15. Display Brightness, Contrast And Uniformity

At MobilityGuru we measure display maximum brightness and contrast using the same luminance meter discussed in the section on MobileMark 2005. Maximum brightness is the average of 64 measures across the display when the display is set at its highest brightness level. To measure contrast we first take readings from 8 black and 8 white checkerboard-like squares covering the entire display. We then compute the ratio between adjacent white and black (white/black) squares. The average of the 8 pairs of ratios is the display's contrast ratio.

In addition to measuring brightness and contrast we present both a plot of the evenness of illumination of the display (spatial uniformity of brightness) using the 64 maximum brightness measures discussed above. We also compute a statistic designed to show spatial uniformity of brightness. This statistic, "known as the adjusted normalized standard deviation of spatial uniformity of brightness," is computed as follows: (standard deviation of 64 brightness measures across a display / average of 64 brightness measures across a display) * 1000.

Here's why we chose this equation. Logically it seems that standard deviation of brightness should tell us all we need to know about the evenness or unevenness of a display. The higher the standard deviation of brightness for a display, the more likely brightness in a particular section of the display will deviate significantly from the display's average brightness. However, things don't work exactly that way. Brighter displays tend to have higher standard deviations of brightness than less bright displays simply because they are brighter. We know this because plots of a bright display's spatial uniformity of brightness displayed in 64 rectangles show it to have better evenness of brightness than the same plots for displays with lower brightness and smaller standard deviations of brightness.

When we divide a display's standard deviation of brightness by the average brightness of the display, we get a number that relates well to what we see in plots of uniformity of brightness. We multiply by 1000 to scale the number to a level that is easier to grasp than the rather small (less than 1) values we get from simply dividing.

Here are the brightness and contrast charts and plots for the WinBook X610. Also included in the charts are the HP Pavilion DV4000, the four 1.2 GHz CPU laptops and Toshiba's Qosmio G25-AV513. As the charts show, the Qosmio is the most impressive performer we've tested when it comes to display brightness and contrast.