Sensitivity analysis of computer-based diameter measurement from digital images

Root diameters less than 0.5 mm are infrequently documented in the literature, yet they are a critical functional component of the root system. Software capable of measuring root diameters from digital images has been used extensively to measure root length, but little is known of its efficacy in diameter measurements. An Epson 1680 scanner with a transparency unit and a Kodak DCS330 digital camera with a 105-mm Macro lens were each used to image sets of random length wires of four diameters from 0.057 to 1.19 mm and human hair of diameter 0.057 nun over a series of pixel sizes from 0.019 to 0.254 mm (50.5-3.9 p mm(-1)). Images were analyzed with the MacRHIZO software package either before or after image sharpening with Adobe Photoshop. Additionally, roots from two experiments were photographed to compare with the wire and hair analyses. Minimum routine mixed object diameter resolution with both this scanner and camera is about 50 mum, though the theoretical minimum is only dependent on lens and scanner technology. With unadjusted images, MacRHIZO pro 3.10b requires a very narrow range of image resolution (7.9-15.7 p mm(-1) with 0.5 to >1-mm diameter objects) for accurate measurement. Higher resolutions cause the software to assume a 1- or 2-pixel thick wire or root is crossing the larger object at points of roughness on the edge of the images. The sharpen filter from Photoshop eliminated this problem at all resolutions tested with scanner and photographic images. Images treated with the maximum sharpening from Photoshop had practical image resolution ranges of 4 to 50 p mm(-1). With sharpening, images of mixtures of roots or other objects with diameters between 50 mum and >1 mm can be accurately analyzed with MacRHIZO. Digital camera and high-resolution scanner appear equally effective in producing digital images for routine length and diameter analysis.