Diapositiva 1

LED bianchi
con nozioni di fotometria e colorimetria
Docente: Mauro Mosca
(www.dieet.unipa.it/tfl)
A.A. 2015-16
Ricevimento: alla fine della lezione o per appuntamento
Università di Palermo – Scuola Politecnica - DEIM
L’occhio umano
Visione umana
NO COLOR
higher sensitivity
Radiometria e fotometria
- Radiometric Units
physical quantities
(ex. optical power)
- Photometric Units
the same quantities
plumber’s
candleeye
as perceived
by human
- Luminous intensity
a monochromatic light source
emitting an optical power of (1/683) watt at 555 nm into the solid
angle of 1 steradian (sr) has a luminous intensity of 1 candela (cd)
Fotometria
- Luminous flux
a monochromatic light source
emitting an optical power of (1/683) watt at 555 nm has a luminous
flux of 1 lumen (lm)
1 cd = 1 lm/sr
An isotropically emitting light source with luminous
intensity of 1 cd has a luminous flux of 4π lm = 12.57 lm.
Fotometria
- Illuminance
area. Measured in lux
1 lux = 1 lm/m2
luminous flux incident per unit
Fotometria
- Luminance (of a surface source)
ratio of the
luminous intensity emitted in a certain direction (measured in cd)
divided by the projected surface area in that direction (measured in
m2). The luminance is measured in cd/m2
If the direction of interest is
normal to the chip surface,
the luminance is the
luminous intensity emitted
along the chip-normal
direction divided by the chip
area
The
luminance
a measureLEDs
of howisefficiently
the valuable
semiconductor
Luminance
ofislambertian
a constant,
independent
of angle
wafer area is to attain, at a given injection current, a certain luminous intensity
Radiometria e fotometria
Riepilogando…
Curva di sensibilità dell’occhio umano
Curva di sensibilità dell’occhio umano
Efficacia ed efficienza luminosa
flusso luminoso
curva
sensibilità
dell’occhio
umano
densità spettrale di potenza
Efficacia ed efficienza luminosa
da ricordare…
Colorimetria: funzioni color-matching
CIE has standardized the measurement
Human
sense
of
vision
is
very
different
of color by means of color-matching
from
the
human
sense
of
hearing
functions and the chromaticity diagram
color-matching functions
con
Colorimetria: funzioni color-matching
tricromaticità
non sono uniche!!
Colorimetria: valori di tristimolo
For a given power-spectral density P(l), the degree of stimulation
required to match the color of P(l) is given by
v
a
l
o
r
i
d
i
T
R
I
S
T
I
M
O
L
O
Colorimetria: coordinate cromatiche
Each
tristimulus
value represents the
approximate
(but primary
not exact)
coordinate
cromatiche
stimulation
of each
degree
of stimulation
that
of retinal
experiences
light (or
of each type
of each
retinaltype
cone)
dividedcone
by the
entire
when
illuminated
byYa+light
stimulation
(X+
Z ) source with spectrum P(l)
Diagramma di cromaticità
Diagramma di cromaticità
colori
monocromatici
Diagramma di cromaticità
equal
energy
point
(X=Y=Z)
distribuzione
spettrale
costante
Diagramma di cromaticità: ellissi di MacAdam
thecalculation
geometric
distance
two
pointscan
in
the
(x,
y) chromaticity
diagram
does
not
total
number
of yields
differentiable
chromaticities
be
obtained
by dividing
the000
area
of the
this
thebetween
result
that
humans
can
discern
approximately
50
distinct
scale linearly
with the
color difference
chromaticity
diagram
through
the average area of the MacAdam ellipses
chromaticities
Diagramma di cromaticità uniforme
it is very desirable for the color difference to be proportional to
the geometric difference
In fact, in the (x, y) chromaticity diagram are spatially very
non-uniform, that is, the color changes much more rapidly
in one direction, e.g. the x-direction, compared with the other
direction, e.g. the y-direction
Coordinate cromatiche uniformi
tanto per la cronaca…
the color difference between two locations in the uniform
chromaticity diagram is (approximately) directly proportional
to the geometrical distance between these points
Lunghezza d’onda dominante e purezza di colore
dominant
color purity
wavelength
100%???
lo spettro della
sorgente si allarga
LED nel diagramma di cromaticità
close to on the perimeter
blue–green and green LEDs are located off the perimeter due to the finite linewidth of
spectral
linewidth
ofthe
about
1.8 kTdiagram
the emission spectrum and
the strong
curvature of
chromaticity
Colore e cromaticità
• A color is defined by its location in the chromaticity diagram
(i.e. by its chromaticity)
HIGH
LIGHTNESS
• The CIE’s definition of color includes chromaticity as well as
brightness (brillanza = intensità luminosa che appare emessa )
• The CIE’s definition of the color of light is given by the
LOW
dominant wavelength,
LIGHTNESS saturation, and brightness
• The color of an object is given by the dominant wavelength,
saturation, and lightness (chiarezza = brillanza di un’area
illuminata )
Lo spettro solare
Planckian black-body radiation spectrum forms
a unique and very useful standard for white light
because it allows to describe the spectrum with
only one parameter: the color temperature
Natural daylight closely resembles the planckian
spectrum but…
- the sun’s spectrum depends on too many parameters
- large IR and UV components
Radiazione di corpo nero di Planck
legge di Wien
Luogo geometrico di Planck
Temperatura di colore e temperatura di
colore correlata
• Color temperature of a white light source, given in units of
kelvin, is the temperature of a planckian black-body radiator
that has the same chromaticity location as the white light
source considered
• The correlated color temperature of a white light source is
and if the
light does not
on the planckian
locus???
defined
aswhite
the temperature
of lie
a planckian
black-body
radiator
whose color is closest to the color of the white light source
e
nel diagramma
diagramma
(u’,v’)(x, y)???
minima distanza dal luogo di Planck
Temperatura di colore e temperatura di
colore correlata
P.S. I colori caldi non hanno niente a che fare con la temperatura di colore!!
Sintesi (color mixing) additiva
How to determine the chromaticity coordinates of the mixture
of three discrete emission bands?
Sintesi additiva
dove:
We assumed that each emission band is much narrower
than any of the three color-matching functions
linear combination of the individual chromaticity coordinates
Color gamut
color gamut
(pertre
treLED
fosfori
(per
RGB)
standard RGB)
(sRGB simile a NTSC)
What is the consequence
on the chromaticity diagram?
linea del porpora
(no colori spettrali)
Resa del colore (color rendering)
The ability to render the colors of an object is measured in
terms of the color-rendering index or CRI
a measure of the ability of the illuminant (i.e. a white
illumination source) to faithfully render the colors of
physical objects illuminated by the source
Indice di resa cromatica
(Color Rendering Index – CRI)
The color-rendering ability of a test light source is evaluated
by comparing it with the color-rendering ability of a reference
light source
Choice of the reference source:
1. planckian black-body radiator with the same color
temperature as the test source (if the test source is
located on planckian locus)
2. planckian black-body radiator with the same correlated
color temperature as the test source (if the test source is
not located on planckian locus)
3. CIE standard illuminant
Indice di resa cromatica
(Color Rendering Index – CRI)
The
black-body
reference
is that
assumed
to
∆Ei*planckian
represents
the quantitative
colorsource
change
occurs
have
color sample
rendering
properties (CRI
100)
whenperfect
a test-color
is illuminated
with, =first,
the
The
pre-factor
4.6 and
had subsequently
been chosen in
such
waysource
that the
reference
source,
with
theatest
TEST
Because
the equals
emission
spectrum
of an incandescent
lamp
general CRI
60COLOR
when a SAMPLES
“standard
warm white”
closely
follows
that
of used
a planckian
black-body
radiator,
such
fluorescent
lamp
was
as a test
source and
a special
planckian
lamps
have radiator
highest
and thus
the
best
Obviously,
ifthe
there
iswas
nopossible
difference
between
the
sources
black-body
used
as CRI
a reference
source
CRIcolor-rendering
properties
of all=artificial
light sources(of 8 test color
∆Ei* = 0 and
special CRI
100
samples)
How to calculate special CRI?
Why 4.6?
Special CRI in the chromaticity diagram
Colore e cromaticità
The chromaticity coordinates take into account of hue
(tonalità del colore) and saturation (purezza del colore)
hue variations
saturation variations
lightness variations
The CIE definition for color includes hue, saturation, and
additionally, brightness (for light) or lightness (for objects)
CRI of different light sources
CRI
PICCOLO TEST
Is it possible for two physical objects of different colors to
appear to have the same color under certain illumination
conditions? YES
What is the color of a yellow banana when illuminated with a
red LED? RED
What is the color of a green banana when illuminated with a
yellow LED? YELLOW
COLOR RENDERING
EFFICIENZA
Generazione di luce bianca con LED
Generazione di luce bianca con LED
Calcolo efficacia luminosa
di un LED dicromatico
Efficacia luminosa di un LED dicromatico
LED dicromatici monolitici
Different approaches:
- using two LEDs (blue and yellow)
- using a semiconductor (e.g. AlGaInN) as a wavelength
converter
- using co-doping
- using different quantum-wells
- using phosphors as wavelength converters
LED dicromatici monolitici
h
e
LED tricromatici
Large number of possible wavelength combinations for trichromatic sources
but… to attain a high efficacy of radiation, sources near the fringes of the
visible spectrum (deep red and deep violet) should be avoided
Efficacia luminosa di LED tricromatici
605 nm
530 nm
DE = 5kT
Efficacia luminosa di LED tricromatici
DE = 8kT
Dipendenza dalla temperatura
(LED tricromatici)
The emission power (P), peak wavelength (λpeak), and spectral
width (∆λ) depend on temperature, each of these quantities
having a different temperature coefficient
instability of trichromatic sources
Dipendenza dalla temperatura
(LED tricromatici)
Tcharacteristic
più
bassa
lpeak
shift
maggiore
Dipendenza dalla temperatura
(LED tricromatici)
> “regola dello 0,01”
Dipendenza dalla temperatura
(LED tricromatici)
The shift in chromaticity can be eliminated by adjusting the
relative power ratio of the three LED sources
1. The spectrum of the light source is constantly measured
and a feedback control is used to adjust the optical power
of the three components
2. The device temperature is monitored and the optical
power of the three components is adjusted using the
known temperature dependence of the different types of
emitters
easier but unable to compensate the device-aging effects
Frequency down-conversion: fosfori
Efficienza dei fosfori
EXTERNAL QUANTUM EFFICIENCY OF l-CONVERTER:
WAVELENGTH-CONVERSION LOSS OF l-CONVERTER:
Stokes shift
quantum-splitting phosphors
Efficienza dei fosfori
WAVELENGTH-CONVERSION EFFICIENCY:
<1
io la preferisco
scritta così…
POWER-CONVERSION EFFICIENCY:
The inherent wavelength-conversion loss is the reason that
In which wavelength range the wavelengthλ-converter-based white LEDs have a fundamentally lower
-conversion losses are higher?
efficiency limit than white-light sources based on multiple
from UV to red
LEDs
Fosfori per la down-conversion
A common phosphor used for white LEDs is cerium-doped YAG phosphor
For Ce-doped phosphors, quantum efficiencies of 75% have been
reported
Dyes per la down-conversion
shorter Stokes shift
lacks the long-term stability
Semiconduttori per la down-conversion
emission lines narrower
than
in phosphors
(linewidth
~2 kT) but…
…semiconductors
must be
deposited
as crystalline
films
LED PRS
(photon recycling semiconductors)
What are phosphors?
Phosphors consist of an inorganic host material doped with
an optically active element
rare-earth elements
Ce (yellow)
Eu (red)
e.g. garnet: A3B5O12
e.g.yttrium-aluminum garnet (YAG): Y3Al5O12
Spettro di emissione del Ce:YAG
The optical characteristics of YAG phosphors can be modified
by partially substituting Gd for Y and Ga for Al so that the
phosphor host has the composition (Y1–xGdx)3(Al1–yGay)5O12
Ce:YAG – Diagramma di cromaticità
high color temperatures
LED bianchi per conversione di fosfori
cup-shaded
depression
LED bianco basato su Ce:YAG
depends on phosphors layer
thickness and concentration
LED bianchi: prima generazione
bluish
LED ad alto CRI
adding an additional red phosphor…
reduced luminous efficiency:
• larger Stokes shift
• low value of the eye’s sensitivity function
• red phosphors less efficient
Uniformità di colore
mineral
(TiOlength
differentdiffusers
optical path
2, CaF2)
Distribuzione spaziale dei fosfori
provide a small emission area
gravity
desirable for applications
that requires point-like
sources (e.g. automotive
headlights, that need use of
lenses)
The probability
that phosphorescence
impinges on the
Problem
of absorption
of phosphorescence
semiconductor
chip is chip
greatly
diminished if the distance
by
the semiconductor
(contacts…)
between chip and phosphor is equal to or greater than the
chip’s lateral dimension
Distribuzione spaziale dei fosfori
Efficienza luminosa:
confronto tra vari tipi di LED
Alcuni dati...
Fwd. Current: 20 mA
 Lum. Flux: 9,37 lm
 Opt. Efficacy: 118,23 lm/W

vs.
TOPLED OSRAM
Alcuni dati...
Fwd. Current: 20 mA
 Lum. Flux: 9,37 lm
 Opt. Efficacy: 118,23 lm/W

vs.
CREE Standard
Alcuni dati...
Fwd. Current: 20 mA
 Lum. Flux: 9,37 lm
 Opt. Efficacy: 118,23 lm/W

vs.
NICHIA White