Since the shutters used in liquid crystal displays for each pixel stay at a steady opacity, they do not flicker, even when the image is refreshed. The backlights of such displays typically operate in the range of 150–250 Hz. But, to save the crystals from deterioration caused by constant current, voltage is constantly reversed, which may cause flicker. "In a pixel on an LCD monitor, the amount of light that is transmitted from the backlight depends on the voltage applied to the pixel. For the amount of light, it doesn't matter whether that voltage is negative or positive. However, applying the same voltage for a long period would damage the pixel.
For example, electricity decomposes water into oxygen and hydrogen gas. A comparable similar effect could happen inside the liquid crystals that are in the pixels. In order to prevent damage, LCD displays quickly alternate the voltage between positive and negative for each pixel, which is called 'polarity inversion'. Ideally, the rapid polarity inversion wouldn't be noticeable because every pixel has the same brightness whether a positive or a negative voltage is applied. However, in practice, there is a small difference, which means that every pixel flickers at about 30 hertz.