How Brightness and Contrast Ratio Affect Notebook LCD Screens

There are many aspects of a liquid crystal display that effect how well your screen displays an image. Of these, brightness and contrast ratio might be the two most important.

Brightness

Notebook LCD screens have a backlight within them that provide the luminance that lights the entire screen. The total luminance of a notebook LCD depends largely on the intensity of the backlight within the display. The typical peak luminance for a notebook LCD screen or a LCD desktop monitor is about 150-400 candela per square meter (cd/m2). This white luminance is the product of the red, green and blue luminance components. The brightness drastically drops off when viewed from angles other than the main angle that most people view when using their laptop or desktop monitor.

Companies, like 3M for example, produce what is called brightness enhancement films, which raise luminance. These were created to enhance brightness of the LCD screen while keeping backlight consumption the same. However, luminance in only increased to the single user, not those that may be viewing the display from the side or from above.

Although the backlight does determine the amount of luminance that is released, other components of the LCD display do as well. Each layer of the display that luminance is passed through, only allows a percentage of that luminance to be transmitted. A typical luminance in a notebook LCD screen starts at roughly 3000 (cd/m2). By the time the light travels through all the layers of a LCD screen, the luminance is down to about 150-300 cd/m2.

The first polarizer transmits about 42% of the luminance. Each pixel has a black matrix opening, which allows about 60% of the light to transmit. Then the light has to travel through the color filter, which transmits roughly 25% of the luminance. In the end, the light that the notebook user sees is only about 5-10% of its starting luminance.

The figure below shows the different layers of the LCD screen and the percentage of luminance each layer transmits.

Contrast Ratio

Contrast ratio can be defined as ratio of the maximum luminance to the minimum luminance. In other words, the brightest a display luminance can achieve to the darkest (full black) luminance a display can achieve.

Contrast ratio is expressed in ratio form. For example if the maximum of white brightness is 500 and the minimum brightness (black) is 1 cd/m2, then the ratio is expressed 500:1. Higher contrast ratios are good to have, but not necessary. It’s a good idea to have high contrast ratios for your television at home, because viewers typically sit a few feet way while viewing the display. For watching movies or gaming, higher contrast ratios should be desired. However, for close viewing, like desktop or notebook LCD screens at work, lower brightness and contrast ratios may be better for the eyes when using the Internet or using applications for work.

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