*3.5. Color Sensing of Humidity*

When transparent film is deposited on absorbing substrate (silicon wafer or metal overlayer in our case) it exhibits a certain color which depends on film's optical thickness. When optical thickness changes due to adsorption of water vapors (as is in our case) from the environment, the reflectance spectrum of the film changes and the color alters. Optical sensing based on perceptual color change in response to analyte of interest offers simplicity and is preferred in color sensing of vapors. So the next step of our investigation was to study the possibility of color sensing of humidity, i.e., monitoring of color at different humidity levels for Ac24 thin film on both types of substrates (Si wafer and glass covered with Au–Pd thin film) post-annealed at 60 ◦C. Figure 6 presents the calculated color coordinates (CIE X and CIE Y) of PVA films with 24% acetal content deposited on selected substrates for low (5% RH) as well as high humidity (95% RH). In the case of film on Si substrate, reflectance spectra are used for calculation, while for polymer film deposited on Au–Pd/glass, transmittance spectra at 5% and 95% RH are used. It is seen from Figure 6 that a substantial change of color takes place in the first case: the two points, for 5% and 95% RH, are well separated in the color space. On the contrary, when transparent substrate is used the change of the color due to humidity is not so distinct: the two points, associated with the sample colors at low and high humidity, respectively, almost overlap.

**Figure 6.** Calculated CIE coordinates for Ac24 thin film deposited on opaque (Si) and transparent (glass covered with Au–Pd thin film) substrates exposed to 5% and 95% RH.
