content_image/296871.jpg
kinked and starts to drop slightly again towards smaller S$_{rel}$ values. Because of the conservation of ´ etendue in free space, 35 this cannot be a geometrical effect. Rather, it is based on greater noise in the spectra with increasing distance, due to longer exposure times of the camera. Assuming I the maximum intensity, i the intensity at the centre of the spot, Δ I the contribution of a constant noise, C the contrast without noise, and ˜ C the contrast with noise, one gets: 

<img src='content_image/296871.jpg'>

Clearly, a non-zero constant noise of our camera system will limit the contrast values, as soon as the measured intensities become comparable to the noise floor. Increasing the driving current would counteract to this effect, but to make the interpretation of the data as clean as possible, it was kept constant over the whole measurement. These results also show that a definite prediction of the beam shape on the screen is only possible within the assumption of ideal measurement conditions. 

## Combining active and passive beam-shaping 

So far we have presented the possibilities offered by active beam-shaping using the planar, bare OLEDs. At the same time, this system offers to be used as a novel, adjustable platform in combination with additional, secondary optics. As an example, we will discuss possible effects using primitive optical elements, in this case a half-sphere and a prism, to demonstrate the variability of this promising concept. 

The difference in the refractive indices especially between the substrate glass and air leads to total internal reflections. Placing a glass half-sphere on top of the substrate, this effect can be suppressed and the device efficiency increased. 36 Additionally, the transition from glass to air at a planar interface results in a fanning out of emitted rays, which gets efficiently reduced by the half-sphere. Thus, a strong enhancement of the contrast is possible, as shown in Fig. 5. In comparison with the bare OLED (see Fig. 1), the forward emission gets more confined and the side emission forms a sharp ring. The distance between OLED and screen was kept constant during all measurements. Studying the brightness profile, the influence is obtained in a more quantitative way (Fig. 5a). In forward emission, the FWHM decreases to 43 ◦ compared to 62 ◦ for the reference. In the sideward emission, the contrast between the centre of the spot and the maximum intensity increases from 26% to 85%. The corresponding spatial distance between the maxima is D$_{max}$ = 82 ◦ and 67 $^{◦}$. 

Due to the physical shape of an OLED, no emission direction ϕ is preferred over the others leading to the rotational symmetry of the beam as visible in Fig. 1 and Fig. 5. However, some applications may require an asymmetric emission pattern. One way to break this symmetry is to add optical elements without circular symmetry, e.g. a prism, to the beam- shaping OLED. As illustrated in Fig. 5a, the prism covers the pixel completely. Due to its orientation, internal reflections in the y-direction increase, whereas they decrease in the x-direction, compared to the reference. This behaviour leads to a strong change in the brightness distribution on the screen, as is visible in Fig. 5c. Indeed, the beam shape now follows a mixture of the intrinsic radial symmetry of the OLED and the symmetry of the prism. The brightness in the forward emission and the x-direction gives a profile with a FWHM of only 40 $^{◦}$. For sideward emission, one obtains a contrast value of 71% and D$_{max}$ = 81 ◦ in the x-direction and a contrast of 60% at D$_{max}$ = 46 ◦ in the y-direction. 

