Range of Vision – ISO/CIE 23539:2023

It has been said by many, that the human eye can provide a visual sense operating in a similar way to a camera, giving a person information received regarding their surroundings, where when light enters the eye, it passes through the cornea, the pupil, and the lens body, before falling on the retina, which is often referred to as the ‘projection screen’.

The retina is the back part of the eye, containing the cells which respond to light, where these specialised cells are referred to as ‘photoreceptors’, of which collectively, there are up to around 130 million. There are two types of visual photoreceptors, ‘rods’ and ‘cones’, with rods being the most sensitive to changes between light & dark, shape & movement, and containing only one type of light-sensitive pigment, which means they are not good for colour vision. The cones, which are less sensitive to light, but are more sensitive to one of the three primary colours (Red / Green / Blue), and where there is around 7 million cones in the eye.

The vision of all individuals is enabled by three primary modes.

‘Scotopic Vision’ is vision that is basically deemed as being monochromatic in colour, occurring when an individual is within an area of extremely low light conditions and is the vision which functions primarily by using the rods located within the eye.

‘Photopic Vision’ is vision which is gained when an individual is within an area of a high illuminated condition, allowing for the provision of colour perception, and is the vision which functions primarily by using the cones located within the eye.

‘Mesopic Vision’ is vision that is a combination of both Scotopic vision and Photopic vision, when an individual is within an area of reasonably low light conditions and is vision which functions by using a combination of both the rods and cones located within the eye.

The international commission on illumination, also known as the CIE from its French title, the ‘Commission Internationale de l´Eclairage’, has offered a definition of each of the three different modes of vision within the CIE 2^nd edition document of the of the international lighting vocabulary, where the numbers provided at the end of each relate to those listed within the CIE document, S017:2020, as being:

Scotopic Vision is ‘vision by the normal eye in which rods are the principal active photoreceptors’ (17-22-017)

Photopic Vision is ‘vision by the normal eye in which cones are the principal active photoreceptors’ (17-22-016)

Mesopic Vision is ‘vision by the normal eye intermediate between photopic vision and scotopic vision’ (17-22-018)

The new ISO/CIE 23539 of March 2023, on the topic of Photometry relating to the CIE system of physical photometry, defines the purpose of photometry as the measure of light as perceived by the human eye, where the brightness of a luminous surface depends not only on the amount of radiation it emits, transmits, or reflects, but also on its spectral composition, and on the visual response function of the observer viewing it. With human visual response varying at different light levels for different individuals, precise photometry requires the definition of representative standard observers.

The document specifies the characteristics of the system of physical photometry established by the CIE and accepted as the basis for the measurement of light, whilst defining the photometric quantities, units and standards that make up the CIE system of physical photometry, which have been officially accepted by the International Committee for Weights and Measures (CIPM – Comité International des Poids et Mesures), encompassing of the definition of photometric quantities, symbols and units, the definition of CIE spectral luminous efficiency functions for photopic vision, scotopic vision, mesopic vision and 10° photopic vision, the definition of CIE photometric observers that conforms to these functions, and the definition of maximum luminous efficacy for photopic vision, mesopic vision, scotopic vision and 10° photopic vision.

Photopic vision relates to or denotes human vision in daylight or other high ambient light levels, with the term coming from the word ‘photo’, meaning ‘light’ and the word ‘opia’, meaning ‘a condition of sight’, such as during daylight conditions, where vision is mediated by the cone cells. For individuals, photopic vision allows colour perception, which is mediated by the cone cells within the eye and offers a significantly higher visual acuity and temporal resolution than that available with scotopic vision. The human eye uses three types of cone cells to sense light in the three bands of colour. The cone cells have maximum absorption values at wavelengths of around 420 nanometres (blue), 534 nanometres (bluish/green), and 564 nanometres (yellowish/green), and their sensitivity ranges overlap to provide vision throughout the visible spectrum, with the maximum efficacy being at a light frequency of 555.17 nanometres, in green and having a luminous efficiency of 683 Lumens/Watt. The photopic vision regime applies to luminance levels which are greater than 5 cd/m².

Scotopic vision relates to human vision in very low ambient light levels as that during the hours of darkness, where in the study of human visual perception, the term comes from the Greek word ‘skotos’, meaning ‘darkness’ and the word ‘opia’, meaning ‘a condition of sight’. When vision is mediated by rods, which have a much higher sensitivity than the cones, the sense of colour is essentially lost in the scotopic vision regime. At these low lighting levels, such as at night, objects lose their colours and only appear to have different levels of tones of grey.

Within the human eye, scotopic vision is produced exclusively through rod cells, as the cone cells are non-functional in these low ambient light levels, with vision achieved by the increased sensitivity of the photopigment in the rods. The rods signal light increments to rod bipolar cells, which are most sensitive to wavelengths of around 507 nanometres in green/blue, whilst being completely insensitive to wavelengths longer than 640 nanometres of reddish/orange, with Scotopic Vision occurring at approximate luminance levels below 0.005 cd/m².

Mesopic vision is a combination of both photopic vision and scotopic vision within low ambient light levels, but not quite dark lighting situations, and the modern illuminated environment at night means that many of the outdoor applications and street lighting scenes are located within the mesopic range, where the eyes may respond to certain light levels differently. This is because under high light levels typical seen during the daylight hours, photopic vision prevails where the eye uses cones to process light, whereas during the hours of darkness under low light levels, corresponding to that of moonless nights without artificial lighting, scotopic vision prevails where the eye uses rods to process light. For night-time scenes, a combination of both cone cells and rod cells will process light to support vision, where some photopic vision facilitate excellent colour perception, and some scotopic vision offer colours which are barely perceptible. Mesopic vision falls between these two extremes, as do many night-time environments, where there is deemed to be sufficient ambient light to prevent true scotopic vision. Mesopic vision occurs in intermediate lighting conditions between that of the photopic and scotopic vision regimes, with a range of the value of luminance from approximately 0.01 cd/m² through to 3 cd/m², however, this level gives inaccurate visual acuity and colour discrimination.