LDS	Laser displacement sensor
$O_1$, $O_2$, $O_3$, $O_4$	Origin points of the laser beam of the $LD{S}_1$, $LD{S}_2$, $LD{S}_3$, $LD{S}_4$
$P_1$, $P_2$, $P_3$, $P_4$	Laer projection points of the laser beam of the $LD{S}_1$, $LD{S}_2$, $LD{S}_3$, $LD{S}_4$
SLP	Structured light probe
$x_0$, $y_0$, $z_0$	Origin coordinates of the laser beam of the LDS
$l,m,n$	Direction vector of the laser beam of the LDS coupling
	the conversion coefficient of voltage to physical distance
d	Thickness of the wafer used for calibration (from calibration)
$A,B,C,D$	Planar equation parameters of the additionally measured plane
v	Output voltage of the LDS
Q	The coefficient matrix of the calibration system of equations
M	Threshold for the number of deleted equations
Thresh	Threshold for the distance between the solved d and measured d (i.e. $d_0$)
$d_0$	Thickness of the wafer used for calibration (from measurement)
d’	Iteration of d
Ratio	Decrease ratio of difference between d and $d_0$
$\alpha$	Maximum value limit of the angle between the laser beam and
	the surface normal to be measured
$\beta$	Minimum value limit of the angle between any two planes in the calibration
$a,b,c,r$	The intermediate variables used to generate the plane parameter
$\lambda$	The smallest angle between the newly generated plane normal (A, B, C)
	and the existing ones
K	The data volume (i.e. the number of equations)
RANSAC	Random sample consensus
vector0	Surface normal of the wafer measured by LDSs when motors rotate
	to an angle of 0
vector0’	Surface normal of the wafer measured by the SLP when motors rotate
	to an angle of 0
$\theta$	Angle between the surface normal of the wafer measured by LDSs and
	vector0 when motors rotate to random angles
${\theta }^{\prime }$	Angle between the surface normal of the wafer measured by the SLP
	vector0’ when motors rotate to random angles