Abstract: A method for detecting a change in a vacuum state within a sealed thermal detector package which is a part of a thermal camera, the package housing a thermal detector array and at least one temperature sensor. The method comprises measuring an initial signal from said thermal detector array; concurrently measuring an initial signal from said at least one temperature sensor; measuring a later signal from said thermal detector array; concurrently measuring a later signal from said at least one temperature sensor; performing a first calculation of a ratio of the difference between the later and initial signals from said thermal detector array to the difference between the later and initial signals from said at least one temperature sensor; and periodically measuring the initial and later signals from said thermal detector array and from said at least one temperature sensor and calculating the ratio to determine changes in the ratio indicative of changes in the vacuum state within the package.