Lighting

Lighting conditions in workplaces are vital for occupational safety and health. Lighting supports visual performance and visual comfort. High lighting quality improves work performance and reduces accidents. The sooner and easier hazards can be detected, the easier it is to avoid them. Therefore, lighting equipment should be designed and installed in such a way that it effectively and comfortably illuminates the work task and the working environment. Besides aspects such as visual performance, visual comfort, work performance and preventing accidents, so-called non-visual effects are also an important focus of occupational safety and health. Activating light-sensitive cells (photoreceptors) in the human eye with blue light increases attention and affects the body's physiological functions. This is why lighting (blue-enriched light) is also used to influence employees' alertness and performance.

Visual effects of light

Overview

Light, also called "visible light", refers to the visible region of the electromagnetic spectrum and comprises the range of wavelengths that can induce brightness and colour perception in the photoreceptors of the human eye. This spectrum is positioned between ultraviolet (UV) and infrared radiation. In general, humans can visually perceive wavelengths from about 400 nanometres (nm) to 780 nm^[1].

Any visual task requires a certain level of lighting in order to be performed in a quick and safe way. In addition to effects on visual performance, the improvement of lighting also has an effect on productivity. This is reflected in higher work performance, lower error rates and less accidents^[2]. Although, in general, accidents occur due to multicausal reasons, it is a well-accepted fact that many occupational accidents could be avoided by means of adequate lighting. For instance, stumbling, slipping and falling are common types of accidents and may be caused by low illuminance levels. The illuminance level determines the amount of light that covers a surface and is measured in lux.

Workplaces with low visual requirements where the guidelines recommend illuminance levels of 50-250 lx^[3] [2] are particularly affected. It should, however, be considered that it is not only the illumination level that plays a role in accident prevention, but rather all aspects of lighting quality. The quality criteria that need to be taken into account for good lighting quality are outlined in the section on lighting requirements. 

In addition to the effects of lighting on visual performance, work performance and accidents, poor lighting can also cause visual discomfort. Symptoms indicating visual discomfort include visual fatigue, eye disorders, migraine or headaches (asthenopic complaints). Visual discomfort is also regarded as a cause for sick building syndrome^[4] . For example, if illuminance levels are too low when demanding visual tasks have to be performed, this may affect visual comfort. Scientific investigations show that humans reduce their blinking rate when the level of lighting is low. As a result, the ocular surface dries out more than usual, which may lead to visual fatigue and eye disorders4 .

Visual performance

Visual performance enables employees to see what needs to be seen for the execution of the work task. The most important approaches to quantify visual performance originated in the 1930s by Weston and Luckiesh. Their studies significantly contributed to the establishment of illuminance values for the workplace. Then, as now, visual performance was defined by the speed and accuracy of processing visual information^{[5] [6] [7] [8]}. Luckiesh took the approach of examining recognition accuracy, whereas Weston performed experiments on detection speed under different lighting conditions. The two approaches differ in that small changes in illuminance have different effects on visual performance. Nevertheless, both approaches are today considered to be correc^[6]. In this context, the type of visual task plays an important role. For example, in emergency lighting, low illuminance levels are sufficient to detect large objects with high contrast (Weston’s approach). In contrast, such as in the case of clock making, a high level of illuminance is needed in order to be able to recognize the fine details in very fine visual tasks with low contrast (Luckiesh’s approach).

Visual comfort

Visual comfort is a high priority in the lighting of workplaces. Visual discomfort does not affect visual performance directly, but rather indirectly. It disturbs the progress of work without causing a measurable reduction in visual performance. Markers for visual discomfort are asthenopic complaints^[9]  (such as, red, sore, itchy and watery eyes or headaches)^[5]. An important characteristic of visual comfort is that it relates to the entire visual field, not only to the task area. According to Boyce^[5]  the following reasons for visual discomfort can be distinguished: 

1. Visual task difficulty: Visual tasks with frequent change of close and far vision or permanent alternating light-dark changes can lead to fatigue of the eye muscles.
2. Under- and over-stimulation: High uniformity and monotonous lighting conditions (such as, completely diffused light in combination with monotonous tasks) can lead to visual discomfort.
3. Distraction: Moving objects or flickering light sources in the visual field can disturb and interrupt work processes with high demands on concentration.
4. Perceptual confusion: Reflexions and glossy surfaces can impede perception and orientation.

Non-visual effects of light

Overview

Photoreception in the eye leads not only to vision but also to effects on human physiology, and behaviour, often summarised as non-visual effects of light^[10]. These non-visual effects describe the impact of light on sleep/wake cycles, alertness, performance patterns, core body temperature and production of hormones.

The starting point for the exploration of non-visual effects of light on human beings was the discovery by Brainard and Thapan ^[11] [12] of light sensitive cells (photoreceptors) that are particularly sensitive to blue light and do not contribute to image-forming visual functions. This class of photoreceptors called intrinsically photosensitive retinal ganglion cells (ipRGCs) are predominantly responsible for driving non-visual responses to light, including biological processes related to circadian rhythms but may also influence many other processes and affect long-term health^{[13] [10][14]} (figure).

Natural daylight contains a strong blue light component that lets the body’s central clock track the alternation of day and night precisely. Thereby bodily functions can be adjusted to the time of day. Interestingly, also white LEDs (Light Emitting Diode) emit much light in the blue spectral range. Exposure to blue enriched light elicits behavioural responses, which include enhancing alertness and performance ^[15] although the supporting scientific evidence remains limited^[16] [17]. 

Figure Non-visual effects of light on circadian regulation and long-term health

Source: ^[14]

Possible adverse health effects

Daylight is an important source of information for the circadian clock which enables the synchronization with the external time. Blue enriched light can take the place of daylight and thus send “incorrect information” about external time to the circadian clock. Improper light exposure, such as "light at the wrong time" can act as a disturbing factor^[18] . This state is referred to as “circadian disruption” ^[19] [20] or “asynchronization” ^[21] [22] .

According to Erren^[20] circadian disruption is “a relevant disturbance of the circadian organization of physiology, endocrinology, metabolism and behaviour, which links light, biological rhythms and the development of cancers with melatonin being a key biological intermediary”. Disruption of the circadian rhythm can lead to health disorders and although there are genetic variables, the environment plays a role in some conditions, which can develop as a result of improper light exposure (i.e. too little light exposure during the day and/or too much at night)^[13]. 

Concerning the disturbance of the circadian system it has to be distinguished between early phases of desynchronisation and long-term effects. Short term effects can result in insomnia, sleepiness during the day, gastrointestinal discomfort or irritation^[23]. On top of that, disruption of the internal clock can cause impaired attention and decreased problem-solving skills, disorientation, loss of sociability, loss of will or motivation, and impair alertness and performance^[22]. As a consequence, this affects safety and health and may lead to serious, sometimes to disastrous accidents^[24]. 

Further risks of improper light exposure for safety and health can be derived from long-term studies. This is often to epidemiological data of shift workers and flight attendants. Shift workers and flight attendants are often exposed to irregular light profiles or have to adjust to different day lengths. For shift workers and flight attendants it is assumed that the disturbance of the circadian system increases the risk for breast or prostate cancer ^[20] [25] . Furthermore, it is believed that obesity, diabetes, cardiovascular disease, and depression may be risks for people who must regularly disrupt their light-dark cycles^[13][26]. 

Cost of poor lighting

Poor lighting in the workplace can also constitute a significant cost to the business. The incorrect design of lighting, the improper installation, maintenance and disposal of lighting or the improper selection of emergency lights can result in costs such as:

• Increased time off work of employees as a result of accidents, injuries or ill health.
• Increased absenteeism
• Reduced staff efficiency and productivity.

Quality criteria

The risks of bad lighting can be ruled out by taking into account the following quality criteria. The quality criteria outline minimum requirements with regard to lighting quality and can be used for both assessment and planning. It is important that all quality criteria are considered separately, but also with their interactions. Even the disregard or insufficient consideration of one attribute can lead to accidents or impair visual performance^[27]. The most important are presented below.

Glare

Glare from sunlight or artificial lighting (direct glare and veiling reflections) is caused by an an unsuitable distribution or range of luminance, or by extreme luminance contrasts ^[28]. It is perceived as unpleasant, can decrease visual performance, and may result in eyestrain. To prevent accidents, glare has to be limited. A distinction can be made between disability glare and discomfort glare, both of which can occur alone or together. Disability glare refers to a reduction in contrast sensitivity, which decreases visual performance. On the other hand, discomfort glare refers to a subjective feeling of disturbance, which is caused by high luminances in the visual field. Glare can also result from glossy and specular surfaces, which reflect details from the environment. These so called “veiling reflections” can super impose visual targets (e.g., on a screen with a veiling luminance), and, thus, reduce contrast.

Illuminance level

A sufficient illuminance level allows people to notice hazards and assess risks. A person will find it difficult to see properly if the level of illuminance is not sufficient for the task they are performing. Illuminance refers to the density of incident luminous flux with respect to area at a point on a real or imaginary surface^[28]. The illuminance is a physical quantity that can simply be measured. The values are stated in lux [lx]. Depending on the point and the direction of measurement, illuminance can be specified as horizontal, vertical or cylindrical illuminance.

The lighting levels specified in the regulations, policies, standards are minimum requirements. They depend on the characteristics of the visual task as well as on production-related particularities. Minimum values of mean illuminance levels are summarised in the tables below and sorted by the type of room/area or activity (e.g. Table 1 based on standard EN 12464-1^[29]). In principle, the greater illuminance values are needed, the higher the visual requirements and the more difficult the visual tasks are. Standard EN 12464-1:2021 identifies the required illuminance levels and also provides a recommended level in case the work circumstances require higher lighting levels. This might be the case as a result of things such as:
- a lack of natural daylight in the working environment;
- a decrease in the eyesight of staff (age);
- low detail perception or poor contrast in the room;
- an excessive duration of tasks to be performed.

Table 1: Minimum requirements for the level of lighting for various visual tasks 

Source: ^[29] 

Prior to the construction of a lighting system a task analysis has to be performed. On the basis of the results the required illuminance has to be determined as function of the visual component and the visual tasks. Thereby it has to be taken into account that the illuminance of the lighting system can decrease due to aging of the lamp, defilement and lamp failure. Even in these cases, the illumination should not drop below minimum requirements.

Uniformity

Uniformity can be characterised by the illuminance distribution. Uniformity relates either to the task area, the immediately adjacent surrounding area or the further environment.

The uniformity of a task area can be calculated from the ratio of the minimum illuminance to the average illuminance^[28]. The uniformity between adjacent areas is defined by the ratio of the average illuminances of each of these areas. Recommendations for uniformity can be found in European standardisation and legislation^[29]. Uniformities specified as either proper fraction (e.g. 1:2) or as a decimal fraction (e.g. 0.5).

Colour

The light colour of a lamp can be derived from the spectral composition of the emitted light. Light colour is defined by the correlated colour temperature (CCT) and the values are stated in Kelvin [K] ^[28]. The colour of daylight is white, and it contains all wavelengths in the visible range. The light colour of artificial light sources is divided into three groups. Colours of light sources with a correlated colour temperature below 3300K are referred to as warm white. Colour temperatures between 3300K and 5300K are referred to as neutral white and with colour temperatures above 5300K are referred to as cool white.

For the identification of the colour characteristics, an international three-digit code is used^[30]. The two last digits represent the approximate colour temperature in units of 100 K and the first digit denotes the colour rendering properties. For example, a lamp with the label 827 has, the colour temperature of 2700 K (warm white) and a colour rendering index (CRI) higher than 80.

In addition to the feature of the colour of light another feature that is important is colour rendering^[31]. The colour rendering characterises the relationship between the perceived colour of a visual object under daylight conditions and their perceived colour under a current light source. The property of a light source to render colours of objects in a natural way is particularly important in workplaces where colours and colour variations must be assessed or where safety colours have to be distinguished. The quality of colour rendering depends on the spectral composition of the illuminated light. If for example, the spectra of a lamp contains little "red" then red body colours could be reproduced imperfectly. The physical quantity of colour reproduction is the colour rendering index (CRI), which is specified by the lamp manufacturer. The best colour reproduction is reached at a CRI of 100. In general, light sources with continuous spectra have better colour rendering properties than those with discontinuous spectra.

Flicker

When lamps are supplied with alternating current, periodic light fluctuations occur that are barely perceptible in general, but can affect the visual comfort of employees ^[32] [33]. This temporal uniformity (flicker) plays a role for primarily in tasks where workers have to observe moving objects or rotating machine parts. In these cases flicker can result in the stroboscopic effect, which can be a trigger for accidents^[27]. The stroboscopic effect is an optical illusion, where moving objects can appear as if they are standing still, move with a lower velocity or reverse the direction of rotation.

Modelling and shadows

For good visibility of bodies, surfaces and structures, light and shadows are required. Shadows enable observers to perceive the shape of objects ^[34], to estimate distances ^[35], to interpret faces ^[36], to evaluate surface roughness ^[37] and to receive information about the motion of objects^[38]. Shading is caused by directional light. This creates a dark area behind an opaque lit object (cast shadow). Highly directional light produces harsh shadows, whereas diffuse illumination creates soft shadows. Neither too hard nor too soft or even missing shadows are perceived as pleasant. A good proportion of diffuse and directional light causes a pleasant shadow^[39] [40]. For example, directed luminaires with a diffuse indirect component are proven to have pleasant modelling^[36] [41]. The light direction should be adjusted to the visual task as it can improve the visibility of details. 

Emergency lighting

When selecting suitable and appropriate emergency lighting for a workplace, there are several factors that need to be considered. These include^[42]:

• The lighting needs to be activated for as long as the danger exists, or until normal lighting is resumed for work activities to continue safely.
• If normal lighting fails, then an immediate light output is necessary. This can be acquired only from certain lamp types such as tungsten, tungsten halogen and tubular fluorescent lamps.
• A mechanism is required for connecting the lamp to an alternative electrical supply when the normal supply fails.
• Necessary illuminance needs to be provided at appropriate places, such as emergency exits and escape routes.
• Direction and fire exit signs need to be illuminated.
• To prevent glare, emergency luminaires need to be mounted at least two metres above the floor but not much higher, as in the event of a fire there is always a risk of smoke reducing the light levels on the escape route.
• It is important that the lighting outside the building is adequate for safe evacuation and that the lighting itself is safe for outside use;

When deciding the position and location of luminaires, there is a need to consider where:

• emergency exits are situated
• escape routes are located; 
• areas that require special attention are situated, such as escalators, plant rooms, direction signs, fire alarm points and fire-fighting equipment.

Assessing lighting in the workplace

It is important that an assessment of lighting conditions in the workplace is carried out so as to determine if work lighting arrangements are satisfactory, or whether they pose any significant risks to staff. Where there is a possible risk to employees, action needs to be taken to remove, reduce or control the risk. Any assessment carried out should consider if lighting in the workplace: 

• allows people to notice hazards and assess risks;
• is suitable for the environment and the type of work (for example, it is not located against surfaces or materials that may be flammable);
• provides sufficient light (illuminance on the task);
• allows people to see properly and discriminate between colours, to promote safety;
• does not cause glare, flicker or stroboscopic effects;
• avoids the effects of veiling reflections;
• does not result in excessive differences in illuminance within an area or between adjacent areas;
• is suitable to meet the special needs of individuals;
• does not pose a health and safety risk itself;
• is suitably positioned so that it may be properly maintained or replaced, and disposed of to ensure safety;
• includes, when necessary, suitable and safe emergency lighting

However, ensuring appropriate lighting focused on safety and health, also needs to take into account individual characteristics and other circumstances. In this context, the term Human-Centric Lighting is often used referring to lighting solutions that considers the traditional elements of lighting quality and also insights about the non-visual effects of light. Human-Centric Lighting considers every user of a lighting system individually in accordance with their age, profession, and current activity as well as external parameters, such as weather, time of day, and the presence of daylight^[10] [43]. These principles can also be found in voluntary certification schemes such as the WELL Building Standard aimed at promoting health and well-being through design interventions and policies. For instance, within the WELL Building standard one of the items requires projects to provide users with appropriate exposure to light for maintaining circadian health and aligning the circadian rhythm with the day-night cycle^[44].

Simple measures for lighting quality criteria

Table 2: Simple measures to eliminate or reduce health and safety risks from lighting

Source: Overview by the authors

Visual Display Units (VDU)

Apart from the requirements in regard to classical visual tasks, such as reading on paper, the lighting in offices must fulfill the requirements for work with visual display units (90/270/EEC)^[47]. Good lighting at workplaces with visual display units allows good visual and work performance and reduces the risk of visual fatigue and eye disorders. The following two aspects play a special role in regard to work with visual display units: 

• A: Ambient lighting that falls onto self-luminous devices reduces the ability to recognise information (e.g. veiling reflections). Therefore, it is necessary to avoid high illuminance levels on displays. This results in a conflict of goals^[5] . The reduction of general lighting improves visibility on the display but worsens the conditions for classical work tasks, whereas increasing the general lighting reduces visibility on the display. In the case of vertical displays, this conflict of goals can be resolved by using lights with a direct light component which is directed downwards (e.g. specular louvre luminaires). The vertical illuminance level on the display can thereby be reduced without having to accept deficits in regard to the horizontal illuminance level.
• B: A second important aspect in regard to work with visual display units concerns uniformity. Significant differences in brightness between display and working environment can mean that the eyes have to permanently compensate for differences between light and dark. Visual fatigue can occur as a result. Therefore, lighting in offices should not exhibit significant differences between light and dark. In this context, attention must be paid to the design of work surfaces (reflectivity) as well as to the positioning of displays with respect to sources of glare.

The preceding paragraphs outline the manifold effects of lighting on occupational safety and health. The risks of bad lighting can be avoided by taking into account the specified quality criteria. In future, it will be necessary to consider non-visual effects, as well as new lighting technologies like LED or OLED, in the assessment and design of lighting equipment.