Temperature distributions inside multilayer filters are often required to assess performance. This subject is rarely treated in the literature. Guidelines were developed that can be used to calculate a thermal model for various applications. Data such as the optical and physical properties of glass and coatings are required to build the model. Calculations can be performed with available thermal analysis software packages. In this study the guidelines are applied to an optical filter. This filter, which is not actively thermally controlled, is the front element in a space experiment devoted to the observation of solar intensity fluctuations. Temperature distributions were calculated for different designs using ESA software (ESATAN). The final design described here minimizes temperature gradients, as well as degradation effects due to UV and particle radiation. The effect of space exposure on temperature distributions inside the filter is also addressed.