Human-invisible but machine-readable fiducial markers. (IMAGE)
Caption
Macroscopic views of fiducial markers made of CSRs with near-IR (columns 1–2) and near-UV (columns 3–4) retroreflection, respectively, as seen by a camera that records across the visible spectrum as well as in the near-UV and near-IR regions (top row) and by a regular mobile phone camera (bottom row). The markers are illuminated by the regular white ceiling light in (a/e) and (c/g), additionally by a 940 nm night vision LED in (b/f) and by a 395 nm black light lamp in (d/h). While the pattern of each marker is difficult to see under ordinary light illumination only, it becomes very clear with the IR/UV-imaging camera when the corresponding near-IR/UV light is turned on. The near-UV marker placed on a printed magazine page is photographed using a regular mobile phone camera in (i), showing that it is nearly fully transparent and difficult to notice by the naked eye (as a guide to the eye, a white frame highlights its location).
Credit
by Jan P.F. Lagerwall
Usage Restrictions
Credit must be given to the creator.
License
CC BY