# Progress in Phase Calibration for Liquid Crystal Spatial Light Modulators
## 5 Discussion  
Two categories of phase calibration methods for SLM are introduced. All of the calibration procedures promote the SLM performance on phase modulation. However, due to the different principles used in the methods, they are different in many ways.  
As shown in Table 1, all the calibration methods for SLM could calibrate global LUT and measure spatial nonuniformity, except for the optical elements-generated method. For the spatial nonuniformity measurement of the self-referenced calibration method, the self-referenced beam from certain zones on SLM is supposed to be uniform in theory. But it may not be ideally uniform in practice. Therefore, the spatial nonuniformity measurement may be carried out with errors. As for the single pixel level phase measurement, the out-referenced interferometric method reaches the accuracy of single-pixel resolution with flexible setups. The single pixel measurement ability comes with more optical elements and a complicated setup configuration compared with other calibration methods. The setups of diffraction methods and self-referenced interferometric methods are compact and robust concerning the environmental disturbance. The most charming feature is that the calibration can be carried out without replacing the SLM. In addition, it is practical for optical alignments with high precision. The proposed polarization methods are like a lever with complexity and spatial resolution on both sides. The method which is able to measure the pixel level spatial nonuniformity comes with a complicated data-processing procedure. The simple one could only realize the global measurement.  
As shown in Figure 9, all the calibration methods mentioned in the article are presented under the consideration of five aspects: system simplicity; spatial resolution; measured region size; environment disturbance isolation; and SLM replacement after calibration. The measured region size is calculated by multiplying the pixel number of spatial resolution by the pixel pitch. The system simplicity,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Calibration** & **Resolution (Pixel** & **Frames Needed 1** & **Basic Alignment 2** & **Reference 3** \\
**Method** & **Pitch)** & & & \\ \hline \multirow{2}{*}{Self-referenced} & – & \multirow{2}{*}{\(<\)20} & \multirow{2}{*}{Directly diffractive} & \multirow{2}{*}{[27]} \\  & 8 \(\upmu\)m & & & \\ \hline \multirow{2}{*}{Out-referenced} & 1-pixel & \multirow{2}{*}{Several} & \multirow{2}{*}{Twyman-Green} & \multirow{2}{*}{[28; 33]} \\  & 8 \(\upmu\)m & & & \\ \hline Diffraction pattern & 8 \(\times\) 8-pixel & \multirow{2}{*}{\(<\)10} & \multirow{2}{*}{Directly diffractive} & \multirow{2}{*}{[11]} \\ analyzed & 14 \(\upmu\)m & & & \\ \hline Optical elements & \multirow{2}{*}{Global} & \multirow{2}{*}{1} & \multirow{2}{*}{Directly diffractive} & \multirow{2}{*}{[45]} \\ generated & & & & \\ \hline Polarization & 1-pixel & \multirow{2}{*}{Several} & \multirow{2}{*}{Retro-reflection} & \multirow{2}{*}{[46; 47]} \\ methods & 11.5 \(\upmu\)m & & & \\ \hline \hline \end{tabular}  
* Images or data groups needed approximately for 512 \(\times\) 512 pixels’ calibration at one grayscale. 2 Basic alignments for the best spatial resolution. 3 The parameters are listed according to the discussed referenced paper.  
\end{table}
Table 1: Phase calibration methods with best spatial resolution.  
SLM replacement, and environmental disturbance isolation are evaluated at three levels: marginal, acceptable, and good. In addition, the diffraction pattern analyzed methods could be more functional with considerations of the five aspects.