Figure 4 | Observers position and contrast. a) Scheme of the transformation needed to compare the measurements obtained on a spectro-goniometer and on a planar screen. b) The contrast is shown with respect to the relative size S$_{rel}$ , which is the ratio of the size of the OLED to its distance from the screen. Black points were obtained by keeping the distance constant; red points by keeping the size constant. The smaller S rel , the better the contrast. The error bars represent the statistical confidence interval of 1 σ . c) With the methods explained herein, it is possible to predict the beam-shape from the spectro-goniometer data. Here, the results for the red OLEDs developed in this contribution are shown. Scale bar is 10 mm. d) To confirm that not only the brightness distribution but also the colours are calculated correctly, samples were made, that emit greenish in the centre of the spot and red to the edges. If the screen is at a distance R ′ = R from the OLED, equation (2) contains an additional factor ( R $^{′}$/ R ) $^{2}$. However, by normalizing the spectra, this factor cancels again. It is worth mentioning that the surface element in spherical coordinates is not of constant size, but dependent on the azimuthal angle θ . Yet, the spectrometer comprises a constant aperture. This situation can be modelled by cylindrical coordinates. In this case the transformation factor becomes 1 / cos $^{2}$θ . However, projecting the respective surface element on a two dimensional plane does not give a filled surface. Hence, the true transformation should lie between those two models. This transformation points out the importance that the integrated radiant intensity for sideward emission has to be strongly enhanced (here, a factor 2.7 to normal emission, cf. Fig. 1d). Otherwise a hollow image, as shown in Fig. 1c for the sideward emission, cannot be realized. 7 R r Relative size S$_{rel}$ Contrast C [%] 20 30 40 50 0 0.2 0.4 0.6 0.1 0.3 0.5 0.7 Distance: 105 mm Pixel size: 24 mm  a b 0 15 30 60 45 15 30 45 60 Angle [°] 0 15 30 45 15 30 45 60 Angle [°] c d Simulation Experiment Experiment Simulation