Disclosed are methods and apparatus for inspecting a photolithographic reticle. A plurality of patch areas of a reticle is defined. Prior to using a reticle in any photolithography process, an optical reticle inspection tool is used during a first inspection to obtain, for each of a plurality of sets of one or more patch areas, a reference average of multiple reference intensity values corresponding to light measured from a plurality of sub-areas of each patch area of the reticle. After using the reticle in a plurality of photolithography processes, the optical reticle inspection tool is used during a second inspection to obtain, for each of the sets of one or more patch areas, an average of multiple test intensity values corresponding to light measured from the plurality of sub-areas of each patch area of the reticle. A same setup recipe for the optical reticle inspection tool is used for both the first and second inspections. A difference intensity map is generated, and such map comprises a plurality of map values that each corresponds to a difference between each average of the test intensity values and average of the reference intensity values for each of the sets of one or more patches. The difference intensity map indicates whether the reticle has degraded over time more than a predefined level.