A vehicle interior rearview mirror system includes an interior rearview mirror assembly having a reflective element and ambient and glare light sensors. A control circuit is operable to establish a reflectance level of the reflective element. At least one of (a) the mirror system includes a charge accumulation device selectively connected with the ambient or glare light sensor and the control circuit establishes the ambient and glare light levels as a function of time for an output of the charge accumulation device to reach a reference level when connected to the respective light sensor, (b) a common element is used to measure outputs of the light sensors sequentially to correspond errors due to component variations, and (c) the mirror system includes temperature compensation of the glare and/or ambient light sensor and the temperature compensation is responsive to a reference light sensor that is substantially not exposed to light.