Smart　windows　have　attracted　considerable　attention　due　to　their　wide　applications　in　optical　data　storage,　switchable　sunroof　and　temperature　sensing.　The　development　strategy　for　smart　windows　is　focused　on　performance　design,　enhancement　and　integration.　However,　developing　integrated　multi-functional　smart　windows　in　a　single　material　remains　a　challenge.　In　this　work,　we　have　successfully　prepared　(K0.5Na0.5)0.95Ba0.04Er0.01NbO3　(4Ba-1Er-KNN)　transparent　ceramics　for　potential　applications　of　temperature　detection　and　optical　information　storage　in　smart　windows.　With　alternating　ultraviolet　(UV)　illumination　and　300　degrees　C　thermal　stimulation,　the　prepared　4Ba-1Er-KNN　ceramics　can　not　only　achieve　non-destructive　luminescence　readout,　but　also　exhibits　an　ultra-high　photochromic　(PC)　contrast　with　rapid　response　time　of　1　s.　Furthermore,　based　on　the　up-conversion　(UC)　photoluminescence　(PL)　intensity　ratio　of　Er3+:　2H11/2/4S3/2　thermally　coupled　levels,　excellent　low-temperature　sensing　performance　with　the　maximum　relative　sensitivity　of　0.023　K-1　at　213　K　is　obtained.　The　integration　between　UC　PL,　PC　response　and　temperature　sensing　performance　makes　it　possible　to　develop　multi-functional　smart　windows.