Improved to Differential Image Motion Monitor

The atmospheric seeing is an important index to measure the atmospheric optical quality of the site. The Differential Image Motion Monitor (DIMM) is widely used for site testing at home and abroad. For example, ESO DIMM was implied in site testing for Europe Southern Observatory. This paper introduced an excellent method of seeing measurement:Improved to Differential Image Motion Monitor (I-DIMM). Firstly, the structure design of I-DIMM and seeing calculation method were described in details. After that, followed by setting the two telescopes, of which one's diameter is 0.36m and the other is 0.12m, this paper compared the calculated values of seeing in these two situations. Both results showed that calculating seeing by method I-DIMM was more accurate than DIMM. Finally, this paper analyzed both simulation results and introduced the advantages of I-DIMM compared to DIMM.