光密度（OD）是生物医学特别是分子生物学实验样品中所要测定的基本参数之一，因为它能间接地反映出样品中阳性物质的相对含量。但OD只是指图象中某一象素点上的OD值，所以它只能反映出该点上阳性物质的相对含量，这在许多课题的研究工作中是远远不够的。例如要观察两组（某一 Optical density (OD) is one of the basic parameters to be determined in biomedical, especially in molecular biology, as it indirectly reflects the relative content of the positive substance in the sample. However, OD refers only to the OD value of a pixel in the image, so it can only reflect the relative content of the positive substance at that point, which is not enough in many research work. For example, to observe two groups (one