Basal Area Wedge Prisms are used for cruising timber. There are two options in choosing a prism: SHAPE: Rectangular (20 mm x 40 mm). Best for use in dense timber Square (40 mm x 40 mm). Best for use in areas having less than a 15% slope. COLOR: Clear - for use in most cruising conditions. Amber - for cruising in hazy, misty or low light conditions. All of our prisms are accurate to +/- 1% of rated BAF (Basal Area Factor). They come in a soft, suede leather protective case. Rectangular Basal Area Prisms Square Basal Area Prisms Need more information? Please feel free to contact us at 1-800-592-6940 - Ask for Extension 547 or Internet Sales.