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Plastic automotive lamp assemblies are vented by a variety of methods to reduce the likelihood of condensation forming in the lamp and to prevent water ingress lamp warranty returns. Previously, it has been shown that the humidity in a vented automotive lamp can be described empirically by a decreasing exponential (i.e. decay). It was also shown that this formula by applying Fick’s Law of Diffusion, particularly the exponential constant (k), can be related to basic physical properties of the lamp system. Specifically, the exponential constant is a ratio of the product of a characteristic cross-section area of the vent and the permeability of water vapor over the product of the lamp volume and a characteristic length of the vent. This description was shown to be less accurate at time t greater than 30 minutes and additional details of the vented lamp system were proposed for better fit to the experimental data.