1、Illumination Design For Automotive Interior Applications,Adam Zysnarski 2002,Electronic Design Seminar Series,Backlighting Design,Outline,The general backlighting problem.,Outline,Spectrum and Color Definition of Spectrum How is Color defined 1931 CIE Chromaticity Diagram,Outline,Photometry Flux Sol
2、id Angle Intensity, “Half-angle” Illuminance Luminance Transmittance/Optical Density “Practical” Definitions of Transmittance Typical Sources,Outline,General Backlighting Revisited Direct Backlighting Indirect (Reflector) Backlighting Indirect (Lightpipe) Backlighting,Outline,Notes on Backlighting U
3、sing LEDs Packages Biasing Thermal Considerations,The General Backlighting Problem,A,A,A,The General Backlighting Problem,Need to Quantify: Source Light Quantity, Directionality, Color Content, Polarization Medium Throughput Geometry, Reflectance, Lightpipe Eff., Polarization Graphic Characteristics
4、 Transmittance, Diffusion, Color “Filtering” , Polarization,Spectrum and Color,Energy=const/wavelength (JOULES) Power = Energy/time (WATTS) Spectrum is POWER/Wavelength Interval,Spectrum and Color,Decompose source spectrum using Tristimulus Functions to obtain x,y color coordinates,1931 CIE Chromati
5、city Diagram,Photometry,R,Photometry,Luminous Intensity, I= df/dW (SI:Candela, cd)Function of angle,Photometry,Illuminance, E=df/dA (lm/m2)Amount of flux per unit area crossing a surface (flux density),D,For D small compared to d: E=I/d2,Photometry,How do we typically quantify the “brightness” of a
6、backlit graphic?,LUMINANCE, L=dI/dA (SI: cd/m2, English: footlambert, fL),Photometry,“Lambertian” An emitter (or reflector) is said to be Lambertian if the luminance from the surface is constant over the hemisphere.,So:,Transmittance,Pin and Pout rarely collimated Pin distribution “source-like” Pout
7、 distribution “diffuse” Absorption and reflection also occurs Optical density,Transmittance,Match transmittance geometry to application as appropriate:,Transmittance,Direct Backlighting,A “pure” green LED is to be used to backlight a button graphic of extent D=8mm at a distance of d=18mm. The graphi
8、c is white (i.e. spectrally flat) with a transmittance in the green of 7%. The luminance is targeted at L=3.4 cd/m2 (=1fL). Determine the intensity and half-angle required to “evenly” backlight the graphic.,Direct Backlighting,Half-angle:Intensity: On the back of the graphic: On the front of the gra
9、phic:,Indirect Backlighting (Reflector),A lightbox is to be constructed to backlight a 14.6cm x 4.9cm LCD. The LCD transmission (diffuse luminance) for LED green is 10.3%. Assuming 100% throughput (I.e. perfect reflecting walls and no light trapping) what is the minimum number of 40 mlm green LEDs N
10、, required if I want to achieve 4 cd/m2 out of the LCD?,Indirect Backlighting (Reflector),Indirect Backlighting (Lightpipe),Throughput of the system can be viewed as the sum:Can still think in terms of the photometric quantities already defined:Need more descriptive physics,Light Rays,Travel in a st
11、raight line until they encounter a surface. Two polarization components, one in the plane of incidence, one perpendicular to the plane of incidence. Initially, the light rays usually contain equal “amounts” of polarization.,Index of Refraction,Describes the optical behavior of a material. Wavelength
12、 dependent (“dispersion”). Complex number. Real part speed of light. Imaginary part attenuation of ray.,Typical Values of n,Polycarbonate, l=550nmn=1.59-0.1i Acrylic, l= 550nmn=1.49-0.01i (Plexiglass V825) Air (vacuum)n = 1.0,When , ray energy will reflect:Portion of the ray will refract:n1n2,Reflec
13、tion,Amount of light reflected at interface, R, is a function of angle, and calculated using Fresnels equation. Perpendicular to interface: For Acrylic, , i.e. 4% of light is reflected back in the direction of the source.,Fresnel Reflectance,Fresnel Reflectance For Acrylic,Average for Both Polarizat
14、ion,At the critical angle, all light is reflected:For acrylic in air:Can only occur if n1 n2.,Total Internal Reflectance (TIR),Bending losses in Lightpipes,R,t,Minimum Radius:,For glass and acrylic:,For Lexan P.C.:,Lightpipe Transmission,Lightpipe Summary,Lightpipe must have a higher n than surround
15、ing ( ie. Air gaps are “good” when trying to reflect light). Critical angle should be maintained to guide light. Example : bend radius; radius of bend should be much larger than thickness of lightpipe.,Raytracing,LightTools (Optical Research Associates, Pasadena, CA) is a Monte Carlo simulation rayt
16、race. Capabilities include calculations of flux, intensity, illuminance, and luminance.,Raytrace,Note on LED Biasing,Current controls luminance; forward voltage above junction voltage ensures current flow.,R,V,IF,VF,R=(V-VF)/IF,Intensity is “reasonably” linear with IF for “normal” ranges.,Note on LE
17、D Biasing,Forward current must be “derated” depending on expected ambient temperatures,References,Introduction to Radiometry and Photometry, W.R. McCluney, 1994 Artech House. The Radiometry of Light Emitting Diodes, Labsphere Technical Guide, . Radiometry and the Detection of Optical Radiation, 1983 John Wiley & Sons. Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd Ed., G. Wyszecki and W. Stiles, 1982 Wiley Interscience.,
