in a method and apparatus for determining runout in a cylindrical object , particularly a cutting or boring tool such as a drill bit or reamer , the object is rotated and a collimated beam of light is directed transversely at the rotating object so as to cast a shadow on a photosensor which produces an electrical signal having a current level representative of the area illuminated . runout in the cylindrical object is manifested as a reciprocal variation in the area of the shadow cast on the photosensor , with a resulting periodic variation in the the output signal of the photosensor . the amplitude of this variation is determined to provide a measure of the total indicated runout of the object . in the preferred embodiment , a pair of photocells is positioned behind the rotating object and a differential amplifier and low - pass electronic filter are employed in series to separate variable signal components arising from runout from variable signal components due to profile variations in the object , for example , flutes in a twist drill .