That equals about, on average, 300 to 400 lines per inch with small grooves per side.
这平均相当于每英寸有 300 到 400 条线，每边都有小凹槽。
英语百科
Lines per inch
Lines per inch (LPI) is a measurement of printing resolution. A line consists of halftones that is built up by physical ink dots made by the printer device to create different tones. Specifically LPI is a measure of how close together the lines in a halftone grid are. The quality of printer device or screen determines how high the LPI will be. High LPI indicates greater detail and sharpness.