What is light?

Light is radiant energy with wavelengths between λ = 380 and 780 nm  that is capable of exciting the human retina and creating a visual sensation. Other parts of the electromagnetic radiation, like the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths), are not visible to the human eye. The definition of light by the International Lighting Vocabulary of the Commission Internationale d’Eclairage (CIE) is ‘any radiation capable of causing a visual sensation directly’ (CIE, 1987).


Light is essential for human vision and health. For only a bit more than two decades it is known that in humans, there are two modes of ocular photoreception: image forming and non-image forming (NIF) photoreception. The image forming, or visual, photoreception enables humans to see; the non-image forming photoreception affects the circadian rhythm and also stimulates parts of the brain directly, influencing aspects such as cognitive functions or the operating capacity.

When humans receive ocular light it is processed by the human brain via two pathways. The three types of photoreceptor cells in human eyes - rods, cones, and photosensitive retinal ganglion cells (pRGCs) - all send their information directly to the Lateral Geniculate Nuclei (LGN). The LGN combine the information signals received from the left and right semi-fields of view captured by each of the two eyes. It activates vision and visual perception, which means the experience and interaction with the surrounding visual environment. Additionally, the pRGCs, interacting with the cone receptors, send signals to the Suprachiasmatic Nuclei (SCN) which is responsible for generating circadian (~ 24h), rhythms.

Image forming effects

Human ocular photosensitive sensors (rods and cones) enable humans to perceive images varying with different light intensities and colors. The image forming sensors are not equally sensitive to different wavelengths, and therefore the human relative sensitivity curve V(λ), see Figure 2, is used to translate light energy into light quantities ‘luminous flux’, ‘luminous intensity’, ‘illuminance’, or ‘luminance’. For the photopic vision - during daytime when the cones are sensitive - the peak value is around 555 nm (yellowish part of the light spectrum). This means that compared to the other colors of the spectrum, the least energy of that wavelength is necessary to create the same stimulation.

Non-image forming effects

The non-image forming (NIF) sensors are not equally sensitive to different wavelengths, and therefore a different human relative sensitivity curve “C(λ)” represents the response to light energy by the pRGCs  , see Figure 2. This curve is defined after the initial research of Brainard et al., 2001 and Thapan et al., 2001, who independently demonstrated that monochromatic light with a wavelength of around 480 nm - the bluish part of the light spectrum - optimally suppresses the hormone melatonin. The NIF- effects are closely related to health-related effects like hormone secretion and consequently indirectly to all circadian related acute aspects. However, not only the pRGCs photoreceptors and its opsin melanopsin is involved in regulating the non-image forming effects.  The process is much more complex, potentially involving all five (human) photoreceptors.

Since a distinction must be made between light for vision and light for NIF effects, international organizations like CIE and multiple researchers (e.g., Lucas et al., 2014) are striving for additional globally accepted nomenclature and potentially accompanying action spectra. Until then, for the visual effects of light the photometric quantities and units as defined by SI and established in the field of lighting technology can only be used. For the non-image forming effects, only the use of basic non-weighted quantities (i.e., a-opic irradiance values) are recommended.

Use of quantities and units

Photometric quantities describing image forming lighting conditions

Photometry is the science of quantification and qualification of light sources and lighting conditions in terms of their perceived brightness related to human vision. Four basic photometric quantities play an important role in lighting technology, and a graphical representation in shown in Figure 3.

Luminous flux

One of the most important basic quantities is the luminous flux Φ. It can be determined by weighting the spectral distribution of the radiant flux Φ with the eye sensitivity curve V(λ) and the maximum of the luminous efficacy of radiation Km (683 lumen/W). The luminous flux is the measure of the perceived power of light and its unit is the lumen (lm).

Luminous intensity

In photometry, the luminous intensity I is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. The unit of luminous intensity is the candela (cd), an SI base unit.


One of the most used photometric quantities is illuminance. Illuminance E is the quantity of light, or luminous flux (Φ), falling on a unit area of a surface (A), and the unit is lux (lx). One lux equals one lumen per square meter (lm/m² = lx). The illuminance is independent of the direction from which the luminous flux reaches the surface.


Luminance L is the luminous intensity (I) emitted per unit of area of a surface (A) in a specific direction. The surface can reflect the light or emit/transmit light itself (like a lamp). The unit of luminance is candela per square meter (cd/m2).

Photometric quantities describing non-image forming lighting conditions

Not long after publication in 2001 of new action spectrum data for melatonin suppression, a discussion started related to the consensus and uncertainty with regard to circadian and neurophysiological photometry. The use of new non-SI units and normalized spectral sensitivity functions  is in conflict  with the guidelines produced by the International Committee on Weights and Measures (CIPM) regarding the International System of Units (SI), and therefore not recommended by Commission Internationale d’Eclairage (CIE, 2015). As a result of an expert work shop held in 2013, a technical note  - CIE TN 003:2015 - was published by CIE containing scientific information supporting the discussion as well as pre-receptoral transmittance and action spectra data accompanying  the proposed representation for  circadian and neurophysiological responses to light (CIE, 2015).

The a-opic  irradiances

The three types of photoreceptor cells in human  (and all vertebras) eyes - rods, cones, and photosensitive retinal ganglion cells (pRGCs) – are sensitive in the wavelengths of visible radiation (light) Each photoreceptive mechanism absorbs light according to its own spectral sensitivity. Five independent representations of irradiance are produced by a photopigment with its own spectral sensitivity profile: rhodopsin for the  rod photoreceptor,  photopsin for the cones (short, medium, and long wavelength), and melanopsin for the ipRGCs photoreceptor.

The CIE recommends to used quantities of the form ‘a-opic spectrally-weighted flux’, with corresponding units based on the watt (W).  An a-opic  irradiance can be determined by convolving the spectral irradiance for each wavelength with the related action spectrum.  The s-cone photoreceptors (peak at 420 nm) generate cyanopic irradiance, the m-cone photoreceptors (peak 530 nm) chloropic irradiance, the l-cones photoreceptors (peak 560 nm) erythropic irradiance, pRGC  photoreceptors  melanopic irradiance, and the rod photoreceptors  rhodopic irradiance. For all irradiance values the corresponding units are in W/m². 

It is not possible to provide a single action spectrum that describes all circadian and neurophysiological responses to light. The balance between the five spectral components may depend or altered by the amount and duration of light, the prior light history, or an individual’s internalized time of day (CIE, 2015).

Light sources


Daylight is the solar radiation, visible to the human eye, emitted by the sun and perceived during daytime. The duration of daytime depends on the location on Earth and the time of year, see Figure 4. Since daylight cannot be artificially replicated, it is often referred to as natural light. Daylight offers continuous variation which is a possible explanation why people prefer daylit spaces. Daylight is characterized by:

  • the high variation in illuminance with exterior horizontal illuminances ranging from 0 to over 120.000 lx,
  • the variation of its always full invisible and visible (380 – 780 nm) spectrum over the day, and
  • the clearly present variation in directionality, especially on days with sunlight.

Electric lighting

An electric light is a device that produces visible light by the flow of electric current. It is the most common form of artificial lighting and is essential to modern society. Electric lighting systems can be categorized based on the way they emit (visible) radiation:

  • Thermal radiators produce light by a filament heated white-hot by an electric current. The most well-known lamps of this type are the incandescent lamps or halogen lamps.
  • Gas-discharge lamps produce light by means of an electric arc through a gas. The most well-known lamp of this type is the (compact) fluorescent lamp, and
  • Light Emitting Diodes (LEDs) produce light by a flow of electrons across a band gap in a semiconductor.
  • Electroluminescence based lamps, or Light Emitting Diodes (LEDs), release energy in the form of photons when electrons are able to recombine with holes across a band gap in a semiconductor. The color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

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