LED bianchi con nozioni di fotometria e colorimetria Docente: Mauro Mosca (www.dieet.unipa.it/tfl) A.A. 2014-15 Ricevimento: alla fine della lezione o per appuntamento Università di Palermo – Facoltà di Ingegneria (DEIM) L’occhio umano Visione umana 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 intensity 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… 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) • The CIE’s definition of color includes chromaticity as well as brightness • The CIE’s definition of the color of light is given by the dominant wavelength, saturation, and brightness • The color of an object is given by the dominant wavelength, saturation, and lightness 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… 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 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) 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 planckian black-body reference source is assumed to have perfect color rendering properties (CRI = 100) special CRI CRI Colore e cromaticità DEi* and uniform color space A uniform color space is motivated by the need for a quantitative color space that includes chromaticity and brightness/lightness. This uniform color space provides direct proportionality between color difference and geometrical distance The CIE has introduced a three-dimensional uniform color space, namely the (L*, u*, v*) space DEi* and uniform color space Next We consider considerathe test-color test-color sample sample iilluminated illuminated with with a white a white test reference sourcesource color stimulus of the test-color sample i when illuminated with reference the test sourcesource color stimulus of the test illuminant white reference illuminant CRI 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 Dl = 5kT Efficacia luminosa di LED tricromatici Dl = 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 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