4.1. Optical transmittance/absorbance curve of Sn, Mn andFe halide films:

The optical transmittance/absorbance curve for Sn, Mn and Fe halides are shown in Figs. 1, 2 and 3 respectively. These curves show that these films are highly absorbing in the ultra violet region (0.3-0.45 pm) 1-3. The films also show very high transmittance in the visible

region (0.4-0.85 gm), while they are reflecting in the near infrared region (0.85-1.0 gm). The maximum transmittance at the end of the visible region are 88%, 86% and 76% respectively for Sn, Mn and Fe halides at 820 nm. On the other hand the films have very low absorbance in the visible. Visible absorbance range from 0.06 to 0.22 x 106 m-1 for Snl2; 0.07 to 0.28x 106 m-1 for MnBr2 and 0.12 to 0.58 106 m-1 for FeCl2. UV absorbance ranges from 0.22 to 0.58 m-1 for Snl2, 0.28 to 0.55 m-1 for MnBr2 and 0.58 to 0.93 m-1 for FeCl2, while NIR absorbance is from 0.06 to 0.33m-1 for Snl2, 0.07 to 0.31m-1 for MnBr 2 and 0.12 to 0.26m-1 for FeCl2.

Thus the behaviour of the spectral curves show that these films will allow transmission of the visible portion of the solar spectrum but will shut off the transmission of the ultra violet and near infrared portions of the spectrum. This is because transmittance of about 60% in the visible region is high enough to allow high level of ‘daylight’ (luminance) into building interiors. Also, an absorbance of about 0.6 per cm in the UV region and about 0.35 per cm in the NIR region are high enough to cause substantial attenuation of both UV and NIR radiation and this will reduce their transmissions, and hence shut off high energy UV and high temperature NIR radiations from entering the building interiors [3].