How We Test Laptop and Notebook Computers for Home and Office

13. Image Quality, Screen Brightness And Contrast, Continued

We also present spatial uniformity of brightness in graphical terms, showing the brightness of each of the 64 rectangles relative to the brightest of the 64. This is a relative and not an absolute view of spatial uniformity of screen brightness. That is, the graphic does not show exactly what the screen looks like; rather, it gives a sense of the range of brightness levels across the screen. A screen with perfect uniformity would show up in the graphic with 64 uniformly bright rectangles. Any deviation from perfection is displayed using a darkened rectangle, with the squared deviation of the rectangle's brightness from the brightest rectangle(s) used to determine the amount of darkening. Thanks to Georg Gerber of the Harvard/MIT Health Sciences and Technology program for developing the software for this presentation.

A luminance meter and a 64 rectangle image are used to measure brightness and spatial uniformity of brightness.

The spatial uniformity of brightness of our baseline notebook, the Acer TravelMate 2403LCi.

To measure contrast, we use an image with 16 alternating white (RGB 255, 255, 255) and black (RGB 0, 0, 0) rectangles. As with our brightness measurement image, the contrast measurement image is resized to fit the computer's screen dimensions as closely as possible . We set the laptop's or notebook's brightness to full and use the luminance meter to measure the brightness of each black and each white rectangle. We compute the ratio of the brightness of adjacent white and black rectangles. This gives us 8 contrast ratio measurements which we then average for an overall contrast ratio for the computer's screen. We report this average for each laptop or notebook.

This is our 16 rectangle image for measuring contrast.

Contrast ratios are always computed as the ratio of white to black, so a contrast ratio of 100 to 1 means that over all of the areas measured, white is 100 times as bright as black. Note that, ideally, a black screen would emit zero nits of brightness, making the computation of a contrast ratio impossible - you cannot divide by zero. This does not happen in the real world, however, when using a properly calibrated luminance meter.

Many who test displays contend that in order for a display to be readable in sunlight, it must have a maximum brightness of at least 500 nits and a contrast ratio of at least 2 to 1. Some manufacturers of outdoor displays go for 1000 or even 1500 nits, but laptop and notebook screen brightness comes no where near 500 nits. So, we base our outdoor readability evaluations on our own experiences with each laptop's or notebook's screen in the sun. We judge outdoor readability both with the sun coming from behind the computer and also with it in front.

Sometimes manufacturers send us computers with displays that have be heavily used. As displays get more and more use, their backlights tend to dim. This results in lower brightness and contrast results the a mobile computer. We often have no idea how bright a display should be. So, we ask vendors to send us test hardware with new or nearly new displays.