Security Printing in pdflatex documents

Question

this is may situation. I generate a test which is printed for 1300 copies to be distributed amongst my students. However I would like to put some feature in the printed pages, such that if someone makes a PHOTOCOPY, it will be self evident to the human eye to notice the difference between the original and the photocopy. I guess my inquiry/requirement goes on the concept of Security Printing. Here is a minimal work example, any suggestions?

\documentclass[10pt]{article}
\usepackage{amsmath}




% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % %
% WATERMARK
\usepackage{draftwatermark}
%\renewcommand{\familydefault}{\sfdefault}
\SetWatermarkText{A}
%\SetWatermarkColor[RGB]{210,217,242}
\SetWatermarkScale{35}
\SetWatermarkAngle{0}
\SetWatermarkLightness{0.9}
% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % %






\begin{document}

\centerline{ \textbf{ Linear Algebra Midterm} }
\centerline{\textbf{April, 18th, 2015} }


 %
%
%
%
\begin{table*}[h!]
%\caption{Notas. Solo Para Uso Oficial}
\centerline{\textbf{Score}}\vspace{5pt}
\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|}
    \hline
1 & 2 & 2 & 4 &  \textbf{TOTAL}& \textbf{GRADE} \\
    \hline
\hspace{0.3in} &  
\hspace{0.3in} & \hspace{0.3in} & 
\hspace{0.3in} & \hspace{1.0in} & \hspace{1.0in}  \\[0.4cm]
    \hline
\end{tabular}
\end{center}

%
\end{table*}
%
%
%
\vspace{-20pt}
%
\noindent \textbf{Instructions:} Mark the options of your choice or fill in the blanks. The test 4 questions to be answered in 60 minutes.  
\newline
\newline
\textbf{IDENTIFICATION}
\newline
\newline
Name: \hrulefill \hrulefill \hrulefill ID Number \hrulefill

\bigskip

Professor: \hrulefill \hrulefill \hrulefill Group \hrulefill

\bigskip




\begin{itemize}

\item $[8 pt]$ Find the real scalars $k$ such that $||kv||=3$ where $v=[-1,2,7,-3]$, 
$k=$\rule{3cm}{0.4pt}

\vspace{2cm}
\item $[8 pt]$ Determine whether the systems have trivial solutions or not.

\begin{equation*}
\begin{gathered}
x_1+3x_2+5x_3+x_4=0   \\
4x_1-7x_2-3x_3-x_4=0  \\
3x_1+2x_2+7x_3+8x_4=0\\
\hrulefill 
\end{gathered} \hspace{3cm}
\begin{gathered}
x+2y+3z=0\\
y+4z=0\\
5z=0\\
\hrulefill 
\end{gathered}
\end{equation*}


\item $[6 pt]$ Let $A,B$ be $4 \times 5$ matrices and $C,D$ and $E$ matrices of size $5 \times 2$, $4 \times 2$ and $5 \times 4$ respectively. Determine if the following matrix operations are well-defined.
\begin {itemize}
\item $BA$ \rule{3cm}{0.4pt}
\item $AC+D$ \rule{3cm}{0.4pt}
\item $AE+B$ \rule{3cm}{0.4pt}
\end{itemize}


\vspace{2cm}
\item $[6 pt]$ Suppose that a $3 \times 3$ matrix has all its entries equal to a non-null constant. What is the range of the matrix? \hrulefill.

%
\end{itemize}
%
\end{document}

Answer 1

I submit to you that this will not copy well. Elements used include:

1. two colors (plus black)

2. contrast of dark and light printing

3. diffraction-grate-like patterns

Here is the MWE. You can play with the colors and their intensity. I had five such figures to see the effect of the shift on the copy outcome. The whole design is contained in the preamble. I use everypage to place it on every page.

\documentclass[10pt]{article}
\usepackage{amsmath,tikz,everypage}
% THESE ARE LaTeX DEFAULTS; CAN CHANGE IF NEEDED.
\def\PageTopMargin{1in}
\def\PageLeftMargin{1in}
\newcommand\atxy[3]{%
 \AddEverypageHook{\smash{\hspace*{\dimexpr-\PageLeftMargin-\hoffset+#1\relax}%
  \raisebox{\dimexpr\PageTopMargin+\voffset-#2\relax}{\textcolor{red}{#3}}}}}
% VERIFIED THAT SETTING \hoffset AND \voffset DO NOT BREAK SOLUTION.
%\hoffset=0.4in
%\voffset=0.2in
\newsavebox\bgrate
\savebox\bgrate{\color{red!20}%
\tikz{%
\node[circle,draw,minimum height=3.25em]{\Huge\textcolor{black}{X}};
\node[circle,draw,minimum height=3.00em]{};
\node[circle,draw,minimum height=2.75em]{};
\node[circle,draw,minimum height=2.50em]{};
\node[circle,draw,minimum height=2.25em]{};
\node[circle,draw,minimum height=2.00em]{};
\node[circle,draw,minimum height=1.75em]{};
\node[circle,draw,minimum height=1.50em]{};
\node[circle,draw,minimum height=1.25em]{};
\node[circle,draw]{};
}}
\newsavebox\rgrate
\savebox\rgrate{\color{red!20!green!15}%
\tikz{%
\node[circle,draw,minimum height=3.25em]{\Huge\textcolor{black}{X}};
\node[circle,draw,minimum height=3.00em]{};
\node[circle,draw,minimum height=2.75em]{};
\node[circle,draw,minimum height=2.50em]{};
\node[circle,draw,minimum height=2.25em]{};
\node[circle,draw,minimum height=2.00em]{};
\node[circle,draw,minimum height=1.75em]{};
\node[circle,draw,minimum height=1.50em]{};
\node[circle,draw,minimum height=1.25em]{};
\node[circle,draw]{};
}}
\def\thegrate{%
\ooalign{\usebox{\bgrate}\cr\kern.7pt\usebox{\rgrate}}%
\ooalign{\usebox{\bgrate}\cr\kern1pt\usebox{\rgrate}}%
\ooalign{\usebox{\bgrate}\cr\kern1.2pt\usebox{\rgrate}}%
\ooalign{\usebox{\bgrate}\cr\kern1.7pt\usebox{\rgrate}}%
\ooalign{\usebox{\bgrate}\cr\kern2.3pt\usebox{\rgrate}}%
}

\atxy{0.1in}{.57in}{\thegrate}

\begin{document}

\centerline{ \textbf{ Linear Algebra Midterm} }
\centerline{\textbf{April, 18th, 2015} }


 %
%
%
%
\begin{table*}[h!]
%\caption{Notas. Solo Para Uso Oficial}
\centerline{\textbf{Score}}\vspace{5pt}
\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|}
    \hline
1 & 2 & 2 & 4 &  \textbf{TOTAL}& \textbf{GRADE} \\
    \hline
\hspace{0.3in} &  
\hspace{0.3in} & \hspace{0.3in} & 
\hspace{0.3in} & \hspace{1.0in} & \hspace{1.0in}  \\[0.4cm]
    \hline
\end{tabular}
\end{center}

%
\end{table*}
%
%
%
\vspace{-20pt}
%
\noindent \textbf{Instructions:} Mark the options of your choice or fill in the blanks. The test 4 questions to be answered in 60 minutes.  
\newline
\newline
\textbf{IDENTIFICATION}
\newline
\newline
Name: \hrulefill \hrulefill \hrulefill ID Number \hrulefill

\bigskip

Professor: \hrulefill \hrulefill \hrulefill Group \hrulefill

\bigskip




\begin{itemize}

\item $[8 pt]$ Find the real scalars $k$ such that $||kv||=3$ where $v=[-1,2,7,-3]$, 
$k=$\rule{3cm}{0.4pt}

\vspace{2cm}
\item $[8 pt]$ Determine whether the systems have trivial solutions or not.

\begin{equation*}
\begin{gathered}
x_1+3x_2+5x_3+x_4=0   \\
4x_1-7x_2-3x_3-x_4=0  \\
3x_1+2x_2+7x_3+8x_4=0\\
\hrulefill 
\end{gathered} \hspace{3cm}
\begin{gathered}
x+2y+3z=0\\
y+4z=0\\
5z=0\\
\hrulefill 
\end{gathered}
\end{equation*}


\item $[6 pt]$ Let $A,B$ be $4 \times 5$ matrices and $C,D$ and $E$ matrices of size $5 \times 2$, $4 \times 2$ and $5 \times 4$ respectively. Determine if the following matrix operations are well-defined.
\begin {itemize}
\item $BA$ \rule{3cm}{0.4pt}
\item $AC+D$ \rule{3cm}{0.4pt}
\item $AE+B$ \rule{3cm}{0.4pt}
\end{itemize}


\vspace{2cm}
\item $[6 pt]$ Suppose that a $3 \times 3$ matrix has all its entries equal to a non-null constant. What is the range of the matrix? \hrulefill.

%
\end{itemize}
%


\end{document}

Here is a zoom of the... the... the blivets:

Here is a scan of the original paper (above) and the color photocopy (below). The contrast (hah hah) is obvious.