General information, optic specifications, index

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L954D955_SRC.QXD 11-03-2004 08:21 Pagina 12.16 Information – Lighting technique The 1 / 2 E 0 angle reflects the angle at which the illuminance has dropped to 50% of the maximum value in the beam centre. 1 / 2 E 0 The beam spread angle Imax reflects the angle over which the luminous intensity drops to 50% of its peak value. 1 / 2 Imax Visual impact diagram Accent factor 100 50 30 15 10 5 2 3m 4m 2m 1m 25 50 100 250 500 1000Eh(lx) Beam width α β E 0 Beam spread Imax The visual impact diagram is a tool to determine the effect of accent lighting by means of the accent factor. The accent factor is defined as:Accent Factor = Espot/Ehorizontaal 12.16 GENERAL INFORMATION, OPTIC SPECIFICATIONS, INDEX Accent factor Effect 2 Noticeable 5 Low theatrical 15 Theatrical 30 Dramatic > 50 Very dramatic For more detailed information on the Accent Factor see the relevant section in this chapter. With the visual impact diagram, the accent lighting effect of a projector can be determined as a function of the average horizontal illuminance and the distance from the projector to the object. The visual impact diagram can be used in two ways: - It can determine the distance from projector to object to achieve specific accent factor at a given horizontal illuminance. Example (see solid line in diagram): an accent factor of 10 (theatrical) at a horizontal illuminance of 300 lux is realised at a distance from projector to object of 4 metres. - It can determine the accent factor when the horizontal illuminance and the distance from projector to object are given. Example (see dashed line in diagram): at a horizontal illuminance of 500 lux with distance from projector to object of 2 metres, an accent factor of approx. 30 (dramatic) is realised. Beam diagram h(m) VBA 2 x 19 o 1 / 2Imax o 2 x 11 h (m) (lx) VBA 1 /2Imax The beam diagram shows the characteristics of the light beam produced by the luminaire / lamp combination (projectors, downlights, reflector lamps, fibre-optic terminations).The diagram gives the Visual Beam Angle (VBA), the beam spread angle (1/2 Imax) and the sharpness of the contour as indicated by the K value. Additionally, it offers the user information about the diameter of the visual light patch and the diameter of the area whose boundary has a luminous intensity equal to 50% of the maximum value. These diameters are available for a range of vertical distances below the luminaire.The illuminance in the centre of the beam (E 0) is available for the same range of vertical distances below the luminaire. The VBA specifies the angle at which the contour of the beam is clearly visible. In contrast to the beam spread angle, the VBA reflects what is perceived when looking at the visual light patch. The beam-spread angle ( 1 / 2 Imax) reflects the angle over which the luminous intensity drops to 50% of its peak value.The beam-spread angle does not reflect the visual appearance of the visual light patch. E0 d(m) K4 1.0 1.0 5250 0.69 0.39 1.5 2333 1.03 0.58 2.0 2.0 1313 1.38 0.78 2.5 840 1.72 0.97 3.0 3.0 583 2.07 1.17 3.5 429 2.41 1.36 4.0 4.0 328 2.75 1.56 5.0 4.5 259 3.10 1.75 3.0 2.0 1.0 0.0 1.0 2.0 3.0 (m)
L954D955_SRC.QXD 11-03-2004 08:21 Pagina 12.17 Lighting of workstations with Display Screen Equipment (DSE) Area 1 - Reflected glare Reflected glare according CEN recommendation Screen classes in accordance with ISO 9241-7 I II III Screen quality Good Medium Poor Average luminaire luminances reflected in the screen ≤ 1000 cd/m 2 Above 65° 45° Area 2 - Direct glare 85° Viewing direction Direct glare UGR according CEN recommendation Glare and glare-reducing techniques are important aspects in interior and especially in office and industrial lighting. Since the 1970s the lighting industry and standardisation institutes have developed various methods to evaluate glare.Additional to this, the lighting industry has developed advanced optical techniques to reduce the glare to required levels. However a clear distinction should be made between: - Direct glare - Reflected glare cause by a combination of a bright source and reflection in a polished surface. (See drawing.) Standards in lighting are developed to define both. In the 1970s methods were developed to standardise the direct glare restrictions. With the introduction of computer screens, especially early models, there were highly reflective dark screens which gave rise to problems in office environments. Subsequently, methods to analyse reflected glare in computer screens have been developed for direct lighting. Direct lighting uses luminaires designed to emit the majority of their light output directly onto the working plane. Direct luminaires can be surface-mounted, recessed into the ceiling or suspended.They are generally viewed as individually lit objects in the space, and for this reason can appear as a distinct and distracting object when reflected on a display screen. If the screen displays light characters (words and numbers, etc) on a dark screen background, (as originally the case with the firstgeneration VDUs) the reflected image will be seen against this dark background. However, if the information is displayed with dark characters on a light background, the reflections will be less visible against the lighter background. Most modern screens and user software programs today are set like this.To avoid glare problems, CEN established luminance limits for luminaires, for typical screen qualities.These are shown in Table 1. Table 1. ≤ 200 cd/m 2 Note: a) The appropriate (CEN) luminance limit for luminaires can be selected when the nature of the screens and software to be used is known. If this information is unknown or subject to doubt, the lower limit of 200 cd/m 2 should be selected. b) The DSE and, in some circumstances the keyboard, may suffer from reflections causing disability and/or discomfort glare. It is therefore necessary to select, locate and arrange the luminaires to avoid high brightness reflections. The designer should determine the mounting zone causing disturbance, then choose equipment and plan mounting positions which will cause no disturbing reflections. Luminaire luminance limits with downward flux Table 1 gives the limits of the average luminaire luminance at elevation angles of 65° and above from the downward vertical, radially around the luminaires for workplaces where display screens, which are vertical or inclined up to 15° tilt angle, are used. 65° L ≤ 1000 cd/m 2 all around the luminaire Note: For certain special places using, for example sensitive screens or variable inclination, these illuminance limits should be applied for lower elevation angles (e.g. 55°) of the luminaire. GENERAL INFORMATION, OPTIC SPECIFICATIONS, INDEX 12.17
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