Technical Terms on Lighting and Light

Accommodation
The eye’s ability to focus on an object at any distance between approximately 6 cm and infinity, allowing the object to be seen clearly.

Adaptation
The process by which the visual functions of the eye adjust to the brightness (luminance levels) within the field of view. The course and duration of adaptation depend on the luminance levels at the start and end of the brightness change. The current state of adaptation determines visual performance.

Ambient light
The perceived light colour in a space is not solely defined by the primary light from luminaires, but also by the reflective properties (e.g. reflectance level, gloss, spectral remission) of walls, ceilings, floors, and furniture. The mixture of direct and reflected light creates the ambient light, which significantly affects well-being. A comfortable space may feel unpleasant if lighting and surface materials are poorly coordinated.

Beam angle
The angle that defines the distribution of luminous intensity in a specific direction. In lighting systems, the beam angle of a luminaire is measured from the vertical axis.

Biologically effective light
The effects of light on humans extend beyond vision. Light regulates the body’s internal clock and influences, for example, the release of the hormone melatonin.

Brightness perception
The perceived intensity of brightness from a visual object. Depending on viewing conditions, brightness perception corresponds to the photometric quantity of luminance. It depends on the luminous flux reaching the retina (retinal illuminance) and is significantly influenced by adaptation and the current state of adaptation.

Colour
A predominantly qualitative sensory impression triggered by physical light stimuli. Colour can be clearly described using three independent parameters: hue, saturation, and brightness.

Colour rendering of lamps
The effect a type of light has on the appearance of object colours compared to how the same objects appear under a reference light source (either consciously or subconsciously).

Colour temperature
A measure used to describe the colour appearance of a light source. When a blackbody radiator is heated, it emits light that changes colour with temperature. If the light colour of a source matches that of the blackbody at a specific temperature, the source is assigned that correlated colour temperature. It is measured in Kelvin [K]. A typical halogen incandescent lamp has a colour temperature of approx. 2,900 K, while daylight ranges from 5,000 to 10,000 K.

Daylight factor D
A value that indicates the relationship between indoor and outdoor illuminance under overcast sky conditions. The daylight factor D is defined as the ratio of the illuminance at a specific point indoors E_p to the illuminance outdoors E_a, both measured under an unobstructed overcast sky. It is expressed as a percentage:
D = (Ep / Ea) × 100%

Dynamic lighting
Often used in the context of biologically effective light. It generally refers to lighting conditions that follow the rhythm of natural daylight. More broadly, it includes any form of lighting in which characteristics and effects (e.g. illuminance and/or colour temperature) can change or be adjusted.

Field luminance
The average luminance within the visual field. It significantly influences both the size of the field of vision and visual performance.

Field of gaze
The total area that can be viewed by eye movements alone, without head movement.

Field of vision
The total area perceived by the eye when it remains fixed. Because cone and rod cells in the peripheral retina have lower resolution and sensitivity, perception decreases from the centre outward. The visibility of objects in the periphery depends on their luminance, size, and contrast with their surroundings.

General lighting
A lighting system that illuminates a space without regard to the specific requirements of individual areas. Additional directional lighting counteracts monotony and supports specific visual tasks.

Glare
A reduction in visual performance and/or discomfort caused by significant differences in luminance within the visual field or by luminance levels that are too high for the current state of adaptation.

Glare angle
An angle with its vertex at the eye, formed by the lines connecting the eye to the visual task and to the glare source.

Glare, direct
Glare caused by actual light sources within the glare zone. The risk of glare increases with lower average luminance in the visual field, larger glare source area, and proximity of the glare source to the line of sight.

Glare, indirect
Glare resulting from reflections of light sources or diffuse reflective surfaces within the visual field. This can occur as reflected glare from windows and other glass surfaces (e.g. monitors), water surfaces, wet roads, coated paper, shiny machine parts, etc. 

Glare, physiological
The effect of a light source that measurably reduces visual performance, e.g. sharp vision, contrast perception, or colour vision.

Glare, psychological
The effect of a light source that causes discomfort within the field of view and leads to visual unease (also known as discomfort glare).

Glare, reflected
See Glare, indirect.

Gloss
A visual sensation caused by reflection highlights on object surfaces under directional lighting at certain angles. The physiological and psychological aspects of gloss are complex and play an important role in the subjective assessment of lighting conditions.

Human Centric Light (HCL)
A term describing lighting systems in which illuminance and colour temperature change throughout the day (see also Dynamic lighting).

Illuminance
A photometric quantity that describes the density of light falling on a surface. It is measured in lux [lx]. Illuminance is the luminous flux (Φ) per unit area (E = Φ / A).

LED controller
To operate one or more LEDs efficiently and with variable brightness, complex electronics are required. LEDs do not require a constant voltage but a constant current. Therefore, brightness (dimming) is achieved by controlling the current.

Lamp
No light without a lamp: a lamp refers to the technical component of an artificial light source—for example, an incandescent lamp, energy-saving lamp, or compact fluorescent lamp. The lamp is installed in a luminaire, which distributes and directs the light and provides glare protection.

Lamp luminous flux
The total luminous flux emitted by a lamp.

Light
Light refers to the visible radiation detectable by the human eye. It represents only a small portion of the electromagnetic spectrum, which also includes gamma rays, X‑rays, infrared, ultraviolet radiation, radio waves, etc. The visible spectrum ranges from 380 to 780 nanometres in wavelength and includes colours from violet through blue, green, yellow to red. Radiation outside this range is invisible to the human eye.

Light colour
The colour appearance of a lamp, typically indicated by its colour temperature (for thermal radiators) or correlated colour temperature (for discharge lamps), measured in Kelvin [K]: warm white ≈ 3,000 K, neutral white ≈ 4,500 K, cool white > 5,000 K.

Light stimulus
Light entering the eye that causes either a noticeable or unnoticeable change in the state of sensory cells (cones, rods). The eye responds to stimuli within a specific intensity range—the lower limit is the threshold, the upper the maximum stimulus.

Lighting, diffuse
A type of lighting where the illumination on the working plane or an object comes from no preferred direction.

Lighting, direct
A type of lighting where luminaires emit over 80% of their luminous flux directly onto the working surface(s).

Lighting, directional
A lighting type in which illumination of the working plane or an object comes predominantly from a specific direction. 

Lighting, indirect
A type of lighting that is primarily generated through the reflection of light from material surfaces (e.g. ceilings, walls), acting as secondary light.

Lighting, secondary
A lighting method where the light source inside the luminaire is not directly visible. Instead, the luminous flux is directed via a reflector system.

Luminaire
The complete lighting fixture, including all necessary components for mounting, operating, and protecting the lamp. A luminaire protects the lamp, directs its light, and provides glare control.

Luminance L

A photometric quantity that describes the brightness of luminous or illuminated surfaces. The brightness impression perceived by the visual system depends solely on the object’s luminance. It is measured in candela per square metre [cd/m²]. Luminance is calculated using the formula:

L = E × ρ / π,
where E is illuminance and ρ is reflectance.

Measurement plane
The plane in which lighting is intended to be effective and where illuminance and luminance are to be measured.

Minimum illuminance
The minimum illuminance level deemed ergonomically acceptable at workstations or in regularly occupied rooms, as defined by norms or standards.

Primary light
Primary light refers to the light emitted directly from a radiant source (e.g. an LED). Only when this light strikes a surface is it reflected in a modified form. The spectral composition of the reflected light defines the secondary light, which enters the eye and determines how a surface or object appears. Since the spectral components of secondary light depend on the primary light, the same object can appear differently under different lighting types.

Rated illuminance
The local average illuminance level specified in standards or regulations for a room, zone, or workstation depending on the visual task.

Reference plane
A plane used to determine measurement values at a workplace.

Reflectance
Reflectance describes the extent to which a material is capable of reflecting incident light or radiation—that is, its ability to return incoming light. A distinction is made between photometric, radiometric and spectral reflectance. Reflectance is expressed either as a dimensionless ratio or as a percentage. Light-coloured surfaces have a high reflectance, while dark surfaces reflect very little (e.g. white wall: approx. 80%; black velvet: approx. 3%).

Room utilisation factor
The proportion of the luminous flux emitted by a luminaire that effectively reaches a specified target area within a space.

Screen luminance
The average luminance produced by data display devices depending on the display type (positive or negative contrast). To avoid screen reflections from lamps, luminaires or bright surfaces, the screen luminance should be adapted to the ambient luminance—and vice versa.

Secondary light
See primary light.

Service life
The lifetime of a light source is defined in various ways. In general, it refers to the number of hours a lamp operates before its luminous flux falls to 70% of its initial value.

Solid angle
A unit describing the angular extent of a surface, defined as the ratio of an area on a sphere to the square of the sphere’s radius.

Spatial ambience
The application of principles from active visual perception, linking luminance (via a theoretical luminance model), lighting type and surface design (including ecological optics) to describe the visually perceived atmosphere of a space—especially in the context of workplaces.

Spectral luminous efficiency curve
The relative sensitivity of the three cone types responsible for colour vision in the human eye under monochromatic light within the visible spectrum. Depending on wavelength, monochromatic light produces different brightness sensations. For example, a stimulus at 555 nm (green-yellow) is perceived as significantly brighter than one at 400 nm (blue) or 700 nm (red), even at the same radiant power.

Spectral sensitivity
A measure of how specific wavelength ranges of light trigger a particular light-related effect. The spectral sensitivity of the human eye is illustrated by the V(λ) curve (for both photopic and scotopic vision).

Stable perceptual state
See Theoretical luminance model.

Surround luminance
The average luminance in candela per square metre [cd/m²] of all objects within the visual surround (see also Theoretical luminance model).

Theoretical luminance model
Describes the relationship between task-area and surrounding luminance levels with regard to the stability of visual perception. The core idea is maintaining a stable state of adaptation luminance, ensuring the eye’s brightness adaptation is fully stabilised. This enables maximum contrast sensitivity and optimal visual performance with minimal information loss and fatigue. The model can be used to determine permissible luminance distributions, reflectance ranges, and luminous flux levels.

Visual fatigue
A decline in alertness caused by prolonged exposure to visual stimuli. Unfavourable lighting conditions reduce the excitability of the eye, increasing the threshold for signal recognition. The onset of visual fatigue is slower when light stimuli deviate less from the eye’s state of adaptation.

Visual perception
Visual perception is the reception and processing of visual stimuli. It involves extracting relevant information, recognising elements and interpreting them based on memory. Thus, visual perception goes far beyond the mere reception of optical input.

Visual performance
Describes the functional capability of the human visual system. According to Schober, the human visual process—comprising the eye, neural processing and relevant brain areas—can be characterised by four simple and four complex tasks. These include brightness perception, colour perception, spatial awareness, motion detection, the perception of luminance differences, colour comparison, visual acuity (the resolution of light stimuli), and the recognition of shapes and objects.

Visual surround, primary
The immediate area surrounding the visual task (task area) within the field of view. The primary surround extends beyond what can be seen using eye movement alone, requiring slight head movements to view specific areas. It still belongs to the direct work area—for instance, a desk where materials are spread out. The angular extent of the primary surround is approximately 90°. Binocular vision is still possible in this range, which is important for depth perception. Also referred to as the narrow ergonomic field.

Visual surround, secondary
The peripheral environment in the traditional sense. In a workroom, the secondary surround is the maximum visible section of the overall visual setting, limited by the field of view from a fixed head position (approx. 190° viewing angle). It includes both binocular and monocular fields of view. When head or body movements are added, the secondary surround encompasses all spatial elements—ceiling, floor, walls, furnishings—each contributing differently to the room’s visual character (appearance, visual atmosphere). The secondary visual surround is crucial for ergonomic design and can be categorised as the extended ergonomic field. It is primarily shaped by materials that affect the overall visual impression of the space through their light-modulating properties.

Visual task
The visual task comprises all visually relevant elements required to complete an activity.

Working plane
The horizontal or vertical surface on which the visual task is performed. It is also used as the reference plane for measuring illuminance, uniformity, etc.

Workplace task lighting
A lighting system that provides additional illumination to the primary working area of a workstation, complementing general lighting and meeting the visual demands of the task. It is typically part of a combined lighting system.