Quality characteristics of illumination

Visual performance is determined by the level of illumination. It is influenced by the degree of illuminance and the reflection properties of an illuminated surface. Here, the following applies: The lower the reflection, the higher the degree of illuminance must be. A white wall has a reflection degree of up to 85%, while a red brick facade only has up to 25%. To compensate for low visual performance, the degree of illuminance must be raised.

Maintained illuminance defines the average degree of illuminance on a reference surface. In the course of the working life of a lighting system, ageing and environmental influences change lamps, luminaires and spaces. The degree of illuminance diminishes. This reduction is described by maintained illuminance. To compensate, new systems should have higher illumination values.

Maintained illuminance = maintenance factor x value as new

The maintenance factor is dependent on the type of lamp, the luminaires, the dirt in the environment and the maintenance intervals.

In a clean environment, for example in an office, a value of 0.67 can be applied for a maintenance cycle of three years; in a dirty room situation, a value of 0.5. The area on which the illuminance is to be realised is used as the basis for calculation. In office workplaces, the measurement is taken at a height of 0.75 m above the ground; in high-traffic areas, a maximum of 0.1 m. The required maintained illuminance values are specified for indoor workplaces for different room types and activities in the standard DIN EN 12464-1 and for outdoor workplaces in DIN EN 12464-2.

Luminance describes the brightness impression that the human eye has of a luminous or illuminated surface. Luminance (L) is measured in candela per square metre [cd/m2]. Here, the luminous intensity is placed in relation to the illuminated or luminous surface. To assess the quality of road lighting, the calculation of luminance is mandatory. On account of the standardised reflection properties of road surfaces and the definition of the location of the viewer, the calculation of luminance is an integral part of the planning of road lighting. 

The planning aid “Light for Europe’s roads” regulates the illumination of roads, paths and squares in accordance with DIN EN 13201. It clarifies that an increase in luminance of 1 cd/m2 to 2 cd/m2, for example, reduces the accident rate by around one third. 

The different luminous flux levels of lamps, different light distribution by the luminaires or varying geometry in lighting systems influence the illumination of a road. Another important factor here is the reflection property of the road surface. In order to evaluate the exact luminance of the carriageway, a flat part of the carriageway must be selected with uniform reflection behaviour. A representative luminaire spacing with two luminaires and an observation location 1.5 m above the centreline of the carriageway is used.

Recommendation: Residential street: 7.5 lx Main road: 1.5 cd/m2 Car park: 15.0 lx

Light defines our spatial environment. Light and shade give room a structure. Objects appear to be plastic, and surfaces are explained. The interplay between light and shade enables us to determine distances and dimensions. Room situations that are visually easy to understand and grasp give us a feeling of safety. There are now two extremes of illumination that cannot be ignored: diffused light, which hardly develops any shadows, and extremely directed light, with strong shadows.

In diffused light, the room does not appear plastic, it appears monotonous. Objects and dimensions are hard to detect. 

In extremely directed light, individual room elements are strongly emphasised and cast high-contrast, hard shadows. The rest of the room remains unilluminated. Both lighting situations can trigger discomfort and insecurity.

A balanced combination of both lends the room a dimensional appearance and gives objects plasticity. For this reason, many luminaires are developed to combine direct with indirect proportions of light. This leads to a significant increase in their possible applications. 

Directed illumination can be used when it is important to emphasise objects, surface structures or persons. Only directed light makes surface structures visible. To avoid mistakes, fatigue and accidents, it is important to limit glare. This applies particularly to perspectives above the horizontal of the field of vision.

The degree of direct glare caused by the luminaires of a lighting system outdoors is called GR (Glare Rating).

Relationship between GR ratings and glare assessment:

GR value: Glare assessment 80 - 90: Unbearable 60 - 70: Disturbing 40 - 50: Just acceptable 20 - 30: Satisfactory 10: Just noticeable

Glare is caused by light surfaces in the field of vision and can be perceived either as psychological glare or as physiological glare. The glare caused by reflections on reflecting surfaces is generally known as veiling reflection or reflected glare.

Reflected glare and direct glare are caused by bright surfaces in the field of vision and are considered to be disruptive factors.

The term reflected glare is used to describe irritations caused by luminaires or windows with high luminance, for example. This occurs particularly frequently on wet asphalt roads, glossy paper or on display screens.

The occurrence of such disruptive factors can be avoided with the right choice and arrangement of luminaires in buildings or outdoor spaces. In order to calculate reflected glare on horizontal shining surfaces, the CRF (Contrast Rendering Factor) is determined using suitable software. As a rule, an office has a minimum CRF value of 0.7, and when glossy materials are being used, a higher value must be assumed.

Besides the reduction in luminance levels reflected in glossy surfaces, it is also possible to change the arrangement, thus minimising the reflective surface. Depending on the degree of direct or reflected glare, physiological or psychological glare may occur. Physiological glare is accompanied by problems in perception caused by a reduction in visual performance. The recognition of shapes and depth perception are made more difficult.

Psychological glare, on the other hand, cannot be quantified in terms of measurements. Only the subjective sensation of the individual is decisive and opinion-forming. Symptoms may be discomfort, insecurity or fatigue. To ensure that such effects do not occur in the first place, it is good policy to avoid glare, particularly above the horizontal of the field of vision.

The evaluation of such physiological glare is effected using the percentage threshold increment (TI). This value specifies by how many percent the visual threshold, that is, the difference in luminance, increases through glare.

With glare-free road lighting, the eye adapts to the average luminance of the carriageway. An object on the carriageway is still visible when there is a difference in luminance compared with the surroundings.

When a car driver is irritated by glare in his field of vision, this glare causes stray light in his eye, which is like a visual veil on the retina. This so-called “veil luminance” leads to greater adjustment in the eye, while the carriageway luminance remains the same. This makes the object invisible, which can lead to dangerous situations in road traffic.

DIN EN 13201-3 describes the relevant calculation formula for the “threshold increment.” For busy roads, a threshold increment (TI) of max. 10% is recommended; for less busy roads a TI of 15% to 20%.

In indoor illumination, psychological glare is determined by the UGR method (Unified Glare Rating). This is based on a glare formula which takes account of all luminaires in the system that lead to the glare impression. To allow a uniform evaluation to take place, UGR tables provided by the luminaire manufacturers are used.

Sunlight contains all of the colours visible to human beings. With lamps, there are various colour rendering properties. To be able to name these, the value CRI measures colour rendering. The higher this is, the better the colour rendering. An optimum value here is CRI 100, for here all colours are rendered naturally.

Human beings experience their environment not only as light and dark, light and shade, but also through colours.

The colour designation of lamps consists of three figures. The first figure identifies the colour rendering, the CRI range, the second and third figures the colour temperature in Kelvin.

People´s moods can be influenced by warm or cold colours. The colour impression is determined by the interaction between colour and the objects viewed (spectral reflectance).

Warm light colours (up to approx. 2,900 K) have a calming effect and ensure a comfortable living situation. Cool light colours, on the other hand, have a higher blue content (over 3,300 K) and have an invigorating effect. These are used in places where concentration or an objective mood is to be encouraged.

Colour temperature in Kelvin (K): warm white: <3300 K neutral white: 3300 - 5300 K daylight white: > 5300 K

Since the light from lamps with the same light colour may have a completely different spectral composition, it is not possible to draw conclusions from the light colour of a lamp about the quality of its colour rendering. The colour triangle defined by the International Commission on Illumination (CIE) illustrates how light sources and body colours are to be classified.

Achromatic, that is, white when brightness is high and grey or black in the dark, is at x = y = 0.333. The other chromatic colours lie around this achromatic point. All spectral colours of sunlight are on the straight line between achromatic and the limiting curve. Colours with the same hue, which gains in saturation in the direction of the limiting curve, can also be found there. The colour triangle therefore contains all real colours. The Planckian locus describes the colours of the “black body” at the specified temperature values in Kelvin.