Quantitative phase mapping is useful for non-invasively quantifying cell properties, such as water concentration. The phase image can be obtained by solving the Transport-of-Intensity equation (TIE) for two brightfield images I1 and I2 taken at two defocus distances. Phase restoration by TIE is based on the gradient of axial intensity, which is approximated by the difference I1-I2. The results of computation, however, are very sensitive to slight fluctuations of intensity between I1 and I2, resulting in a spatially variable background and difficulty analyzing the data. This is especially true for spherical cells, driving our current focus on finding additional ways to increase the accuracy of the phase image calculated for these spherical objects. Thus far, we have found that the quality of phase images can be drastically improved by manually equalizing the intensities of the input images before processing. The tolerance of the computed phase to the input focal planes can be further enhanced by averaging multiple time-lapse images to represent each of I1 and I2. With this simple modification, TIE microscopy can be easily applied to various biological problems.