Fig. 2. The bending steel can be considered as a part of an imaginary circle with a radius R. When the steel bended, the steel length is fixed and is considered as an arc of the imaginary circle. Only the chord of the imaginary circle becomes shorter as the curvature of the imaginary circle increases. The central angle can be obtained from the ratio of arc to chord, and the radius of the imaginary circle and the arc can be calculated from the central angle. The inset shows that as the steel is flat, the laser-irradiated area is a circle with a radius of “a”. Under upward bending, the laser-irradiated area is no longer a circle but an ellipse with semi-minor axis “a” and semi-major axis “b” with larger area.