There are other ways to determine root length also. The classical method is the line-intercept method, where root length is estimated on the basis of the number of intersections of roots with a grid which is superimposed on them. For a complete explanation, see:

A Test of a Modified Line Intersect Method of Estimating Root Length

D. Tennant, The Journal of Ecology, Vol. 63, No. 3. (Nov., 1975), pp. 995-1001

Another approach might be to use the best scanner available and use Image J, a free image analysis program from the US NIH. This approach is presented in:

Root length and diameter measurement using NIH Image: application of the line-intercept principle for diameter estimation

K. Kimura and S. Yamasaki, Plant and Soil, Vol 234, No. 1. (July, 2001), pp. 37-46

We also find image J to be useful for analysis of microscope images and particle counting (eg, counting Arabidopsis seed)

Delta T scan is another image analysis program which we have used to estimate root length.