Understory light is essential to the establishment and growth of understory plants and varies temporally and spatially within gaps, therefore, quickly measuring light intensities at different positions within gaps is important. Hitherto, there are three types of methods for measuring light intensities within canopy gaps. (1) Direct measurement methods use light sensors to measure light intensities within canopy gaps. It is expensive for measuring light heterogeneity in gaps because direct measurement methods are only possible over a limited time span and number of points and the measurements are laborious to take. (2) Model methods can quickly assess light intensities at any particular position within canopy gaps and, thus, is suitable for studies on light heterogeneity within canopy gaps. However, these models have low accuracy in assess light intensities because they simplify canopy gaps into cylindrical or ellipsoid shaped gaps. (3) Photographic methods are based hemispherical photographs taken with a skyward-facing fish-eye lens fixed on a camera within a canopy gap, and then calculate light intensity at photo point according to one of three indices: the gap fraction, weighted canopy openness or gap light index (GLI). Of them, GLI has the largest accuracy and has two kinds of methods: hemispherical photograph-based and geometric calculation-based. Hemispherical photograph-based GLI are only available for single-point assessments based on one hemispherical photograph and are therefore also time- and labor-consuming for measuring light intensities for numerous points in a gap, particularly in a large gap. Geometric calculation-based GLI includes four processes: the measurement of tridimensional shape of canopy gap and the aspect and slope of ground, the calculation of gap coordinates based on these three parameters, the transformation from gap coordinates to hemispherical photograph, and the computation of GLI, direct light and diffuse light. The approach can quickly measure light intensities at any position within canopy gaps. Due to these advantages, the geometric calculation-based GLI is a powerful tool for studies on the leveled spatial pattern, the vertical structure and the components (direct and diffuse lights) of light intensities within canopy gaps.