Light-Emitting Diodes

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445.664 Light-Emitting Diodes Chapter 16.

(circadian receptor) Cones: provide color sensitivity Rods : color insensitive Color perception

1 Light-Emitting Diodes Chapter 16. Human eye sensitivity and photometric quantities Euijoon Yoon Human vision Ganglion cell (circadian receptor) Cones: provide color sensitivity Rods : color insensitive Color perception depends on light intensity. Photopic vision regime: High light level vision regime (during daylight) Scotopic vision regime: Low light level vision regime (at night) 1

level between the photopic and scotopic vision regime Normalized spectral sensitivity Night-time

2 Three different vision regime Photopic vision : human vision at high ambient light level (daylight condition) Scotopic vision : human vision at low ambient light level (at night) Mesopic vision : light level between the photopic and scotopic vision regime Normalized spectral sensitivity Night-time vision is weaker in the red spectral range Stronger in the blue spectral range as compared to day-time vision 2

History of photometric units Photograph shows plumber s s candle. A plumber s s candle emits a luminous intensity of 1 candela (cd( cd). The cd is historical origin of all photometric units.

3 History of photometric units Photograph shows plumber s s candle. A plumber s s candle emits a luminous intensity of 1 candela (cd( cd). The cd is historical origin of all photometric units. photometric unit is relevant when it comes to light perception by a human being. First definition (now obsolete) : The luminous intensity of a standardized candle is 1 cd. Second definition (now obsolete) : 1 cm 2 of platinum (Pt) at 1042K (temperature of solidification) has a luminous intensity of cd. Third definition (current) : 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 cd. Candlepower and candle are obsolete units. Candlepower and candle measure luminous intensity and are approximately equal to 1 cd. Luminous flux, illuminance,, and luminance Luminous flux : A light source with a luminous intensity of 1 cd emits a luminous flux of 1 lm into a solid angle of 1 steradian. An isotropic light source with a luminous intensity of 1 cd emits a total luminous flux of 4 4π lm = lm. Illuminance : If a 1m 2 surface receives a luminous flux of 1 lm, then the illuminance of the surface is 1 lux. lm/m 2 Ex) Moonlight 1 lux; ; street light 10 lux; ; home lighting lux, office desk light lux; ; surgery light 10 4 lux; ; direct sunlight 10 5 lux Luminance is the luminous intensity emitted per unit area of a light source. Luminance is a figure of merit for displays. Typical displays have a luminance of cd/m 2. 3

4 Exercise: Photometric Unit A 60 W incandescent light bulb has a luminous flux of 1000 lm. Assume that the light is emitted isotropically from the bulb (a) What is the luminous efficiency (i.e., the number of lumens emitted per Watt of electrical input power) of the light bulb? A: 1000 lm / 60 W = 16.7 lm/w (b) What number of standardized candles emit the same luminous intensity? A: luminous intensity per steradian =1000 lm /4p = 80 (c) What is the illuminance, E lum, in units of lux, on the desk located 1.5 m below the bulb? A: 1000 lm / 4p (1.5) 2 m 2 = 35.4 lm/m 2 = 35.4 lux (d) Is the luminous level obtained under (c) sufficiently high for reading? A: Yes. Exercise: Photometric Unit (e) What is the luminous intensity, I lum, in units of candela, of the light bulb? A: 1000 lm / 4p sr =79.6 lm/sr = 79.6 cd (f) Derive the relationship between the illuminance at a distance r from the light bulb, measured in lux, and the luminous intensity, measured in candela? A: E lum * 4p r 2 / 4p = I lum E lum r 2 = I lum (g) Derive the relationship between the illuminance at a distance r from the light bulb, measured in lux, and the luminous flux, measured in lumen? A: E lum 4p r 2 = F lum (h) The definition of the cd involves the optical power of (1/683) W. What, do you suppose, is the origin of this particular power level? A: Historical origin: 1/683 of the 555 nm light source has the same intensity as the candle. 4

5 Typical illuminance in different environments Illumination condition Full moon Street lighting Home lighting Office desk lighting Surgery lighting Direct sunlight Illuminance 1 lux 10 lux 30 to 300 lux 100 to 1000 lux lux lux The projected surface area A projected = A surface cosθ (Θ is the angel between the direction considered and the surface normal) The luminous intensity of LEDs with lambertian emission pattern also depends on the angle Θ according to a cosine lqw Thus, the luminance of LEDs is a constant, independent of angle 5

Common units Conversion between common Si and non-si units for luminances Unit Common name Unit Common name 1 cd/cm 2 1 stilb (1/π) cd/m 2 1 apostilb (1/π) cd/cm 2 1 lambert (1/π) cd/ft 2 1

6 Common units Conversion between common Si and non-si units for luminances Unit Common name Unit Common name 1 cd/cm 2 1 stilb (1/π) cd/m 2 1 apostilb (1/π) cd/cm 2 1 lambert (1/π) cd/ft 2 1 foot-lambert 1 cd/m 2 1 nit Device Display Luminance (cd/m 2 ) 100 (operation) Device Organic LED Luminance (cd/m 2 ) Typical values for the luminance of displays Display (max) III-V LED 1 M-10M Photometric Unit Luminous flux Luminous intensity Dimension lm lm/sr = cd Radiometric unit Radiant flux (optical power) Radiant intensity Dimension W W / sr Photometric and corresponding radiometric units Illuminance lm/m 2 = lux Irradiance (power density) W / m 2 Luminance lm/(sr m 2 )=cd/m 2 Radiance W / (sr m 2 ) CIE eye sensitivity function CIE 1931 function : current photometric standard in US The CIE 1978 function, which can be considered the most accurate description of the eye sensitivity in the photopic vision regime 6

Eye sensitivity function and luminous efficacy The conversion between radiometric and photometric units is provided by eye sensitivity function, V( λ) Visible range: 390-720 nm Definition of Lumen: -

7 Eye sensitivity function and luminous efficacy The conversion between radiometric and photometric units is provided by eye sensitivity function, V( λ) Visible range: nm Definition of Lumen: - Green light (555 nm) with power 1W has luminous flux 683 lm Efficacy of radiation gives number of lumens per optical Watt. Among LEDs with same output power, green LEDs are brightest. Luminous flux and efficiency Luminous flux (unit: lm) lm Φlum = 683 V ( λ) P( λ) dλ W λ ( V( λ ) : eye sensitivity function, P( λ) : the power spectral density) Luminous efficacy (unit: lm/w) Φ lm = lum = 683 V ( λ) P( λ) dλ / P( λ dλ P W ) λ λ - the conversion efficiency from optical power to luminous flux Luminous efficiency (unit: lm/w) Φ = lum IV - The luminous efficiency is the product of luminous efficacy and electrical-to to-optical optical power conversion efficiency. - The luminous efficiency is highly relevant figure of merit for visiblev isible-spectrum spectrum LEDs. 7

8 Luminous efficiencies of different light sources Incandescent sources Fluorescent sources HID (Highintensity discharge) Light sourse Edison s first light bulb (C filament) Tungsten filament light bulbs Quartz halogen light bulbs Fluorescent light tubes Mercury vapor light bulbs Metal halide light bulbs High pressure sodium vapor light bulbs Luminous efficiency 1.4 lm/w lm/w lm/w lm/w lm/w lm/w lm/w Summary of photometric, radiometric, and quantum performance measures for LEDs Figure of merit Luminous efficacy Luminous efficiency Luminous intensity efficiency Luminance Power efficiency Internal quantum efficiency External quantum efficiency Extraction efficiency Explanation Luminous flux per optical unit power Luminous flux per input electrical unit power Luminous flux per sr per input electrical unit power Luminous flux per sr per chip unit area Optical output power per input electrical unit power Photons emitted in active region per electron injected Photons emitted from LED per electron injected Escape probability of photons emitted in active region Unit lm/w lm/w cd/w cd/m 2 % % % % 8

Brightness and linearity of human vision Brightness: lack of standardized scientific definition Brightness is an attribute of visual perception and is frequently used as synonym for luminance and

9 Brightness and linearity of human vision Brightness: lack of standardized scientific definition Brightness is an attribute of visual perception and is frequently used as synonym for luminance and (incorrectly) for the radiometric term radiance For point source, Brightness (in the photopic vision regime) can be approximated by the luminous intensity (cd) For surface source, Brightness can be approximated by luminance (cd/m 2 ) Standard CIE Assumption: human vision is linear within the photopic regime Isotropically emitting blue point source and red point source have the same luminous intensity Circadian rhythm and circadian sensitivity Circadian rhythm of circadian cycle The human wake-sleep rhythm has a period of approximately 24 hrs. The wake-sleep rhythm of humans is synchronized by the intensity and spectral composition of light. Exposure to inappropriately high intensities of light in late afternoon or evening can upset the regular wake-sleep rhythm. Retinal ganglion cells have an optical sensitivity. 9

Lighting Terminologies Introduction

Lighting Terminologies Introduction A basic understanding of lighting fundamentals is essential for specifiers and decision makers who make decisions about lighting design, installation and upgrades. Radiometry