The principle upon which registration of water and hydrocarbon condensation on the chilled mirror’s surface is based involves exploiting the effect of total refraction.
Total refraction is the effect that occurs when light waves that are polarized in the longitudinal plane encounter the interface between dissimilar media in such a manner that the result is the absence of a reflected wave. This effect is only possible when these waves encounter the interface at Brewster’s angle.
In order to achieve this effect, a vertically polarized laser is used as a light source and the mirror is made using a dielectric material.
When the mirror is clean, the laser beam falls on the mirror’s surface at Brewster’s angle and is fully refracted. As a result, a zero signal is obtained at the photodetector output.
When a thin hydrocarbon film with a different refraction index forms on the mirror’s surface, the conditions for total refraction are not met and a new wave reflected from the gas-film media boundary is formed. Moreover, due to the optical transparency of the condensate film, another wave reflected from the film-mirror media boundary is formed as well. As a result, the photodetector captures two reflected beams, which then combine to form an interference pattern.
The hydrocarbon dew point measurement occurs when the thickness of the film is about 5 - 10 nm.
When the mirror cools and water droplets condense onto the mirror’s surface, intense scattering of light occurs. Condensation of water vapor onto the mirror results in an increase in the photoelectric signal of the registration system. The signal level from the photodetector depends on the amount of water condensed on the mirror’s surface.