We have developed a coated fabric which can be used as UV protector. We are in a process to optimize the parameters of the fabric for the highest UV light absorbency. Surprisingly, we are getting less than 1% transmittance. As per Beer-Lambert Law, to measure absorbency accurately, transmittance should be higher than 10%. Please suggest the way to measure absorbency in the above condition.
In principle: measure the reflectance instead! From that you can derive the index of absorption by either a dispersion or Kramers-Kronig analysis if you have a smooth surface and high specular reflection. Otherwise it would be diffuse reflection and Kubelka-Munk...
From your description I guess that you are measuring on the coated fabric, i.e. the complete system of both the fabric and the coating. If so, I would have measured both the total (specular + diffuse) and diffuse transmittance and reflectance using an integrating sphere in your spectrophotometer, in order to obtain as complete picture as possible of the UV protection of your system. A removable (exclusion) port in the specular beam path will enable you to measure both the total and the diffuse transmittance and reflectance (recording the spectra with and without the exclusion port in place, respectively). Comparison of all these four spectra, i.e. total transmittance, diffuse transmittance, total reflectance and diffuse reflectance, will tell you much about your system. It is important to measure on the complete and real-life system in order to obtain a more correct value for your UV protection, i.e. also measuring the fabric and not only the coating. Naturally, for UV protection behind a protective fabric, what is coming through of UV rays (and often also short-wave visible solar radiation) is in principle what is most important, i.e. it is the transmittance which is most important, whereas the reflectance will in most cases not be so important (with some exceptions), although the reflectance values of course are telling you what is not entering your coated fabric at all, i.e. neither absorbed or transmitted, and that is of course also interesting to know. If you have measured specular transmittance on your complete coated fabric system, it is not strange that you have got transmittance values below 1 % instead of your expected values above 10 %, as you then have a lot of diffuse radiation which is not collected by the detector(s). So in conclusion, use an integrating sphere and measure both the total and diffuse transmittance (and reflectance) of your coated fabric. If you send me a sample, I could measure and check this for you. In addition, if possible, you could also measure the coating itself (without the fabric) to learn more about the UV protective properties of the coating (specular, total and diffuse transmittance and reflectance). Finally, based on your measured transmittance spectra, solar material protection factors (SMPF) and solar skin protection factors (SSPF) could be calculated, see e.g. the following article (which may be requested through Research Gate): B. P. Jelle, A. Gustavsen, T.-N. Nilsen and T. Jacobsen, ”Solar Material Protection Factor (SMPF) and Solar Skin Protection Factor (SSPF) for Window Panes and other Glass Structures in Buildings”, Solar Energy Materials and Solar Cells, 91, 342-354, 2007.
Dear Dr. Thamas,
Thank you very much for your answer to my query. The details of roughness of the coated fabric is attached. Please suggest.
Regards
Kajal Sarkar
SRF, IIT Kharagpur, India
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