Electric Lighting for Offices and Educational Buildings PDF

4. LIGHTING FOR OFFICES AND EDUCATIONAL BUILDINGS 4.0 INTRODUCTION People working in today's offices spend just as much of their time on telephone and in communicating with the computer and office colleagues as they do reading and writing. Each of these tasks h...

4. LIGHTING FOR OFFICES AND EDUCATIONAL BUILDINGS 4.0 INTRODUCTION People working in today's offices spend just as much of their time on telephone and in communicating with the computer and office colleagues as they do reading and writing. Each of these tasks has its own specific visual requirements and it is necessary that the lighting be designed accordingly. Moreover, since feelings of well being, interest and enthusiasm, which are so important to productivity, are affected by the office environment, it is clear that the lighting should be designed to make 'this as pleasant as possible. 4.1 GENERAL RECOMMENDATIONS Illuminances Levels For the vast majority of office tasks, the "working plane is usually horizontal at a height of between 0.75m and 0.85m above floor level. The recommended average illuminances on this plane for various types of office interiors are given in IS:3646 (Part-II) 1992. Research into the illuminances preferred in working interiors has shown that there is no value at which everyone is satisfied. Added to this is the fact that various office tasks are often being performed simultaneously in one and the same office. Clearly, each individual should be able to decide for him or herself exactly what level of lighting is needed at a given moment, provided that this can be arranged without causing disturbance to others. Such control, ideally should be possible from the desk or work station concerned, and can be achieved by either switching or dimming individual luminaires. Uniformity In the case of general lighting, the ratio of the minimum to the average illuminance (Emin/ Eav) should be greater than 0.8. In the case of localised general lighting or local lighting plus general lighting, the average illuminance in non-working areas should normally not be less than 50% of the level for the working areas, with a minimum of 350 lux. The ratio between the average illuminances for any two adjacent areas (eg. Office and adjacent corridor) should not exceed 5:1 and the area with the lower level should have an illuminance of at least 150 lux. 4.2 LUMINANCES Excessively large differences in luminance in an office can create visual adaptation problems or, in extreme cases, glare. Conversely, if the luminance differences are too small, the surroundings will appear dull and uninteresting. Both these effects can be avoided by ensuring that the luminance ratios in the field of view are proper. The luminance ratio between the visual task itself and its immediate background (viz. the desk) should be smaller than 3:1, but greater than 1:1. Some recommended luminances are listed in the table below: Surface Luminance (cd/m2) Walls 50-150 Ceiling 100-300 Task area 100-300 26  The above recommendations can be satisfied by adhering to the reflectance ranges listed in the table below: Surface Reflectance Ceiling 0.7 or above Walls 0.5-0.7 Partitions or screens 0.4-0.7 Floors 0.1-0.3 Furniture and equipment 0.2-0.5 Window shades 0.4-0.6 4.3 GLARE CONTROL Glare is experienced if lamps, luminaries, windows or other areas are too bright compared with the general brightness. Glare is defined as a condition of vision in which there is discomfort or a reduction in the ability to see significant objects or both due to an unsuitable distribution of intensity or extreme contrasts in range of luminance in the field of vision. Glare index is a number representing the amount of discomfort glare in a lighting installation to which upper limits are given for variety of room sizes and surface reflection factors. For an office the permissible glare index is given as 19. The glare index for interior spaces where light fittings with equal spacing are provided can be obtained with the help of photometry data and IS3646. The glare increases when the glare index is more. 4.4 LAMP COLOUR 4.4.1 Colour appearance The choice of the colour appearance of the lamps employed in a particular office depends on the illuminance level, the presence or absence of daylight, climatic conditions and, of course, personal preference. The lamps normally employed in interior lighting may be divided into three groups according to their colour appearance as described by their colour temperature. Correlated colour Colour appearance temperature (CCT) > 5300 K Cool (bluish white) 3300 - 5300 K Intermediate (white) < 3300K Warm (reddish white) Research has indicated that a higher colour temperature (colder colour appearance) is preferred at higher lighting levels, and that in the absence of day light, a low colour temperature (warm appearance) is preferred. In warmer climates, the personal preference is usually for higher colour temperatures, while in cooler climates the preference is more for low colour temperatures. Light sources should not be mixed without first establishing that their colour appearances match. 4.4.2 Colour rendering Light sources with a colour rendering index (Ra) of at least 80 are recommended for all offices. 27 4.5 LIGHTING FOR VDU WORK-STATIONS In general, the visual requirements for office lighting listed above are equally valid, for video-display-unit work-stations for both word processors and CAD systems. However, there are certain additional features that have to be taken into account when planning the light if visual performance and comfort are not to be impaired. 4.5.1 Reflections In the first place, the screen of the VDU constitutes an important, additional part of the task area and light sources such as luminaires and windows that reflect in the screen can produce a-considerable deterioration in the legibility of the characters. Such reflection can best be minimised by taking into account the position of VDU workstations and light sources in relation to one another at the initial planning stage. But care in the choice of the lighting installation can also help to obviate this problem. Both indirect and direct lighting systems can be used, the best choice depending on the height of the room, the type of VDU and the sort of luminaire screening. 4.5.2 Indirect lighting Good indirect lighting employing uplighters creates more or less equal brightness on walls, ceiling and furniture. There will therefore, be no bright spots to cause reflections in the screens of the monitors or VDUs. This means that, at least as far as the artificial lighting is concerned, these can be moved about at will. But indirect lighting does have certain disadvantages' The luminaires and ceiling become dirty sooner. Some of the dust collecting in the Iuminaire is carried upward by the warm air currents to settle on the ceiling. The lighting level is thereby reduced, as is the efficiency of the installation. In low-ceilinged areas, the ceiling brightness will be uneven, creating reflectors in the VDU. The very uniform lighting is devoid of shadows, which is not natural. 4.5.3 Direct lighting Generally speaking, direct lighting is more efficient and does not get dirty so quickly as indirect lighting. Moreover, the heat from the (recessed) luminaires can be drawn away via. the ceiling, keeping the heat loading in the room to a minimum. The degree of freedom allowed in the sitting of the VDUs with direct lighting will depend on the type of screen involved-where dark-screen (positive screen) monitors are in use. Strict requirements are placed on the luminaire screening employed. This should be such as to give a very directional downward light distribution, so lessening the chance of the luminaire causing reflections in the screen. But this also means that they produce quite high contrasts (highlights and deep shadows) on the objects illuminated whilst leaving the walls in relative darkness and such illumination creates a rather sombre impression Where coated, light-screened VDUs are employed, on the other hand (which is generally the case), minor or prismatic luminaires giving a broader light distribution can be chosen without the risk of producing disturbing reflections. Such lighting is more acceptable in terms of the modelling effect produced. Where high ceilinged areas are concerned (3.5m and above), pendant luminaires providing a combination of direct and indirect lighting offer a suitable alternative for both types of screens. However, especially with dark screens, it is still necessary to ensure that the ceiling does not exhibit great differences in brightness. And the "screening” of the direct component of lighting should also be critically examined. 28 4.5.4 Illuminance level The choice of the correct illuminance at the workstation is very important. If it is: too low, it will reduce visual performance for conventional work, while if it is too high it can result in a reduction in character contrast on the VDU screen Excessive jumps in luminance between the documents and the screen must also be avoided if adaptation problems are to be avoided. Research and experience indicate that the optimum illuminance will lie somewhere between 400 lux for light screens and 250 lux for dark screens. 4.6 OFFICE TYPES 4.6.1 General offices General (Fixed) Lighting Most general offices are moderate to large and their layout is seldom fixed: office furniture is re-arranged from time to time, and partition walls may be added, shifted or removed. One way of ensuring that all work positions will be adequately illuminated, no matter what the office layout, is to install a system of general lighting in which luminaires (surface mounted or recessed) are arranged in a regular, fixed pattern over the ceiling If the building is designed with provisions for the future erection of partition walls along the axis of its modules, the lighting arrangement should be planned with this possibility in mind. This normally means that the layout of the luminaires will be a function of the modular building structure and window spacing. Two feet T5 luminaires (4 x 14 W) with suitable reflectors and screening are ideally suited for equally spaced light fittings for general office lighting Flexible lighting Energy savings can be obtained by employing a flexible approach to the design and use of the lighting. There are two alternatives namely: lighting controlled by switching and/or dimming and task lighting with uplighters. A further possibility is supplementary local lighting, which allows the level of general lighting to be reduced. Desk lights (decorative or otherwise) fitted with incandescent lamps are not really suitable, not only are they insufficient, with high heat emission, but they also create a far from uniform luminance distribution over the task area. It is better to employ desk-mounted reflector luminaires equipped with (compact) fluorescent lamps. These have the broad, even light distribution needed for an even luminance distribution, and the heat emission is negligible. They should be mounted above the line of sight (viz, higher than 0.6m above the desk-top) and equiped with an adequate louvre to screen the lamp(s) from view. Adjustable (pivoted) luminaire mounting is appreciated because it allows for easy repositioning for various tasks. These lighting systems should be carefully planned if an irregular, and possibly unpleasant luminance pattern is to be avoided. The local lighting should, of course, allow a task to be performed in comfort for all possible positions of the worker, and the low-level general lighting should provide approximately 50% of the total task illuminance 4.6.2 Drawing offices As drawing involves the accurate discrimination of fine detail, the illuminance in drawing offices should be at least 1000 lux. Ceiling reflections can occur on the surfaces of drawing boards that are badly positioned with respect to the luminaires. The easiest way of minimising such reflections is to position the boards accordingly. Disturbing shadows and all forms of glare can be avoided by using luminaires that have a large luminous area. 29  The problem of providing suitable lighting is much simplified if it can be arranged that all occupants of the office face one way, and the office layout is limited to fixed rows of work positions. Rows of fluorescent luminaires can then be mounted parallel to the direction of view on both sides of the drawing boards. With this arrangement the absence of any luminaire directly above a given work position serves to keep glare and ceiling reflections to a minimum, while the lighting coming from both sides of a worker effectively eliminates hard shadows. 4.7 EDUCATIONAL BUILDINGS The visual environment in an educational building, apart from providing the comfortable seeing conditions needed for the performance of a wide variety of work tasks, must also satisfy the psychological and emotional needs of learners. Good lighting can enhance pleasant and attractive surroundings, stimulate learning and influence behaviour in a positive way. Educational buildings are usually designed to be lit by day light. Whenever and wherever possible, lighting controls should therefore ensure that the artificial lighting can be easily adjusted to suit the prevailing daylight conditions 4.7.1 Class-rooms In a conventional class-room the window wall will be parallel to the direction of view (ideally to the left of the students). Good, glare-free lighting can then be provided by employing low-brightness or well screened ceiling luminaires arranged in parallel rows running parallel to the window wall. The rows should be spaced sufficiently far apart to allow the rows of desks to be positioned between the rows. This last precaution serves especially to reduce reflected glare from the work surface. The Black board should be "provided with its own lighting system in which both direct glare and ceiling reflections in board are eliminated. The simplest arrangement is to have one or more fluorescent luminaires mounted on the ceiling and screened from the students so as to light the board at an angle of not less than 60° from the normal to any part of its surface. Where necessary window blinds can be employed to prevent reflected glare from windows occurring in the Black board and to eliminate sun on desks. These same blinds, perhaps, in conjunction with a dimming device for the artificial lighting, will in any case be needed if it is planned to use the class-room for slide /film shows. Depending on the depth of the class-room it may be necessary to arrange that the luminaires remote from the window wall can be switched on independently of the remainder of the installation to compensate for fading day light. 4.7.2 Lecture Halls In lecture halls and auditorium day light is often shut out altogether and there is complete dependence on artificial lighting. The following points may be mentioned as being of importance in such areas. Reading and writing require an illuminance of more than 300 lux, preferably 500 lux Special care has to be taken to prevent glare. Equipment for dimming the lighting during demonstrations, film and slide shows has to be provided. Local lighting is required near the chalk board. 30  Ideally a control panel should be installed at the lecturer's position to enable him to switch the various groups of lights on and off, operate the dimming equipment, and possibly control also an automatic projection system. It is important that careful attention be paid to the provision of emergency lighting and exit lighting. Steps and stairs should, also be equipped with local orientation lighting. -------------------------------------- 31