Innovative photographic technique reveals hidden detail in historic inscriptions

7 May 2014

Pam and Greg setting up their PTM apparatus

Pam and Greg setting up their PTM apparatus

A central aim of the Quarantine Project is to develop a detailed inventory of all known historic inscriptions at the North Head Quarantine Station. In order to document this unique heritage archive as comprehensively as possible, we have enlisted a number of recording techniques including mapping, photography, drawing, 3-D laser scanning and tracing. In addition to the more conventional methods of daylight photography, we have recently trialled a technique called polynomial texture mapping (PTM). Researchers Pam Forbes and Greg Jackson joined the field team specifically for this recording exercise and spent 6 sunrises and sunsets recording as many inscriptions as possible using their specially developed equipment.

Polynomial texture mapping is an image-capture and processing technique that utilises a compilation of multiple digital photographs acquired from a single camera position. The process involves lighting the subject (in our case an inscription) with an artificial light source that is adjusted over 24 different angles. A photograph is taken when the light is in each of the 24 positions, so that a variety of lighting conditions are simulated. The key to this technique is that the artificial light system has to be brighter than the ambient light, hence the need to record the inscriptions at dawn and dusk. Using freeware developed by Hewlett Packard Labs, these photographs are combined to create a composite image file in which the illumination can be adjusted interactively. This effectively means that the inscription can be viewed under a variety of lighting conditions and angles.

Greg, Lotty and Pam directing light onto an inscription

Greg, Lotty and Pam directing light onto an inscription

Although it may sound complex, the technique is actually quite easy to use once a consistent workflow is established. Knowing that some of our inscriptions occur on the vertical plane while others are horizontal, Pam and Greg developed a toolkit that allowed them to test the technique on both. The lighting system that they created had the benefit of being relatively light and portable so that it could be carried in the field. This comprised a rope framework with the eight lighting positions marked onto it. In addition, a three-part light stand allows the light to be cast at an angle of 14, 30 or 60 degrees. This allows for 24 different photographs to be taken of the single subject while the camera remains stationary on a tripod. The shutter of the camera is operated with a radio remote control to minimise any shake or movement that would result in blurring.

All in all, the PTMs that resulted from this trial were quite impressive. After viewing some inscriptions using the PTM software, we were able to make out letters and words that were barely visible to the naked eye. Additional benefits of the software such as enhancement filters and the capacity for taking ‘snapshots’ gave us the opportunity to see these inscriptions in a new light. This is particularly evident when we compare snapshots taken from the PTMs against photographs taken under normal or raked daylight conditions.

A notable example is one of the striking pictographic inscriptions recorded on a slate drain cover. Prior to the application of the PTM process we had not observed the faint hairs indicating a moustache on the British man pictured. After viewing the PTM we were able to discern small lines and a cravat, that were much harder to make out under natural daylight.

Smyrna inscription in normal daylight photography  Smyrna inscription enhanced by a snapshot of PRM rendering
  Smyrna inscription in normal daylight
  photography
  Smyrna inscription enhanced by
  a snapshot of PRM rendering
Revealing careful inscription work on a slate drain cover

Headstone detail recaptured by PTM

Headstone detail recaptured by PTM

 

  Revealing careful inscription work
  on a slate drain cover