Mirror contamination is a prominent problem in dew point meter. Its impact is mainly manifested in two aspects. One is the Raoul effect, and the other is changing the level of specular background radiation. The Raoul effect is caused by water-soluble substances. If this substance (usually soluble salts) is carried in the measured gas, dew condensation will occur on the mirror surface in advance, which will cause a positive deviation in the measurement result. If the pollutants are water-insoluble particles, such as dust and so on, the background scattering level will be increased, and the photoelectric dew point meter will have zero drift. In addition, it is needless to say that the vapour of some easily condensable substances (such as organic matter) having a boiling point lower than water will interfere with the measurement of the dew point. Therefore, for any type of dewpoint meter, measures should be taken to prevent contamination of the mirror surface. Generally speaking, the impact of industrial process gas analysis pollution is more serious. However, even in the measurement of pure gas, the pollution of the mirror surface will accumulate over time.