Experimental evaluation of the dependence of the optical properties of glass on the angle of incidence and wavelength

The refractive index and extinction coefficient of a particular glass are functions of the wavelength of incident electromagnetic radiation and the properties of the glass. The reflectance and transmittance of a glass pane are functions of its thickness and the angle of incidence of an incident electromagnetic wave with the reflectance tending to unity and the transmittance tending to zero as the angle of incidence tends to 90o. Variations of the optical properties with wavelength imply that the optical performance of transparent homogeneous materials will also depend on the spectrum of an incident electromagnetic beam and will vary as the spectrum varies as it does with solar irradiance. An investigation was carried out to determine the effects of wavelength and angle of incidence on the reflection and transmission of an electromagnetic beam incident on a 6 mm thick sample of a transparent homogenous commercial glass. Measurements of the reflectance and transmittance were made from 0o to 90o using a Pyrheliometer to measure the direct unpolarised components of incident, reflected, and transmitted beams. The variations of refractive index from 300 nm to 2500 nm were determined from reflectance and transmittance measurements made using a spectrophotometer with the incident beam normal to the glass sample. Good correlation was obtained between measured and calculated values of reflectance and transmittance over all angles of incidence. The standard deviation between the measured and calculated reflectance was 0.01 and that for the transmittance was 0.006. The variation of reflectance and transmittance with angle of incidence is significant where angles of incidence are greater than 40o. It was found that the refractive index varied from 1.467 to 1.580 over wavelengths of interest and a weighted index was required for accurate analysis of the transmission of full spectrum solar irradiance. Large variations of transmittance with wavelength were measured with the transmittance dropping to zero at approximately 300 nm and 0.15 at 2800 nm. The maximum transmittance of 0.9 for the glass tested occurred at a wavelength of 550 nm. The dependence of glass properties on wavelength was supported using ellipsometry to recalculate the variation of reflectance and transmittance using the refractive index and extinction coefficient obtained from the spectrophotometer readings. Comparison of these calculations with the original measurements from the spectrophotometer gave a standard deviation for the reflectance of 0.04 and for the transmittance of 0.007. The results clearly demonstrated the viability of the methods used to measure the optical properties of glass and the need to take variations of its properties with angle of incidence and wavelength into account when calculating the optical performance of glass.