Background and Aims: Most optical devices for indirect measurement of leaf area index (LAI) from canopy-transmitted light are tailored for homogeneous canopies, thus limiting their application to discontinuous canopies such as vertically trained vineyards. This study evaluates the influence of sun position on the reliability of LAI estimates provided by a ceptometer and proposes a measurement protocol for use of such instruments on vineyards under direct illumination. Methods and Results: Ceptometer readings at several sun elevation and azimuth angles were recorded in two fields. LAI estimated at different sun positions using several measurement protocols were then compared against destructive leaf area measurements. The best results when the sun position departs from the zenith (sun elevation<40°) were achieved by reading the transmitted, photosynthetically active radiation in all the inter-row spaces, whereas measurements below the vines were suitable only when the sun illuminates close to the zenith. Conclusions: The homogeneity of the canopy fraction measured along the ceptometer at each individual reading is a major requirement in order to obtain non-biased LAI estimates. Therefore, the protocol followed to measure the transmitted photosynthetically active radiation at every sun position proved critical for the accuracy of LAI determination. Significance of Study: This study provides guidelines for reducing LAI uncertainties associated with vineyard canopy structure in LAI estimation with linear array optical devices such as ceptometers. .