Nowadays, office work is mostly done on computers. This is why extra care should be taken about the interactions between the light sources and the computer screens, in an early stage of design. Special regulations have been elaborated to help designers to avoid glaring situations in offices where VDU work is carried out.
A light source, either primary or secondary (window, luminaire, light wall), can cause either a specular or diffuse reflexion on a computer screen. When working on a VDU (Video Display Unit) we have all noticed the impact that can have veiling reflexions on the screen on our visual performance. And the darker the screen is, the more visible and disabling are the veiling reflexions
When a VDU screen receives an extra amount of light, the contrast between the background and the characters decreases, and so does the visual performance. In the 1940s Blackwell carried out some experiement about the contrast thresholds [AFE, 1993]. His work has been adapted to give the value of the maximum veiling luminance that a VDU screen can tolerate to keep VDU workers in satisfying visual conditions. If the screen background is white and the letters are black, the contrast is very close to 1. This graphic gives the veiling luminance threshold against the background luminance (measured in the dark) of the VDU screen.
So the bighter a VDU screen is, the more light it will tolerate before the worker sees his visual performances impaired. With the introduction of the new flat-panel TFT screens (over 150 cd/m²), the problem of veiling reflections has somewhat improved. To avoid veiling reflections from windows, it is recommended that VDU screens are placed perpendicularily to the window.