etc and is focused onto a focal plane when the reference lens 410 is not positioned within the optical path the scene radiation is focused as shown by solid lines in fig5 onto an intended focal plane 520 in which the detector array 420 is situated for ordinary scene viewing however when the reference lens 410 is positioned within the optical path the scene radiation is directed as shown by dashed lines in fig5 to an effective focal plane 530 which is positioned behind the detector array 420 this shifting effect in combination with the rough polished lens surface 430 results in a blurred image and a uniform flux level at the detector array 420 as desired in infrared implementations the lens is typically made at least 0 03 approximately 0 762 mm inches thick in order to obtain sufficient diffusion and image blurring fig6 and 7 depict detector calibration methods in which the exemplary system of fig4 and 5 can be utilized specifically fig6 depicts steps in an exemplary single point calibration method 600 and fig7 depicts steps in an exemplary two point calibration method 700 both methods 600 700 can be carried out continuously and in real time during operation of the imaging device 400 so that.