Public health issues related to intensive usage of CRT monitors in the workplace were not slow in arising. This ended in 1990 with the introduction of an international standard for the reduction of electrostatic emissions - the MPR2. 1990 was also the year of the new TCO standards introduced by the Swedish professional employees confederation, which were subsequently updated and are currently defined as TCO92, TCO95 and TCO99. They cover regulations relating to visual comfort, recycling of parts, the use of certain chemical components (including CFC) and mandatory use of biodegradable materials. TCO99 is the latest standard to which most monitors comply, and it completes the directives by adding a minimum refresh rate of 85 Hz (100 Hz recommended) and restricting the external light reflection rate as well as the screen's magnetic field. But above all, the TCO95 and TCO99 logos on a monitor are the guarantee of regularity in both contrast and brightness on the screen surface.
In CRT monitors, purity refers to color. Each beam is theoretically designed to reach a single dot for a single color (one of the three colors). Generally speaking, any defect in purity is due to poor alignment of one of the RGB beams. If the beam is incorrectly aligned, it will not only touch the dot that it is supposed to hit, but also one of the two other neighboring colors. There will then be a color rendering defect at this point. This will be quite visible when a single color is displayed on the entire screen. It sometimes happens that, in one or more spots, the red takes on a slight yellow or pink shade, which means that the red beam is incorrectly aligned and is therefore touching the blue and green dots.
On a shadow mask monitor, purity defects are often due to a deformation in the perforated sheet. Such deformation can be caused in the long term by metal fatigue. The result is an elongation or deformation of the holes in the mask, which in turn results in their poor alignment in relation to the electron beam. Invar shadow mask monitors are less prone to such deformation.
In an aperture grill monitor, purity defects are mainly due to two causes - either a violent shock that shifts the mask or an external electromagnetic influence. The latter is often just caused by natural environmental magnetism. Fortunately, most monitors these days are equipped with purity adjustment.