Technical terms on the subject of human centric lighting

This glossary and abbreviation list is intended to help understanding terms of different disciplines mainly in the context of lighting and non-visual effects of light on humans. There is no claim that the term explanation has definition characteristics nor that the list is fully comprehensive or complete. Some explanations have been simplified.

Accommodation: Adjustment of the eye’s focus to see nearby or distant objects. Bending of the eye’s lenses is changed by contraction or relaxation of the ciliary muscles.

Action spectrum: Spectral sensitivity for an effect triggered by optical radiation, especially by light. The action spectrum for the sensitivity of the eye is the V(λ) curve. Since 2014 the sensitivity of the photopigment melanopsin for excitation by light Smel(λ) is used as action spectrum for non-visual effects of light. It is now assumed that the other biological effects of light essentially follow this action spectrum as well.

Adaption: Adjustment of the eyes sensitivity to the luminances in the visual field. Based on change of pupil diameter and on biochemical processes in the retina.

Adrenal cortex: Gland tissue that produces hormones, e. g. cortisol.

Adrenaline: Hormone with stimulating effect (e. g. on heart and respiration), produced by the adrenal medulla.

Age Dependent Macular Degeneration (AMD): Damage of central parts of the retina (fovea) associated with partly loss of visual function with person age. The causes are unclear, besides a genetic predisposition and behavioral circumstances (e.g. smoking), also constantly high luminances of short wavelength optical radiation are discussed.

Agitation: State of being nervous or excited, e. g. comes along with depression and dementia.

Alzheimer's disease: Most common form of dementia, comes along with a progressive reduction of brain volume.

Antioxidant: Compound which prevents oxidation processes (chemical change due to electron transfer). “protective compound”.

Autonomous nervous system: Nervous system that controls the body functions that were not affected by our will (e. g. heart or breathing rate). Syn. vegetative nervous system.

Biological clock: Internal time control in organisms with genetically fixed schedules. In humans it is located in the suprachiasmatic nucleus of the brain and can be modulated by external influences (esp. phases of light)

“Biological darkness”: Term that stands for a level of light which is enough to see, but is insufficient for significant non-visual effects.

Biorhythm: Unspecific term for a natural rhythm of biological cycles in organisms. The term is often used in esoteric context and should be avoided in scientific use. Better is to use the term “biological rhythm”. Often biological rhythm is used as a synonym for circadian rhythm.

Blue Light Hazard (BLH, Retinal blue light hazard): Irreversible photochemical damage of photoreceptors in the eye by high radiances of short wavelength optical radiation (range from UV - ~500 nm).

Candela (Cd): Unit of -> luminous intensity. 1 cd = 1 lm/sr. Lumen output per unit of solid angle. A candle has typically 1 cd. An incandescent lamp ~ 50 cd. A modern 1W, single chip LED ~ 100 cd. The sun ~ 3*1027 cd.

Cardiovascular: refers to the heart and blood circulation system.

Care home (nursing home): Facility for people who need support in daily life, typically home for the elderly.

Chiasm: (optic chiasm): Crossing of optic nerves in the brain.

Chloropic: Relating to human green cone photopigment absorption. Chloropic lux is an illuminance level weighted to a photopigment with λ(max) 534 nm. Should no longer be used because the term “chloropic lux” is no acceptable SI unit for use.

Chronobiology: Discipline which studies time dependence of biological processes, often studying biological rhythms.

Circadian: Spanning a period of approximately 24 hours.

Circadian rhythm: A biological cycle with a period of approximately 24 hours (Latin: circa = about, dies = day), like the human sleep-wake-cycle. Light is the most important “zeitgeber” for the circadian rhythm in humans.

Circalunar: Spanning a period of approximately one month.

Circannual: Spanning a period of approximately one year.

Circaseptal: Spanning a period of approximately one week.

Cognitive: Refers to different forms of mental capabilities.

Color rendering: Ability of a light source, to enable the human eye to perceive colors of objects correctly, as it would in natural daylight. The color rendering index (CRI) is a measure of the correspondence between the color of an object (its “self-luminous color”) and its appearance under a reference light source. A light source with a CRI value of 100 displays all colors exactly as they appear under the reference light source. The lower the CRI value, the worse the colors are rendered. For color temperatures below 5000 K the reference light source is defined as black body radiator, while for higher color temperatures the matching daylight phase is used as reference.

Color temperature: Unit: Kelvin [K]. Color of a light source, corresponding to the color of a black body at that temperature. Color Temperatur is usually only given for lamps that are more or less “white”, that means with color coordinates close to the black body curve.

Cones: Light sensitive cells in the eye. Blue-, green- or red sensitive. S-cone, short wavelength, blue. M-cone, medium wavelength, green. L-cone, long wavelength, red. See more at -> photoreceptors.

Cortisol (hydrocortisone): "Stress hormone". Hormone stimulating different functions of the body.

Cyanopic: Relating to human blue cone photopigment absorption. Cyanopic irradiance is weighted to a photopigment with λmax=420 nm with pre-receptoral filtering based on a 32 year old standard observer. No longer to be used; instead: S-cone.

Dawn simulation: Mimicking the slow increase in natural light levels before and during sunrise with artificial lighting. Used e. g. in therapy of depressions.

Daylight system: Construction to enable daylight to enter deeper into a building than through windows or openings in the stonework.

Daylight zone: Part of interior rooms which is virtually always illuminated sufficiently by daylight.

Dementia: Pathological decrease of the mental capacity, more common in the elderly. Reasons are degeneration processes in the brain (Alzheimer dementia) or insufficient blood circulation (vascular dementia).

Depression: Massive feeling of gloom as pathological condition, therapy is needed.

Disability glare: Glare with impairment of visual perception, may not be experienced as uncomfortable.

Discomfort glare: Glare which is perceived as uncomfortable, does not necessarily impede visual perception.

Dusk simulation: mimicking of slow decrease of natural light levels before and during sunset with artificial lighting. Used e. g. in therapy of depressions.

Electroencephalogram (EEG): Recording of the electrical activity of the cerebral cortex of the brain.

Endocrine gland: Gland which secretes its product (e. g. a hormone) into the bloodstream.

Endogenous: Originating within an organism, not by an external cause.

Epiphysis (pineal gland, pineal organ, med. Corpus pineale): endocrine gland in the brain, between the cerebrum and the cerebellum. The epiphysis produces the hormone melatonin, which is secreted in the dark into the circulation system.

Erythropic: Relating to human red cone photopigment absorption. Erythropic irradiance is weighted to a photopigment with λ(max) =564 nm with pre-receptoral filtering based on a 32 year old standard observer. No longer to be used; instead: L-cone.

Field of view: This is the extent of the observable world that lies before our eyes and can be formed on the retina without moving our eyes. The field of view of an eye is limited by the size and optical properties of the components of the eye (cornea, lens, iris, vitreous body) and the population of the retina with visual cells (cones and rods).

Free-running period: eEndogenous rhythm of the biological clock without external synchronization. The period length of human circadian rhythm is individually genetically fixed and lasts approximately 24 hours.

FWHM: Full-width half maximum. The wavelength range of the spectral power distribution for a narrowband light source, based on the width of the SPD at half maximum.

Ganglion cell: Nerve cell in a ganglion which processes and conducts signals. With regard to the biological effect of light the retinal ganglion cells are of particular importance. Besides their function in processing information from the visual receptors – the cones and the rods - around 2 to 3 % of these ganglion cells are themselves sensitive to blue light. They act as “third photoreceptors”.

Geniculohypothalamic tract (GHT): Nerve tract between the suprachiasmatic nucleus and the intergeniculate leaflet, connects the optic nerve to the NIF tract.

Harvard protocol: Mathematical model to determine the phase shift of the biological clock on the basis of time, duration and light intensity.

Hormone cycle: Regular rhythmical change in hormone production and release. In most cases this is a circadian or circalunar cycle.

Human Centric Lighting: Lighting concept which sets human needs in the center of lighting design. Not only biological effects of light are respected, but also requirements on good vision as well as the emotional effects of light.

Hypophysis (pituitary gland): Endocrine gland in the brain which produces a number of important hormones, including ACTH which stimulates the adrenal cortex to produce cortisol.

Hypothalamus: Centre at the base of the brain, which contains the suprachiasmatic nucleus (SCN).

Illuminance: E - Unit: lux [lx]. Illuminance E is the ratio between the luminous flux and the area being illuminated. An illuminance of 1 lx occurs when a luminous flux of 1 lm is evenly distributed over an area of 1 m².

Infradian: Spanning a period longer than 24 hours.

Internal clock: Also known as the biological or physiological clock. The internal clock is the central element of the timing system. Its progress is genetically predetermined but is essentially controlled by the SCN and therefore influenced significantly by light. Light is consequently the most important “zeitgeber” for the internal clock; it synchronizes the internal clock with the external day. The status of the internal clock, in other words the phase of the biological clock, is the sum of overlaying all the body’s circadian rhythms. The phase can be deduced from the curve showing the melatonin level.

Karolinska Sleepiness Scale (KSS): Established scale to measure sleepiness, graded in 9 steps.

LED: (light emitting diode): Semiconductor which emits light. The semiconductor emits almost monochromatic light with a narrow bandwidth of ~5-50 nm. LEDs with a broader spectrum (e.g. white LEDs) use phosphors which convert a part of this light to longer wavelengths – comparable to fluorescent lamps.

Leucoderma: Missing pigmentation of skin due to previous inflammatory skin responses, associated with increased reaction to optical radiation (important for therapy and protection).

Light color: The light color of a lamp can be defined in terms of color temperature. Despite having the same light color, lamps may have very different color rendering properties depending on the spectral composition of their light.

Light therapy, bright light therapy (BLT): Exposure of patients to high-intensity light to treat certain disease – especially depressions. Only effective through the eyes. Around 5-10000 lux are used for illumination.

Lipofuscine: Is a product of lipometabolism which accumulates in the retinal area especially with elderly people. L. can be cracked by blue light and is then building products that can damage the photoreceptors in the retina. L. is seen as a potential catalyzer for AMD.

Lumen (lm): Unit of -> luminous flux

Luminance L: Unit of measurement: candela per square meter [cd/m²]. The luminance L of a light source or an illuminated area is a measure of the impression of brightness.

Luminous efficacy η: Unit of measurement: lumens per watt (lm/W). Luminous efficacy η indicates the efficiency of the conversion of electrical power into visible light.

Luminous flux F: Unit: lumen [lm]. Luminous flux F is all the radiated power emitted by a light source evaluated with the spectral sensitivity of the eye and the photometric radiation equivalent km = 683 lm/W.

Luminous intensity I: Unit: candela [cd]. Generally speaking, a light source emits its luminous flux F in different directions and at different intensities. Luminous intensity is the luminous flux radiated in a particular direction (solid angle Ω).

Lupus (L. vulgaris, L. erythematodes): chronic ulcerative skin diseases, associated with increased sensitivity to optical radiation (important for therapy and protection).

Lux (lx): Unit of -> illuminance. 1 lx = 1lm/m2.

Maintained Illuminance: Minimum value of illuminace between routine maintenance cycles.

Melanopic: Relating to human melanopsin photopigment absorption. Melanopic irradiance is weighted to a photopigment with λmax 480 nm with pre-receptoral filtering based on a 32 year old standard observer.

Melanopsin: Photopigment which is responsible for the light sensitivity of the retinal ganglion cells. Its light sensitivity is defined as the action spectrum non image forming effects of light with a maximum at around 485 nm.

Melatonin (suppression): Melatonin is the most important hormone of the pineal gland and can be described as the body’s signal for the nocturnal dark phase. It is often called “sleeping hormone” as it promotes sleep in humans (but activity in nocturnal animals). Melatonin is produced and stored continuously in the pineal gland from serotonin, and is released only in the dark. Light exposure at night causes secretion of melatonin to be suppressed. If no melatonin is released the level of melatonin in the bloodstream falls as a result of decomposition processes. As melatonin is the most important measurable marker for the circadian phase of the internal clock the melatonin-suppressing effect is generally equivalent to the circadian effect, i.e. the biological effect of light on the internal clock.

Metabolism: Sum of chemical processes in organisms.

Mesopic vision: Vision in the luminance region between photopic and scotopic vision. Is relevant during dusk or dawn as well as for street lighting, automotive headlamps and other low illuminance levels.

Minimal Erythema Dose (MED): 1 MED is the minimal dose of UV-B radiation, that will produce noticeable effects of skin reddening in people of different skin phototypes. MED is depending on skin phototype. For skin type I, 1 MED is around 150-300J/m2, for skin type IV around 450-600 J/m2.

Monochromatic light: light of a single wavelength (ideal) or with a very narrow spectral bandwidth. Produces an impression of strongly saturated color in humans.

Neuroendocrine system: Information transfer system of the body, composed of nervous and hormone system.

Non Image Forming cells (NIF cells): Intrinsic photosensitive retinal ganglion cells in the eye, which mediate the external brightness, but not a picture of the environment to the central nervous system.

Nursing home: see --> care home

Ocular light: Light which is received by the eye and reaches the retina.

Optic nerve: Nerve tract between the eye and the brain used for visual perception.

Paraventricular nucleus (PVN): Group of nerve cells in the brain that gets input from the SCN and controls epiphysis and hypophysis.

Photobiological effects: Effects of (non-ionizing) optical radiation on living tissue. Visual effects are usually not denominated as photobiological effects.

Photochemotherapy (PUVA therapy): Treatment with combined medication and radiation, the medication increases the sensitivity of the skin (or the radiation can convert the chemical substance into an active form).

Photopic vision: Vision mainly with help of the cones in the eye. Therefore, average luminance of 3.5 cd/m2 and higher is needed. With inferior luminance, mesopic vision is relevant, followed by scotopic vision.

Photoreceptors: These are light-sensitive sensory cells that convert quantums of light (photons) into the electrical signals for the nervous system. The photoreceptors in the retina of the human eye are called cones, rods and melanopsin-containing ganglion cells. The cones are responsible for vision of colors and brightness. They are at their highest density in the center of the eye, known as the macula lutea. This is the area of the retina providing the highest spatial resolution. The rods are responsible for vision at low light levels and have a higher sensitivity than cones. They are therefore important for night vision. They are mainly found at the periphery of the field of view. The melanopsin-containing ganglion cells are not considered to be visual cells as they do not transmit images to the central nervous system but merely provide brightness information for controlling the internal clock.

Phototherapy: Treatment of certain diseases with optical radiation, including UV- or IR-radiation. Usually the skin is treated ( e. g. treatment of psoriasis). The term is usually not used for light therapy.

Pineal gland: see --> Epiphysis.

Pituitary gland: see --> Hypophysis.

Post-lunch dip: Period of tiredness and reduced concentration shortly after lunch.

Profile Of Mood State (POMS): Established method for psychological assessment of mood. 5 point scale to measure six different aspects: Stress - discomfort - anger - willpower - fatigue - confusion. Can be used in lighting studies to assess effects of light on well being.

Psoralene: Drug to sensitize the skin against UV radiation, used in photochemotherapy.

Psoriasis: Chronic skin disease leading to patches with silvery scales over inflammatory red spots, not curable but treatment with ultraviolet radiation or PUVA therapy often alleviates symptoms.

Psychosis: Severe mental disorder with different characteristics, amongst them e. g. schizophrenia or depressions.

Pupil response: Reflex to change the diameter of the pupil due to the amount of light reaching the retina.

Purkinje effect: Shift of the spectral sensitivity of the eye during the transition from photopic to scotopic vision or vice versa.

PUVA therapy: see -> Photochemotherapy".

Quantum energy: Energy of a radiation particle, depending on the wavelength of the radiation.

Retina: Layer of cells on the back wall of the eye that contains the visual cells and the information processing nerve cells, including ganglion cells.

Retinal Blue Light Hazard: see -> Blue Light Hazard.

Retinal Thermal Hazard: Irreversible damage of photoreceptors in the eye due to high radiances of light or infrared radiation (400 – 1400 nm).

Retinohypothalamic tract (RHT): Nerve tract between the eyes and the SCN in the brain.

Rhodopic: Relating to rod photopigment absorption. Maximum sensitivity is at λ(max) 498 nm.

Scotopic vision: Vision in very dark surroundings, when only the rods are still active. The average luminances have to be below 0.035 cd/m2. Color vision is not possible under this condition. (All colors will agree in the dark).

Seasonal affective disorder (SAD, winter depression): Many people suffer mood changes during the winter months which may even be serious enough to qualify as clinical depression. Triggering factors for SAD are generally given as short daylight hours and low light levels during the “dark season”. Other (endogenous) factors must also be considered as the causes of winter depressions. Phototherapy with very bright light helps in many cases. A Summer depression is also known but infrequent.

Serotonin: Messenger molecule that transmits information between nerve cells (neurotransmitter). The molecule melatonin is built from serotonin. S. can only be built in the body – mainly in the brain. Bright light supports the building of Serotonin.

SPD: Spectral Power Distribution of a light source as a function of wavelength. Measurement of the SPD is required for assessment of light with respect to effects which are dependent on wavelength.

Stroboscopic effect: Illusion in flickering light where regularly reappearing occurrences were lightened always in exact or nearly the same phase and therefore seem to stand still or move slower than in reality. A temporal light artefact, occurring up to flicker frequencies of max. 2000 Hz.

Sub-SAD (Winter blues): Seasonally elicit mood deterioration which is not as severe as a seasonal affective disorder (SAD).

Suprachiasmatic nucleus (SCN): Collection of several thousand nerve cells situated above (supra) the intersection of visual nerves (chiasma opticum). Now considered as the main regulator of the internal clock. Each of the nerve cells in the SCN has its own internal clock. Normally the internal clocks of these cells are in synchronism and therefore amplify the “zeitgeber” signal to form a common strong control signal for the internal clock.

Task area: In context of lighting: part of the work place where the visual task is carried out.

Third photoreceptor (intrinsic photosensitive retinal ganglion cells, ipRGC): sSensory cells in the human eye that transmit information about the brightness of the surroundings to the central nervous system. Other photoreceptors are the rods and cones, which were responsible for image perception. ipRGC contain the pigment melatonin with a sensitivity in the blue spectrum and transmit lighting information to the SCN and pineal gland. By this means, they affect the inner clock and the production and secretion of the hormone melatonin. The light sensitivity of the retinal ganglion cells is approximately factor 100 less than that of the cones responsible for color vision.

Ultradian: Spanning a period of less than 24 hours.

Unified Glare Rating (UGR): Standardized Method for quantitative assessment of discomfort glare in a room from a luminaire. UGR < 19 is regarded as good for office work.

Veiling luminance: Light which impairs vision due to scattering of light in the dioptric apparatus of the eye (cornea, lens, vitreous body). Increasing problem with person age.

Visual acuity (Visus): The ability to distinguish closely adjacent points. Typically measured with a chart with Landolt rings (rings with a gap) of decreasing size.

Visual cortex: To the rear (occipital) part of the head oriented brain region which processes visual information.

Vitamin D: Vitamin D3 oder 25(OH)D3 is regarded as hormone. Fat-soluble molecule which is taken up by nutrition and can be produced in the body with help of UV-B radiation. It regulates among others concentration of calcium in the blood. A lack of vitamin D can lead to osteoporosis and rachitis (rickets). If direct exposure to sunlight is too low, Vitamin D can be supplemented orally (min. 1000 I.E. = 25 µg per day). By exposure of the whole body to solar radiation with a minimal erythema dose (MED), about 10 times this minimal dose is produced. Vitamin D is supposed to have also positive effects against carcinogenesis.

Vitiligo: Skin disorder with unpigmented areas. Often treated with ultraviolet radiation or photochemotherapy.

Vitreous body: Gelatinous eyeball as part of the dioptric apparatus of the eye between the lens and the retina.

Winter blues: see --> sub-SAD.

Winter depression: see --> Seasonal Affective Disorder.

Zeitgeber: “Zeitgeber” is the German word for timer. It is used to denote any factor that may set the internal clock. The most important zeitgeber is light, which acts on the SCN through the eye via the retinohypothalamic tract. In the SCN is the central internal clock that controls a large number of circadian rhythms. Other zeitgebers are in the liver for example and these too control circadian rhythms.