Chromaticity of white light volume holographic cell arrays for divergent illumination with phosphor converted LEDs in automotive headlights

Holographic optical elements (HOEs) offer the possibility to replace conventional optical elements like lenses, reflectors and apertures in illumination lighting systems such as vehicle headlights. In this context reflection-type volume holographic optical elements (vHOEs) represent a promising approach. Although illuminated by incoherent light emitting diodes (LEDs), they provide good beam-shaping quality due to their high wavelength and angular selectivity. However, to achieve high efficiency in operation the vHOE must utilize the full spectrum of the illumination systems light source. Therefore, when using phosphor converted LEDs, vHOEs with multiple recording wavelengths are required. In addition, the generated light distribution must comply with the chromaticity requirements of the ECE regulations for vehicle headlights. Therefore, a major goal in development of vHOEs as optical elements for vehicle lighting is to produce a white light distribution for vehicle headlights while achieving high efficiency. Previous work based on the Lippmann color process [1] attempted to record full-color holograms to produce highly realistic three-dimensional images by using multiple wavelengths in the recording process. Bjelkhagen and Mirlis [2] for example focused on undersampling in the wavelength domain and the resulting false coloration of the recorded objects. Similar problems occur, when a noncontinuous white light LED spectrum is used to illuminate vHOEs, which must produce a white light distribution. In this paper, simulations based on Kogelnik’s coupled-wave-theory (CWT) [3] are presented for selecting appropriate laser wavelengths for hologram recording and white light LED illumination. The holographic recording medium and the recording angles of the reference and object beams are first defined. Then, several commercially available laser wavelengths are chosen to calculate the spectral diffractive efficiency of each laser wavelength. The results are then convolved with the spectral power distribution of white light LEDs and the chromaticity coordinates are calculated accordingly. Various combinations of laser wavelengths and LED spectra are presented as the results. Suitable combinations of recording lasers and LEDs could be found to achieve the color locus required in the ECE regulations.


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