Transmission spectra of glasses such as BK7 and fused silica (FS) provide excellent reference points in the range of 91 to 94 %T. These materials have been studied extensively and their theoretical spectral values are widely known. Transmission spectra of BK7 and FS are shown in the lower panel of the figure. However, if reference spectra are needed for a higher transmission range, for the study of antireflective coatings for example, there are no basic materials available. Use of the two glasses together can solve this problem.
The first step is to create an instrument baseline (100 %T) with FS in the sample beam. Then scan BK7 as a sample, which will generate a spectrum similar to that in the upper panel of the figure, with transmission around 98.5%. Higher spectral values can be obtained by using two glasses with a smaller refractive index (RI) difference.
Another way to generate the high T spectrum is to simply divide the original spectrum of the BK7 (higher RI) by the FS spectrum (lower RI).