Detecting Indentations on Documents Pressed by Pen Tip Force Using a Near Infrared Light Emitting Diode (NIR LED) and Shape from Shading

Abstract

We proposed the new method that detected indentations pressed by pen tip force on paper using an oblique near infrared (NIR) emitting by light emitting diodes (LEDs). According to conventional methods indentations were observed by document examiners' eyes using a microscope. However it is difficult to estimate depths of the indentations because human eyes only can observe shades and brightness made by indentations instead of measuring the depths of indentations. Using a confocal laser microscope is able to directly measure the depths of indentations. However this method needs to take long time and instruments are expensive. Using a NIR LED and an optical model called shape from shading resolved the issues of time and cost. It is useful for forensic document examiners to approximately evaluate the depths of indentations of handwriting by our proposal method because the method will be lead to convenient discrimination between forgery handwriting and genuine one.