An optical system including one or more optical sensors and a processing unit capable of performing built-in tests of the optical sensor(s) and methods thereof are disclosed. The processing unit includes a built-in test module configured to detect a reduction of an optical quality of at least some of the images generated by at least one of the optical sensors with respect to an expected optical quality thereof. The built-in test module may be configured to determine whether the reduction of the optical quality thereof is due to the “external optical disturbances” (EOD) and/or failure of the optical sensor(s)/system. The processing unit may include a relative built-in test module configured to compare at least some images generated by at least two of the optical sensor(s) and to determine which of the at least two optical sensors thereof, if any, is subjected to at least one of the EOD and/or failure.