Although our understanding of the effects of UV-B radiation on annual and crop plants has improved considerably over the past three decades, knowledge of effects on perennial plants, particularly trees, remains much more limited. Understanding the effects of enhanced UV-B radiation on forest trees has been hampered by an inability to develop realistic dose-response relationships, which in turn, has resulted from lack of instrumentation sufficient to accurately determine biologically effective UV irradiances within the canopy. Traditional spectroradiometers equipped with spherical domes have sensor heads that are too large to make within-canopy measurements, especially at individual leaf locations or other small targets. Broad-band sensors, which are smaller, lack the wavelength discrimination necessary to calculate biological effective UV-B radiation. Measurement of UV radiation above a plant canopy provides little information about the radiation environment of individual leaves within the canopy. This is important because whole plant response to UV-radiation is an integrated response of all of the leaves, though obviously not all leaves contribute equally. Within leaves, stress stimuli may induce compensatory responses in unaffected portions of the same leaf, in other adjacent leaves, and even in remote leaves. It is not known if this is true for W radiation; the only way of finding out is to have a probe small enough to measure diminutive and precise locations within the canopy. In this study a StellarNet EPP2000C spectrometer equipped with a 400 μm optical fiber and Teflon diffuser was used to determine solar UV-B irradiances above and within the canopy of representative forest trees. Data was collected at different times during the day, for equivalent leaf and canopy positions in several representative trees. Data collected in the open and below-canopy positions are compared with an Optronics OL-754 spectroradiometer and data obtained at the USDA UVB Monitoring and Research Program field site at Albion, Washington. Results are considered in the context of precision, accuracy and logistical attributes. Sufficient precision and accuracy were shown to suggest that, with proper calibration, this inexpensive and highly portable instrument can be used to make precise measurements of solar UV-B radiation within tree canopies.