If you write something and alter it with the same pen it is not possible to detect. If you used a different pen, you would be able to detect the difference with specific light sources. Each ink has a different response to light, such as UV-light or Infrared light.
Jessy forensic scientist the Netherlands
Jan 30, 2008 Rating
Video Spectral Comparitor by: David
The following information is taken from the Office of Crime Laboratories - Questioned Document Section.
"Altered documents can be of several different varieties. Erasures can be detected with proper lighting techniques, although identification of the original information may not be possible. Obliterations may take several forms, including the use of opaquing material, marker pens or simply using a ballpoint pen to mark over earlier writing. Special lighting and magnification techniques and the use of the Video Spectral Comparitor (VSC) can frequently reveal the information underneath the obliteration. On checks, the amount may have been altered. The VSC can reveal the presence of different inks in the amount area."
