This paper presents a new ray-tracing acceleration technique called improved pixel selected ray-tracing (IPSRT), which is based on the reduction of ray-traced pixels using adaptive undersampling. In PSRT, the intensities of coarsely sampled pixels are calculated by regular ray-tracing, and the intensity of the pixels between sampled pixels is calculated by intensity interpolation if the characteristics of the sampled pixels are similar. However, in IPSRT, the similarities of the sampled pixels are classified into four levels, and the intensity of the pixels between sampled pixels is calculated by either intensity interpolation, ray-tracing along the ray path of a sampled pixel, or regular ray-tracing according to the results of the similarity check. Moreover, the danger of missing an object due to undersampling is reduced by using an efficient sampling algorithm. With these improvements, IPSRT can generate images of better quality than those generated by PSRT in the same or a shorter amount of time.