  This paper presents a new approach for embedding authentication information
into image on printed materials based on optical projection technique. Our
experimental setup consists of two parts, one is a common camera, and the other
is a LCD projector, which project a pattern on personnel's body (especially on
the face). The pattern, generated by a computer, act as the illumination light
source with sinusoidal distribution and it is also the watermark signal. For a
color image, the watermark is embedded into the blue channel. While we take
pictures (256 *256 and 512*512, 567*390 pixels, respectively), an invisible
mark is embedded directly into magnitude oefficients of Discrete Fourier
transform (DFT) at exposure moment. Both optical an d digital correlation is
suitable for detection of this type of watermark. The decoded watermark is a
set of concentric circles or sectors in the DFT domain (middle frequencies
region) which is robust to photographing, printing and scanning. The unlawful
people modify or replace the original photograph, and make fake passport
(drivers' license and so on). Experiments show, it is difficult to forge
certificates in which a watermark was embedded by our projector-camera
combination based on analogue watermark method rather than classical digital
method.
