Grid frequency in x-ray

What is Grid frequency in x-ray.

The grid frequency is the number of lead strips per unit distance ( cm or mm) is call grid frequency. It is also called grid lattice. The range of 40-50 lines/cm is call low frequency grids. The range of 50- 60 lines/ cm is called medium frequency grids. The above 70 lines/ cm is called high frequency grids. Manly the range of 60- 110 lines/ cm grid frequency used in radiography.