입경측정을 위한 영상처리기법에서 입자 초점면 존재 판단 기준의 설정

Abstract

In the present image processing technique, the concept of the gradient indicator(GI) has been introduced to find out the depth-of-field in sizing large particles ranging from 30μm to 300μm where using of the concept of the normalized contrast value(VC) is not appropriate. The gradient indicator is defined as the ratio of the local value to the maximum possible value of the gray-level gradient in as image frame. The gradient indicator decreases with the increases of the particle size and the distance from the exact focal plane. A particle is considered to be in focus when the value of the gradient indicator at its image boundary stays above a critical value. this critical gradient indicator(GI orltlcal) is defined as the maximum gradient indicator(GI max) subtracted by a constant (ΔGI) which is to account for the particle-size effect. In the present case, the value of ΔGI was set to 0.28 to keep the standard deviation of the measured particles mostly within 0.1. It was also confirmed that, to find the depth-of-field for small particles(≤30μm) with the same measurement accuracy, the concept of the critical normalized contrast(VC orltlcal) is applicable with 85% of the maximum normalized contrast value(VC max). Finally, the depth-of-field was checked for the size range between 10μm and 300μm when the both in-focus criteria (GI orltlcal and VC orltlcal) were adopted. The change of the depth-of-field with the particle size shows good linearity in both the VC-applicable and the GI-applicable ranges with a reasonable accuracy.