Light Emitting Diodes (LED) are classified as Solid State Lighting (SSL) light sources. The principle of operation consists in radiative recombination (light generating) of medium in a semiconductor structure called the chip. A standard LED emits light in a specific colour only, depending on semiconductor material used.
A combination of primary colours (red, green and blue) is required to generate a white light. LED design includes a luminophore – a material converting high-energy light (blue or UV) to a low-energy light (green to red). Three method are generally used to emit white light:
- A blue LED with a single – luminophore emitting yellow light (combination of red and green light). This system is easy and cheap to manufacture and features high efficiency and relatively low Colour Rendering Index (CRI). It is most commonly used by LED manufacturers in applications where high efficiency is required and light quality is of lesser importance.
- A blue LED with a dual – luminophore emitting red and green light. A combination of blue light emitted by the LED chip and a light emitted by the luminophore in a specific ratio allows white light from warm, similar to standard incandescent light (~2700K) through neutral (~4000K) to cold white (>5000K) light. This systems are used whenever a balance between high efficiency and high CRI is required.
- A UV LED with a triple – luminophore emitting green, red and blue light. This is the most complex and advanced system available. It emits white light with the highest CRI at the expense of efficacy and lifetime.
Spectral characteristics of white LED, fluorescent lamp and incandescent lamp are shown below.
LED power supply requires current stabilisation as opposed to other sources which require voltage stabilisation only. It is a result of a negative temperature coefficient of the LED. When the LED, in particular PowerLED, is supplied with direct voltage, a current flow determined by a current – voltage (I-U) characteristics of the LED is forced. The current flow results in structure heating, reducing LED’s forward voltage and thus increasing current flowing through the LED and further heating which as a consequence may cause LED damage due to overheating. DC power supply eliminates this problem – as the LED heats, the forward voltage and thus the power input and generated heat are reduced, and operating conditions are stabilised (with proper heat removal).
Color Rendering Index (CRI) – is a measure on a scale of 0-100 of the ability of a light source to reproduce the colours of various objects faithfully. The closer the value is to 100, the more natural is the colour perception. Incandescent lamps and the sun have a maximum CRI. Fluorescent lights range from about 65 to 90, while LEDs by leading manufacturers have about 75 to 95 CRI.
Generally, the difference in CRI between type II and III LEDs is slight. The LEDs with dual – luminophore range from about 90 – 95 CRI, typical LEDs with single – luminophore have 80 – 85 CRI, whereas UV LED have 95 – 98 CRI. CRI of at least 80 is generally recommended for most lighting applications. CRI above 95 is required whenever the highest colour rendering level is required (museums, art galleries and operating theatres).
A hue of light emitted by a light source is defined by a colour temperature corresponding to the temperature of an ideal black body radiator. The ideal black body theory defines an object heated to a certain temperature in the unit of absolute temperature, the kelvin, emitting a light which can be perceived as warm (<3300 K), neutral (3300 K to 5000 K) or cold (>5000 K). The perfect example is a standard incandescent light with colour temperature of approx. 2,700 K (warm white), since the filament is heated to this exact temperature.
A luminous efficacy is the ratio of luminous flux to 1 W of power consumed by the light source expressed in lumen per watt unit (lm/W). The higher the value, the more cost-effective the light source is. Standard luminous efficacy of an incandescent lamp is 10-12 lm/W, 35 – 50 lm/W for a compact fluorescent lamp, 50 – 100 lm/W for a fluorescent lamp and 100-150 lm/W for a standard LED.
Most luminaires with standard incandescent or fluorescent lamps have lower effective efficacy than the light source itself due to the reflections inside the luminaire. For luminaires with LEDs, a light beam is directed precisely on an illuminated surface.
The classification is simple: high power LED – PowerLED – power above 1 W, mid power LED – power approx. 0.5 W and general class of other LEDs with power below 0.25 W.
The most important parameter determining how long the LED will maintain its properties for a specific application by providing a required luminance. LEDs, in particular used in lighting applications, have a limited lifetime. Most often, the manufacturers specify L70 lifetime. For example, LED lifetime specified by the manufacturer as L70= 50,000 h represents the time of the average lumen maintenance to drop to 70% of its initial value. For lighting applications, this value is a threshold, below which the light source does not meet the specifications and requires replacement.
Standard LED lifetime ranges from 50,000 to 100,000 h. The lifetime of a luminaire is limited by the component with the shortest lifetime.
LEDs in our designs
Luxon products feature two types of OSRAM Duris E5 LEDs. It is one of the most advanced mid-power LEDs available today. Its luminous efficacy exceeds 130 lm/W and L70 lifetime in all LUXON LED application exceeds 50,00 h, calculated in accordance with the Commission Regulation (EU) No 1194/2012. Duris E5 LEDs are used in Smart:LED and Intel:LED luminaries, whenever a uniform and an elegant illumination is required.
OSRAM Oslon Square – is a 700 mA supplied LED with extreme efficacy up to 150 lm/W and L70 lifetime of up to 100,000 h. It features 2 mm2 chip and ceramic package with high mechanical strength and is used in Highbay:LED luminaires and custom track-light designs.