A method of inspecting a photolithographic reticle, the method comprising: defining a plurality of local areas of a reticle; using an optical reticle inspection tool during an inspection to obtain, for each local area, an average of multiple reflected intensity values corresponding to light reflected from a plurality of sub-areas of each local area of the reticle; using the optical reticle inspection tool during the inspection to obtain, for each local area, an average of multiple transmitted intensity values corresponding to light transmitted through the plurality of sub-areas of each local area of the reticle; and generating a combined intensity map, using a controller, by combining, for each local area, the average of multiple reflected intensity values and the average of multiple transmitted intensity values received at the optical inspection tool such that a reticle pattern of the reticle is cancelled out of the combined intensity map if the reticle has not degraded and such that the reticle pattern of the reticle is not cancelled out of the combined intensity map if the reticle has degraded; wherein the average of multiple reflected intensity values for each local area is obtained from a reflected intensity image obtained by the optical reticle inspection tool and the average of multiple transmitted intensity values for each local area is obtained from a transmitted intensity image obtained by the optical reticle inspection tool, and wherein the combined intensity map is in the form of an image that combines both the reflected and transmitted intensity images; wherein each local area corresponds to a patch of the reflected and transmitted intensity images, wherein each patch has a plurality of pixels, wherein each patch is scanned to obtain multiple intensity values from both the reflected and transmitted intensity images, wherein the plurality of intensity values obtained from each patch from each of the reflected and transmitted intensity images are averaged together to determine the average of multiple reflected and the average of multiple transmitted intensity values respectively; wherein the inspection is performed while a pellicle is mounted on the reticle and wherein the presence of a spatially varying signature across the combined intensity map indicates whether a pellicle of the reticle has degraded over time more than a predefined level. defining a plurality of local areas of a reticle; using an optical reticle inspection tool during an inspection to obtain, for each local area, an average of multiple reflected intensity values corresponding to light reflected from a plurality of sub-areas of each local area of the reticle; using the optical reticle inspection tool during the inspection to obtain, for each local area, an average of multiple transmitted intensity values corresponding to light transmitted through the plurality of sub-areas of each local area of the reticle; and generating a combined intensity map, using a controller, by combining, for each local area, the average of multiple reflected intensity values and the average of multiple transmitted intensity values received at the optical inspection tool such that a reticle pattern of the reticle is cancelled out of the combined intensity map if the reticle has not degraded and such that the reticle pattern of the reticle is not cancelled out of the combined intensity map if the reticle has degraded; wherein the average of multiple reflected intensity values for each local area is obtained from a reflected intensity image obtained by the optical reticle inspection tool and the average of multiple transmitted intensity values for each local area is obtained from a transmitted intensity image obtained by the optical reticle inspection tool, and wherein the combined intensity map is in the form of an image that combines both the reflected and transmitted intensity images; wherein each local area corresponds to a patch of the reflected and transmitted intensity images, wherein each patch has a plurality of pixels, wherein each patch is scanned to obtain multiple intensity values from both the reflected and transmitted intensity images, wherein the plurality of intensity values obtained from each patch from each of the reflected and transmitted intensity images are averaged together to determine the average of multiple reflected and the average of multiple transmitted intensity values respectively; wherein the inspection is performed while a pellicle is mounted on the reticle and wherein the presence of a spatially varying signature across the combined intensity map indicates whether a pellicle of the reticle has degraded over time more than a predefined level.