A CMOS optical sensor which measures the number and the size of objects has been designed and successfully tested. The sensor contains an array of 24×14 pixels in a chip area of 6.8mm×4.6mm. The sensor first performs image digitization by comparing a photogenerated current with a given threshold. A Winner-Take-All (WTA) operation is carried out next in each object region which is defined as a collection of illuminated pixels mutually connected in either a horizontal or a vertical direction. Realization of the WTA circuit with sharp selectivity makes it possible to select only one winner from each object by utilizing inherent mismatch in transistor characteristics. The output current of the winning pixel represents the object size; the number of winning pixels represents the number of objects. The use of an analog WTA circuit contributed to simple implementation. The sensor operated successfully for images in which the sizes of objects were equal to or smaller than 4×4 pixels.