Minimizing fingerprints and reflections
Another consideration for front lens aesthetics is the use of antiglare (AG) films to lessen the effect of fingerprints and reduce the clarity of specular reflections. AG films must be used with 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.
Figure 4: The antiglare film can blur the TFT image as shown on the right image
Performance and conformance
One of the challenges is to ensure vehicle center display visibility under all lighting conditions. Visibility problems occur when engineering principles coupled with human factor studies are not properly applied. The fundamental geometric requirement is that the display must be positioned and tilted so no window can be seen in a mirror placed on the display surface. If a window can be seen, a potentially unsafe specular sunlight condition could result in increased driver recovery time and possible retinal damage from seeing the sun’s reflection. No amount of antireflective films or antiglare treatments can solve the sunlight specular angle problem since the luminance of the sun is approximately 1.6x109 cd/m².
To determine the required display luminance, the reflection components must first be assessed from various sources.
Since superposition applies, the process involves considering each source separately to obtain the total background reflection luminance. These three major lighting conditions must be considered: Hemispherical illumination comes from the cockpit interior where the diffuse reflectance of the display may be estimated by using an integrating sphere specular component excluded (SCE) measurement. Specular illumination comes from sources like white shirt, seats, etc. Once the specular reflectance, ζ
, for the system is measured, the specular object luminance seen by the driver can be calculated per Equation 1
The haze reflection component of the display, due to direct sunlight illumination, is perhaps the least understood component of reflection and is the cause of many poor visibility implementations. Using a classical Bi-Directional Reflection Distribution Function (BRDF) method to measure the reflectance as a function of angle from the specular position can cause some of the reflection components to miss the detector due to multiple reflection surfaces.
Figure 5: Small signal reflection measurement method
It is not uncommon to calculate very high required display luminances on the order of 3000cd/m² out of the TFT for dead-front touch-lens systems. This value is much larger than typical automotive TFT luminances of 500-850cd/m² that do not have a “dead front” neutral density lens in front of the TFT. The luminance reduction of the display at center stack viewing angles also needs to be considered for different backlight brightness enhancement film configurations. By considering thermal maintenance from a system level, high luminance 3000 cd/m² displays can be achieved, which dissipate approximately 20 Watts of backlight power for an 8” TFT.