Aperture photometry is a method of determining the apparent magnitude of a star using a CCD. All the pixel counts affected by the star are summed and from it is subtracted the sum of same count of average surrounding pixels, which yields every excess ccd pixel count due to light from the target. "Aperture" refers to a ring around the target chosen to include all the pixels possibly affected by the target.
Since all the light from the target forms the Airy disk, the sum of all that light is all the light from the target. Taking into account the length of the imaging, an accurate measure of the brightness is determined. With passband filters, accurate color indices can be determined as well.