One embodiment provides a method of testing humidity, comprising: determining a property of a device which encloses a plurality of interferometric modulators; and determining a relative humidity value or a degree of the relative humidity inside the device based at least in part upon the determined property. In one embodiment, the property of the device includes one of the following: i) a weight of the device, ii) a color change of a desiccant enclosed in the device, iii) a resistance inside the device, iv) whether frost formed in an inside area of the device which is contacted by a cold finger device, v) whether a desiccant enclosed in the device, when water vapor is provided into the device, is working properly, and vi) combination of at lest two of i)-v).