The angle defined by the numerical aperture of the illumination and projection optics at the DMD optical area is typically the same. Ensure this angle does not exceed the nominal device micromirror tilt angle unless appropriate apertures are added in the illumination or projection pupils to block out flat-state and stray light from the projection lens. The micromirror tilt angle defines DMD capability to separate the "ON" optical path from any other light path, including undesirable flat–state specular reflections from the DMD window, DMD border structures, or other system surfaces near the DMD such as prism or lens surfaces. If the numerical aperture exceeds the micromirror tilt angle, or if the projection numerical aperture angle is more than two degrees larger than the illumination numerical aperture angle (and vice versa), contrast degradation and objectionable artifacts in the display border or active area may occur.