Tài liệu Lighting with Artificial Light 01 doc

LINK DOWNLOAD MIỄN PHÍ TÀI LIỆU "Tài liệu Lighting with Artificial Light 01 doc": http://123doc.vn/document/1039793-tai-lieu-lighting-with-artificial-light-01-doc.htm

3
04
Most of the information we receive about
our surroundings is provided by our eyes.
We live in a visual world. The eye is the
most important sense organ in the human
body, handling around 80% of all incoming
information. Without light, that would be im-
possible – light is the medium that makes
visual perception possible.
Insufficient light or darkness gives rise to a
sense of insecurity. We lack information, we
lose vital bearings. Artificial lighting during
the hours of darkness makes us feel safe.
So light not only enables us to see; it also
affects our mood and sense of wellbeing.
Lighting level and light colour, modelling
and switches from light to dark impact on
momentary sensations and determine the
rhythm of our lives.
In sunlight, for instance, illuminance is
about 100,000 lux. In the shade of a tree it
is around 10,000 lux, while on a moonlit
night it is 0.2 lux, and even less by starlight.
People nowadays spend most of the day
indoors – in illuminances between 50 and
500 lux. Light sets the rhythm of our biolog-
ical clock but it needs to be relatively in-
tense to have an effect on the circadian
system (Ͼ 1,000 lux), so for most of the
time we live in “chronobiological darkness”.
The consequences are troubled sleep, lack
of energy, irritability, even severe depres-
sion.
As we said above, light is life. Good lighting
is important for seeing the world around us.
What we want to see needs to be illumi-
nated. Good lighting also affects the way
we feel, however, and thus helps shape our
quality of life.
Around 300,000 years ago, man began to
use fire as a source of warmth and light.
The glowing flame enabled people to live in
caves where the rays of the sun never pen-
etrated.
The magnificent drawings in the Altamira
cave – artworks dating back some 15,000
years – can only have been executed in arti-
ficial light. The light of campfires, of kindling
torches and oil and tallow lamps radically
changed the way prehistoric man lived.
But light was not only used in enclosed
spaces. It was also harnessed for applica-
tions outdoors. Around 260 BC, the Pharos
of Alexandria was built, and evidence from
378 AD suggests there were “lights in the
streets” of the ancient city of Antioch.
Ornamental and functional holders for the
precious light-giving flame appear at a very
early stage in the historical record. But the
liquid-fuel lamps used for thousands of
years underwent no really major improve-
ment until Aimé Argand‘s invention of the
central burner in 1783.
That same year, a process developed by
Dutchman Jan Pieter Minckelaers enabled
gas to be extracted from coal for street-
lamps. Almost simultaneously, experiments
started on electric arc lamps – fuelling
research which acquired practical signifi-
cance in 1866 when Werner Siemens suc-
ceeded in generating electricity economi-
cally with the help of the dynamo. But the
real dawn of the age of electric light came
in 1879, with Thomas A. Edison’s “re-
invention” and technological application of
the incandescent lamp invented 25 years
earlier by the German clock-maker Johann
Heinrich Goebel.
With each new light source – from campfire
and kindling to candle and electric light
bulb – “luminaires” were developed to
house and harness the new “lamps”. In re-
cent decades, lamp and luminaire develop-
ment has been particularly dynamic, draw-
ing on the latest technologies, new optical
systems and new materials while at the
same time maximising economic efficiency
and minimising environmental impact.
licht.wissen 01 Lighting with Artificial Light
4
From nature‘s light to artificial lighting
Light is life. The relationship between light and life cannot be stated more simply than that.
5
[05] The light of the sun determines the pulse
of life and the changing alternation of day and
night throughout the year.
[06] The light of the moon and stars has only
1/500,000th of the intensity of sunlight.
[07] In a rainbow, raindrops act like prisms.
[08] Advances in the development of electric
discharge lamps, combined with modern lumi-
naires, has led to high-performance lighting sys-
tems.
[09] For the majority of people today, life with-
out artificial lighting would be unimaginable.
[10] For more than 2,000 years, artificial light-
ing has illuminated the night and provided secu-
rity and orientation for human beings.
10
05 06
07 08 09
For example, since no connection could be
discerned between a flame and the object
it rendered visible, it was at one time sup-
posed that “visual rays” were projected by
the eyes and reflected back by the object.
Of course, if this theory were true, we
would be able to see in the dark
In 1675, by observing the innermost of the
four large moons of Jupiter discovered by
Galileo, O. Römer was able to estimate the
speed of light at 2.3 x 10
8
m/s.
A more precise measurement was obtained
using an experimental array devised by
Léon Foucault: 2.98 x 10
8
. The speed of
light in empty space and in air is generally
rounded up to 3 x 10
8
m/s or 300,000 km/s.
This means that light takes around 1.3 sec-
onds to travel from the Moon to the Earth
and about 8
1
⁄3 minutes to reach the Earth
from the Sun. Light takes 4.3 years to
reach our planet from the fixed star Alpha in
Centaurus, about 2,500,000 years from the
Andromeda nebula and more than 5 billion
years from the most distant spiral nebulae.
Different theories of light enable us to de-
scribe observed regularities and effects.
The corpuscular or particle theory of light,
according to which units of energy (quanta)
are propagated at the speed of light in a
straight line from the light source, was pro-
posed by Isaac Newton. The wave theory
of light, which suggests that light moves in
a similar way to sound, was put forward
by Christiaan Huygens. For more than a
hundred years, scientists could not agree
which theory was correct. Today, both con-
cepts are used to explain the properties of
light: light is the visible part of electromag-
netic radiation, which is made up of oscillat-
ing quanta of energy.
It was Newton again who discovered that
white light contains colours. When a narrow
beam of light is directed onto a glass prism
and the emerging rays are projected onto a
white surface, the coloured spectrum of
light becomes visible.
In a further experiment, Newton directed
the coloured rays onto a second prism,
from which white light once again ap-
peared. This was the proof that white sun-
light is the sum of all the colours of the
spectrum.
In 1822, Augustin Fresnel succeeded in
determining the wavelength of light and
showing that each spectral colour has a
specific wavelength. His statement that
“light brought to light creates darkness”
sums up his realization that light rays of the
same wavelength cancel each other out
when brought together in corresponding
phase positions.
Max Planck expressed the quantum theory
in the formula:
E = h · ␯
The energy E of an energy quantum (of
radiation) is proportional to its frequency
␯,
multiplied by a constant h (Planck‘s quan-
tum of action).
licht.wissen 01 Lighting with Artificial Light
6
The physics of light
Man has always been fascinated by light and has constantly striven to unravel its mysteries. History has produced
various theories that today strike us as comical but were seriously propounded in their time.
7
[14] If the artificial light of a fluorescent lamp
is split up, the individual spectral colours are
rendered to a greater or lesser extent, depend-
ing on the type of lamp.
[15] Both the particle and the wave theory of
light are used to provide a succinct description
of the effects of light and how these conform to
natural laws.
[11] Within the wide range of electromagnetic
radiation, visible light constitutes only
a narrow band.
[12] With the aid of a prism, “white” sunlight
can be split up into its spectral colours.
[13] The prism combines the spectral colours
to form white light. Sunlight is the combination
of all the colours of its spectrum.
15
13 14
1211
long waves
medium waves
short waves
ultra-short waves
television
radar
infrared rays
light
ultraviolet rays
x-rays
gamma rays
cosmic radiation
The Earth‘s atmosphere allows visible, ultra-
violet and infrared radiation to pass through
in such a way that organic life is possible.
Wavelengths are measured in nanometres
(nm) =10
-9
m = 10
-7
cm. One nanometre is
a ten-millionth of a centimetre.
Light is the relatively narrow band of elec-
tromagnetic radiation to which the eye is
sensitive. The light spectrum extends from
380 nm (violet) to 780 nm (red).
Each wavelength has a distinct colour
appearance, and from short-wave violet
through blue, green, green-yellow, orange
up to long-wave red, the spectrum of
sunlight exhibits a continuous sequence.
Coloured objects only appear coloured if
their colours are present in the spectrum of
the light source. This is the case, for exam-
ple, with the sun, incandescent lamps and
fluorescent lamps with very good colour
rendering properties.
Above and below the visible band of the
radiation spectrum lie the infrared (IR) and
ultraviolet (UV) ranges.
The IR range encompasses wavelengths
from 780 nm to 1 nm and is not visible to
the eye. Only where it encounters an object
is the radiation absorbed and transformed
into heat. Without this heat radiation from
the sun, the Earth would be a frozen planet.
Today, thanks to solar technology, IR radia-
tion has become important both techno-
logically and ecologically as an alternative
energy source.
For life on Earth, the right amount of radia-
tion in the UV range is important. This ra-
diation is classed according to its biological
impact as follows:
> UV-A (315 to 380 nm), suntan, solaria;
> UV-B (280 to 315 nm), erythema
(reddening of the skin), sunburn;
> UV-C (100 to 280 nm), cell destruction,
bactericidal lamps.
licht.wissen 01 Lighting with Artificial Light
8
[16] A prism makes the colour spectrum of
light visible.
[17+18] Compared with its appearance in
daylight, a red rose looks unnatural under the
monochromatic yellow light of a low-pressure
sodium vapour lamp. This is because the
spectrum of such light contains no red, blue
or green, so those colours are not rendered.
Despite the positive effects of ultraviolet
radiation – e.g. UV-B for vitamin D synthe-
sis – too much can cause damage. The
ozone layer of the atmosphere protects us
from harmful UV radiation, particularly from
UV-C. If this layer becomes depleted
(ozone gap), it can have negative conse-
quences for life on Earth.
16
17 18
The image-producing optics consist of the
cornea, the lens and the intervening aque-
ous humour. Alteration of the focal length
needed for accurate focusing on objects at
varying distances is effected by an adjust-
ment of the curvature of the refractive
surfaces of the lens. With age, this accom-
modative capacity decreases, due to a
hardening of the lens tissue.
With its variable central opening – the pupil
– the iris in front of the lens functions as an
adjustable diaphragm and can regulate the
incident luminous flux within a range of
1:16. At the same time, it improves the
depth of field. The inner eye is filled with a
clear, transparent mass, the vitreous hu-
mour.
The retina on the inner wall of the eye is the
“projection screen”. It is lined with some
130 million visual cells. Close to the optical
axis of the eye there is a small depression,
9
[19] The eye is a sensory organ with extraordi-
nary capabilities. Just a few highly sensitive
“components” complement each other to form
a remarkable visual instrument:
a cornea
b lens
c pupil
d iris
e suspensory ligaments/ciliary muscles
f vitreous humour
g sclera
h retina
i blind spot
j fovea
k optic nerve
[20] Curve of relative spectral sensitivity for
day vision (cones) V(␭) and night vision (rods)
V‘(␭).
19
20
The physiology of light
The optical components of the eye can be compared to a photographic camera.
the fovea, in which the visual cells for day
and colour vision are concentrated. This is
the region of maximum visual acuity.
Depending on the level of brightness (lumi-
nance), two types of visual cell – cones and
rods – are involved in the visual process.
The 120 million rods are highly sensitive to
brightness but relatively insensitive to
colour. They are therefore most active at
low luminance levels (night vision); their
maximum spectral sensitivity lies in the
blue-green region at 507 nm.
The 7 million or so cones are the more sen-
sitive receptors for colour. These take over
at higher levels of luminance to provide day
vision. Their maximum spectral sensitivity
lies in the yellow-green range at 555 nm.
There are three types of cone, each with a
different spectral sensitivity (red, green,
blue), which combine to create an impres-
sion of colour. This is the basis of colour
vision.
400 500 600 700 800
Wavelength (nm)
Spectral light sensitivity V (␭)
1,0
0,8
0,6
0,4
0,2
The ability of the eye to adjust to higher or
lower levels of luminance is termed adapta-
tion. The adaptive capacity of the eye ex-
tends over a luminance ratio of 1:10 billion.
The pupils control the luminous flux enter-
ing the eyes within a range of only 1:16,
while the “parallel switching” of the ganglion
cells enables the eye to adjust to the far
wider range.
The state of adaptation affects visual per-
formance at any moment, so that the
higher the level of lighting, the more visual
performance will be improved and visual
errors minimized. The adaptive process and
hence adaptation time depend on the lumi-
nance at the beginning and end of any
change in brightness.
Dark adaptation takes longer than light
adaptation. The eye needs about 30 min-
utes to adjust to darkness outdoors at night
after the higher lighting level of a workroom.
Only a few seconds are required, however,
for adaptation to brighter conditions.
Sensitivity to shapes and visual acuity are
prerequisites for identification of details.
Visual acuity depends not only on the state
of adaptation but also on the resolving
power of the retina and the quality of the
optical image. Two points can just be per-
ceived as separate when their images on
the retina are such that the image of each
point lies on its own cone with another
“unstimulated” cone between them.
Inadequate visual acuity can be due to eye
defects, such as short- or long-sighted-
ness, insufficient contrast, insufficient illumi-
nance.
Four minimum requirements need to be
met to permit perception and identifica-
tion:
1. A minimum luminance is necessary to
enable objects to be seen (adaptation lumi-
nance). Objects that can be identified in de-
tail easily during the day become indistinct
at twilight and are no longer perceptible in
darkness.
2. For an object to be identified, there
needs to be a difference between its bright-
ness and the brightness of the immediate
surroundings (minimum contrast). Usually
this is simultaneously a colour contrast and
a luminance contrast.
3. Objects need to be of a minimum size.
4. Perception requires a minimum time. A
bullet, for instance, moves much too fast.
Wheels turning slowly can be made out in
detail but become blurred when spinning at
higher velocities. The challenge for lighting
technology is to create good visual condi-
tions by drawing on our knowledge of the
physiological and optical properties of the
eye – e.g. by achieving high luminance and
an even distribution of luminance within the
visual field.
licht.wissen 01 Lighting with Artificial Light
10
4
3
2
1
22 23 2421
11
[21] Schematic structure of the retina:
1 ganglion cells
2 bipolar cells
3 rods
4 cones
[22 – 24] Adaptation of the eye: On coming out
of a bright room and entering a dark one, we at
first see “nothing” – only after a certain period of
time do objects start to appear out of the dark-
ness.
[25] Where two points 0.3 mm apart are iden-
tified from a distance of 2 m, visual acuity is 2.
If we need to be 1 m from the visual object to
make out the two points, visual acuity is 1.
[26 – 32] Four requirements need to be met to
permit perception and identification: a minimum
luminance, minimum contrast, minimum size,
minimum time
26
27
28
29
31
32
25
30
Luminous flux ⌽
is the rate at which light is emitted by a lamp. It is measured in lu-
mens (lm). Ratings are found in lamp manufacturers‘ lists.
The luminous flux of a 100 W incandescent lamp is around 1,380
lm, that of a 20 W compact fluorescent lamp with built-in elec-
tronic ballast around 1,200 lm.
Luminous intensity I
is the amount of luminous flux radiating in a particular direction. It is
measured in candelas (cd).
The way the luminous intensity of reflector lamps and luminaires is
distributed is indicated by curves on a graph. These are known as
intensity distribution curves (IDCs).
To permit comparison between different luminaires, IDCs usually
show 1,000 lm (= 1 klm) curves.
This is indicated in the IDC by the reference cd/klm. The form of
presentation is normally a polar diagram, although xy graphs are
often found for floodlights.
licht.wissen 01 Lighting with Artificial Light
12
The language of lighting technology
⌽⌱
33 35
34 36