We present a new approach for estimating printer model parameters that can be applied to a wide variety of laser printers. Recently developed "model-based" digital halftoning techniques depend on accurate printer models to produce high-quality images using standard laser printers (typically 300 dpi). Since printer characteristics vary considerably, e.g., write-black versus write white laser printers, the model parameters must be adapted to each individual printer. Previous approaches for estimating the printer model parameters are based on a physical understanding of the printing mechanism. One such approach uses the "circular dot-overlap model," which assumes that the laser printer produces circularly shaped dots of ink. The circular dot-overlap model is an accurate model for many printers but cannot describe the behavior of all printers. The new approach is based on measurements of the gray level produced by various test patterns and makes very few assumptions about the laser printer. We use a reflection densitometer to measure the average reflectance of the test patterns and then solve a constrained optimization problem to obtain the printer model parameters. To demonstrate the effectiveness of the approach, the model parameters of two laser printers with very different characteristics were estimated. The printer models were then used with both the modified error diffusion and the least-squares model-based approach to produce printed images with the correct gray-scale rendition. We also derived an iterative version of the modified error diffusion algorithm that improves its performance.