Polarizer Alignment Card™

Making images of artworks is the most difficult photographic task. The image must be an exact reproduction of the original, which is made more difficult by specular reflections, mirror-like reflections of the photographic lighting. These specular reflections can be caused by the artwork surface, the canvas texture, or varnishes, among other things. One technique that can remove these reflections is to use cross-polarized imaging.

This is a technique where polarizers are used on the lighting and another polarizer on the camera lens. Usually the light polarizers are fixed and aligned so their polarization direction is in the same direction. The camera polarizer is usually rotatable so it can be turned until it is crossed at exactly 90 degrees from the light polarization direction.

Since the specular reflections maintain the polarization from the lights, the crossed camera polarizer will block the specular reflections. The parts of the image without specular reflections will pass through the camera polarizer unchanged.

Getting the camera polarizer exactly crossed to the polarized lighting can be difficult because some reflections disappear at slightly different rotation angles. So what is the true crossed angle?

By placing the Polarizer Alignment Card™ (PAC) on the copy board instead of the artwork the card acts as a specular reflectance standard, allowing for precise crossed alignment of the camera polarizer. Simply rotate the camera polarizer until the parallel lines on the PAC turn dark. The lines will appear darkest at the precise crossed polarization angle.

Uncrossed		Crossed

The Polarizer Alignment Card™ also has a second function. It can serve as a reference for setting the camera's focus.

Camera focus systems need high contrast edges to focus properly. Once the crossed-polarization has been aligned, the parallel lines of the Polarizer Alignment Card™ provide dark lines which stand out in high contrast. With the card at the desired focus point, the lines provide just the needed high constrast edges for the camera's focus system.

The ability to align both the camera's polarizer and focus makes this card doubly useful for your art reproduction imaging.