What Is Sunlight Readable?
1. What constitute a sunlight readable or outdoor readable LCD?
First, the display screen on a sunlight readable/outdoor readable LCD should be bright enough so that the display is visible under strong sunlight. Second, the display contrast ratio must be maintained at 5 to 1 or higher.
Although a display with less than 500 nits screen brightness and a mere 2 to 1 contrast ratio can be read in outdoor environments, the quality of the display will be extremely poor. At Clarion Displays, a truly sunlight readable display is typically considered to be an LCD with 1000 nits or greater screen brightness with a contrast ratio greater than 5 to 1. In outdoor environments under the shade, such a display can provide an excellent image quality.
2. What is Luminance?
Luminance is the scientific term for “Photopic Brightness” which specifies the visual brightness of an object. In layman’s terminology, it is commonly referred to as “brightness”. Luminance is specified in candelas per square meter (Cd/m2) or nits. In the US, the British unit Foot-lamberts (fL) is also frequently used. To convert from fL to nits, multiply the number in fL by 3.426 (i.e. 1 fL = 3.426 nits).
Luminance is a major determinant of perceived picture quality in an LCD. The importance of luminance is enhanced by the fact that the human mind will react more positively to brightly illuminated scenes and objects. Users are typically more drawn to brighter displays that are more pleasing to the eye and easier to read. In indoor environments, a standard active-matrix LCD with a screen luminance around 250 nits looks good. However, a sunlight readable LCD with a screen luminance of 1,000 will look even more beautiful.
3. What is Contrast Ratio?
Contrast ratio (CR) is the ratio of luminance between the brightest “white” and the darkest “black” that can be produced on a display. CR is another major determinant of perceived picture quality. If a picture has high CR, you will judge it to be sharper and more crisp than a picture with lower CR. For example, a typical newspaper picture has a CR of about 5 to 7, whereas a high quality magazine picture has a CR that is greater than 15. Therefore, the magazine picture will look better even if the resolution is the same as that of the newspaper picture.
A typical AMLCD exhibits a CR between 300 to 700 when measured in a dark room. However, the CR on the same unit measured under ambient illumination is drastically lowered due to surface reflection (glare). For example, a standard 200 nit LCD measured in a dark room has a 300 CR, but will have less than a 2 CR under strong direct sunlight. This is due to the fact that surface glare increases the luminance by over 200 nits both on the “white” and the “black” that are produced on the display screen. The result is that the luminance of the white is slightly over 400 nits, and the luminance of the black is over 200 nits. The CR ratio then becomes less than 2 and the picture quality is drastically reduced.
Clarion Displays sunlight readable LCDs with 1500 nits screen brightness will have a CR over 8 with the same amount of glare under the same strong sunlight, making the picture quality on these units extremely good.
4. What is Uniformity and how do we measure it?
Uniformity is the maintenance of luminance from one display area to the next. Non-uniformity can be viewed as a gradual shifts in brightness from one area to the next on the display, making the display inconsistent and less pleasing to the eye. Most of the LCD manufactures specify uniformity by measuring the screen luminance at 9 points on the display.
At Clarion Displays, uniformity is measured as follows
Uniformity = (Lmax – Lmin)/ (Lmax + Lmin)
where Lmax (Lmin) is the maximum (minimum) luminance measured with a 10 mm diameter meter aperture over the LCD active area except the last 10 mm area from the edges.
Clarion Displays products consistently maintain a uniformity level of better than 20%.
5. What is Viewing Angle and why does it matter?
The viewing angle is the angle at which the image quality of an LCD degrades and becomes unacceptable for the intended application. As the observer physically moves to the sides of the LCD, the images on an LCD degrade in three ways. First, the luminance drops. Second, the contrast ratio usually drops off at large angles. Third, the colors may shift. The definition of the viewing angle of an LCD is not absolute as it will depend on your application.
Most LCD manufacturers define viewing angle as the angles where the CR (contrast ratio) ³ 10. For LCDs designed for less demanding applications, the viewing angle is sometimes defined as the angles where the CR ³ ?5.
For LCDs used in outdoor applications, defining the viewing angle based on CR alone is not adequate. Under very bright ambient light, the display is hardly visible when the screen luminance drops below 200 nits. Therefore, Clarion Displays defines the viewing angles based on both the CR and the Luminance.
6. What is dimming range and why is it important?
The dimming range or dimming ratio of an inverter specifies its capability of performing backlight luminance adjustment. For inverters used in notebook computers and LCD monitors, the backlight luminance can be adjusted typically over a dimming range of less than 10:1. That is, the luminance is adjusted from 100% down to about 10%.
For very high brightness backlights used in Clarion Displays sunlight readable LCD modules, the inverters must be able to provide a much wider dimming range. Otherwise, the LCD screen will be too bright during night time conditions. Therefore, our inverters provide a typical dimming ratio of 200:1, meaning that the luminance can be adjusted from 100% down to 0.5%.
Dimming capabilities are beneficial because lowering the backlight luminance will result in a lengthening of the backlight life. It also lowers the power consumption and the related thermal management issues.
