Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 132

dots are applied. Where for example, traditional rare-earth phosphors also produce
some infrared radiation that does not contribute to visible radiation, the emission in
infrared can be better controlled with quantum dots. Quantum dots, either used as
the sole wavelength conversion method or in combination with phosphors, may in
the future also be used in LED systems for road lighting.
Research is also going on to produce single, multi-layer LED chips, each layer
producing a specific colour of light so that the combined result is white light.

10.3.2 LEDs

10.3.2.1 Construction

Different steps, or levels, are commonly distinguished in the construction of LED
systems, ranging from level 0 to level 3.

• Level 0: LED chip.

• Level 1: LED package. Chip packed with its electrical connection, mechanical

connection and protection, heat dissipation device and basic optical components.

• Level 2: LED cluster. Since, for most applications the light output of a single LED

is not sufficient, multiple LEDs are assembled on a printed circuit board (PCB).

• Level 3: LED module. A module with LED cluster, heat sink, electrical driver

and sometimes an optical device. The LED module functions as a lamp.

Sometimes a level 4 and 5 is used to characterize the LED luminaire and the LED
lighting installation respectively.

LED Chip The semiconductor, crystalline, p and n sandwich material, the basis
of the LED chip or LED die, is deposited on a substrate of suitable composition in
a complex industrial process called epitaxial growth. The sandwich and substrate
material together determine the quality of the LED chip. The LED chip resulting
from this process is a “photon or light trap”: that is to say, much of the light emitted
within the chip is internally reflected by its surfaces (borders between the material
and air) and ultimately, after multi reflections, absorbed in the material (heating this
up). Only light that hits the outer surface more or less perpendicularly (approximately
20 ⓦ ) can leave the material. By giving the chip a specific shape, the so-called light-
extraction efficiency can be improved. Figure 10.19 gives an example of such a
specifically-shaped chip.

LED package The design of LEDs is progressing rapidly, and construction of LEDs
will consequently change. Figure 10.20 which shows the construction of a high power
LED package, should be seen as a typical illustrative example. The LED chip is placed
in a reflector cup, which because of its shape, helps to direct the light in an upwards
direction. Highly reflective metal or ceramic material is used. All high-power LEDs
have a heat sink of high-thermal conductivity material to conduct the heat away from
the chip. Non-successful recombinations of p and n-atoms at the chip's junction, and