REVIEW OF ILLUMINATION SYSTEM - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/77996/10/10...unsound circuit design and high drive current can cause premature failure of LED

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Experimental and Simulation Study of Optimal Illumination Systems 16

CHAPTER 2

REVIEW OF ILLUMINATION SYSTEM

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2.1 Introduction

A wide range of illumination applications demand different specifications as

discussed in chapter 1. One has to verify whether the proposed illumination

system design fulfils the design goals of an application before its installation.

Moreover one has to check for optimized design of source system. The

source system analysis helps in finalising the source design. As system

performance depends upon number of parameters like number of sources,

their geometrical placement, optical characteristics of sources, source to

object distance etc., most of the times computational tools are used for

analysis of system and decide the source design. Computational tool forms a

bridge between system description and actual system design goals.

Figure 2.1 Computational tool as bridge between

system description and design goals

The need for a dedicated computational tool was felt, while addressing

the problem of development of reading light for patients with Low Visual

Acuity (LVA). These patients are unable to read comfortably under normal

lighting conditions and need specific light source. It was decided to build a

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multispectral luminaire with multiple low power LEDs which would allow

adjustment of illuminance and spectral contents of the source. During design

and development, optimization of variables like selection of LEDs and their

placement generated the need of understanding the factors determining the

performance of an illumination system.

To minimise the development time a dedicated computational tool was

required to perform analysis of illumination system. Specific experimental

setups were also needed to characterize the system and to measure realistic

optical characteristics of sources. The experimental setups are expected to

support the simulation tool by creating entries in the database of optical

characteristics of LED sources.

Considering this application requirement and wide acceptance to solid

state lighting as future of lighting [1,2], the task of development of

computational tool and experimental setups for multielement LED illumination

system was taken up. The development required thorough knowledge of

illumination system and its design & analysis aspects.

The present chapter reviews the information about the conventional

light sources and LED as a light source. Further the system classification and

terminologies used for characterization of illumination system are discussed.

Various aspects for optimization of LED source design are also described. In

the end, commercially available computational tools and their computational

methodologies are briefed.

2.2 Light sources

Nowadays various commercial light sources are available in the market

having different wattage, lumen output and color. They are available in

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different sizes and shapes. According to working technology they are divided

into three general classes: incandescent, discharge, and solid-state lamps.

Incandescent lamps produce light by heating of a filament, discharge lamps

produce light by ionizing a gas through electric discharge inside the lamp

while solid-state lamps use a phenomenon called electroluminescence to

convert electrical energy directly to light [3]. Following subsection describes

these sources in brief.

2.2.1 Incandescent Lamps

The incandescent lamp was one of the widely used light sources since last

200 years. It generates light when an electric current passes through a thin

wire. This wire glows white-hot and radiates energy in all directions. A

consequence of generating light using this technology is that only 15% of the

energy produced is visible light; the remaining 85% is dissipated as heat. This

fact reduces light efficacy of these bulbs. At present the product-lifecycle of

the incandescent lamp seems to come to an end, primarily because of the

inherent low luminous efficacy. More efficient light sources are available and

the expansion of environmental awareness has resulted in restrictive

governmental legislation [4].

2.2.2 Discharge Lamps

Amongst discharge lamps, common are fluorescent lamps/tubes.

Fluorescent lamp technology consists of a thin glass tube filled with

argon/mercury vapor. At each end of the tube are metal electrodes coated

with an alkaline-earth oxide that gives off electrons easily. When a current is

passed through the ionized gas between the electrodes, the fluorescent lamp

emits ultraviolet radiation. The phosphors, coated at inside surface of tube,

absorb the ultraviolet radiation and re-radiate the energy as visible light. To

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start a fluorescent lamp the unit requires a "boost" in the form of a starter and

ballast that provide high voltage. These lamps produce more uniform

illumination.

Recently developed compact fluorescent lamp (CFL) is the efficient

alternative to incandescent bulbs. However the disadvantages of CFLs are

similar to incandescent bulbs, CFLs also create unfocused light, and produce

an uneven illumination. So reflectors are needed to redistribute the output

which creates an additional level of complexity and cost to the product. The

performance and lifespan of CFLs is dramatically degraded by extremes in

ambient temperature.

2.2.3 Light Emitting Diodes (LEDs)

In 1970s only colored LEDs were available and were mainly used as

indicators. The use of LEDs for signaling applications started about 35 years

ago. Neither the total luminous flux nor the spatial intensity distribution was of

great interest to the users. Even the spectral distribution was never of

importance for the initial applications.

Previously LEDs were available with flux of only few lumen and didn’t even

achieve the luminous efficiency of standard incandescent bulbs. High quality

GaN crystal was deposited on Sapphire substrate in 1986 [5] and high

brightness blue LEDs were developed in 1993 and have been continuously

improved [6-8[. White LEDs were developed using blue LEDs combined with

yellow phosphors and commercialized in 1996 [9]. The availability of higher

luminous efficiencies among the coloured LEDs as well as the launching of

the first white colored OLEDs around 1990 made the LEDs more and more

interesting for a wide range of products in display and in illumination

applications [10]. Nowadays LEDs, colored or white, surpass the

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incandescent and halogen bulbs in their luminous efficiency. Efforts are on to

reach 200 lm / W.

Spatial and spectral characteristics of LEDs make them more suitable in

certain applications as compared to others. However, price, thermal issues

and reliability of the high power LEDs call for tremendous extra efforts for the

efficient design of LED based illumination systems. The various advantages,

limitations and challenges of LED as a light source are listed below:

Advantages of LED

Advantages of LED as a light source are well known now a days. Main

advantage is higher energy efficiency. Because unlike incandescent bulbs or

even fluorescent lamps, almost all of the energy used by LEDs is converted to

light. It has long lifetime of 1,00,000 hours, that is more than 11 years [11].

Wide varieties of color LEDs are available which allows much greater

flexibility in spectral power distributions than do tungsten / halogen lamps

[12-13]. Being solid state devices their intensity and color of illumination is

adjustable by using programmable digital driving circuit. LEDs are said to be

environment-friendly since toxic substance such as mercury is not used in the

device. They are small size, lightweight and more compact elements. Also

these are strong solid state devices and do not break easily. They can

withstand normal vibrations and shocks. Their maintenance cost is less.

Interesting thing about LED as light source unlike other conventional sources

they don’t have sudden breakdown. Moreover since LED luminaire have

multiple LEDs, breakdown of few has less influence on illumination.

Limitations of LED

Till today initial cost of LED is high due to row material. Development

of higher efficacy power LED is still under research. Rendering property of

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LED is poor as compared to conventional light sources especially in case of

LEDs with yellow phosphor (due to lack of red and green components). LED

chip gets damaged by electric surges and spikes so appropriate prevention is

necessary. Color shade changes with ageing. Luminaries need to be

designed properly so as to avoid breakdown of LED due to thermal runaway.

Excessive heat causes deformation of epoxy encapsulation which leads to

optical distortion and permanent fresnel light loss. Improper and Technically

unsound circuit design and high drive current can cause premature failure of

LED. Power LED packages are different from low power LEDs. Incorrect

assembly procedure and excessive soldering during assembly damages the

LED. Improper bending of LED leads during assembly causes stress in the

lead.

Challenges

Technical challenges in SSL are to improve efficacy in visible range and

reduce cost of SSL [14]. Optical characteristics of LED are different from

other conventional sources and needs developments of new models to design

and analyze the SSL lamp. In the past 3-4 years, publications have pointed

out the inadequacies of CRI metric with respect to the modern light sources.

For example, the existing incandescent based color rendering index (CRI)

metric applied to LED does not correlate well with people’s preference for the

color of an illuminated space. Moreover technology challenges include many

issues. In the development process of LED’s, manufacturers have to sacrifice

luminous efficacy to gain CRI. Contrary to general belief, recent studies at

LRC (Light Research Centre, New York) show that a low CRI, LED source

was more preferred as reading and task light than a high CRI halogen or

incandescent light [15]. Secondly being a new technology, manufacturers of

LED luminaires, drivers and controls are limited. One has to carefully handle

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the thermal management problems. Thus there are lot of opportunities for

researchers to work in manufacturing and designing in SSL field.

2.2.4 Comparison of Light Sources

The following table compares performance of conventionally used light

sources with LED. Kerosene and conventional light sources are inefficient

and expensive as seen from the table. The long life and low power

requirements result in more cost effective light source over the time.

Table 2.1 Comparison between light sources [2]

Parameter

Kerosene

Wick

Lamp

Incandescent Compact

Fluorescent

Luxeon

K2

WLED

Lamp Consumption 0.05L/h 25W 7W 1W

Lamp Cost(Rs) 20 10 100 150

Lamp Luminous

Output (lm) 10 250 250 60

Lamp Lifetime(hours) 5000 1000 6000 +50 000

Lifetime Energy

Consumption 2500L 1250kWh 350kWh 50kWh

Based on rated life, it would take at least 50 incandescent bulbs and 6-10

compact fluorescent lamps to last as long as one LED which has an average

life of 50,000 hrs [16]. As life of LED is significantly greater there is no need of

purchasing new one for a long period while incandescent and CFL require

frequent replacement and cost increases as time passes by. Moreover the

energy consumption is much lower in the case of LED for producing

equivalent illumination compared to the CFL and incandescent bulb. The cost

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of LED, incandescent and CFL bulb versus life is plotted in figure 2.2

considering their replacement cost. It is seen that though initially cost of

LED is higher, over the years it is maintained while cost of luminaire

consisting of either incandescent or CFL increases.

Figure 2.2 Plot of cost comparison of light sources

Above comparative study shows why LEDs are said to be better light

source amongst all. An experimental comparison of the performance of the

illumination systems using three types sources is performed during the

present work and is presented in chapter 5. The world of lighting is changing

with invention of high power Light Emitting Diodes and solid state lighting is

becoming the next generation of lighting industry.

2. 3 Illumination Systems

Illumination system consists of one or many primary light sources. Its

purpose is to create a visual environment for visibility of objects and to extract

features of the object. Illumination is achieved either by natural illumination of

interiors from daylight or may be by artificial sources such as incandescent

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bulbs, compact fluorescent tubes etc. Artificial illumination constitutes

luminaire and light sources. Luminaire provides support and electrical

connections to sources within it. It also helps in distributing or directing the

light. A wide variety of luminaries and light sources are available in different

shapes and sizes. The selection mainly depends upon the application.

Each application has its own specifications of illumination. Efficient

illumination system can be designed by optimizing the configuration

parameters which affect the Quality of Illumination. To precisely quantify the

system parameters, design specifications of application, the optical

characteristics of sources, and their effect on illumination system performance

needs investigation. The present section provides an overview of these

aspects.

2.3.1 Classifications

Design specifications of an Illumination system are mainly dependent upon

the purpose of illumination that is the type of work to be carried out under

illumination. Broadly one can classify the illumination system based on the

place of deployment or on interpretation of application. Based on place of

deployment, systems are categorized as (1) residential (2) office/corporate (3)

hospitality/foodservice (4) institutional (5) retail store (6) industrial application

(7) medical applications etc. Lighting requirements at each work place has

wide variation as discussed in subsequent paragraphs.

According to interpretation, illumination applications are categorized as (1)

General lighting (2) Task lighting.

General lighting

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General lighting is typically employed to increase illumination above ambient

levels. It can be further categorized based on area to be illuminated and on

color of illumination. Lighting at residences, offices, hospitals, hotels, stage

lighting, retail stores, institutional / health care lighting etc. are covered under

this category. In each application illuminance level is different. Even when we

refer residential lighting various rooms such as hall, kitchen, bedroom,

corridor etc. need illumination with different lux levels. Same is true with office

and hospital lighting. On areas such as working table or at inspection desk

etc. lux level between 250 - 500 lx is required with minimum glare. In

decorative lighting such as in malls or stage lighting color of illumination, color

uniformity and controllability of luminous intensity and color are desirable [17].

Cost, visual comfort, maintenance and aesthetic look are important as well.

General illumination applications such as flood lighting, stadium lighting cover

much larger area. Here lux level requirement is much higher, above 1000 lx

and also it should provide substantially uniform level of illumination

throughout the area along with personal comfort and minimum glare.

Specific lighting is needed for growth of plants and animals. There have been

several reports on plants responses to monochromatic light to wide spectrum

light [18]. Light intensity is a key factor in poultry operations because minimal

threshold intensity is required to obtain optimal production performance.

While in fishing, fishing light must be more energy efficient and reduce

environmental impact [19]. Lot of research is going on in this field to increase

productivity by using adjustable multispectral LED light source.

Task lighting

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In task lighting a lot of variation is observed in specifications. Area of

illumination is small and varies from nm2 to tens of cm2 while illuminance may

vary from 1000 lx to 50,000 lx. Task lighting can further be categorized as

machine vision illumination applications, medical field illumination or

illumination required in forensic laboratories etc.

Machine vision illumination required in industry includes applications such as

surface inspection, optical character recognition (OCR), material or product

inspection, print quality verification, edge detection etc. In all these

applications diffuse, high intensity, uniform and stable illumination with

minimum shadows is expected. Illumination for visual display demands flat

and uniform intensity distribution along with efficiency [20,21]. Microprojection

system applications such as microdisplays, exposure tools for mask-free

photolithography require focused spot with small central lobes, high central

intensity and low side lobes over micro dimensional area [22]. Intense

illumination over small areas can be used to get depth information from object

while inspecting 3-D objects or in topography measurements.

Correct illumination is critical to an image system and improper

illumination can cause a variety of problems. Hot spots can hide important

image formation. Non-uniform lighting makes thresholding more difficult.

In medical field focused light with good color rendering property is

required on surgical table without any glare [23]. Dentist need intense and

focused light for faster curing process of resin [24].

For above applications small spectral bandwidth is unfavorable

demanding for white light. But there are some machine vision applications

such as spectroscopic applications which demand either narrow spectral

width or monochromatic source [25]. Forensic laboratory demands

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monochromatic illumination to meet variety of forensic inspection needs. It

inspects variety of objects like Biological stains and latent fingerprints, Blood

and other bodily fluids, Bruises/bite marks/pattern wounds, Hair and fibers,

Grease, oil and other petroleum-based stains etc. Therefore it demands a

versatile light source emitting in full visible spectrum with interchangeable

filter. Different requirements of illumination for various applications mentioned

above are summarized in figure 2.3.

Figure 2.3 Classifications of Illumination Applications

Lighting standard recommendations for different illumination applications are

specified by Illuminating Engineering Society of North America (IESNA),

Commission International de I’Eclairage (CIE), Chartered Institution of

Building Services Engineers (CIBSE) and other organizations. Each university

campus or governing agency constructs their own standards usually based on

IESNA / CIE / CIBSE suggestions. Different security requirements, lighting

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conditions, climate and other factors lead to the variation of standards used in

the country. The countries are currently improving the lighting standards

adopted 20-30 years ago. Awareness of light pollution and the need of energy

conservation are reshaping the codes of many institutions and are saving

energy for future use by adopting LED as light source. The important

terminologies used for specifications of illumination system are listed in next

section.

2.3.2 Terminologies used in Illumination System

The technical terminologies of illumination system are listed here since

they are used extensively throughout this work. These are the parameters

which decide the performance of the system.

1. Illuminance: Illuminance is a measure of photometric flux per unit

area or visible flux density. It is expressed in lux (lumens per square meter) or

footcandles (lumens per sq. foot). Design of lighting system must achieve

recommended illuminance level. More details are covered in chapter 3.

2. Uniform illumination: It is expected that illumination area under

consideration is illuminated with same lux level through out the surface. This

characteristic is specified by terms uniformity ratio and diversity ratio which

are defined as:

eilluminanc Maximum

eilluminanc Average RatioUniformity = …….. 2.1

eilluminanc Minimum

eilluminanc Maximum RatioDiversity = ……… 2.2

Perfect uniform illumination is said to be achieved when maximum,

average and minimum illuminance levels match. Under this condition

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uniformity and diversity ratio become one. Designer will try to achieve

uniformity value as maximum as possible. In general illumination applications

uniformity ratio upto 0.6 is tolerable while task illuminations need uniformity

ratio greater than 0.9. Diversity ratio of 0.66 is acceptable.

3. Hot spot and dark spot: The areas with lux level greater than the

recommended produce brighter area called as hot spots. These areas

responsible for creating glare. The visibility of task greatly depends upon the

glare. IESNA definition of glare is the sensation produced by luminances

within the visual field that are sufficiently greater than the luminance to which

the eyes are adapted, which causes annoyance, discomfort, or loss in visual

performance and visibility. It may injure the eye, disturb the nervous system,

cause discomfort and fatigue, reduce efficiency, interfere with clear vision and

increase risk of accident. The magnitude of the sensation of glare depends

upon factors such as the size, position, and luminance of a source, the

number of sources, and the luminance to which the eyes are adapted. Glare

is caused by faulty lighting installations and can be reduced by proper

placement of light sources.

On the contrary, the areas with less illuminance level than the desired,

are referred to as dark spots. The presence of dark spots create disturbance

similar to the hot spots and also lead to artifacts.

4. Correlated Color Temperature (CCT) & Color Rendering Index (CRI):

It is a metric used to describe the "whiteness" of a light source. CCT is the

temperature at which a Planckian Black Body Radiator and a source’s

appearance match, usually specified in Kelvin ( ⁰K ). Depending upon CCT,

white sources are classified as warm white (2700 – 3000 ⁰K), neutral white

(3300 – 5300 ⁰K) and cool white (Above 5300 ⁰K).

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CRI of a light source is a measurement of how well a light source represents

color compared to an ideal source. Color rendering index (CRI) is measured

on a scale of 0 to 100. The higher the value the higher is the ability to render

the colors. Sunlight has a CRI of 100. Common lighting sources have a large

range of CRI. CRI values below ~ 55 are regarded as illumination with poor

color rendering properties; values between 55 and 85 are regarded as good

and above 85 as excellent. Typical fluorescent office lighting has a CRI of 80

to 85.

5. Luminous Flux: Luminous flux describes the quantity or amount of light

emitted by a source during a given unit of time and is measured in Lumens. In

case of multielement source total flux output is addition of individual flux

output.

6. Efficacy (lm/W): The ratio of the total luminous flux emitted by light

source to the total amount of electrical power it consumes is called efficacy of

light source. It is specified in lm / W. Higher the efficacy better is the source.

Luminous flux and efficacy are the characteristics of sources but

source is a part of illumination system and hence these are characteristics of

illumination system as well.

7. Cost: Overall cost of illumination system includes cost of sources,

fixture, power supply, heat sink, installation, maintenance etc.

2.3.4 Illumination System Design

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As seen in section 2.3.1 different applications impose different design criteria

to be emphasized [26] and needs efficient design of system. Design of an

illumination system is defined as the selection and placement of light sources

to achieve the desired goals of the illumination application. One has to

optimize number of light sources, their geometry and optical characteristics.

These source configuration parameters are finalized based on the

performance of designed illumination system. The various aspects considered

in evaluation of performance of system are shown in figure 2.4. Designer has

to balance between human needs, architectural context, economics and

environment [27].

Figure 2.4 Parameters for Illumination System Characterization

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Many practical applications of illumination must meet the primary

lighting requirements such as recommended lux level, uniformity, sufficient

reduction of glare or hot spots, cost and less energy consumption. The

secondary issues like thermal effects, life, cost, installation, safety and

security etc., impose further restrictions on optimization of the source

configuration parameters. Many researchers have attempted the geometrical

and optical characteristic optimization of sources to fulfill the need of the

application [28-33].

Since objective of the present work is to design an optimal illumination system

using computational tool, the effect of variation of source density, source-

source distance, source to target distance and spatial distribution of sources

on system performance was studied. It is summarized in next two

paragraphs.

Illuminance level and uniformity increases with use of more number of

elements. If the source density is increased by decreasing inter-element

distance (so as to have merging of irradiances because of adjacent sources)

then the luminaire creates extra brightness on target surface, which gives rise

to uncomfortable glare and causes loss in the efficiency of the eye. It also

increases energy consumption and the cost of the luminaire. Having fewer

sources means insufficient light, which leads to dullness in the working

environment, strain on the eyes, and inefficient working conditions. Even an

optimal source density with improper placement of sources can create hot

spots in some areas, while some other areas can have low illuminance.

Uniform illumination can also be obtained by using sources with typical

radiation intensity pattern and expand the beam angle much larger than the

area to be illuminated. Another way to increase uniformity is by increasing the

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source-to-target distance. But, both these lead to a decrease in the

illuminance level on the target plane. It also decreases the efficacy of the

source, as only a fraction of the source flux can be utilized for illumination of

the surface under consideration.

The above concepts are applicable to multiple LED illumination system as

well. However the characteristics of LEDs are different from conventional

sources and needs special considerations. Majority of the attempts claim to

provide optimal solutions to have uniform illumination over a plane surface

using LED source. Papers have reported the optimization of the distance

between two LEDs using mathematical equations based on inverse square

law and sparrow’s criteria to obtain uniform illumination [34-35]. Different

configurations of LED arrays were studied by Moreno to find the maximum

LED density and the minimum LED-to-detector distance that can produce

satisfactory uniformity [36]. However, greater LED density is limited by cost,

available space, and by thermal considerations. Guttsait has discussed

analysis of LED modules considering illumination level and uniformity for local

illumination [37].

Above discussion shows that uniformity, optimal lux level, power efficiency,

spatial distribution of the sources and source geometry are interrelated. For

optimal system design each and every aspect is required to be considered.

The optimization of system considering primary aspects like illuminance,

glare, minimum source flux etc. other than uniformity were not been found in

literature. This task is taken up in the present work.

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2.4 Simulation Tools for Illumination Design

Section 2.3 has illustrated how illumination applications have varied

range of specifications and demands unique design for each and every

application. It also describes how optimized design requires consideration of

various parameters. As the importance of lighting has become more explicitly

understood, particularly in terms of its effect on day-to-day activities, lighting

designers are more frequently retained for the lighting design in office, health

care, education and in industry sector. It is obvious that special lighting design

is essential in the design of uncommon applications like zoos, museums,

exposition and convention centers etc. It is the role of the lighting designer to

provide an optimal solution for the placement and the power of the light

sources for adequate illumination effects. For the design of an efficient

illumination system fulfilling human, economical and environmental needs

many professionals use lighting simulation tools. The tools help in optimized

illumination system design based on specifications of various applications.

2.4.1 Necessity of Simulation Tool

Despite being long known that the luminous flux on a given working

area not only depends on the power of the light sources, but also on their

placements and the effects of the absorbing and reflecting surfaces of the

enclosure, the first systematic methods for the analysis and design of artificial

lighting were established only in the first half of the twentieth century.

Harrison and Anderson [38, 39] proposed an experimental procedure, the

now called lumen method, in which the luminous flux on a working plane was

determined from a combination of a series of proposed assembling of light

sources. Moon [40] and Moon and Spencer [41,42] proposed the

interreflection method for the design of three-dimensional rectangular

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enclosures having any aspect ratio and being formed by diffuse surfaces. The

method presented the advantage of allowing the calculation of the brightness

of a surface accounting for the reflection of light. Due to the complexities of

the required calculations, the method requires the use of tables. The lumen

method [43] is probably the most widely employed method for the design of

illumination, since its algebraic relations provide a rapid, simple procedure to

determine the power of the lamps, although the method lacks precision. A

more elaborate solution can be achieved by the WinElux code [44], which

contains a database of different types of lamps. In spite of their widespread

use, both the lumen method and the WinElux code are not, in general,

capable of providing solutions that can assure uniformity of luminous flux on

the design surface [45].

With the increased capability of computers, in terms of speed and

memory capacity the use of simulation tools for designing illumination system

has become common. The tools have enabled relatively accurate predictions

of performance of illumination system. These can determine light levels,

brightness and uniformity. Some programs incorporate rendering and

generate lighting patterns. However lighting software can’t choose an

appropriate lighting system based on the designer’s requirements. First the

designer must develop design and then analyze it with the tool. Illumination

depends on so many parameters that, for a given specification, more than

one solution can be proposed. User has to select the one which is best suited

for his needs considering results of the analysis.

2.4.2 Software for Illumination System Design

The newest generation of illumination design and analysis software reduces

design cycle time and cost, and also enables the production of more

sophisticated and efficient systems. Lighting programs calculate the lighting

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effects caused by specific luminaires on specific targets at specific points [46].

Numerous programs are available for lighting simulation, aimed at a range of

end-users with varying levels of complexity. The programs are distinguished

mainly by the capabilities of their photometric translators, the speed and

accuracy of their calculation engines, the way in which data is input to the

program and the presentation of calculated results. In general input and

output data forms of simulation tools are as follows:

INPUT PARAMETERS:

• Dimension of work plane, it’s height and luminaire mounting height

• Surface reflectance

• Detail luminaire photometric data in IESNA format. Photometric data files

are supplied by manufacturers in IESNA format via CD-ROM or website

• Precise location and orientation of luminaires using x,y,z coordinates or

CAD drawing. The more complex illumination geometries are specified by

a 10-Mbyte CAD file.

• Light loss factors and any other multiplying factors to adjust the lamp and

ballast output from the assumptions used in the luminaire photometry

OUTPUT PARAMTERS:

• Illuminance (lux or footcandles) on a horizontal work plane at selected

points on the target surface; summary statistics such as average,

maximum, minimum and standard deviation of illuminance values.

• Surface Illuminance (candelas per unit surface area) or existence (lumens

per unit surface area).

• Lighting power density ( watts / m2 or watts / ft2 )

• Visible pattern and glare rating are the principal metrics computed by

some of the programs.

Output from these programs is usually a chart of calculated values, an

illuminance plot, or a shaded plan with color or gray scales representing a

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range of light levels. All programs display the results directly on the screen or

print them.

Computational Engines:

Depending on the goal of the analysis, software tools utilize different

methods to achieve results and renderings. Computer modeling of an

illumination system is based on the same basic principles as imaging optical

system design. The categories of computer based tools are differentiated by

the calculation methods they use:

• Radiosity: It is a computer graphics method to calculate diffuse light

distribution and reflection in three dimensional environments. Its basis is in

the theory of thermal radiation. Radiosity algorithms compute how photons

emitted from source affect surfaces. This method is computationally intense,

due to the use of linear systems of equations and the spatial complexity of

large scenes. Some of the packages which are using radiosity algorithm are

Agi32, AutoDesk VIZ, Lumen Designer etc.

• Ray-tracing: To create more realistic images, a few programs use a

far more precise form of calculation called ray-tracing. Ray-tracing is a

rendering technique that calculates an image of a scene by simulating the

way rays of light travel in the real world. In this method light rays are traced

through the system, and either reflect, refract, or both at each material

interface. This changes the direction and flux of each ray as it propagates to

the next surface. The analysis and optimization of most imaging optical

systems is accomplished by tracing several hundred or several thousand

rays. Sometimes performing a useful analysis of a relatively simple

illumination system may require the tracing of millions of rays for accurate

results. Process becomes time consuming. It is only with the recent advent of

powerful desktop computers that Ray Tracing method has become

commonplace for computer-aided illumination system design. Software which

are using ray-tracing are: Zemax, Radiance, Tracepro, Ecotect, DAYSIM,

SPOT, Adeline etc.

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Many commercial available software tools can be used for analysis of

illumination system. But very few are intended only for analysis of LED

illumination system. Almost all use ray-trace method for analysis. Accuracy of

results depends upon how many rays are traced which in turn decides

execution time. Also they require photometric data in IESNA format. If

datasheet does not have optical data of light sources in this format, extra

software is required to generate ray data format. If optimized system is to be

built using LED sources, there is need to develop new methods for analysis of

light system because of peculiar spatial and spectral features. The job

becomes more critical because of requirement of LED cluster to achieve the

desired lux level and color. The need was felt to have dedicated

computational tool to characterize and optimized multiple LED illumination

system.

2.5 Conclusion

Chapter provides an overview of illumination system and its design aspects.

Illumination plays an important role in several applications, including

manufacturing processes, medical and forensic lighting, plants growth and

domestic animal reproduction. Various studies have been carried out to find

illumination specifications for different applications. Based on these findings

the illumination applications are categorized as general illumination and task

illumination. Each application domain demands unique luminaire design.

Optimized illumination system design is needed to avoid adverse effect of

lighting on health, economy and environment. Now a days designers use

simulation tools for analysis and optimization of the illumination system. On

the basis of literature review it is found that the available software tools

simulate and optimize the standard luminaries available in the market for

conventional sources either using radiosity or ray-tracing algorithm. Use of

analytical equations however gives more accurate results.

_____________________________________________________________________


Literature survey and comparison between conventional light sources show

that solid state lighting is the future of lighting industry. LEDs are replacing

conventional light sources due to various advantages like low power

consumption, cost, long life, ecofriendly etc. Many research papers have

reported developments in fabrication of power LEDs and their modelling and

design of efficient LED system for specific applications. Researchers found

that LEDs are different from conventional sources in geometrical and optical

characteristics and need modifications in source modelling for designing of

LED luminaire. The luminaries of LEDs are to be specially designed since

multielement LED sources positioned at multiple places are required for

efficient lighting system. Moreover spatial and spectral characteristics of

LEDs are different from conventional light sources. The dedicated software

for LED luminaire design and analysis is not been reported so far. If system is

to be built using LED sources, there is a need to develop new methods for

analysis of light system. The job becomes more critical because of low lumen

output of an LED and requirement of cluster to achieve the desired lux level

and colour.

Realizing the enormous potential of LED lighting, work is focused on

development of multielement LED source system. The task has been taken

up to develop a user friendly tool for simulation of multielement LED source

based on analytical equations. Though primary focus is on LED as light

source, it can be used for other sources as well provided its geometry, spatial

and spectral characteristics are known.

Next chapter describes various source models reported in literature and

selection of best suited models for OPTSIMLED.

_____________________________________________________________________


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