Three types of broadband ultraviolet-B (UVB) radiometers were shown to display different irradiances from the same light source. Also, natural light and different lamp types were shown to have different vitamin D–synthesizing potential. Equations relating the irradiance readings from UVB radiometers from Gigahertz-Optik Inc., UVP Inc., and Spectronics Corp. to in vitro vitamin D–synthesizing potential are reported for four UVB sources. Zoo Biol 23:355–363, 2004. © 2004 Wiley-Liss, Inc.