A number of different transparent displays have been developed, each suitable for a subset of the applications. The simplest type is the head-up display that projects into the viewer’s eyes through reflecting the image off a beam splitter. Head-up displays are well suited to certain occasions, but the narrow viewing angle limits the position of the viewer. Diffusive screens achieve wider viewing angles by light scattering; such screens do not have wavelength selectivity, so stronger scattering is necessarily accompanied by lower transparency. Frequency-conversion screens achieve higher transparency using molecules that convert the projected ultraviolet light to visible light ,or infrared light to visible light; the conversion, however, is challenging to implement with high efficiency. Instead of relying on projection, one can also make electronic flat panel displays transparent, for example by combining organic light-emitting diodes with transparent electronics. There is active research in developing this type of transparent displays, but scaling to large display sizes remains challenging.