Process for preparing perylene-3,4,9,10-tetracarboxylic diimide in transparent pigment form

A process for preparing transparent perylimide pigments of the formula (I):

• in which
• u is a number from 0 to 8 and, if u>0,
• E is a chlorine or bromine atom and, where u>1, is a chlorine or bromine atom or a combination thereof, which comprises wet-grinding a crude perylimide pigment in a liquid medium in a stirred balImill operated at a power density of from 1.0 kW per liter of milling space and at a stirrer tip speed of more than 12 m/s under the action of grinding media having a diameter of less than or equal to 0.9 mm, and isolating the resulting pigment,
• wherein the crude perylimide pigment has been prepared by melting 1,8-naphthalenedicarboximide with an alkali metal hydroxide to give a leuco form and oxidizing the leuco form, or by condensing perylene-3,4,9,10-tetracarboxylic acid, its salt or its anhydride with ammonia, and
• wherein, before or during preparing the crude pigment, before or during wet grinding, before or during the isolation of the pigment, or to the isolated pigment, or at two or more of said points in time, an auxiliary is added selected from the group consisting of surfactants, pigment dispersants, fillers, standardizers, resins, defoamers, antidust agents, extenders, shading colorants, preservatives, drying retardants and rheology control additives, or a combination thereof.