Analysis of the measurement method of optical path difference of VA liquid crystal display

introduction

With the continuous development of liquid crystal displays, higher requirements are put forward for display contrast. Using vertically arranged liquid crystal displays, namely VA liquid crystal displays, can better reduce bottom light leakage and greatly improve display contrast. The arrangement of liquid crystal molecules in VA liquid crystal display boxes is vertical, so the existing commonly used optical path difference test equipment such as CG-200 cannot measure the optical path difference of VA liquid crystal displays. According to the display mechanism of VA liquid crystal displays, this paper summarizes a simple method for measuring the optical path difference of VA liquid crystal displays by testing and analyzing the high-voltage photoelectric curves corresponding to different optical path differences, which is convenient for further research and analysis of VA liquid crystal display devices.

1 Test Principle

In the unpowered state, the VA LCD uses the upper and lower polarizers to cross orthogonally obtain an extremely low dark state; in the powered state, the vertically incident visible light and the liquid crystal molecules have an angle of 45°, and the best bright state display is obtained when the effective optical path difference is equal to 1/2 of the visible light wavelength. The effective optical path difference refers to the optical path difference reflected by the entire liquid crystal molecules after the liquid crystal molecules rotate under the action of the electric field. The effective optical path difference is different from the theoretical optical path difference of the liquid crystal. The theoretical optical path difference (abbreviated as optical path difference) refers to the optical path difference reflected by the entire liquid crystal molecules when the liquid crystal molecules are completely rotated from the vertical state to the parallel state under the action of the ideal electric field.

It can be concluded from Figure 1 that when the effective optical path difference is small, the wavelengths of visible light are relatively concentrated, and the colors are only black and white. A bright state maximum value appears near the effective optical path difference of 300nm, and then as the effective optical path difference increases, the phase difference of different wavelengths becomes larger, and its bright state peak value will become smaller and smaller.

Apply driving voltage to the VA LCD from zero. From the display principle above, we know that under a certain driving voltage, when the effective optical path difference reaches 300nm, the display brightness will reach the maximum value. When the driving voltage is large enough, the liquid crystal molecules completely rotate from the vertical state to the parallel state. For a VA LCD with a fixed optical path difference, the ratio of the display brightness corresponding to the high-voltage state to the maximum display brightness is certain. Therefore, by testing VA LCDs with known different optical path differences, the corresponding ratios of different display brightness can be obtained and organized into charts to provide accurate judgment basis for future testing of products with unknown optical path differences. [page]

2 Experimental plan

2.1 Test materials and measuring instruments

The experiment selected 8 small units of VA liquid crystal display, and appropriately adjusted the liquid crystals so that the optical path differences from small to large were 320nm, 460nm, 544nm, 572nm, 600nm, 612nm, 624nm and 648nm respectively.

The measuring instrument adopts German photoelectric testing equipment DMS301.

2.2 Test methods

The high voltage photoelectric curve of each VA LCD with known optical path difference is measured by the photoelectric test equipment DMS301. The test waveform uses the square wave SW waveform, and the test voltage gradually increases from 0V to 30V. The high voltage is 30V, mainly because when the driving voltage reaches 30V, the photoelectric curve is flat, at this time the liquid crystal molecules basically rotate into a parallel state, and the effective optical path difference is close to the theoretical optical path difference.

The display brightness corresponding to voltages from 0V to 30V was tested, and the corresponding high-voltage photoelectric curves were drawn, and the high-voltage photoelectric curves corresponding to different optical path differences were further sorted out and analyzed.

3 Experimental analysis

The ratio of the corresponding high-voltage display brightness to the maximum display brightness for VA liquid crystal displays with different optical path differences is shown in Table 1.

Table 1 Ratio of high-voltage display brightness to maximum display brightness corresponding to different optical path differences.

The following curve can be drawn from the above table, where the horizontal axis Δnd represents the optical path difference, the unit is nm; the vertical axis Rel luminance represents the ratio of the high voltage display brightness to the maximum value display brightness, the unit is percentage (%).

4 Conclusion

By using the above Rel luminance- Δnd curve summarized in this experiment, for a VA LCD display with an unknown optical path difference, it is only necessary to simply test the high-voltage photoelectric curve to obtain the ratio of the high-voltage display brightness to the maximum display brightness, and then compare it with the curve shown in Figure 2 to accurately obtain the corresponding optical path difference.