An organic EL display device including a substrate, a transparent electrode, a luminescent layer, and a metal electrode layer in this order from a light emission side, a circularly polarizing plate disposed on the light emission side of the substrate, the circularly polarizing plate including a polarizing film and phase difference films that stacked on each other. The method includes calculating and comparing a long-term error rate and a short-term error sample. The nanoparticle ink may be any suitable nanoparticle ink which includes stabilized nanoparticles such as silver, gold or copper nanoparticles.