The　blue　laser　diode　(LD)　illuminator　in　this　study　was　composed　of　a　cerium　doped　yttrium　aluminum　garnet　(Ce:YAG)　crystal,　a　450-nm　laser　diode　and　an　optical　fiber.　After　a　first-principles　calculation,　the　energy　gap　of　the　Ce:YAG　crystal　was　found　to　be　4.71　eV,　which　was　less　than　that　of　the　YAG.　The　luminescent　properties　of　the　Ce:YAG　were　determined　by　the　electronic　distribution　of　the　Ce　(d)　and　Ce　(f)　orbits.　The　Ce:YAG　crystal　had　two　characteristic　absorption　peaks　of　Ce3+　at　332　nm　and　455　nm.　Hence,　the　450-nm　LD　excited　fluorescence　spectra　of　a　Ce:YAG　crystal　can　be　used　for　laser　illumination　We　discovered　that　the　luminous　efficiency　of　the　LD　increased　with　increasing　color　temperature　for　4000　K,　5000　K,　6000　K　and　6500　K　but　not　for　3500　K,　due　to　the　low　light　transmittance　of　the　thickest　Ce:YAG　crystal.　The　highest　color-rendering　index　was　about　70.0.　Also,　the　blue　laser　without　the　430-nm　light　from　spectral　radiance　was,　compared　to　an　LED,　a　more　serious　eye　hazard.　We　calculated　that　the　permissible　exposure　time　of　the　LD　was　longer　than　that　of　an　LED.　We　also　discovered　that　LD　illumination　is　more　secure　than　LED　illumination.