PURPOSE: To determine the proper reference stepwedge for digital Cu-Equivalent Image analyzing systems for measurement of bone density. METERIALS AND METHODS: Radiograms of lumbar vertebrae phantom (1 g/cm2) with 3 test copper stepwedges of 0.03, 0.05 and, 0.1 mm thickness unit were taken and analyzed using NIH image software on a Macintosh personal computer. Measured densities of the lumbar areas in the Cu-Equivalent images made by utilizing 3 different copper stepwedges were compared with a known bone density. RESULTS: The values of r2 for all copper equivalent images were over 0.99. The mean Cu-Eq value of lumbar in copper equivalent image made by a 0.1 mm copper stepwedge was 0.22+/-0.06 mm and converted to hydroxyapatite density of 1.03 g/cm2. The stepwedges of 0.03 and 0.05 mm produced results having higher values than the actual known bone density. They did not show the blue and green color level that appeared in lumbar on color enhanced image. CONCLUSION: A copper stepwedge of adequate thickness and range of steps which can express the range of density of bone being measured should be used.