There are a certain number of pores in the film. Before the film is not absorbent, the pores in the film are filled by air and the refractive index is 1. After absorbing water, the pores are filled with water, which has a refractive index of 1.33. Due to the change of refractive index in the pore, the average refractive index of the film increases, and the optical thickness of the film increases with the increase of refractive index, so the peak wavelength of the filter shifts to long wave.
With the wide application of narrow band optical filter in aviation, aerospace, national defense, communication, environmental protection, analytical instrument and other fields, higher requirements are put forward for its performance. Temperature and hygroscopicity are two main factors affecting the performance of narrow band optical filter.
The characteristic is sensitive to temperature change. For the material with positive temperature coefficient, when the temperature change is very small, with the increase of temperature, the most important influence is that the wave peak moves to the direction of long wave. When the temperature changes greatly, the moisture of optical filter film will be released, which may cause the wave peak to move to the direction of short wave.
The relationship between optical property and aggregation density, temperature variation and hygroscopicity was analyzed. According to the thickness errors of each film of narrow band optical filter caused by hygroscopicity and temperature, the model of spectral drift before and after hygroscopicity and temperature change are made. Through the analysis and preliminary experiment of the optical stability of narrow band optical filter, the measures to improve the optical performance of narrow band optical filter are put forward.
After the all-medium narrow band optical filter is exposed to the air, macroscopically water absorption appears soon. First, there are many water-absorbing spots on the surface of the film, then gradually expand and connect into a piece, and finally form a uniform water-absorbing layer. When the water absorption of membrane reaches stability, the transmittance of the filter is measured and found that not only the peak wavelength shifts to the long wave, but also the transmittance of the peak wavelength increases.
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