Fundamental Issues in Automotive Veiling Glare

The veiling glare effect in automotive vehicles consists of diffuse and specular scattering of sunlight onto and from the windshield. This effect occurs over a wide range of solar elevation angles and increases with increased degree of inclination of the windshield. Thus its effect on visual acuity must be considered in automotive design. The present research on the subject of veiling glare only addresses scattering from a clean windshield and ignores the larger effect of scattering from dust, dirt or haze on the front and back faces of the windshield since the latter is operator dependent (can be removed by cleaning the windshield). In this paper, we present an analysis of autmotive veiling glare that takes into account windshield reflectivity without and with coatings, and the characteristics of dashboard cover materials. The analysis develops a new reflection model for rough surfaces, based on laboratory measurements of reflectivity as a function of wavelength and angle of incidence, and on reflection and/or scattering angle. In the case of windshield reflectivity, single-film, 3-film and graded-index coatings are considered for both s and p polarizations and a range of windshield rake angles, and calculations are compared to laboratory and full scale measurements. In the case of dashboard reflectivity, surface texture and material ligthtness are considered. Specular and diffuse reflection components are analyzed in terms of angle of incidence, color and polarization direction. It is found that texture has a large effect on the reflected light, especially at large angles of incidence and causes a noticeable forward scattering component which includes specular reflection. Experimental measurements and computer-generated models show that this forward scattering effect can be as large as 10-20 degrees. The presence of forward scattering has the result of increasing the range of solar elevation angles that produce veiling glare. Windshield and dashboard reflectivity effects are then combined and analyzed to determine how they impact the veiling glare process.