1. LED Basics
2. LED Optics
3. LED Thermal
A light-emitting diode (LED) is a semiconductor device that emits light on certain wavelength (color). A die (active area of LED) is encased in plastic or ceramic housing. The housing may incorporate one or many dies.
Figure 1.1.1: When LED is switched on, electrons recombine with holes within the device, releasing energy in the form of photons with certain wavelength (color).
Photon: Unit of light
The term SSL (Solid State Lighting) is common term for LED technology being used for lighting applications. It refers to technology in which the light is emitted by solid-state electroluminescence as opposed to incandescent bulbs (where the light is emitted via thermal radiation in visible part of spectrum – incandescence).
White LED working principle
The most common method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphor (Figure 1.1.2a) of different colors to form white light; the resultant LEDs are called phosphor-based white LEDs. The “blue” photons emitted by High-brightness LED (HB LED) (Figure 1.1.2b) either passes through the phosphor layer without alteration, or they are converted to the “yellow” photons in the phosphor layer (Figure 1.1.2c). The combination of “blue” and “yellow” photons leads to white light (Figure 1.1.2d).
Figure 1.1.2: a) Cross-section of standard phosphor-based white HB LED. b) Recombination of electrons with holes results to “blue” photons. c) “Blue” photons either passes through the phosphor layer without alteration, or they are converted to the “yellow” photons in the phosphor layer. d) Combined together, they create white light.
Spectrum of a phosphor-based white LED clearly showing blue light directly emitted by the LED die and the more broadened yellow light emitted by the phosphor (Figure 1.1.3).
Figure 1.1.3: White light can be produced by combining blue and yellow light only. Sir Isaac Newton discovered this effect when performing color-matching experiments in early 1700s.
Figure 1.2.1 depicts basic parameters of LED light source compared with the most common traditional light sources. LED shows better or at least comparable numbers for all important parameters than traditional light sources.
Figure 1.2.1: Various light sources versus LED.
Efficacy of LED luminaires
The efficacy (energy efficiency) of LED lighting fixtures is ratio between net lumen output (in lumens) and input power (in watts) of a luminaire, or lm/W. LEDs with the highest efficacy are the coolest whites – 5000 K and above.
Color rendering index – CRI
Color rendering index measures the ability of a light source to render colors of illuminated objects faithfully in reference to an ideal light source – sun or incandescent bulb.
Figure 1.2.2: Standardized color samples set.
Correlated color temperature – CCT
Color temperature of a light source is the temperature of an ideal black-body radiator (solid object with certain properties heated up to point of incandescence) that radiates light of comparable hue to that of the light source, and its temperature is expressed in Kelvins (K). As a black body gets hotter, wavelenght of light emits progress through a sequence of colors from red to blue (Figure 1.2.3). Sequence of colors is described by curve (Planckian locus) within a CIE 1931 color space (Figure 1.2.4).
Figure 1.2.3: Example of LED color temperature correlation.
For colors based on black body theory, blue occurs at higher temperatures, while red occurs at lower temperatures. This is the opposite of the cultural associations attributed to colors, in which “red” is “hot”, and “blue” is “cold”.
Figure 1.2.4: The black-body curve (Planckian locus) defines the range of color temperatures, from warm (reddish) to cold (bluish), within the CIE 1931 color space.
2. LED Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it.
Luminaire is a device that changes light distribution of a light source, diffuses the light or eventually changes its spectral composition. This goal is reached by luminous active parts of luminaire (reflector, diffuser, lens, etc.) and auxiliary parts (socket, leads, starter, ballast, etc.) that are designed and constructed for specific light sources. Luminaire is also equipped with parts that are used for fixation, protection of a light source and feeder.
Figure 2.1.1: Simplified functional scheme of LED-based luminaire.
Reflector is an optical part that regulates luminous flux from the light source by reflection from reflector material – a mirror reflection, a diffuse reflection, and a mixed reflection.
Reflectors are basically divided into two groups: first group includes conic reflectors of four basic geometries – elliptical, zonal, hyperbolic, and parabolic (Figure 2.2.1); second group includes non-conic reflectors, such as square or asymmetric ones of the same basic geometries of reflection surfaces.
Figure 2.2.1: Four basic geometries of a reflector.
Figure 2.2.2: Various reflectors for LED light sources.
Diffuser directs the light by diffuse-scattering through its material. Diffuse light is also obtained by making light to reflect diffusely from a white surface. Based on diffusion mechanism diffusers are divided into the following types: Opal, Gaussian, and Prismatic diffuser.
Figure 2.3.1: Types of diffusers.
Lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam.
Figure 2.4.1: Two basic types of lenses – converging and diverging.
A simple lens consists of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis. The use of multiple elements allows more optical aberrations to be corrected than it is possible with a single element. Lenses are typically made from glass or transparent plastic.
Figure 2.4.2: Different types of lenses used with LED light sources.
Various optical parts need various optical materials. Aluminium with various types of finish and powder-coated metal sheets are used for reflectors. Clear polycarbonate (PC), polystyrene, and Polymethylmethacrylate (PMMA) are used for micro-prismatic diffusers and lenses.
Materials for reflective optics – with various characteristics are available in order to achieve different types of reflection. There are three basic types of reflection: mirror, diffuse and mixed reflection. Difference between types of reflection is in the proportion of mirror and diffuse part of reflection.
|Ideal mirror reflection||Real mirror reflection||Ideal diffuse reflection||Mixed reflection|
|Lambertian reflection||Lambertian with mirror reflection|
Figure 2.5.1: Different types of reflection from material.
3. Led Thermal
LEDs, like all electronic devices, are temperature dependent. LED performance strongly depends on the ambient temperature. Operating LED-based luminaires at high ambient temperatures without proper thermal design may overheat LED packages, eventually lead to short lifespan or device failure in the worst case. Well-designed cooling system is required to maintain long lifetime and high efficacy of the luminaire (Figure 3.1.1). Therefore thermal management is the most critical part within LED-based luminaire design.
Figure 3.1.1: a) Thermal simulation results. b) picture of the same luminaire taken by thermo-camera.
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy and heat between physical systems. It is classified into three main mechanisms (Figure 3.2.1):
• Thermal Radiation
Figure 3.2.1: Three main heat transfer mechanisms.
All light sources convert electric power into light and heat in various proportions. Incandescent bulbs emit mainly in infrared (IR) region with only approx. 8% of light emitted. Fluorescents emit higher portion of light (21%) but also emit IR, UV, and heat. LEDs generate little IR and convert up to 40% of the electrical power into the light (see Figure 3.3.1). The rest is converted to heat that must be conducted from the LED active area to the underlying printed circuit board, cooling system, housing, and atmosphere.
Figure 3.3.1: Power conversion rates for “White” Light Sources.
In case of LED light source, there is no heat removal by thermal radiation, thus dissipative heat has to be withdrawn only by conduction and convention. However LED has the best efficacy, thermal management of this light source is the most challenging and proper design of cooling system is crucial for LED-based luminaire.
The use of LEDs has been increasing dramatically over the last few years. At the beginning, heat dissipation from LED junction was not a problem because low-power LEDs were used. However, modern high-power LEDs dissipate much higher portion of heat which has to be removed from the junction in order to maintain high efficacy, reliability and lifetime of LED-based light source.
Basic parameters to evaluate LED performance are (Figure 3.4.1):
• Junction temperature – Tj
• Thermal resistance – Rj-a
|Junction Temperature||Thermal resistance|
Figure 3.4.1: Basic parameters to evaluate LED performance.
From the thermal design point of view, typical LED-based luminaire consists of an LED light source, a printed circuit board (PCB) and a cooling system. The LED light source incorporates semiconductor die (active part), optics, casing, and heat slug which is used to withdraw the heat away from the die. Heat slug is soldered to the PCB (mostly metal core PCB – MCPCB).
Figure 3.5.1 shows simplified setup of typical LED luminaire being simulated with basic input/output parameters.
Figure 3.5.1: Simplified setup of typical LED luminaire being simulated with basic input/output parameters.