Commercial white light-emitting diodes (LEDs) are optimized for the photopic (daytime) spectral sensitivity of eye that is characteristic of average ambient luminance above 10 cd/m^2. However in the half-darkness conditions (0,1-3 cd/m^2), which are typical of outdoor (street, road, landscape) lighting, the spectral sensitivity of eye is different and depends on luminance (mesopic), therefore, light generated by common white LEDs becomes less efficacious. The Project MesoLED is devoted to the novel energy-saving technology of outdoor lighting. The Project aims at the optimization of solid-state light sources in respect of mesopic eye sensitivity and at the development of such sources. During the Implementation of the Project, the spectral power distributions (SPDs) containing two, three, and four colour components will be found for various colour temperatures within the mesopic ambient luminance range using a numerical optimization method. Based on the optimization results, the following high-efficacy mesopic light sources will be modelled, developed and patented: a cluster of three coloured LEDs, a LED with a partial conversion in phosphor, and a LED with complete conversion in phosphor. The developed LED clusters will be used for the establishing of the limits of known mesopic vision models by the investigation of reaction time and contrast and colour discrimination. Also, a solid-state source of light with a tuneable SPD capable of generating metameric blends with different mesopic luminous efficacy of radiation will be modelled, developed, and patented. The tuneable light source will be used in real outdoor conditions for the subjective selection of colour blends that are preferential in terms of feeling of safety and aesthetic judgement. The Project implementation is intended to stimulate the evolvement of technologically advanced lighting industry and the installation of new-generation energy-saving street and road lighting systems in Lithuania.