licht.wissen No. 01 "Lighting with Artificial Light"

licht.wissen 01
Lighting with Artificial Light
Light generation today: LEDs
Light generation in tiny electronic components
or in surfaces with eco-friendly materials:
controllable, durable and efficient.
Today’s LEDs are suitable for almost every
lighting task.
The LED is an electronic semiconductor
element that emits light when current
passes through it. The solid crystal is stimulated
to radiate light by means of direct
current: the electronic chip emits light as
the result of compensation between an excess
of electrons at the junction between
the n-type semiconductor region and the
p-type semiconductor with its lack of electrons.
Depending on the material of the
semiconductor crystal, monochromatic light
is produced in a narrow waveband.
White LED light can be generated using different
methods. The most common method
is “luminescence conversion” which is also
used in fluorescent lamps. Here, a thin layer
of fluorescent phosphorus is evaporated
above a blue LED chip. Some of the blue
light is converted into white light by the
yellow phosphorus. Another way of obtaining
white LED light is to mix coloured light:
RGB colour mixing. This additive colour
mixing of red, green and blue (= RGB) can
produce white light in addition to all other
light colours.
In practice, almost all colour temperatures –
from 2,700 to 7,000 kelvins – and very
good colour rendering values of R a > 90
can be achieved. If a uniform light colour is
important in all LEDs for the user, strict binning
(see page 26f.) is essential.
There are three basic LED construction
types:
Wired (radial) LEDs represent one of the
earliest forms of LED. They are often used
as simple signal indicators due to their
low luminosity; they play no role in lighting.
SMD (= surface-mounted device) LEDs
are glued directly onto a printed circuit
board and contacted in a solder bath. The
LED chip sits on a housing or wafer with
contacts.
COB (= chip on board) LEDs are used for
very powerful, tightly packed LED modules;
the LED chip is mounted directly on
the board.
The chips are mostly offered as premounted
modules on circuit boards. Complemented
with a power supply unit, an
outer bulb and e.g. an E27 base, the LED
lamp can be used directly in the form of a
retrofit. There are now many LED luminaires
on the market, offered as complete systems
including non-replaceable LED modules.
Ever higher luminous efficacy claims of
more than 200 lm/W are regularly published.
These are, however, lab values which cannot
be obtained in practical operation due
to electrical, optical and especially thermal
losses. The service life is also significantly
influenced by environmental factors: only
good thermal management (i.e. good heat
dissipation) can ensure high values.
Innovative: OLEDs
The glass pane of a window which lets in
daylight in the middle of the day and acts
as a light source at night might sound
rather futuristic at present, yet the first
OLED-based luminaires are already on the
market. The extremely thin panels, the
shape of which can be flexibly adapted to
[61] The lamp jackets of the first incandescent
lamps were evacuated, i.e. the tungsten
wire of the coil glowed in a vacuum; migrating
tungsten molecules blackened the glass bulb.
In modern halogen lamps, noble gases limit
the freedom of movement of the tungsten
molecules. Thermal radiators are relatively inefficient;
conventional incandescent light
bulbs have already been withdrawn from the
market in Europe.
61
62
32
63
64
[62] In 230V and low voltage halogen lamps,
the halogen cycle yields higher luminous efficacy
and longer life.
[63] Fluorescent lamps are low-pressure
discharge lamps and work with low-pressure
mercury vapour. The composition of the luminescent
material on the tube wall can be
used to alter the light colour and colour rendering
of fluorescent lamps.
[64] High-pressure discharge lamps contain
a burner in which the light is produced by
electrical discharge in gases or metal
vapours, or a mixture of both. The metalhalide
lamp shown has a burner made of
transparent ceramic which ensures consistent
colour quality.