The brightness uniformity is a parameter typical of LCD monitors – it is considerably less important for CRTs. The problem is that when the entire screen is filled with the same color its brightness in different screen spots may be different, for example, the corners may be darker than the center. There may be multiple reasons for that: starting with uneven backlighting and finishing with unevenness of the matrix itself (for instance, small warps that occurred during matrix installation into the case).
The peculiarities of LCD matrices design imply that this brightness unevenness may vary for white and black colors. For example, if we take TN panels, the white color on them corresponds to 0 voltage on liquid crystal cells, when the crystals line up along special grooves on the inside of the panel glass plates. For the block color the voltage is sent to the cells and the crystals twist at a certain angle according to the applied electric field. So, there are two factors that determine the positioning of the crystals at the correct angle, which results in different types of unevenness for white and black colors.
To measure the brightness uniformity with a sensitive photo-sensor we take the readings for screen brightness with 3cm increments for 19-inch monitors and proportionally larger increments for larger models. For example, here is the grid marking for this sort of measurements for a 20-inch monitor:
The measurements are taken in two modes: when only white color is displayed and only black. After that the deviation in % for each spot is calculated for both data arrays: the white color deviates below the maximum value, while the black – above the minimal. The obtained deviation results are used to build two brightness diagrams showing the brightness distribution over the entire screen surface which are then applied to the schematic monitor image (for more illustrative picture). This will help you get a better idea what this unevenness we talked about looks in reality on your display.
Here it is important to understand that we do not try to emulate the exact look of the monitor on these diagrams – these are just diagrams with some reference colors, and not monitor photos. Some reviewers may use different colors – green, yellow, orange, red, etc - for different deviation levels (with 5% or 10% increments), however, we believe it makes things very hard to perceive, because you will have to remember all the way through the article that the yellow color stands for darker areas on the screen, while red – for lighter areas. That is why in our reviews we will use the closest to natural representation: lighter areas will be colored lighter, while darker areas – darker. However, we changed the brightness scale to make the images more illustrative, i.e. if the brightness of two dots on the diagram differs by the factor of 3, it doesn’t mean that in reality their brightness is also 3 times different – please check the scale showing the actual brightness deviation percentage and the colors we use.
So, the diagrams above serve to estimate the brightness uniformity: the distribution over the monitor screen, what areas are darker – corners or center, etc. For the sake of quantitative comparison between different monitor models we always provide percentage values in the text of the review: average deviation and maximum deviation. At first we calculate the arithmetic mean of the screen brightness and then find the average and maximum deviations.
Speaking of particular numbers, if the deviation is within 5% it is considered a good result, within 7-8% - acceptable, and over 8% poor. As for the maximum deviation, the range is more lenient: if it doesn’t exceed 15% - good, 20% - acceptable, over 20% - poor.
I would also like to say that the degree of brightness deviation may vary greatly – more than any other parameter – between particular samples of the same monitor. Unfortunately, it’s not easy to check this parameter out when shopping. The folks at the store are unlikely to turn out the lights and give you the opportunity to scrutinize the monitor in such conditions.