According to Fresnel’s law an antireflective (AR-) coating on glass is realized by a AM — layer with a refraction index of niayer = (ngias)1/2. To get such a low refraction index the density of the coating has to be lowered, e. g. by increasing the porosity. The thickness of the A /4-layer is matched with the wavelength of the most intensive part of the solar spectrum, i. e. 560 nm. Whereas reflectance in this part of the spectrum is nearly zero, it increases towards smaller or higher wavelengths in the solar spectrum. Therefore (including an extremely high transmittance of the 3 mm basic glass close to 92 %) an overall transmittance of the antireflective coated glass envelope of more than 96 % was achieved.

The SCHOTT AR-coating is the result of a dipping process using the sol-gel technique to produce the required porosity. The thickness of the AR-layer in the range of > 100 nm will be controlled by the viscosity of the sol and the velocity of withdrawing from the solution.

The weak point of competing AR-coatings is their low adhesion to the borosilicate glass substrate. They are normally removable with a handkerchief. To overcome this weakness SCHOTT has developed a new composition of the AR-layer on silicate basis which guarantees long-term stability against abrasion.

The abrasion resistance of the AR-layer has been measured using a standard method developed by SCHOTT. A cylindrical standard eraser (MIL-E-12397) with a cross-section of 5 mm is moved under pressure (0,5N/mm2 = 72 PSI) and the number of strokes needed to remove the layer are counted. According to this the SCHOTT AR-coating tolerates at least 50 strokes whereas competing AR-coatings are already removed after 5 strokes.