dif·frac·tom·e·ter \ˌdiˌfrakˈtäməd.ə(r)\noun Etymology:diffraction + -o- + -meter : an instrument for measuring the diameters of small particles in a microscope field by means of the diffraction rings which appear to surround them • dif·frac·to·met·ric\di|fraktə|me.trik\adjective • dif·frac·to·met·ri·cal·ly\-trə̇k(ə)lē\adverb