A method of producing a transparent diffusive OLED substrate includes lapping one face or both faces of a flat translucent glass substrate with an abrasive slurry, so as to obtain a flat glass substrate with at least one roughened surface having a roughness profile with an arithmetical mean deviation Ra of between 0.1 μm and 2.0 μm; coating the roughened surface or one of the roughened surfaces with a high index glass frit having a refractive index of at least 1.7, the amount of the high index glass frit being sufficient to completely cover the roughness profile of the roughened surface after melting of the frit, and heating the coated substrate to a temperature above the melting temperature of the high index glass frit and below the softening temperature of the underlying substrate, so as to form high index enamel on one of the roughened surfaces.