Spatial frequency The term spatial frequency refers to the number of lines or other detail. within a given length. A technical line drawing, 天津大学精仪学院 for example, may have four lines, and four intervals adjoining these lines, per millimeter. The spatial frequency, then, is lines/mm 天速大学作 14 2414 Spatial frequency The term spatial frequency refers to the number of lines, or other detail, within a given length. A technical line drawing, for example, may have four lines, and four intervals adjoining these lines, per millimeter. The spatial frequency, then, is 4lines/mm