Figure 3 | Simulation of the cavity mode and the SRI for beam-shaping OLEDs. Calculated cavity modes are shown for an OLED in the optical maximum (a) and the optical minimum (c), respectively. b) shows the normalized PL spectra of the red emitters used. The peak intensities of both emitters are marked in the cavity modes to clarify the choice of the emitter selection. Multiplying the cavity modes with the PL-spectra explains the measured SRI as shown in d) for forward emission and e) for sideward emission. The spectral difference between the maxima is 48 nm. Both the simulation and the measurement using a spectro-goniometer follow a radial symmetry with the OLED in the centre of rotation, mimicking a spherical surface of observation. However, for many conceivable applications this does not represent a very realistic model. Therefore, as introduced earlier, the analysis of the beam-shaping performance is carried out using a planar, diffusive screen (see Fig. 1b). A subsequent image analysis allows us to obtain the brightness distribution. The results of spectro-goniometer and imaging screen measurements are in direct correlation to each other via an azimuthal transformation, as sketched in Fig. 4a. This case, where the projection source is in the centre of the sphere, is called gnomonic projection. Assuming the OLED to be a point source and the aperture of the spectrometer as d A at the distance R , the corresponding area on the screen is d A $^{′}$, which is at a distance r from the spheres north pole ( θ$_{north pole}$ = 0 $^{◦}$). Because the diameter of d A is much smaller than R , it can be interpreted as the surface element in spherical coordinates and analogue d A ′ in planar coordinates. Simple calculation yields for the irradiance E : Various planes of observation E$_{Goniometer}$ = 1 cos $^{3}$θ · E$_{Screen}$. (2) 6 0 10 20 30 40 50 60 70 80 Angle [°] 500 600 550 650 700 750 Wavelength [nm] 0 0.2 0.4 0.6 0.8 1 Norm. spectral rad iant intensity 0 10 20 30 40 50 60 70 80 Angle [°] 500 600 550 650 700 750 Wavelength [nm] 0 10 20 30 40 50 60 70 80 Angle [°] 0 0.2 0.4 0.6 0.8 1 Norm. cavity mode  Ir(piq) 3  Ir(MDQ) 2 (acac) 0.5 1  608 630 Norm. s$_{PL}$ 0 10 20 30 40 50 60 70 80 Angle [°] 500 600 550 650 700 750 Wavelength [nm] 48nm a b c d e Forwa rd Side 0