  Refractive freeform components are becoming increasingly relevant for
generating controlled patterns of light, because of their capability to
spatially-modulate optical signals with high efficiency and low background.
However, the use of these devices is still limited by difficulties in
manufacturing macroscopic elements with complex, 3-dimensional (3D) surface
reliefs. Here, 3D-printed and stretchable magic windows generating light
patterns by refraction are introduced. The shape and, consequently, the light
texture achieved can be changed through controlled device strain. Cryptographic
magic windows are demonstrated through exemplary light patterns, including
micro-QR-codes, that are correctly projected and recognized upon strain gating
while remaining cryptic for as-produced devices. The light pattern of
micro-QR-codes can also be projected by two coupled magic windows, with one of
them acting as the decryption key. Such novel, freeform elements with 3D shape
and tailored functionalities is relevant for applications in illumination
design, smart labels, anti-counterfeiting systems, and cryptographic
communication.
