There is only one file type that uses the .TN extension:
LG Phone Image
Image format used by some LG mobile phones, including the LG Env3 and LG EnV Touch; saves a picture taken by the phone and is saved to the memory card (MicroSD) on the device; cannot be viewed with traditional image viewers, but instead should be sent from the phone to the user's email address to view on a computer as a .JPG file (Media Center → Picture & Video → My Pictures → Select Picture → Send → Picture Message).
Some users have had success opening TN files on their local computer manually by renaming the ".tn" extension to ".jpg" and opening them with IrfanView.
NOTE: While some information indicates that files with the ".tn" extension are Tracor Northern images, this is unlikely the case. The Tracor Northern (TN) format, as described by the U.S. Geological Survey Open-File Report OF 98-248, is used by Tracor Northern (now NORAN Instruments) electronic microscopy test equipment for logging images with a 512x512 pixels, 8-bit grayscale format. These images use the ".sem" extension, not the ".tn" extension, and can be converted to .GIF files using the program SEM2GIF.EXE, or to a proprietary ".bin" format using the program SEM2BIN.EXE. However, the ".sem" image format is not the format used by modern mobile devices.
Updated: April 23, 2012