Keeping in mind the measured transmission spectra and availability of lasers, it seems that the He–Ne laser generating at 3.39 μm wavelength, strongly absorbed by the alcohol, is the best option. For reference beam, a laser diode generation at 1.5 μm wavelength was chosen. Using only two wavelengths is not enough to detect alcohol because of the window pane, which has different transmission for these wavelengths as well as different thicknesses in different cars. If the thickness was the same for all cars or the transmission was the same for both wavelengths, there would not be any problem with the detection. However, the differences are quite big and using the third wavelength, which will enable measurement of the thickness of the windowpane, becomes necessary. The third wavelength should have a transmission different from that of 1.5 μm wavelength and lie outside the absorption bands of alcohol, , and . Laser diode generation at 1.3 μm wavelength seems to be a reasonable solution. The chosen wavelengths are indicated in Fig. 1 by straight vertical lines.