Design work at screens is one of the most visually demanding tasks. Capillary lines, letters and symbols as well as areas with varying colours and contrast levels have to be recognised securely and thus require high levels of concentration.
CAD offices
Besides the visual task on the screen (inclined visual task area), the working area contains drawing templates, sketches, table books and other conventional office equipment with horizontal visual tasks.
The CAD workstation should not be illuminated with excessive illuminance, EN 12464-1 specifies 500 lx (q.v. table). Reflections of bright surfaces such as windows and walls off the screen surface should be avoided, just as reflections of luminaires, which usually require more shielding than normal VDU workstation room lighting.
Figure 3.100: The design office, visualised
(a) Illuminance (isolux lines, values in lx) on the drawing board. The centre line of the drawing board is situated 1,25 m above the floor. The drawing board is inclined by 75° from the horizontal.
(b) Design office lighting with direct-indirect-distribution luminaires 2 x 54 W/T5 in the drawing board area and 1 x 54 W/T5 at room depth, isolux lines (values in lx) for horizontal illuminance at 0,75 m above the floor.
Figure 3.101: Illuminance distribution in a classic construction office.
Table contains the maximum admissible luminance for luminaires and room surfaces (e.g. walls, furniture, partitions, windows, skylights) which can reflect on the screen from the user's point of view.
The requirements apply for screens with an inclination of up to 15° from the vertical and for screens with a screen diagonal of up to 48 cm (19").
For screens with bigger screen diagonals and screens with reflection-sensitive surfaces – often features of CAD screens – special measures to avoid such reflections must be taken, e.g.
use of luminaires with low luminance values in the direction of the work surface
utilisation of luminaires where the average luminance values specified in table are also met at distribution angles below 65°, e.g. starting at 55°,
particular arrangements of luminaires in relation to the screen,
or the lighting concept "partial area-based lighting".
With brightness-controlled (dimmable) and direct-indirect-distribution luminaires, it is possible to set the individual lighting level as well as reduce average luminance of luminaires and ceiling and thus reduce reflections on the screen to a barely noticeable level.
The example in the figure shows a solution for lighting of a CAD office with a classic arrangement of visual task areas: the desks are located at the window, and the CAD workstations at room depth, meaning removed from windows and daylight. The viewing direction onto the screen is parallel to the window. At the sides of the CAD workstations, drawing stands can be arranged additionally, which can also serve to block off excess daylight or, with brightness-controlled lighting, excess light from the neighbouring CAD workstation. Predominantly indirect-distribution workstation luminaires with special optic (figure) for high contrast rendering and low reflected glare are especially suitable for lighting screen working areas. They are arranged across the viewing direction. The luminaires assigned to each workstation are dimmable individually.
Example:
In the CAD office, there is a suspended, direct-indirect-distribution dimmable LED work zone luminaire with special optic above every CAD workstation. This way, the desired lighting level and thus individually perceived ideal visual conditions can be adjusted for every CAD workstation – independent of other workstations. Along with that, average luminance at light-emitting surfaces and ceiling as well as reflections on the screen are reduced. In the window area, work desks for supporting design work are arranged..
(a) The illuminance distribution at 0,75 m above the floor at 100% luminous flux, depicted as isolux lines (values in lx).
(b) Ceiling luminance distribution (curves of identical luminance, value in cd/m²). At maximum luminous flux setting, the luminance above the CAD workstations does not exceed 400 cd/m² at any point of the ceiling, the average being ≤ 160 cd/m². Depending on the control state of the luminaires, luminance can be further reduced.
Figure 3.102: Illuminance and luminance in a CAD office.
Figure 3.103: Photometrics of a work zone luminaire