ASST. PROF. IVA PETRINSKA

Transcription

1 ASST. PROF. IVA PETRINSKA 1 Faculty of Electrical Engineering

2 The Technical University of Sofia. Faculty of Electrical Engineering; R&DTCL Lighting Technics. Electric Lighting. Solid State Lighting. LED structure. Advantages and drawbacks. LED Drivers. Flicker. Thermal Management. Thermal measurements. Photometry and Colorimetry of LEDs. Quality of LED light. Glare our experience and investigation. 2

3 The Technical University of Sofia is the largest educational and scientific complex in Bulgaria in the field of technical and applied science with an institutional accreditation grade of 9.5 (on the scale of 10) for the period As the first and largest polytechnic center, which supported the establishment of most of the higher technical colleges in the country, it sets the educational standards and national priorities for the development of engineering education and science. 3

4 The University structure includes 14 faculties in Sofia, three departments, a Center of Information Resources, a Library and Information Center, a Center for international meetings, a Center for Continuous Education, a Center for Educational and Innovative Projects, a University Science and Research Complex for innovations and knowledge transfer in the field of micro and nano-technologies and materials, energy efficiency and virtual engineering, Post graduate schools, Scientific and Research Sector, small enterprises, a branch in the city of Plovdiv with two faculties, a Faculty of Engineering and Pedagogy and a college in the town of Sliven, a school for power engineering and electronics and two high schools School of Electronic Systems Technologies in Sofia and a Vocational School for Computer Technologies and Systems in the town of Pravetz. 4

5 The Faculty of Electrical Engineering (FEE) was established in The faculty prepares highly qualified specialists for the needs of the electrical industry, electrical power engineering, electrical power supply and electrical equipment in industry, transport and household appliances, lighting equipment and sources of light and other applications of electricity. There are 5 departments in the faculty of total academic staff about Doctors of Sciences and 345 Doctors have obtained their degrees from the faculty. 5

6 The faculty is responsible for education on Bachelor, Master and Doctor degrees on the specialities: Electrical Engineering; Electrical Power Engineering and Electrical Equipment, as well as Master degree in Electrical Energy from Renewable Energy Sources There are five departments in the faculty of electrical engineering: Department of electrical apparatus, Department of electrical machines, Department of electrical power engineering, Department of Electrical Power Supply, Electrical Equipment and Electrical Transport, Department of General Electrical engineering. 6

7 The department of Electrical power supply, electrical equipment and electric transport has four majors: electrical power supply, electrical equipment, lighting technology and electric transport. The department provides education in 48 subjects. The educational process takes place in 10 educational laboratories. During the last 30 years more than 2000 Bulgarian and 250 foreign electrical engineers have graduated in the field of Electrical power supply and electrical equipment of industrial enterprises" and more than 300 in the field of Electric transport. Under the supervision of members of the chair, more than 30 PhD dissertations have been defended. Lecturers and PhD students from the department develop and participate in the activities of the Scientific and research testing and calibration laboratory of lighting technology. 7

8 R&DTCL Lighting Technics is a leading lighting technology center of the TU - Sofia. For 30 years it has been a place where research was carried out, unique photometrical and measuring equipment has been constructed, representative lighting systems have been designed and young scientists have been trained. The laboratory is the only accredited lighting laboratory in Bulgaria, equipped with: photometric sphere with diameter of 2 m, automated mirror goniophotometer for measurement of light distribution curves of luminaires and light sources, automated gonioreflectometer for measurement of reflection characteristics of pavements and other surfaces, specialized digital camera for lighting measurements, colorimeter, spectroradiometer ect. 8

9 9

10 10

11 11

12 12

13 In 1879 Thomas Edison demonstrated the first light bulb. Over the past 125 years, incandescent and gas discharge technologies have provided many shapes and sizes of light sources for a variety of lighting applications. 13

14 Filament heating produces light Only 5% of the total energy input is converted to light and the rest is heat Very inefficient Efficacy Generally around 15 lm/w Color CRI = 95+ CCT = 2500K 3000K Life (average rated) hours Dimming can extend life 14

15 A halogen lamp contains an inert gas and a small amount of halogen. Efficacy PAR and MR Lamps (line or low voltage) 9 10 to 25 lm/w IR PAR Lamps (Infrared reflector) 9 20 to 30 lm/w Color CRI 95+ CCT Typically 3000K Life (average rated) 2000 hours Shortens if consistently dimmed below 80% 15

16 A fluorescent lamp is a low-intensity gas-discharge lamp that uses electricity to excite mercury vapor to produce ultraviolet (UV) radiation that causes a phosphor to fluoresce and produce light. Linear fluorescent lamps (LFL) and compact fluorescent lamps (CFL) are popular choices for conserving energy. About 20% to 30% of the total energy input is converted to light. 16

17 Types: Pin-base for dedicated fixtures Screw-base self-ballasted Efficacy: 25 to 60 lm/w Color CRI = 82 typical CCT = 2700K, 3000K, 3500K, 4100K, 5000K Life 6,000 to 10,000 hours Frequent on-off switching can reduce life significantly Dimming is possible but can reduce life 17

18 Lamp Efficacy Ranges from 65 to 105 lm/w Color CRI = 82 typical CCT = 2700K to 5000K Life 20,000 to 30,000 hours Frequent on-off switching can reduce life significantly Dimming can reduce life 18

19 Three types of HID lamps: Mercury vapor lamp Metal halide lamps High-pressure sodium (HPS) lamp Metal halide lamps produce light by passing an electric arc through a mixture of gases, which causes a metallic vapor to produce radiant energy. It contains a high-pressure mixture of argon, mercury, and a variety of metal halides in a compact arc tube. About 24% of the total energy input is converted to light. 19

20 Now, solid-state light (SSL) sources LEDs and OLEDs are evolving to displace some of the traditional light sources in some applications. Light-Emitting Diode (LED) Light-Emitting Polymer (LEP) Organic Light Emitting Diode (OLED) 20

21 LEDs are semiconductor diodes or electronic devices, that permit current to flow in only one direction. The diode is formed by bringing two slightly different materials together to form a PN junction. In a PN junction, the N side contains negative charge carriers, that is electrons, and the P side contains positive charge carriers, that is holes, which indicate the absence of electrons. When a forward voltage is applied to the PN junction electrons move from the N side towards the P side and holes move from the P side towards the N side and combined in the depletion zone between these regions. Some of these recombinations are radiative in which energy is released in the form of light. This principle of light generation offers many advantages and new opportunities. 21

22 The non-radiative recombinations occurring in the PN junction result in heat being generated in the semiconductor material. The ratio of the radiative to non-radiative recombinations in the PN junction determine the internal efficiency of the LED. Some of the light generated is lost within the semiconductor material due to effects such as total internal reflection, absorption, and shadowing of contacts resulting in only a certain portion of the light exiting the package. This is the extraction efficiency of the LED. To improve the overall efficiency of an LED package it is important to improve both the internal and extraction efficiency of the LED. 22

23 Within a cavity of the LED is a semiconductor chip mounted on to a leadframe which is housed in a premould package. The leadframe acts as a thermal path to dissipate the heat from the semiconductor chip and also serves as the electrical and mechanical interface to the Printed Circuit Board. A gold wire bond is used to connect to the top side of the chip to the leadframe The cavity is filled with epoxy resin and serves as a reflector to extract the maximum amount of light out of the package. 23

24 A monochromatic LED emits light in a narrow spectral band. The Spectral Power Distribution is a representation of the radiant power emitted by a light source as a function of wavelength. The semiconductor material used in an LED determines its wavelength or color of light. InGaN and InGaAlP are the two primary semiconductor materials and slight changes in the composition of these alloys changes the color of the emitted light. 24

25 One approach to generating white light utilizes a combination of three primary color red, blue and green LEDs. Another approach is to use a blue and yellow LED chip together in a certain ratio in sum to produce white light. Third approach would be to use a blue chip and a yellow phosphor to generate white light. OR by Utilizing an ultraviolet LED to excite - a red, green and blue phosphor. The most widely used approach to create a white LED is to use a blue LED chip combined with a phosphor. The phosphor layer absorbs a portion of the blue light and emits light at longer wavelengths. The phosphor concentration defines how much of the blue light is up converted. 25

26 OLED (Organic Light Emitting Diodes) is a flat light emitting technology, made by placing a series of organic thin films between two conductors. When electrical current is applied, a bright light is emitted. OLEDs can be used to make displays and lighting. 26

27 LED lights use about 50 percent less electricity than traditional incandescent, fluorescent and halogen options, resulting in substantial energy cost savings, especially for stores with lights that are on for extended periods. LEDs aim light in a specific direction unlike conventional bulbs, which emit light and heat in all directions (because LEDs are mounted on a flat surface, they emit light hemispherically rather than spherically). This directional lighting capability reduces wasted light and energy. 27

28 Unlike incandescent lighting, LEDs don t burn out or fail, they merely dim over time. Quality LEDs have an expected lifespan of 30,000 50,000 hours or even longer, depending on the quality of the lamp or fixture. A typical incandescent bulb lasts only about 1,000 hours; a comparable compact fluorescent lasts 8,000 to 10,000 hours. With a longer operational life, LEDs can reduce labor costs of replacing bulbs in commercial situations, achieving a lower maintenance lighting system. 28

29 LEDs love the cold unlike fluorescent lamps. At low temperatures, higher voltage is required to start fluorescent lamps, and luminous flux (the perceived power or intensity of light) is decreased. In contrast, LED performance increases as operating temperatures drop. This makes LEDs a natural fit for refrigerated display cases, freezers and cold storage spaces in addition to outdoor applications such as the parking lot, store perimeter and signage. 29

30 Without filaments or glass enclosures, LEDs are breakage resistant and largely immune to vibrations and other impacts. Traditional lighting is usually contained in a glass or quartz exterior, which can be susceptible to damage. LEDs, on the other hand, tend not to use any glass, instead they are mounted on a circuit board and connected with soldered leads that can be vulnerable to direct impact, but no more so than mobile phones and similar small electronic devices. 30

31 Most fluorescent and HID lamps do not provide full brightness the moment they re switched on, with many requiring three minutes or more to reach maximum light output. LEDs come on at 100-percent brightness almost instantly however, and with no restrike delay. For retailers, this can be advantageous following a power outage or anytime employees open the store during early morning hours when it is still dark outside. 31

32 Traditional light sources tend to have a shorter lifespan the more they re switched on and off, whereas LEDs are unaffected by rapid cycling. In addition to flashing light displays, this capability makes LEDs well suited for use with occupancy or daylight sensors. 32

33 It can take more than a few dollars to make commercial fluorescent lighting systems dimmable, but LEDs, as semiconductor devices, are inherently compatible with controls. Some LEDs can even be dimmed to 10 percent of light output while most fluorescents only reach about 30 percent of full brightness. LEDs also offer continuous, opposed to step-level, dimming (where the shift from 100-to-10-percent light output is smooth and seamless, not tiered). 33

34 Less than 10 percent of the power used by incandescent lamps is actually converted to visible light; the majority of the power is converted into infrared (IR) or radiated heat. Excessive heat and ultraviolet radiation (UV) presents a burn hazard to people and materials. LEDs emit virtually no IR or UV. Rapid advancements in LED lighting technologies, with more improvements on the horizon, have resulted in lowered costs and increased reliability of LEDs. And while it may be tempting to assume LEDs are the right choice for all applications because of their energy efficiency, selection should be based on a combination of factors, including light quality and distribution, dimmability, and expected lifetime. 34

35 High initial price Voltage sensitivity Hard to mimic the light distribution that other forms of light produce; LEDs are direct focus lights Thermal dependency (requires effective cooling to prevent overheating of the LED lamp) Quality Issues. 35

36 To operate the advantageous LED lighting in the grid system, special circuits are necessary which convert the mains voltage to a precisely matching voltage for the connected LED luminaire. These circuits are power supplies optimized for LEDs and are commonly known as "LED drivers". An LED luminaire usually consists of several LEDs, the number of which depends on the required light output. The LEDs are connected to a PCB in series or parallel circuits. When connecting the LEDs, series circuits are to be prioritized to ensure that the same current is applied to all LEDs. In a parallel circuit the same voltage is applied to each LED. Theoretically the same current would pass through each LED if they do not have any production tolerances and if a constant temperature of all LEDs is guaranteed. A luminaire with LEDs connected in series may best be operated with a constant current driver. LEDs and drivers for this mode are usually marked "CC" (constant current). 36

37 There are applications for which a parallel circuitry is required (if the maximum voltage is limited by the protection class). Even if the LED luminaire can be expanded modularly, a parallel circuitry would be the best choice (LED strip which can be cut by the user as desired, or a system which allows single modules to be turned into a larger panel by pins). Such a structure consists of small, equal-sized groups of uniform, series-connected LEDs. All groups contain the same quantity of LEDs, they can be operated with the same voltage. The threshold voltages of the individual groups may vary in terms of production tolerances and temperature differences so applied currents may also vary significantly. This unequal load must be compensated. For this purpose, an element is added to each group which counteracts the impact of tolerances. In the simplest case this would be a resistor, connected in series to the LED group. If the forward voltage of a group is smaller than the calculated value, a distinctly higher current would pass through this group. Current and voltage are proportionally linked together within the resistor so that a higher current reduces the voltage available with the LEDs. Such a resistor converts some of the energy into heat, thus reducing the luminaire's efficiency. These luminaires need to be operated at a constant voltage to ensure an even current for all LEDs, which is why they are marked "CV" (constant voltage). 37

38 If several panels are to be operated with an LED, only LED luminaires of the same type should be used. It is therefore not possible to mix CC and CV luminaires. If CC luminaires are chosen, they will be connected in series by ensuring that all panels are designed for the same current. There is, however, a risk that all panels connected to the driver remain dark if a single LED fails. If several CV panels are operated by a single driver, they will be connected in parallel. It must be observed that all panels are designed for the same current. In case an LED fails, this will only affect part of the system's LEDs. 38

39 Flicker is also nothing new with SSL. Most of the attention has focused on the ripple frequency that occurs on the output of the LED drivers, which is typically two times that of the input. The light output of an LED correlates closely with the output waveform of its driver. Driver output ripple current Measured light output 39

40 Flicker is also present with pulse-width modulation (PWM), a technique commonly used to dim LEDs. The Figure on the right shows flicker index versus duty cycle for a square wave at three different modulation percentages. The worst case flicker index, with the value approaching 1.0, would be for a light that flashes in short, lowfrequency bursts. 40

41 If designing a custom driver for a luminaire, capacitance should be added to the output of the driver to filter out the AC ripple component - this can potentially decrease system reliability, especially if low-quality capacitors are used. In many applications, such as replacement lamps, it may not be possible to add sufficient capacitance because of physical space constraints. If a luminaire designer chooses to use a commercially available driver, a driver that minimizes the amount of driver ripple current should be selected. One cause of flickering is compatibility issues with dimming and control circuitry. It is important to specify and verify that the products are indeed compatible with the dimmers or other control circuits used in the lighting system. Furthermore, random, intermittent flickering could be an indication of some other problem in the lighting system such as loose wiring and interconnections. Possible problems with the quality of the electrical supply can also result in power fluctuations. If those causes are suspected, it is important to investigate further to prevent any potential safety hazards 41

42 Main cause of LED failures is improper thermal management. Many performance characteristics of LED components are influenced by the operating temperature. Some performance characteristics experience a recoverable change, such as light output, color and voltage, while others, such as lifetime, can experience a nonrecoverable degradation due to high operating temperatures. Exceeding the maximum operating temperature specification, which is typically a 150 C junction temperature, can cause permanent and/or catastrophic damage to LEDs, so care must be taken to operate LEDs below this limit 42

43 Elevated junction temperatures cause recoverable light output reduction. As the junction temperature increases, the light output of the LED decreases, but recovers when the LED cools. With increasing junction temperatures, the color of all LEDs shifts. Forward voltage decreases as the junction temperature of an LED increases. The reliability of any LED is a direct function of junction temperature. The higher the junction temperature, the shorter the lifetime of the LED. CCx and CCy shifts relative to solder point temperature 43

44 LEDs generate visible light when current passes across the junction of the semiconductor chip. LEDs are not 100% efficient; much of the power running through an LED is output as heat, which thus needs to be dissipated. That is, under normal operating conditions, approximately 50% to 60% of the input power is output as heat, while the rest of the input power is converted to light. Thermal power is estimated through the equation: Pt = 0.75 Vf If Pt is the thermal power (W) - this is the amount of power the system/heat sink must dissipate. Vf is the forward voltage of the LED (V) If is the source current to the LED (A) 44

45 There are three basic modes of heat transfer: conduction, convection and radiation. Each plays a role in LED performance and final system design and must be understood for proper thermal management. Conduction is the transfer of heat through a solid material by direct contact. This is the first mode of heat transfer to get thermal power from the LED junction to the heat sink. Metals are typically the best conductors of heat. The heat conduction potential of all materials can be expressed as thermal conductivity, typically abbreviated as k. The equation below shows how to calculate the quantity of heat transferred via conduction. Qcond is the amount of heat transferred through conduction (W) k is the thermal conductivity of the material (W/m K) A is the cross sectional area of the material through which the heat flows (m2 ) ΔT is the temperature gradient across the material ( C) Δx is the distance for the heat must travel (m) 45

46 Convection is the transfer of heat through the movement of fluids and gases. In LED systems, this is typically the transfer of heat from the heat sink to the ambient air. There are two sub-categories of convection: natural and forced. Natural convection occurs with no artificial source of field movement and is due to the buoyancy forces induced by thermal gradients between the fluid and solid. Forced convection occurs when an external instrument such as a fan, pump, or other device is used to artificially move the fluid or gas. In LED cooling systems, convection is the main mode of heat transfer to remove the generated heat from the LED system and heat sink. The equation below shows how to calculate the quantity of heat transferred via convection. Qconv = h A T Qconv is the amount of heat transferred through convection (W) h is the heat transfer coefficient (W/m2 K) A is the surface area (m2 ) ΔT is the temperature gradient across the material ( C), typically the difference between the surface temperature and ambient air temperature 46

47 The transfer of thermal energy through an electromagnetic field is the third component of heat transfer, radiation. The magnitude of radiation heat transfer is based on the emissivity of the material, which is the ratio of how closely the surface approximates a blackbody. In an LED system, radiation typically has a very small effect on the net system heat transfer since the surface areas are typically fairly small and surface temperatures are relatively low, to keep the LED junction temperatures below the maximum rated temperature of 150 C. The equation below shows how to calculate the quantity of heat transferred via radiation. Qrad is the amount of heat transferred through radiation (W) ε is the emissivity of the surface (dimensionless) σ is the Stefan-Boltzmann constant (5.67 x 10-8 W/m2 K4 ) A is the surface area (m2 ) Ts is the surface temperature of the material ( C) Tf is the fluid temperature of the medium ( C), typically referenced to the ambient air temperature 47

48 The thermal path of an LED system can be illustrated by a simple resistor network similar to an electrical circuit. Thermal resistances are represented by the resistors, the heat flow is approximated by the electrical current, and the corresponding temperatures within the system correspond to the electrical voltages. The figure is a resistor network representation of a multiple-led system on a printed circuit board (PCB) mounted to a heat sink in ambient air. T is the temperature at each corresponding location ( C) Θa-b is the thermal resistance from point a to point b ( C/W) n is the number of LED components on a single PCB 48

49 Heat is conducted from the LED junctions through the LED components to the PCB, through the thermal interface material (TIM) to the heat sink and then convected and radiated to the ambient air. The nodes in the circuit represent the individual sections within the system and the locations where temperatures may be measured. The resistors represent the thermal resistances of the individual contributors. The system is divided into a network of parallel connections for the multiple LEDs and series connections for the singular components. If the system includes only one LED, or n =1, the entire thermal path is simply in series. 49

50 The individual thermal resistances described above can be calculated from: Θa-b is the thermal resistance from point a to point b ( C/W) Ta is the temperature at point a ( C) Tb is the temperature at point b ( C) Pt is the thermal power The thermal resistance of the entire system can also be compared to an electrical circuit in series, where the system thermal resistance can be calculated as shown below: Θsys,a-z is the system thermal resistance from point a to point z ( C/W) Θa-b is the thermal resistance from point a to point b ( C/W) Θb-c is the thermal resistance from point b to point c ( C/W) Θy-z is the thermal resistance from point y to point z ( C/W) 50

51 Accurate temperature measurements are required to appropriately design a thermal system and to evaluate and assess an existing design. Whether for a final design or a prototype, the measurement process is the same and requires due diligence to make sure realistic and accurate measurements are made. Various methods to measure temperature exist, and for LED systems the most common methods are simple thermocouples, infrared (IR) microscopes, and pulsed voltage/transient response monitoring. IR camera measurements can be useful to get a quick visual representation of the heat spreading through an LED system. Simple thermocouples are the most common and simplest method to obtain accurate data and are recommended for precise absolute LED system measurements. For early design validation, before investing in expensive prototyping and multiple design revisions, it is highly recommended to perform thermal simulations. 51

52 Many aspects of traditional photometry are not applicable for evaluating LED products. With conventional sources of the past, the science of photometry sought to decouple the characteristics of the lamp from the performance of the fixture. Thus fixture and lamp performance can be evaluated separately, giving the opportunity for different lamp-fixture combinations. The methodology for this type of testing is known as relative photometry. Decoupling lamp from luminaire in an LED product is not practical since the LED performance is tied to the thermal design of the luminaire. This method is termed absolute photometry, since the results are actual or absolute, not relative or scaled to a source that was not actually tested. The limitation of absolute photometry is that it is applicable only for the exact luminaire / light source combination tested. 52

53 Absolute is more difficult because: There is a need to maintain flux standards and calibrate equipment (Calibrate with incandescent, measure other light sources?) Sampling concerns How many luminaires to be tested? How to choose them? Are samples representative? Environmental and operating conditions must be reproduced while maintaining calibrated equipment (temperature, input voltage or current). 53

54 Total Luminous Flux and Luminous Efficacy measurement: Integrating sphere with a spectroradiometer; Integrating sphere with a photometer head; Goniophotometer. Luminous Intensity Distribution measurement: Goniophotometer. Color Characteristics Measurement: Sphere-spectroradiometer system; Spectroradiometer or colorimeter spatially scanned. 54

55 Light quality is indicated by two measurements, correlated color temperature (CCT) and the color rendering index (CRI). CCT is measured in Kelvins ; cool white light is 5000K, while warm white light has a low CCT at about 2700K. Until recently, most white light LEDs had very high CCTs, often above 5000K, but warm white LEDs are now available. They are slightly less efficient than cool white LEDs. The CRI is a measure of how color appears when illuminated by a light source compared to reference sources such as incandescent light or daylight. A CRI of 100 is identical to the reference source, so the higher the CRI the better. Phosphorconverted warm white LEDs are being produced that claim to have a CRI of 80, a value most people find quite acceptable. Others exceeding 90 also have been reported. 55

56 To create more consistency of light quality in the manufacture of LEDs, a process called binning is used. The CRI has been found to be inaccurate for white light RGB LEDs, and there is controversy in the industry as to the reliability of the rating for other lighting types as well, so a new measurement system is under development. 56

57 Binning is how LED manufacturers reconcile manufacturing process variations and the exacting sensitivities of the human vision system. Hot binning is binning the LED lamps at a higher temperature than the conventional 25 C. The LED manufacturers who have decided to launch new products binned at an elevated temperature have converged on 85 C as the new conventional binning temperature. 85 C binning is an arbitrary number, like 25 C binning before it but its advantage is that 85 C is a lot closer to the typical operating temperature of many Solid State Lighting products. 57

58 Designing LEDs into general illumination requires a choice between designing either a complete luminaire based on LEDs or an LED-based lamp meant to install into an existing fixture. Generally, a complete luminaire design will have better optical, thermal and electrical performance than the retrofit lamp, since the existing fixture does not constrain the design. The convenience of a retrofit lamp is more important in the target application. 58

59 Optics is one of the key factors in the operation and duration of the LED lights. Optics in LED lighting design is based on mirrors, lenses and elements that help control the projected light. Optics is essential to the efficiency of the light. Optical components help direct light, increase intensity or protect from glare. Types of optics for LEDs Primary, is placed directly on the LED and protects each chip individual. Secondary, in charge to mold the light depending on the type of lens installed; lens types can be: divergent, to expand the light beam, or convergent, to concentrate the light beam at a particular point. Such optical elements are secondary lenses, reflectors, diffusors, refractors. Tertiary is composed of other external components that collect light from diverse LED lamps. 59

60 Where the light goes is important for two reasons. One is providing light where it is needed and preventing it going where it should not go. The other is with the very good optical control of well- designed LED products, additional energy can be saved avoiding the typical excessive blob of light underneath or in front of most other lighting technology products. A cosine distribution directs most of the light straight down or straight in front, while a batwing distribution shines more angled light. 60

61 One of the issues that stays as a challenge in front of the lighting producers and designers concerns the light distribution of LEDs - lately a nearly batwing curve has been reached. The more serious problem concerning the indoor LED luminaires is the glare that they cause. The term glare includes two different types of glare: disability glare and discomfort glare. o Disability glare is the one that leads to reduction of the visibility due to scattered light in the eye and does not necessarily lead to physical discomfort. o Discomfort glare is defined as glare which causes discomfort without necessarily impairing the vision of objects. o These different effects are brought about by different mechanisms in the human visual system and as a consequence the discomfort and the disability are actually independent, even though they usually occur together. 61

62 The established systems for estimation of glare are: Unified Glare Rating (UGR) (EN /2011); Visual Comfort Probability (VCP) (IESNA 2000); British Glare Rating System (CIBSE 1997). They cannot be used for LED luminaires estimation, because they give inaccurate results. The problems that occur when glare is estimated for rooms illuminated with LED luminaires concern the definition of the exact size of the source of glare and the brightness of its immediate background. This problem comes from the fact that for some existing LED luminaires the bright LEDs are apparent through the optical system, which leads to doubt what is the size of the apparent area of the luminaire that of the LED array chip, that of a single LED or that of the whole luminaire. The second concern is the Guth position index used in the UGR formula the problem here comes from the fact that the LED luminaires where the LED arrays are visible usually do not have uniform luminance distribution and constant spectrum. The third issue is the light distribution of the LED luminaires nearly batwing curve have been achieved, but this leads to significant losses in the optical system. 62

63 The UGR system, developed in 1995 by CIE is used for evaluation of the discomfort glare of indoor lighting systems. L b is the background luminance (cd/m 2 ), Li is the luminance (cd/m 2 ) of each luminaire, ω i the solid angle [sr] in the direction of the observer, P i the position index of each luminaire. This glare estimation approach is based on results for fluorescent lighting fixtures and for small sources (< 0.005m 2 ) the UGR values overestimate glare. UGR estimation method aimed for small light sources is established: I is the luminous intensity (in cd) toward the eyes, r is the distance (in m) between the observer s eye and the light sources. 63

64 In order to estimate the glare for a matrix LED luminaire and to verify the applicability of the UGR formula an experimental setup is used, utilizing a LED luminaire with size 600х600mm. It is equipped with 32 LED matrixes arranged as four lines with 8 matrixes per line. The correlated color temperature of the luminaire is 4000K, and its luminous flux is 4000lm. 64

65 The question that occurs is what is the area of the glare source, that that should be considered, so that the luminance L i can be calculated or which intensity should be considered. The other question is which expression should be used at all, because apparently the luminaire considered consists of number of small illuminating matrixes. In order to clear this problem the UGR is calculated once using equation for small sources of light considering the emitting area of a single LED matrix, once considering a whole line consisting eight LED matrixes, once for a line of LED matrixes and its surrounding area and once for the whole luminaire. For the last three cases since the area of the light source is bigger than 0.005m 2, equation (1) is used. 65

66 Area under consideration Area, m 2 I, cd L, cd/m 2 UGR calc. UGR estimation EN / x x x eq.(1) 23.4 eq.(2) x x x x x x

67 An experiment on the visual acuity of test participants under LED illumination has been conducted in order to obtain subjective estimation of the discomfort glare. 20 independent subjects 16 men and 4 women with normal vision have participated in the experiment. During the experiment each subject has a fixed position in the room (7x4x2.5), illuminated by a single LED luminaire, placed on the ceiling, which is in the visual field of the observers, 4 meters from his position and additional diffuse component, placed so that they are not glare sources. At the beginning of the experiment every test subject is left in the test room for 15 minutes to adapt to the lighting conditions and instructed. After that time a Landoldt ring with random orientation and diameter of 40 mm and 8 mm gap width (with two different contrasts between the ring and its background) is shown to the test subject, so that he becomes familiar with it. In order to mimic the usual activities done in an office space the test objects are shown a Landoldt ring with random orientation and then asked to mark this orientation on a piece of paper. Landoldt rings with five different sizes, eight different orientations and two different ratios L ring /L background are shown to the test subjects. 67

68 Landoldt ring Outer diameter, mm Gap width, mm С С С С С Every Landoldt ring is shown to the participants in the experiment on a monitor for one second, after that they answer the question about its orientation for 30 seconds. The brightness of the monitor changes to cd/m 2 (R=186, G=186,B=186). и 0.95 cd/m 2 (R=96, G=96, B=96), while the luminance of the Landoldt rings is constant 0,57 (R=0,G=0, B=0) cd/m 2. 68

69 Participants in the experiment Number of mistakes Size of the Landoldt ring and luminance of its background overall % diameter of the Landoldt ring 30mm, cd/m diameter of the Landoldt ring 25mm, cd/m diameter of the Landoldt ring 20mm, cd/m diameter of the Landoldt ring 15mm, cd/m diameter of the Landoldt ring 10mm, cd/m diameter of the Landoldt ring 30mm, 0.95 cd/m diameter of the Landoldt ring 25mm, 0.95 cd/m diameter of the Landoldt ring 20mm, 0.95 cd/m diameter of the Landoldt ring 15mm, 0.95 cd/m diameter of the Landoldt ring 10mm, 0.95 cd/m Subjective evaluation Number of participants Imperceptible 1 Noticeable 8 Disturbing 9 Intolerable 2 69

70 Because of the unsufficient experimental data gathered so far the experiment on the glare produced by LED office luminaires has to be continued. There are also additional issues and problems, that influence glare and must be considered. The influence of light distribution and spectrum of the LEDs on glare has to be estimated and the latest findings in the colorimetry of LEDs has to be considered. Also when diffuse LED luminaires are used an estimation of the losses in the optical system and their real efficiency has to be estimated. An optimal degree of shielding of the LED chips by different optical elements has to be estimated. Some preliminary research of the authors has shown that the discomfort glare caused by LED luminaires show that it can be reduced through: increase of the background luminance L b (especially on the working plane and the wall or surface against the observer); decrease of the luminance of the luminaire in direction towards the observer (cut off in the area above 45⁰ and especially above 65⁰ to the vertical line of vision); increase of the area of the luminaire or use of glowing periphery of the luminaire. The existing methods for estimation of glare are not straight forward when applied for LED luminaires. This leads to the need of new critearia and research in the area. 70

71 71