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1
Mikroskopische Methoden
& Anwendungen WS 12/13
MLS Master 1. Semester
Strukturanalytik, Teil D
Prof. Rainer Duden, Prof. Alfred Vogel, Dr. Gereon Hüttman
Lichtquellen und Detektoren
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Sir Isaac Newton Christian Huygens Augustin Fresnel James Maxwell Albert Einstein
What is Light?
The question of the nature of light has puzzled philosophers
and scientists for more the 2000 year and is connected with
main discoveries in physics.
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History of Optics • Ancient philosophers in Greece (Pythagoras, Demokrit,
Empedokles, Plato, Aristoteles)
What is light? Basic geometrical optics, refraction of light, primitive
lenses (ice), mirrors
• Middle ages: arabic scientists e.g. Alhazen (1000 n. Chr.)
reflection, spherical and parabolic mirrors, anatomy of human eye
• Roger Bacon (1215-94)
Lens for correction of sight → first glasses
• Leonardo da Vinci (1452-1519)
describes the „Camera obscura“
• Galileo Glailei (1564 - 1642)
First telescope
• Willebrod Snellius (1591 - 1621)
Law of refraction
• Robert Boyle (1626 - 1691)
Robert Hook (1635 - 1703)
Diffraction, Interference, colours form thin layers
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• Isaac Newton (1642 - 1727)
Colours of white light 1666, light as particles
• Ole Christensen Rømer (1644 - 1710)
Calculation of the velocity of light by the shading of
the moons of Jupiter
• Christian Huygens (1629 - 1695)
Light as waves in the aether
• Thomas Young (1773 - 1829)
Interference principle
• Jean Fresnel (1788 - 1827)
Description of light propagation by diffraction and
interference
• Michael Faraday (1791 - 1867)
Experimente zur magnetischen Induktion und
Drehung der Polarisation
• James Clark Maxwell (1831 - 1879)
Theory of electromagnetic waves
• Heinrich Hertz (1857 - 1894)
Generation and detection of electromagnetic waves
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Light is an Electromagentic Wave…
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Spectrum of Electromagnetic Waves
m/s103 8 cf
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Light is quantitized!
h = 6,6 10 Js -34 c = 3 108 m/s
/chhE
The photoelectic effect showed the
quantization of light. For a certain
metal only below a certain
wavelength light can remove
electrons, irrespectively of the
irradiance.
Experimental set up
• Albert Einstein (1879 –1955)
quantisation of light
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Sun
H
Na
Cu
CN
CO2
solid or fluid
• Niels Bohr(1885 - 1962)
quantisation of energy state
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• Albert Einstein (1879 –1955)
Stimulated Emission
All these 50 years of conscious brooding have brought me no nearer to the answer to the question „What
are light quanta?“. Nowadays every Tom, Dick and Harry thinks he knows it, but he is mistaken.
(Einstein)
The first working laser, a ruby laser,
constructed by Theodore Maiman in 1960
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In the second half of the 20th century applied optics flourished:
• Communication (Laser (1960), optical fibers, ...)
• Image projection (Displays, laser TV...)
• Cameras (surveillance , IR-cameras, CCD, photography,...)
• Thin film technology (dielectric mirrors, filter,...)
• Laser (communication, medicine, data recording, bar-code
scanner,CD, material processing,...)
• Optical crystals (opt. modulators, LCD-Displays, LED,...)
• Holography (non-destructive testing, credit cards,...)
• Quantum computer
• .......
The 21st century as the century of the photon?
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Sources of Optical Radiation
1. Sunlight, skylight
2. Incandescent sources
a) Blackbody sources
b) Nernst glower and globular
c) Tungsten filiament
3. Discharge lamp
a) Spectral sources (Hg, D2, He)
b) High-intensity sources (Hg, Xe)
c) Flash lamps
4. Fluorescent lamps
5. Light emitting diodes (LED)
6. Lasers
7. Synchrotron sources
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T
Tungsten filiament (Black Body Radiation)
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Gas Discharge Atoms or gas molecules are excited by
an electrical discharge. On return to their
ground state they emit light of a
characteristic wavelength (photon
energy)
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Gas discharge lamp for microscopy
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Emission von Halbleitern (LEDs)
Eg: band gap EA: Excitation energy of doped atom
External voltage lifts Fermi-energy level in n-region
creates inversion in transition zone
Electron-hole recombination creates photons (or phonons)
High conversion efficiency electric energy light (30-50%)
holes
free electrons
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Light Emitting Diode (LED)
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Comparision of Gas Discharge Lamp with LEDs (1)
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Comparision of Gas Discharge Lamp with LEDs (2)
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LASER: Light Amplification by Stimulated Emission of Radiation
- Nobel Price 1964 (CT) and 1981 (AS) -
Key properties of laser irradiation
• Narrow spectral bandwidth
(“monochromacity”)
• Small divergence
• Coherence
High (spectral) brightness
What is this good for?
Charles H. Townes
*1915, age 93
Arthur Schawlow
1921-1999
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laser physics 27 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
background
radiative transfer
between energy
levels
H - atom
energ
y 13.6 eV
0 eV
+
-
Lyman Balmer Paschen
spectral series
DE = h n
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laser physics 28 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
“keeping Einstein´s track”
two level system
idealized material:
assembly of Ntot atoms
with just two energy levels
Ntot = N1 + N2
radiative transfer between two levels
is allowed: h n21 = E2 – E1
E2
E1 N1, g1
N2, g2
atoms are in thermal equilibrium
with radiation field
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laser physics 29 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
“keeping Einstein´s track”
two level system E2
E1 N1, g1
N2, g2
what will happen?
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laser physics 30 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
emission and absorption
(induced) absorption E2
E1 N1
N2
(g1=g2=1)
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laser physics 31 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
emission and absorption
spontaneous emission
E2
E1 N1
N2
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laser physics 32 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
emission and absorption
stimulated (induced) emission
E2
E1 N1
N2
photons / field in phase: coherent
Probabilities for absorption and stimulated
emission are equal!
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laser physics 33 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
emission and absorption
two level system rate balance
E2
E1 N1
N2
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laser physics 37 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
elements of a laser
1. Amplifying medium
2. Pump
3. Resonator
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laser physics 38 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
pumping mechanisms
• Optical excitation (laser, arc-
lamp, flash-lamp)
• Electrical gas discharge (gas
laser)
• Radio frequency excitation of
gases
• Chemical reaction (F + H HF*
+ H)
• Injection of charges (diode laser)
• Acceleration of electrons in
amagentic field (free electron
laser)
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laser physics 39 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
laser process
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laser physics 40 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
pump energy
many atoms, ions, or molecules excited in laser medium
laser process
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laser physics 41 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
high
reflecting
mirror
outcoupling
mirror
laser process
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laser physics 42 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
lasing condition
Amplification > Loss
pump powerpump power
resonator losses
inversion
photon density
resonator losses
inversion
photon density
Electronic oscillator
Laser oscillator
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laser physics 43 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
different laser types
The first laser build
Historic Overview
•1917 Postulation of the stimulated
emission Einstein
•1928 Experimental proof of the
stimulated emission Ladenburg,
Kopfermann
•1950 Experimental proof of an
inversion Purcel, Pound
•1951/1955 Suggestion to use stimulated Fabrikant, Weber,
emission for amplification Basov,Prochorov
•1954 NH3-Maser Townes
•1958 Suggestion to use stimulated
emission for amplification in the
optical reagion Schawlow, Townes
•1959 Suggestion to build
a gass laser Javan
•1960 First laser build (Ruby laser) Maiman
•1961 First HeNe-laser Javan,Benett, Herriott
•1962 First semiconductor laser Nathan,Duncke,
Burns, Dill, Lasher
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laser physics 44 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
fundamental (TEM00 ) mode
phase front
2
w0
(confocal resonator L=R)
z
stable cavity,
(e.g. confocal resonator)
0w
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laser physics 45 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications
Die Schlüsseleigenschaften der Laserstrahlung?
Schlüsseleigenschaften der Laserstrahlung
• Schmale spektrale Bandbreite
(“Momochromasie”)
• Geringe Divergenz
⇒ Kohärenz
Hohe (spektrale) Strahldichte Le
Bsp. - 1 mW Laserpointer, 650 nm Le = 2.37 • 105 W/cm²
- 100 W Glühbirne W = 4 , w0 = 3 cm
Le = 7.2 • 10-3 W/cm²
Unterschied von 3 • 107 !
22
0
2
0
22
0
W
eL
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Monochromacity, small divergence spectrometry
Enormous gain in
spectral brightness by
• Small divergence
of laser light source
• Tunable, narrow
spectral bandwidth
“Brightness” (Radiance)
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Semiconductor lasers (3)
Gain guided Gain and refractive index guided
laser diode stack (for optical pumping) Individual laser diode
auf Reflektor
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Semiconductor
lasers (4)
Laser diode
material
(active region
/ substrate)
Typical emission
wavelengths Typical application
InGaN / GaN,
SiC
380, 405, 450,
470 nm data storage
AlGaInP / GaAs 635, 650, 670 nm laser pointers, DVD players
AlGaAs / GaAs 720–850 nm CD players, laser printers
InGaAsP / InP 1000–1650 nm optical fiber
communications
Advantages
• Small
• Inexpensive
• High efficiency (30-50%)
• High power (kW)
• Can be modulated up to 10 GHz
• Long lifetime
Disadvantages
• Asymmetric beam profile
(e. g. θx=10°, θy=30°)
• High divergence
• Low pulse energies
High-power
diode lasers
Band gap determines laser wavelength
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Modes of Light Detection
Optical detectors
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Photon Tube
Outer electrooptical effect
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Quantum Efficiency
Responsivity (Sensitivity)
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Spectral Sensitivity
A/Wm000.806R
A/Wm000.806/
000
PPhc
ePRIP
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Spectral Sensitivity
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Photomultiplier Tube Detector
Anode
• High sensitivity at
low light levels
• Cathode material
determines spectral
sensitivity
• Good signal/noise
• Shock sensitive
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The Photodiode Detector
• Wide dynamic range
• Very good
signal/noise at high
light levels
• Solid-state device
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Θ Θ
Spektrale Empfindlichkeit
Photomultiplier (PMT) Photodiode (PD)
ElektronenneI Photonenn
hc
hc
enn
hc
eIR
nn
PhotonenElektronen
PhotonenElektronen
// :hkeitEmpfindlic
/ :beuteQuantenaus
Θ Θ Θ
Θ Θ
Θ Θ Θ Θ Θ Θ
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Avalanche Photodiode
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Schematic Diagram of a
Photodiode Array
• Same characteristics
as photodiodes
• Solid-state device
• Fast read-out cycles
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CMOS Sensor
(Complementary Metal-Oxide-Silicon)
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Charge Coupled Device (CCD)
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Die CCD-Typen
L – lichtempfindliche Pixel,
T – Transfer-Register,
A – Ausleseverstärker.
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Types of CCD-Sensors
FF CCDs = Full Frame CCDs
Application: astronomy, spektroskopy,
Advantages: high resolution, large arrays are possible
(7000x9000 Pixel by Philips)
Draw-back: mechanical shutter necessary
FT CCDs = Frame Transfer CCDs
Advantages: high resolution, high aperture, internal
shutter possible, homogeneous sensitivity
Draw-back: strong smearing effects, large chip areas
IL CCDs = Interline CCDs
Application: video cameras, low cost
Advantages: small chip
Draw-back: small aperture (can be increased by on-chip
lenses), strong smearing effects
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Einchip Farb-CCD-Kameras
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3-Chip Farb-CCD
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Wieviele Pixel sind notwendig?
Struktur
Pixel
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Wieviele Pixel sind notwendig?
Struktur
Pixel
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Wieviele Pixel sind notwendig?
Struktur
Pixel
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Wieviele Pixel sind notwendig?
Struktur
Pixel
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Wieviele Pixel sind notwendig?
Struktur
Pixel
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Wieviele Pixel sind notwendig?
Die kleinsten auflösbaren Strukturen sollten mit mindestens
4 Pixeln abgetastet werden!
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Abtastung der Bilder
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Kenngrößen für CCD- Kameras • Die Quantenausbeute, also die Wahrscheinlichkeit, dass ein einfallendes Photon ein
Elektron auslöst. Die Quantenausbeute von CCDs hängt von der Wellenlänge des Lichts
ab und kann über 90 % betragen (Fotografischer Film zum Vergleich: 5 bis 10 %).
• Der Dunkelstrom der lichtempfindlichen Zellen. Der Dunkelstrom ist stark
temperaturabhängig und führt aufgrund seiner statistischen Eigenschaften zu
Dunkelstromrauschen. Er ist für alle Pixel individuell verschieden und eine Quelle des
Bildrauschens. Weiter können einzelne „hot pixels“, also Pixel mit besonders hohem
Dunkelstrom auftreten.
• Die Anzahl der Ladungen, die in einem Pixel gespeichert werden können (engl. full well
capacity oder well depth'').
• Das Verhalten, wenn durch Überbelichtung in einzelnen Pixeln mehr Ladung erzeugt wird,
als gespeichert werden kann. Tritt die Ladung in benachbarte Pixel über, spricht man von
„Blooming“. Viele CCD-Kameras vermeiden diesen Effekt, indem die überschüssigen
Ladungen abgeleitet werden („anti-blooming gate“), dadurch kann aber auch schon
Ladung verloren gehen, bevor ein Pixel wirklich voll ist. Der Zusammenhang zwischen
Lichtmenge und Ladung ist dann nicht mehr linear, und genaue Messungen sind nicht
mehr möglich.
• Die Effizienz des Ladungstransports zum Ausleseverstärker (Charge Transfer Efficiency).
• Das Rauschen des Ausleseverstärkers (Ausleserauschen, engl. readout noise).
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Empfindlichkeit
104
Bildrauschen
=
Ausleserauschen
+
Photonenrauschen
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Typische Parameter für CCD-Kameras
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Korrektur der Bilder
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Electron Multiplying CCD
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Photon Detectors
Semicondutor Vaccum tube
internal
ampli.
no
internal
ampli.
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Radiometrische Größen
Lichtquelle bestrahlte
Fläche Raumwinkel
Größe Symbol Einheit
Strahlungsenergie (Radiant energy) [J] eQ
3
Energie einer Anzahl von Photonen
Strahlungsdichte (Radiant energy density) / [J/m ] volumetrische Energiedichte
Bestrahlung (Radiant exposure)
e ew dQ dV
2/ [J/m ] pro Fläche empfangene Energie
Strahlungsfluss (Radiant flux) / [W] Strahlungsenergie pro Zeit
spezifische
A
e e
e e
H dQ dA
dQ dt
2
2
ustrahlung (Radiant exitance) / [W/m ] Strahlungsfluss pro Emitterfläche
Bestrahlungsstärke (Irradiance) / [W/m ]
e e
e e
M d dA
E d dA
Strahlungsfluss pro Empfängerfläche
Strahlstärke (Radiant intensity) / [W/sr] Strahlungsfluss pro Raumwinkel
Strahldichte (Radiance)
e eI d d W
2 / cos [W/(sr m )] Strahlungsfluss pro Raumwinkel pro Emitterfläche
e eL dI dA
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Photometry
Photometry measures the brightness of light
Relavive spectral sensitivity of the eye for
photopic (V) and scotopic (V‘) vision
photometric unit = 685 lm/W V() radiometric unit
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Spectral Radiometry and
Photometry
For polychromatic light all quantities can be related to the
wavelength :
dds /)()(
ds
2
1
)(
Total quantities are calculated by integration :
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Radiometrische Größen Größe Symbol Einheit
Strahlungsenergie (Radiant energy) [J] eQ
3
Energie einer Anzahl von Photonen
Strahlungsdichte (Radiant energy density) / [J/m ] volumetrische Energiedichte
Bestrahlung (Radiant exposure)
e ew dQ dV
2/ [J/m ] pro Fläche empfangene Energie
Strahlungsfluss (Radiant flux) / [W] Strahlungsenergie pro Zeit
spezifische
A
e e
e e
H dQ dA
dQ dt
2
2
ustrahlung (Radiant exitance) / [W/m ] Strahlungsfluss pro Emitterfläche
Bestrahlungsstärke (Irradiance) / [W/m ]
e e
e e
M d dA
E d dA
Strahlungsfluss pro Empfängerfläche
Strahlstärke (Radiant intensity) / [W/sr] Strahlungsfluss pro Raumwinkel
Strahldichte (Radiance)
e eI d d W
2 / cos [W/(sr m )] Strahlungsfluss pro Raumwinkel pro Emitterfläche
e eL dI dA
Größe Symbol Einheit
Lichtmenge (Luminous energy) [lm s] vQ
2
gewichtet Energie einer Anzahl von Photonen
Belichtung (Luminous exposure) / [lm s/m ] pro Fläche empfangene Lichtmenge
Lichtstrom (Luminous
v vH dQ dA
2
flux) / [lm] Lichtmenge pro Zeit
Spezifische
Lichtausstrahlung (Luminous exitance) / [lm/m ] Lichtstrom pro Emi
v v
v v
dQ dt
M d dA
2
tterfläche
Beleuchtungsstärke (Illuminance) / [lm/m ] Lichtstrom pro Empfängerfläche
Lichtstärke (Luminous intensity) /
v v
v v
E d dA
I d d
W
2
[lm/sr], [cd] Lichtstrom pro Raumwinkel
Leuchtdichte (Luminance ) / cos [cd/m ] Lichtstrom pro Raumwinkel pro Emitterfläche
v vL dI dA
Photometrische Größen
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What limits the irradiance we can
achieve?
It is the Radiance (Strahldichte)
of the light source!
2211
2211
WW
AA
HH
/ cose eL Q t A W