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EETE DECEMBER 2012

DESIGN & PRODUCTS DISPLAYS & INTERFACES principles coupled with human factor Focusing on aesthetics studies are not properly applied. The hiding the display behind a continu- fundamental geometric requirement ous curved surface that provides is that the display must be positioned partial or complete display hid- and tilted so no window can be seen in ing provides a high craftsmanship a mirror placed on the display surface. appearance that is a trend in the If a window can be seen, a potentially automotive space. This exciting unsafe specular sunlight condition could styling opportunity gives the Tier 1 result in increased driver recovery time supplier or system integrator one of and possible retinal damage from seeing the biggest challenges in the center the sun’s reflection. No amount of an- stack of the vehicle. addressing tireflective films or antiglare treatments this challenge requires the ability to can solve the sunlight specular angle quantitatively assess the visibility of problem since the luminance of the sun the display for a given optical sys- is approximately 1.6x109 cd/m2. tem and vehicle packaging, optimize To determine the required display the design of the optical system, luminance, the reflection components and provide the best combination of must first be assessed from various display and efficient backlight. Fig. 3: The contrast sensitivity function. sources . Providing a dead-front or black since superposition applies, the panel appearance requires an understanding of how the human process involves considering each source separately to obtain eye works to optimize the light transmission while providing the total background reflection luminance. These three major the desired hiding effect. Hiding the display opening when the lighting conditions must be considered: display is off involves human factor studies. The contrast sensi- hemispherical illumination comes from the cockpit interior tivity function (CSF) per figure 3 shows that a contrast of about where the diffuse reflectance of the display may be estimated 0.01 is necessary to hide the display opening. by using an integrating sphere specular component excluded Using the CSF as a guide, different optical constructions may (sce) measurement. be considered to control the reflection level of the display open- specular illumination comes from sources like white shirt, ing versus the non-display area to minimize the contrast (also seats, etc. Once the specular reflectance, ζ, for the system is known as Michelson contrast, Cm) while maximizing display measured, the specular object luminance seen by the driver can visibility. There are many interrelated factors to consider when be calculated per Equation 1. selecting the lens optical configuration, such as transmission, polarization, retardation, display luminance and reflected back- The haze reflection component of the display, due to direct ground luminance. Since reflections can be used beneficially to sunlight illumination, is perhaps the least understood compo- hide the display opening, the lowest reflection lens configura- nent of reflection and is the cause of many poor visibility imple- tion may not always be the best solution. Taking the visibility mathematical function into account, vari- ous optical configurations may be examined to ascertain the required display luminances. Lower display luminances (higher optical system efficiencies) can be achieved with different optics mentations. Using a classical Bi-Directional Reflection Distribu- other than the traditional neutral density filter (ND). However, the tion Function (BRDF) method to measure the reflectance as a background reflection level must be considered to pick the best function of angle from the specular position can cause some of optical configuration based on actual in-vehicle jury evaluations. the reflection components to miss the detector due to multiple reflection surfaces. Minimizing fingerprints and reflections A more suitable method is to use a small signal reflection another consideration for front lens aesthetics is the use of an- measurement method, shown in figure 5, where the collimated tiglare (AG) films to lessen the effect of fingerprints and reduce light source beam size is large enough to encompass all of the the clarity of specular reflections. AG films must be used with reflection surfaces in the optical stack. caution due to unwanted speckle and decreasing the sharpness of the TFT image – see figure 4 - as the film is moved further from the TFT. The blurring performance of AG films may be quantitatively determined by obtaining the line-spread-function and associated modulation transfer function via FFT techniques. Performance and conformance One of the challenges is to ensure vehicle center display visibility under all lighting conditions. Visibility Fig. 4: The antiglare film can blur the TFT image as shown on the right image. problems occur when engineering 18 Electronic Engineering Times Europe December 2012 www.electronics-eetimes.com


EETE DECEMBER 2012
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