【Objective】This study aims to evaluate the simulation accuracy of two measures of understory diffuse radiation: the scaled weighted diffuse shading degree and the weighted inside-boundary neighbor diffuse shading degree. These measures, based on easily measurable tree factors, are implemented within a forest understory radiation simulation system. The study also assesses these measures' performance relative to opening degree and seeks to identify model structural characteristics that can accurately estimate radiation at any understory location.【Method】A stand pixel model was created using survey data from a broad-leaved Korean pine forest (Pinus koraiensis). Understory diffuse transmittance was simulated for two scenarios: a homogeneous environmental background for research plots within the stand, and a heterogeneous environmental background for plots including the forest edge. The simulation accuracy of the diffuse radiation index measures was evaluated based on these scenarios. Optimization of the local-scale circle radius and background-scale circle radius for the scaled weighted diffuse shading degree, as well as the local critical value and background critical value for the weighted inside-boundary neighbor diffuse shading degree, were performed using a hill-climbing algorithm. The performance of the three indices-opening degree, scaled weighted diffuse shading degree, and weighted inside-boundary neighbor diffuse shading degree-were compared using Pearson and Spearman correlation coefficients and linear regression determination coefficients.【Result】Under a homogeneous environmental background, the scaled weighted diffuse shading degree showed the best fit for understory diffuse transmittance with a local-scale circle radius of 5.401 m. The weighted inside-boundary neighbor diffuse shading degree achieved the best fit with a local critical value of 2.800. Under these conditions, Pearson and Spearman correlation coefficients were 0.397 and 0.425 for opening degree, -0.716 and -0.692 for the scaled weighted diffuse shading degree, -0.730 and -0.694 for the weighted inside-boundary neighbor diffuse shading degree, respectively. The linear regression determination coefficients were 0.158 for opening degree, 0.514 for scaled weighted diffuse shading degree, and 0.533 for the weighted inside-boundary neighbor diffuse shading degree. Compared to the opening degree, the variance explanation rates for the weighted inside-boundary neighbor diffuse shading degree and the scaled weighted diffuse shading degree increased by 237.3% and 225.3%, respectively. In a heterogeneous environmental background, with the same local-scale circle radius and local critical value, the background-scale circle radius for the scaled weighted diffuse shading degree was 15.521 m, and the background critical value for the weighted inside-boundary neighbor diffuse shading degree was 0.875 when achieving the best fit for understory diffuse transmittance. In this case, the Pearson and Spearman correlation coefficients were -0.930 and -0.719 for the scaled weighted diffuse shading degree, -0.927 and -0.820 for the weighted inside-boundary neighbor diffuse shading degree. The linear regression determination coefficients were 0.866 and 0.860, respectively.【Conclusion】(1) With appropriate parameter selection, the scaled weighted diffuse shading degree and the weighted inside-boundary neighbor diffuse shading degree indices effectively simulate understory diffuse radiation and better account for variations in small-scale tree structure and large-scale environmental background compared to the opening degree method. (2) Accurate simulation of understory diffuse radiation requires consideration of large-scale environmental background differences, and the uniformity of neighboring tree distribution around research sites is an indispensable influencing factor in these simulations.