthe beam splitter must focus on the next optical fiber.Therefore, we chose the prism and optimized the in-cident surface and exit surface by using a Fresnellens to control the beam angle rather than using amirror. We designed a Fresnel surface at the incidentsurface so that the incident beam would be changedfrom an expanding beam into parallel beams. Afterthe parallel light passes through the dichroic prism,the parallel light is divided into three colors andthree directions. Finally, the three lights separatelyfocus on the three optical fibers.

A. System

According to the system concept, any mechanism canbe divided into three parts: input, processing, andoutput, as shown in Fig. 16. The dimension of the di-chroic prism is 52 mm × 52 mm × 52 mm, and theradius of the fibers is 9 mm. The cube has Fresnellenses on the top, bottom, left, and right surfaces. Ithas two dichroic optical coatings, and its material isBK7. There are four fibers near the cube: one is inputand three are for output.

Fig. 10. (Color online) To find the best length and width rate, wechanged the length and width ratios from 1∶1 to 10∶1.

Table 2. Best Coupling Efficiency is 50.28% while Length Width is 1∶1

Ratio 1∶1 2∶1 3∶1 4∶1 5∶1 6∶1 7∶1 8∶1 9∶1 10∶1Efficiency 50.28% 50.17% 48.86% 48.36% 48.14% 47.54% 46.28% 46.74% 46.85% 45.65%

Fig. 11. (Color online) Cubic structure under changing directioncondition.


B. Optical Thin Film

In this simulation, we used low-pass and high-passcoatings on the prism. Because we used 600, 550,and 480 nm for red, green, and blue light, the cutoffwavelengths of the coatings are 575 and 515 nm.

C. Simulation

The sunlight enters the system from the left opticalfiber, and then the sunlight is divided into red, green,and blue colors. The red, green, and blue beamsseparately enter the top, right, and bottom opticalfibers, as shown in Fig. 17.

D. Simulation Results

The red line is the spectrum of red light in the nat-ural light illumination systemwith a beam splitter inwhich the wavelengths are higher than 575 nm. Thegreen line is the spectrum of green light with a beamsplitter in which the wavelengths are from 515 to575 nm. The blue line is the spectrum of blue lightwith a beam splitter in which the wavelengths arelower than 515 nm [12].

7. Lightpipe

A. Illumination Lightpipe

In this paper, we aimed to show a design for lightingin a natural light illumination system. Because the

goal of the design is to replace traditional sourcesin the same environment, we designed a lightpipeto meet our needs. The lightpipe can be divided intoinput, processing, and output parts. For the input, wedesigned a coupler to improve the coupling efficiencywith natural light. In the processing section, the sun-light is locked by total internal reflection (TIR). Forthe output, we designed a dot pattern to scatter thelocked sunlight for uniform lighting [9].

B. Structure Parameters

We wanted the lightpipe to have the same distribu-tion of intensity as traditional sources. We use aHitachi FML model fluorescent tube as a reference.The parameters of the lightpipe are 16 mm in diam-eter and 150 mm in length. In the simulation, wetook a fiber with a 10 mm diameter as the sourceto be plugged into the end of the lightpipe. Schematicdiagrams of the components are shown in Table 3.

C. Design of Input Part

The lightpipe can be separated into three parts—input, processing, and output—as designed. First,we discuss the input part of the lightpipe with a cou-pler. The coupler not only acts as a holder for the fiberbut also guides sunlight into the lightpipe with theleast amount of loss possible. To guide the sunlightinto the lightpipe, we simulated coupled efficiencywith a coupler and obtained 91% coupled efficiency.

D. Design of Output Part

In the output section, we consider how to emit thecollected sunlight uniformly. Since the sunlight isguided into the lightpipe, most of the light will be

Fig. 12. (Color online) Rays of the original optical switch.

Fig. 13. (Color online) Rays of the optical switch after secondoptimization.

Fig. 14. (Color online) The ray tracing of the optical switch underreal conditions.

Fig. 15. (Color online) Pictures of dichroic mirror (left) anddichroic prism (right).


locked in by the TIR. Only a small amount of sunlightwill radiate out of the lightpipe; to radiate it out uni-formly, we needed to destroy the TIR gradually. Wedesigned several types of dot patterns on the surfaceof the lightpipe to compare possibilities. In general,dot patterns have two functions. The first is to de-stroy TIR so the sunlight will radiate out of the light-pipe; and the second is to control the emitted energyin each area of the lightpipe to obtain uniform illu-mination. The diagram of the simulation is shownin Fig. 18. The ray tracing is shown in Fig. 19.

After the simulation, we can easily observe that itis more uniform, as shown in Table 4.

We use three different shapes of dot patterns inthree lightpipes with the proportion of dot patternsarea to total area, 20%, 25%, 33%, 50% and 100%.The results of our simulations are shown in Table 5.

In this study, we derived 91% coupled efficiencywith the designed coupler and the best position forthe fiber. The design of the dot pattern on the surfaceof the lightpipe can destroy TIR to make the sunlightuniformly radiate out of the lightpipe. In the future,we will focus on the design of the dot pattern or mi-crostructure of the lightpipe to improve efficiencyand uniformity [13].

8. Dynamic Light Module

Many of the lighting parts in the natural light illu-mination system only use passive optical elements,such as diffusers, reflectors, and lenses, to redistri-bute the intensity of the collected sunlight. Becausethe Sun is a dynamic source, its intensity depends ontime, weather, and season. Therefore, passive light-

ing modules have previously always been used inunimportant areas. To expand the application field,we have offered a dynamic lighting module that in-cludes: the lighting part of the natural light illumi-nation system, LED auxiliary sources, and an opticalsensor.

A. Structure

We used optical software to simulate the dynamiclighting module, as shown in Fig. 20. The dimensionsof the module are 100 cm × 100 cm × 4 cm. In themodule, there are two light sources. First, the fivelightpipes emit the collected sunlight whose inten-sity distribution is close to Lambertian. The radii ofthe lightpipes are 9 mm and the lengths are 100 cm.We assume that the total flux of each lightpipe is1500 lm and use 20,000 rays from each lightpipe tocarry out the simulation. There are five parabolic re-flectors to improve the illumination efficiency of thesunlight. Second, the four LED lines include 100LEDs whose intensity distribution is also Lamber-tian. We assumed that the total flux of each LEDis 75 lm and use 1000 rays from each LED to carryout the simulation. The LEDs are managed by feed-back from an optical detector that is set in the centerof the module and faces the mid lightpipe. When thesunlight is strong, the LEDs will be turned off. Whenthe sunlight is insufficient, the LEDs will be turnedon by current from the feedback signal, adjusted to

Fig. 16. (Color online) Model of the beam splitter.

Fig. 17. Simulation.

Table 3. Schematic Diagrams of the Components

Name of Components Diagram of Simulation

Lightpipe

Coupler

Lightpipeþ Coupler

FiberFiberþ Lightpipeþ Coupler

Ray Tracing

Fig. 18. (Color online) Diagram of the simulation.


react to changes in incident light. In the simulation,the surface area is 16 m2, and the distance betweenthe dynamic lighting module and the surface is2:4 m.

B. Simulation

We simulated three different conditions using the op-tical software to prove that the dynamic lightingmodule can offer steady illumination.

The first condition is sunshine strong enough sothat the collected sunlight is sufficient to light upthe ground. In this condition, the detected flux ofthe optical sensor is 0:35765 lm and the feedback sig-nal will turn the LED array off. The simulation thatis based on this condition is shown in Fig. 21. Theaverage illumination is 258:04 lx. In the simulation,the area of ground is 16 m2.

The second condition is sunshine that the collectedsunlight can only offer half the energy needed to lightthe ground. In this condition, the detected flux of theoptical sensor is 0:18019 lm and the feedback signalwill turn on the LED array. According to the current–flux curve of the LED, the LEDs will be driven by anadjusted current and emit 37:5 lm per LED. The si-mulation that is based on Condition 2 is shown inFig. 22. The average illumination is 237:96 lx.

The third condition is sunshine so weak that col-lected sunlight is near zero and cannot light theground. In this condition, the detected flux of the op-tical sensor is 0:1 lm and the feedback signal willturn the LED array on. The simulation that is basedon Condition 3 is shown in Fig. 23. The average illu-mination is 217:27 lx.

The summary of the simulations in each conditionis shown in Table 6. Although sunlight is a dynamiclight source, the total flux of the dynamic lightingmodule is the same in each condition. The constant

Fig. 19. (Color online) Ray tracing of the simulation.

Table 4. Comparison of Uniformity With and Without Optical Design

Without Design With Design

Table 5. Comparison of Different Dot Patterns

Type of the Dot Pattern Integrated Power (lm) Improvement Uniformity

1 Without Design 95.8 1 Not Uniform2 Top 399.406 4.175 Uniform

Side

3 Top 333.5607 3.445 Uniform

Side

4 Top 650.3473 6.785 Uniform

Side


total flux is based on the feedback signal from theoptical sensor. When the detected flux is weaker,the emitted flux of the LED array is stronger. With

a dynamic lighting module, the efficiencies of sun-light only, LED only, and sunlight with an LED lightare similar [14].

Fig. 20. (Color online) Dynamic lighting module with five lightpipes and 100 LEDs.

Fig. 21. Illumination distribution (left) and detected flux (right) for Condition 1.


Table 6. Summary of Simulations for Each Condition

Sunlight Level Collected Sunlight LED LED Flux Total Flux Average Illumination Efficiency

Condition 1 Strong 5 × 1500 lm Turn off 100 × 0 lm 7500 lm 258.04 lux 55.05%Condition 2 Mid 5 × 750 lm Turn on 100 × 37:5 lm 7500 lm 273:96 lx 50.76%Condition 3 Weak 5 × 0 lm Turn on 100 × 75:0 lm 7500 lm 217:27 lx 46.35%

Fig. 24. (Color online) Illustration of the ecological illumination system.

Table 7. Parameters of Traffic Tunnels

OpeningNumber

TunnelDirection

TunnelLength

TunnelWidth

TunnelAltitude

Form of TunnelSection Speed

RoadWidth

RoadAltitude

SidewalkWidth

Single hole East and West 12;900 m 5:2 m 5:4 m U shaped 80 km=h 3:5 m 4:9 m 0:85 m

Table 8. Requirements for Illumination

TunnelDirection Speed

Conversion Factorof Average Illumination

MaintenanceFactor

ReflectionFactor RoadWidth Height of Lamp

East and West 80 km=h 15 lx=cd=m2 0.6 0.4 3:5 m 4:9 m

Table 9. Chinese National Standards of Illumination in Traffic Tunnels

Case General Tunnel Illumination (Length: 12:9 km)

Design Value

Illumination Section

BorderSection

MotionSection #1

MotionSection #2

TransitionSection

Base LightingSection

ExportSection #1

ExportSection #2

Brightness ðcd=m2Þ 120 60 30 15 6 15 30Section Length (m) 50 60 50 50 12610 40 40Number of Lamps 25 30 25 25 6305 20 20Total Flux of Single Lamp (lm) 26250 13125 6562.5 3281.25 1312.5 3281.25 6562.5

Table 10. Numbers of Collecting Units in Each Section of the Traffic Tunnel

Design Value

Illumination Section

BorderSection

MotionSection #1

MotionSection #2

TransitionSection

Base LightingSection

ExportSection #1

ExportSection #2

Number of Collecting Unit 58334 35000 14584 7292 735584 5834 11667


9. Application

A. Ecological Illumination System

To offer a steady, healthy illumination in traffic tun-nels, save energy, and meet safety requirements, theecological illumination system includes a naturallight illumination system and an auxiliary lightingsystem that includes LEDs and photo detectors. Asshown in Fig. 24, there are two types of input—sunlight and electric power—and the output of thesystem is the redistributed light that is sunlight,auxiliary light, or both. When the sunshine is strongenough that it can provide enough illumination inthe traffic tunnel, the auxiliary lighting system willturn the LEDs off, and the photo detectors will re-main on standby. At night or when the weather iscloudy or rainy, the auxiliary lighting system willturn on the LEDs using an adjusted current thatis dependent on the strength of the signals of photodetectors.

B. Traffic Regulation

There are many famous long tunnels in the world. Ifwe can combine their illumination systems with nat-ural light illumination systems, there will be manybenefits, such as healthy lighting and energy conser-vation. In this paper, we utilized the common speci-fications of traffic tunnels as examples for evaluatingthis system. The traffic tunnels’ parameters and illu-mination requirements are shown in Tables 7 and 8.

According to the lighting criterion for traffic tun-nels of the Chinese National Standards (CNS) regu-

lated by the Bureau of Standards, Metrology andInspection, the brightness of the tunnel in differentareas should conform to the standards, as shown inTable 9.

In Table 9, the parameter, the total flux of a singlelamp, is calculated from theparameter, brightness, bythe average coefficientmethod, as shown inEq. (1). InTable 8, we can find the maintenance factor, M, theconversion factor of average illumination, K , and theroad width, W. In Table 9, brightness requirements,L, and thenumber of lamps,N, are shown.Weassumethat thedistance between the two lamps,S, is2 mandthe illumination rate, U, is 0.4. Therefore, we cancalculate the total flux of a single lamp, F, for eachillumination section from Eq. (1):

S ¼ F ×U ×M ×NK × L ×W

: ð1Þ

In Taiwan, the illumination of sunshine is around80; 000 lx at noon in the springtime. According toFig. 6, the collecting efficiency of each unit is 33%.The assumption coupling efficiency is 75%, and the il-lumination area for each unit is 3:75 cm × 3:75 cm.Therefore, the collected flux of each unit is 11:25 lm.Table 10 shows the number of collecting units in eachillumination section.

C. Simulation Results

This is a frontal view of the tunnel, as shown inFig. 25. The lamp altitude is 4:9 m and the roadwidth is 3:5 m. In our simulation section, we makean initial simulation. This is the simulation struc-ture. The road width is 3:5 m, and the lampaltitude ¼ 4:9 m.

The simulation results for one lamp and six lampsare shown in Figs. 26 and 27. Light cannot be usedefficiently; the illumination is too low, so the lensmust be redesigned. We used optimization to opti-mize the lens and improve the average illumination.

10. Conclusion

The natural light illumination system includes col-lecting, transmitting, and lighting parts. For thecollecting area, we designed a static prismatic con-centrator that can directly collect sunlight, and weused plastic fibers to transmit the light. We offered

Fig. 25. (Color online) This is a frontal view of the tunnel.

Fig. 26. Simulation results for one lamp.

Fig. 27. Simulation results for six lamps.


a stepped-structure coupler whose efficiency is50.28% to maintain the divergence angle of the exitbeam and reduce the large number of fibers.

For transmitting, we designed a cubic structuredlight switch and a prismatic structured beam split-ter. The light switch can change the path of the col-lected sunlight so that the light can light differentareas or be saved as electricity by a solar cell. Ithas four Fresnel surfaces. The efficiency of the lightswitch is 32.23% in actual operation. A prismaticstructure beam splitter can separate the sunlightinto red, green, and blue light, based on an x-cubeprism. We prepared the surface of the element withan aspheric surface and Fresnel surface to reduce thebeam angle of the exit light.

In the lighting area, we achieved 91% coupled ef-ficiency using the designed coupler and the best po-sitions of fibers. The design of the dot pattern on thesurface of the lightpipe can destroy the TIR to causethe sunlight to radiate uniformly out of the lightpipe.

In this system, we also provided a dynamic lightingmodule that includes the lighting device of a naturallight illumination system,LEDauxiliary sources, andan optical detector. With the feedback from the detec-tor, the dynamic lighting module will adjust the LEDintensity to provide steady illumination.

We can also use this system as an ecological illu-mination system to offer steady illumination in traf-fic tunnels to provide healthy illumination, saveenergy, and meet safety requirements [15].

References1. A. J. W. Whang, Y. Y. Chen, and B. Y. Wu, “Innovative design of

cassegrain solar concentrator system for indoor illuminationutilizing chromatic aberration to filter out ultraviolet and in-frared in sunlight,” Solar Energy 83, 1115–122 (2009).

2. E. Andre and J. Schade, “Daylighting by optical fiber,”Mastersthesis (Luleå University of Technology, 2002).

3. Y. Wu, R. Jin, D. Li, W. Zhang, and C. Ma, “Experimental in-vestigation of top lighting and side lighting solar light pipesunder sunny conditions in winter in Beijing,” Proc. SPIE 7157,1–6 (2008).

4. Y. Wu, “Research and development of solar light pipes inChina,” in 2008 International Conference on InformationManagement, Innovation Management and IndustrialEngineering (IEEE, 2008), Vol. 3, pp. 146–149.

5. A. J. W. Whang, C. C. Wang, and Y. Y. Chen, “Design of cascad-able optical unit to compress light for light transmission usedfor indoor illumination,” Renew. Energy 34, 2280–2295 (2009).

6. K. L. Martin, “An overview of daylighting systems,” Sol. En-ergy Mater. 73, 77–82 (2002).

7. B. Bouchet and M. Fontoynont, “Day-lighting of undergroundspaces: design rules,” Energ. Buildings 23, 293–298 (1996).

8. A. Rosemann and H. Kaase, “Lightpipe applications fordaylighting systems,” Sol. Energy Mater. 78, 772–780 (2005).

9. S. H. Yang, Y. Y. Chen, and A. J. W. Whang, “Using prismaticstructure and brightness enhancement film to design cascad-able unit of static solar concentrator in natural light guidingsystem,” Proc. SPIE 7423, 74230J (2009).

10. P. H. Pan, Y. Y. Chen, and A. J. W. Whang, “An optical couplerof natural light guiding system based on prismatic structureand microlens array,” Proc. SPIE 7423, 74230I (2009).

11. K. H. Chou, Y. Y. Chen, and A. J. W. Whang, “An optical switchof natural light guiding system based on cubic structure withFresnel surface,” Proc. SPIE 7428, 74280O (2009).

12. Y. C. Li, Y. Y. Chen, and A. J. W. Whang, “A beam splitter ofnatural light guiding system based on dichroic prism for eco-logical illumination,” Proc. SPIE 7429, 742909 (2009).

13. Z. Y. Lee, Y. Y. Chen, and A. J. W. Whang, “Design and opti-mization of dot pattern in illumination lightpipe of naturallight guiding system,” Proc. SPIE 7429, 74290A (2009).

14. C. A. Chen, Y. Y. Chen, and A. J. W. Whang, “An active lightingmodule with natural light guiding system and solid statesource for indoor illumination,” Proc. SPIE 7422, 74220Z(2009).

15. C. N. Chen, Y. Y. Chen, and A. J. W. Whang, “Design andevaluation of natural light guiding system in ecologicalillumination of traffic tunnel,” Proc. SPIE 7423, 742300(2009).


natural light illumination system

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