不明な日付, but before 1911 when subject (Bohr) died. Further, photograph was likely taken after 1880, when subject would have been 25 years old. Supporting this lower limit is the web page of the Niels Bohr Archive Photograph Reproduction Collection, which lists a "start date" of 1880.
Licensed by Felicity Pors, representative of the Niels Bohr Archive, on 2008-06-25 under GNU Free Documentation License (GFDL) and Creative Commons Attribution - Share Alike 3.0 Unported (cc-by-sa-3.0).