2.2 Optical measurements and choice of manufacturer
where T is the transmission of the sample. The photon spectrometer allows the measurement of the transmission in steps of 1 nm, from 190 to 900 nm. Figure 2-1 shows a schematic of the Elmer-Perkins Lambda 15 spectrometer. It consists of light source that is split into two parallel and collimated beams. The selection of the wavelength is done by means of a prism controlled by a precise step-motor. One of the beams is used as a reference while the sample is positioned in front of the other. A set of mirrors is used to direct the light of both beams to two photon detectors (photomultipliers). The current of each is measured and the transmission, T, is given by:
. . . 2-2
where, I0 is the current measured at the reference photon detector and IT is the current measured at the photon detector which is behind the sample. The absorption coefficient for each sample can be calculated from:
where T is the transmission, l is the thickness and m is the absorption coefficient. Before using eq. 2-3, one has to apply corrections due to surface reflectivity losses of transmission. However, since the index of refraction of the samples is very low, the corrections are, in the first order, negligible.
Figure 2-1 Schematic of the Elmer-Perkins Lambda 15 photon spectrometer used in the measurements.
All three Novosibirsk samples have the same absorption coefficient (m). We have repeated the transmission measurement of the Novosibirsk sample on December 3rd, 1996 after leaving the sample in an environment uncontrolled for humidity for more than 3 months. Figure 2-2 shows the transmission curves measured in August and December 1996. After 3 months the transmission cutoff was shifted to 340 nm (instead of 290 nm) and the overall transmission dropped by ~15%.
Figure 2-2 Transmission of the same Novosibirsk sample measured on 29 August and 3 December 1996. The sample had been in uncontrolled environment for this period.
This effect, which is due to the known hygroscopicity of aerogel, makes necessary the storage of the full quantity in a humidity-controlled storage area.
. . . 2-1
. . . 2-3