Pen-type drug injection device with particular dose scale and dose display window comprising optical filter for unambiguous display of set dose value

A drug delivery device for dispensing a dose of a medicament, the drug delivery device comprising:

• a drive mechanism comprising a piston rod configured to operably engage with a piston of a cartridge containing the medicament;
• and
• a display arrangement comprising:
• a housing defining a longitudinal shape and extending in an axial direction and having a window covered by an optical filter, and
• a sleeve-shaped display member configured to be displaced relative to the housing between a retracted position, in which the display member in its entirety is located inside the housing, and an extended position and overlapping with the window, wherein when in the extended position at least a proximal portion of the display member protrudes from a proximal end of the housing,
• wherein the display member comprises an information surface with an illegible pattern, wherein the illegible pattern comprises an information content and a coding pattern, wherein the information content comprises one or more of incidence, symbols, and numbers to display a size of a dose actually set or dialed, and wherein the optical filter matches and corresponds with a reflective optical behavior of the illegible pattern and wherein at least one of the information content and the coding pattern is arranged such that light reflected by or emanating from the illegible pattern interacts with the optical filter in such a way that the information content is recognizable through the optical filter, and
• wherein the drive mechanism and the display member are arranged in the housing.