We demonstrate use of the ratio of red to far-red (R:FR) light in assessing understory light levels in tropical forests. We determined the sensitivity of the ratio to bias when used under a variety of light conditions. We also calibrated the relationship between percent transmittance, as measured with traditional paired-instrument methods, and the R:FR ratio under a wide range of canopy cover conditions. Finally, we used the relationship to measure light levels under a closed canopy in a secondary tropical wet forest and assessed the usefulness of the measurements in determining spatial structure of light in a 1 ha plot, compared with percent transmittance values. Across a wide range of canopy conditions, red:far-red ratios explained 97% of the variation in percent diffuse transmittance under cloudy skies. Predictions of percent transmittance made under sunny skies were consistently lower than those made during cloudy conditions, even under a closed canopy, indicating that the method can be used reliably only with diffuse light. We used the R:FR ratio to characterize the spatial structure in understory light levels in a wet tropical forest in Costa Rica, explaining more variation in spatial structure than did a model fit to percent transmittance at a height of 1.5 m. The ratio provides a quick way to assess with high sensitivity the quantity and quality of light without the inconvenience and potential errors that can be introduced with the use of paired instruments or hemispherical photographs.