Linear systems methodology is proposed which quantifies flare as part of the spread function or Modulation Transfer Function (MTF) of a microdensitometer with general application to any optical detection system. Because the irradiance system, the sample's scattering properties and the optical components alter the flare portion of these functions, an empirical method, such as edge-gradient analysis, should be used to determine their profiles. Applying straightforward linear systems theory, one has a method of correcting for flare which is usually considered to be a "nonlinear and unpredictable" factor. More importantly, this method is applicable for any object distribution (for a given optical system and sample scattering profile). Partial spread functions and partial MTFs are introduced which permit quantifying the degradation produced by flare, this quantity being denoted as the partial flare factor. The partial flare factor is compared to a more restrictive black-overlay flare factor. Examples are discussed for optical detection systems with and without flare.