Key influences on the cavity mode need to be considered to allow for effective beam-shaping as demonstrated here. First, the thickness of the transparent contacts in the device have a determining role. In analogy to a Fabry-P´ erot interferometer, a higher reflectance of the mirrors leads to a higher finesse, resulting in a narrower cavity mode and, consequently, a reduced full width at half maximum (FWHM) of the emission spectra. 24 Those arguments hold true for both the ultra-thin wetting layer metal electrodes and metal oxides like indium tin oxide, which are used in the device layout (cf. Fig. 1a). 25,26 Moreover, a change of the cavity length, meaning the resonance wavelength, 27 strongly influences the emission of OLEDs. 20,24,28 In this study, the thickness of the device is adjusted through thickness changes of the highly conductive and transparent doped transport layers. Importantly, OLEDs are different than passive cavities, because the active layer where EL originates, the emission layer (EML), accounts only for a small fraction of the total cavity thickness (around 20 nm typically) and can be placed freely within the cavity. Thus, changes can be achieved for a given cavity length and variable EML position. For an unambiguous description, it is necessary to consider the distribution of the electromagnetic field in layered media. 10,29,30 OLEDs enclosing one field maximum are denoted as OLEDs of first order. Analogue OLEDs of second order show two maxima. The latter architecture allows various ways of positioning of the EML: in one of the two field maxima (optical maximum) or even in the minimum (optical minimum). Simulations demonstrate that OLEDs of higher order show narrower cavity modes, which bend to shorter wavelengths with increasing angles (compare Fig. 3a and Fig. 3c). The curvature follows the cosine-like behaviour, typical for a Fabry-P´ erot interferometer. 27 This effect is used for the desired angularly-confined emission. Primarily forward emission emerges from placing the EML close to the optical maximum. In our study, the cavity length is optimized for the dark red emitter Ir(piq)$_{3}$. The resulting experimentally measured spectra (cf. Fig. 3d) show maximum intensity at 0 ◦ and 626 nm, dropping to half intensity at 48 $^{◦}$. Outcoupling of another branch of the cavity mode gets visible at higher angles, but can be ignored due to its low intensity. In order to realize a condition where the 0 ◦ emission is suppressed effectively, one has to place the EML in the respective optical minimum of the cavity (cf. Fig. S1, Supplementary Information). The calculation of the respective cavity mode (cf. Fig. 3c) shows that indeed one can expect emission at higher angles, around 50-80 $^{◦}$. A comparison of Fig. 3a and Fig. 3c indicates an offset between the respective modes, with the sideward emission shifted to a longer wavelength. To compensate for this effect, another emitter is chosen, namely the red emitter Ir(MDQ)$_{2}$(acac). Its PL-spectrum resembles the one of Ir(piq)$_{3}$, but is blue-shifted about 22 nm (see Fig. 3b). This approach may seem counter-intuitive but it is one of the most important design steps towards colour stability between forward and sideward emission. The experimentally obtained SRIs are shown in Fig. 3d) and e). Because of the aforementioned mode curvature, the resulting SRI for sideward emission peaks at higher viewing angles of around 60 $^{◦}$. These SRIs of forward and sideward emission are compared to simulated values, calculated by multiplying the cavity modes with the respective PL spectra of the emitters, in Fig. S4 (Supplementary Information). The difference in wavelength between the experimental maxima in forward and sideward emission is 48 nm. This is slightly smaller than the value obtained from the spectra in Fig. 2c, which is due to the fact that the latter refers to the maximum of the integrated radiant intensity. Without compensation through emitter choice, the peak differences would be as high as 66 nm, as simulated for the case of the emitter Ir(MDQ)$_{2}$(acac). With the right emitter molecules at hand, this colour difference can be further minimized. 5