Methods

A rock sample is typically prepared for microscopic examination as a thin section (30 μm thick slice of rock). This makes it possible to send polarized light through the sample (transmitted light), which is refracted in a manner which is characteristic for a specific phase. Thus, it is possible to recognize and determine different phases and their relationship to each other. Additionally, crossed polarizers can be used to make further details visible.

The PICAT (Petrographic Image Capture and Archiving Tool; Crawford & Bennight, 2014) is capable of shooting high-resolution overview images of thin sections. This system is designed to digitally photograph petrographic thin section samples, capturing the entire thin section in both plane and crossed polarized light. The upper polarizer can be folded to the side, facilitating easy switching between polarized light and crossed polarizers. Instead of using the sample stage, as is usual with microscopes, the polarization direction is changed using a rotary wheel. The two polarization filters are coupled by a magnet.

Conventional microscope-based systems create thin section images with time-consuming mosaic displays. The PICAT, on the other hand, can acquire a publication-quality image within seconds, as only a single image is required to cover the entire thin section. A live image is displayed on a computer monitor, enabling immediate detection and adjustment of errors in exposure, sharpness, colour balance and cropping.

The most important components at a glance

(1) Camera Body - Sony Alpha A7R II 42.4MP Mirrorless

(2) Camera lens - Sony FE 90mm f/2.8 Makro G OSS Lens SEL90M28G

(3) Custom Filter Module with Magnetic coupling (PICAT)

(4) Light source - Walimex pro Soft LED 300 Round Bi Color

(5) Camera stand - Kaiser RS 2 XA

(6) Computer with Sony Imaging Edge Desktop Software for Live-Bild

Reference: Crawford, W. & Bennight, C. (2014). Petrographic Image Capture and Archiving Tool (PICAT): User Guide. IODP Publications