Lighting is provided within interiors in order to satisfy the following requirements:

• to assist in providing a safe working environment
• to assist in the performance of visual tasks
• to develop an appropriate visual environment.

The provision of a safe working environment has to be at the top of the list of priorities, and, in general, safety is increased by making hazards clearly visible. The order of priority of the other two requirements will depend to a large extent upon the use to which the interior is put. Task performance can be improved by ensuring that task detail is easier to see, while appropriate visual environments are developed by varying the lighting emphasis given to objects and surfaces within an interior.

Our general feeling of well-being, including morale and fatigue, is influenced by light and colour. Under low lighting levels, objects would have little or no colour or shape and there would be a loss in perspective. Conversely an excess of light may be just as unwanted as too little light.

In general, people prefer a room with daylight to a room which is windowless. Furthermore, contact with the outside world is considered to aid the feeling of well-being. The introduction of automatic lighting controls, together with high-frequency dimming of fluorescent lamps, has made it possible to provide interiors with a controlled combination of daylight and artificial light. This has the added benefit of saving on energy costs.

Perception of the character of an interior is influenced by both the brightness and colour of visible surfaces, both interior and exterior. The general lighting conditions within an interior can be achieved by using daylight or artificial lighting, or more likely by a combination of both.

Evaluation of Lighting

General requirements

Lighting systems used in commercial interiors can be sub-divided into three major categories—general lighting, localized lighting and local lighting.

General lighting installations typically provide an approximately uniform illuminance over the whole of the working plane. Such systems are often based upon the lumen method of design, where an average illuminance is:

Average illuminance (lux) =

Localized lighting systems provide illuminance on general work areas with a simultaneous reduced level of illuminance in adjacent areas.

Local lighting systems provide illuminance for relatively small areas incorporating visual tasks. Such systems are normally complemented by a specified level of general lighting. Figure 1 illustrates the typical differences between the systems described.

Figure 1. Lighting systems

Where visual tasks are to be performed it is essential to achieve a demanded level of illuminance and to consider the circumstances that influence its quality.

The use of daylight to illuminate tasks has both merits and limitations. Windows admitting daylight into an interior provide good three-dimensional modelling, and though the spectral distribution of daylight varies throughout the day, its colour rendering is generally considered to be excellent.

However, a constant illuminance on a task cannot be provided by natural daylight only, due to its wide variability, and if the task is within the same field of view as a bright sky, then disabling glare is likely to occur, thereby impairing task performance. The use of daylight for task illuminance has only partial success, and artificial lighting, over which greater control can be exercised, has a major role to play.

Since the human eye will perceive surfaces and objects only through light which is reflected from them, it follows that surface characteristics and reflectance values together with the quantity and quality of light will influence the appearance of the                                                                                                                                       environment.

When considering the lighting of an interior it is essential to determine the illuminance level and to compare it with recommended levels for different tasks (see table 1).

Table 1. Typical recommended levels of maintained illuminance for different locations or visual tasks

Lighting for visual tasks

The ability of the eye to discern detail—visual acuity—is significantly influenced by task size, contrast and the viewer’s visual performance. Increase in the quantity and quality of lighting will also significantly improve visual performance. The effect of lighting on task performance is influenced by the size of the critical details of the task and upon the contrast between task and surrounding background. Figure 2 shows the effects of illuminance upon visual acuity. When considering visual task lighting it is important to consider the ability of the eye to carry out the visual task with both speed and accuracy. This combination is known as visual performance. Figure 3 gives typical effects of illuminance on the visual performance of a given task.

Figure 2. Typical relationship between visual acuity and illuminance

Figure 3. Typical relationship between visual performance and illuminance

The prediction of illuminance reaching a working surface is of prime importance in lighting design. However, the human visual system responds to the distribution of luminance within the field of view. The scene within a visual field is interpreted by differentiating between surface colour, reflectance and illumination. Luminance depends upon both the illuminance on, and reflectance of, a surface. Both illuminance and luminance are objective quantities. The response to brightness, however, is subjective.

In order to produce an environment which provides visual satisfaction, comfort and performance, luminances within the field of view need to be balanced. Ideally the luminances surrounding a task should decrease gradually, thereby avoiding harsh contrasts. Suggested variation in luminance across a task is shown in figure 4.

Figure 4. Variation in luminance across a task

The lumen method of lighting design leads to an average horizontal plane illuminance on the working plane, and it is possible to use the method to establish average illuminance values on the walls and ceilings within an interior. It is possible to convert average illuminance values into average luminance values from details of the mean reflectance value of the room surfaces.

The equation relating luminance and illuminance is: 

Figure 5. Typical relative illuminance values together with suggested reflectance values

Figure 5 shows a typical office with relative illuminance values (from an overhead general lighting system) on the main room surfaces together with suggested reflectances. The human eye tends to be drawn to that part of the visual scene which is brightest. It follows that higher luminance values usually occur at a visual task area. The eye acknowledges detail within a visual task by discriminating between lighter and darker parts of the task. The variation in brightness of a visual task is determined from calculation of the luminance contrast:

where

L_t = Luminance of the task

L_b = Luminance of the background

and both luminances are measured in cd·m^–2

The vertical lines in this equation signify that all values of luminance contrast are to be considered positive.

The contrast of a visual task will be influenced by the reflectance properties of the task itself. See figure 5.

Optical Control of Lighting

If a bare lamp is used in a luminaire, the distribution of light is unlikely to be acceptable and the system will almost certainly be uneconomical. In such situations the bare lamp is likely to be a source of glare to the room occupants, and while some light may eventually reach the working plane, the effectiveness of the installation is likely to be seriously reduced because of the glare.

It will be evident that some form of light control is required, and the methods most frequently employed are detailed below.

Obstruction

If a lamp is installed within an opaque enclosure with only a single aperture for the light to escape, then the light distribution will be very limited, as shown in figure 6.

Figure 6. Lighting output control by obstruction

Reflection

This method uses reflective surfaces, which may vary from a highly matt finish to a highly specular or mirror-like finish. This method of control is more efficient than obstruction, since stray light is collected and redirected to where it is required. The principle involved is shown in figure 7.

Figure 7. Light output control by reflection

Diffusion

If a lamp is installed within a translucent material, the apparent size of the light source is increased with a simultaneous reduction in its brightness. Practical diffusers unfortunately absorb some of the emitted light, which consequently reduces the overall efficiency of the luminaire. Figure 8 illustrates the principle of diffusion.

Figure 8. Light output control by diffusion

Refraction

This method uses the “prism” effect, where typically a prism material of glass or plastic “bends” the rays of light and in so doing redirects the light to where it is required. This method is extremely suitable for general interior lighting. It has the advantage of combining good glare control with an acceptable efficiency. Figure 9 shows how refraction assists in optical control.

In many cases a luminaire will use a combination of the methods of optical control described.

Figure 9. Light output control by refraction

Luminance distribution

The light output distribution from a luminaire is significant in determining the visual conditions subsequently experienced. Each of the four methods of optical control described will produce differing light output distribution properties from the luminaire.

Veiling reflections often occur in areas where VDUs are installed. The usual symptoms experienced in such situations are reduced ability to read correctly from the text on a screen due to the appearance of unwanted high-luminance images on the screen itself, typically from overhead luminaires. A situation can develop where veiling reflections also appear on paper on a desk in an interior.

If the luminaires in an interior have a strong vertically downward component of light output, then any paper on a desk beneath such a luminaire will reflect the light source into the eyes of an observer who is reading from or working on the paper. If the paper has a gloss finish, the situation is aggravated.

The solution to the problem is to arrange for the luminaires used to have a light output distribution which is predominantly at an angle to the downward vertical, so that following the basic laws of physics (angle of incidence = angle of reflection) the reflected glare will be minimized. Figure 10 shows a typical example of both the problem and the cure. The light output distribution from the luminaire used to overcome the problem is referred to as a batwing distribution.

Figure 10. Veiling reflections

Light distribution from luminaires can also lead to direct glare, and in an attempt to overcome this problem, local lighting units should be installed outside the 45-degree “forbidden angle”, as shown in figure 11.

Figure 11. Diagrammatic representation of the forbidden angle

Optimal Lighting Conditions for Visual Comfortand Performance

It is appropriate when investigating lighting conditions for visual comfort and performance to consider those factors affecting the ability to see detail. These can be sub-divided into two categories—characteristics of the observer and characteristics of the task.

Characteristics of the observer.

These include:

• sensitivity of the individual’s visual system to size, contrast, exposure time
• transient adaptation characteristics
• susceptibility to glare
• age
• motivational and psychological characteristics.

Characteristics of the task.

These include:

• configuration of detail
• contrast of detail/background
• background luminance
• specularity of detail.

With reference to particular tasks, the following questions need to be answered:

• Are the task details easy to see?
• Is the task likely to be undertaken for lengthy periods?
• If errors result from the performance of the task, are the consequences considered to be serious?

In order to produce optimal workplace lighting conditions it is important to consider the requirements placed upon the lighting installation. Ideally task lighting should reveal colour, size, relief and surface qualities of a task while simultaneously avoiding the creation of potentially dangerous shadows, glare and “harsh” surroundings to the task itself.

Glare.

Glare occurs when there is excessive luminance in the field of view. The effects of glare on vision can be divided into two groups, termed disability glare and discomfort glare.

Consider the example of glare from the headlights of an oncoming vehicle during darkness. The eye cannot adapt simultaneously to the headlights of the vehicle and to the much lower brightness of the road. This is an example of disability glare, since the high luminance light sources produce a disabling effect due to the scattering of light in the optic media. Disability glare is proportional to the intensity of the offending source of light.

Discomfort glare, which is more likely to occur in interiors, can be reduced or even totally eliminated by reducing the contrast between the task and its surroundings. Matt, diffusely reflecting finishes on work surfaces are to be preferred to gloss or specularly reflecting finishes, and the position of any offending light source should be outside the normal field of vision. In general, successful visual performance occurs when the task itself is brighter than its immediate surrounds, but not excessively.

The magnitude of discomfort glare is given a numerical value and compared with reference values in order to predict whether the level of discomfort glare will be acceptable. The method of calculation of glare index values used in the UK and elsewhere is considered under “Measurement”.

Measurement

Lighting surveys

One survey technique often used relies upon a grid of measuring points over the whole area under consideration. The basis of this technique is to divide the whole of the interior into a number of equal areas, each ideally square. The illuminance at the centre of each of the areas is measured at desk-top height (typically 0.85 metres above floor level), and an average value of illuminance is calculated. The accuracy of the value of average illuminance is influenced by the number of measuring points used.

A relationship exists which enables the minimum number of measuring points to be calculated from the value of room index applicable to the interior under consideration.

Here, length and width refer to the room dimensions, and mounting height is the vertical distance between the centre of the light source and the working plane.

The relationship referred to is given as:

Minimum number of measuring points = (x + 2)²

where “x” is the value of the room index taken to the next highest whole number, except that for all values of RI equal to or greater than 3, x is taken as 4. This equation gives the minimum number of measuring points, but conditions often require more than this minimum number of points to be used.

When considering the lighting of a task area and its immediate surround, variance in illuminance or uniformity of illuminance must be considered.

Over any task area and its immediate surround, uniformity should be not less than 0.8.

In many workplaces it is unnecessary to illuminate all areas to the same level. Localized or local lighting may provide some degree of energy saving, but whichever system is used the variance in illuminance across an interior must not be excessive.

The diversity of illuminance is expressed as:

At any point in the major area of the interior, the diversity of illuminance should not exceed 5:1.

Instruments used for measuring illuminance and luminance typically have spectral responses which vary from the response of the human visual system. The responses are corrected, often by the use of filters. When filters are incorporated, the instruments are referred to as colour corrected.

Illuminance meters have a further correction applied which compensates for the direction of incident light falling upon the detector cell. Instruments which are capable of accurately measuring illuminance from varying directions of incident light are said to be cosine corrected.

Measurement of glare index

The system used frequently in the UK, with variations elsewhere, is essentially a two-stage process. The first stage establishes an uncorrected glare index value (UGI). Figure 12 provides an example.

Figure 12. Elevation and plan views of typical interior used in example

The height H is the vertical distance between the centre of the light source and the eye level of a seated observer, which is normally taken as 1.2 metres above floor level. The major dimensions of the room are then converted into multiples of H. Thus, since H = 3.0 metres, then length = 4H and width = 3H. Four separate calculations of UGI have to be made in order to determine the worst case scenario in accordance with the layouts shown in figure 13.

Figure 13. Possible combinations of luminaire orientation and viewing direction within the interior considered in the example

Tables are produced by lighting equipment manufacturers which specify, for given values of fabric reflectance within a room, values of uncorrected glare index for each combination of values of X and Y.

The second stage of the process is to apply correction factors to the UGI values depending upon values of lamp output flux and deviation in value of height (H).

The final glare index value is then compared with the Limiting Glare Index value for specific interiors, given in references such as the CIBSE Code for Interior Lighting (1994).

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