1. An inspection system configured to block specular reflection and suppress modulation in an image of a specimen, comprising: an illumination subsystem configured to illuminate the specimen with a predetermined pattern of spatially incoherent light, wherein periodic features are formed on the specimen, wherein the periodic features are periodic in at least one dimension, wherein the illumination subsystem comprises one or more parallel open slits configured to produce the predetermined pattern of the spatially incoherent light, and wherein a width of the slits is less than a value determined by multiplying a wavelength of the snatially incoherent light by a focal length of a condenser lens configured to focus the predetermined pattern of the spatially incoherent ht onto the specimen and dividing results of said multiplying by a pitch of the periodic features in a direction perpendicular to a length of the slits; an optical element positioned in a path of light from the specimen, wherein the optical element is configured to block light reflected from the periodic features and at least some light diffracted from the periodic features; a detector configured to detect light that passes through the optical element and to generate an image of the specimen in response to the detected light, wherein the optical element blocks specular reflection and at least partially suppresses modulation in the image due to the periodic features; and a processor configured to detect defects on the specimen using the image.