Mesopic vision and public lighting – A literature review and a face recognition experiment
report
Purpose : The Ministry of Infrastructure and Environment (I & M), has followed the advice of the - by her in 2007 established - Lighting Taskforce, and started an investigation to determine to what extent energy can be saved in public lighting by using light sources that are optimized for mesopic vision. The trigger for this investigation is a recent CIE publication on mesopic vision and the emergence of led lighting, whose light spectra are relatively easy to optimize.
The light spectrum optimized for mesopic vision contains a relatively high amount of bluish light (high S/P-ratio) and is therefore effective for peripheral visual tasks at mesopic light levels. Since the current scientific knowledge concerning mesopic vision is insufficient for changing standards for public lighting, we investigated a number of issues concerning mesopic vision and public lighting are investigated. We performed a literature review on light adaptation, effect of age, atmospheric scatter and face recognition and a face recognition experiment at various light spectra.
Methods : In the literature review we focused on the modelling of adaptation luminance, effect of observers age on S/P-ratio, atmospheric scatter and face recognition for light sources optimized for mesopic vision.
In the face recognition experiment in a simulated residential area we measured the face recognition distance of target persons for various positions and light levels. Six different light sources were used for illumination: a warm white fluorescent lamp (S/P=1.26),
a high pressure sodium (HPS; S/P=0.52), two white led lamps (3000 K, S/P=1.16 and 4500 K, S/P=1.61), and two led lamps with a high S/P-ratios of 2.73 and 3.16.
Two groups (young, mean: 16.5 years; old, mean: 60.2 years) of in total 45 subjects assessed the recognition of the faces of the target persons. We measured the vertical illuminance and semi-cylindrical illuminances at the faces of the target persons. The mesopic illuminances were calculated with the CIE-model.
Results : Literature review
• The visual adaptation system is complex, but there are models available for the calculation of the time course of the adaptation process, which can last for 3 minutes for luminance levels in public lighting (<2 cd/m2). The adaptation is affected by eye movements and disability glare and is never optimal in practical situations. There is no reliable model that takes into account eye movements and luminance distributions in real traffic situations.
• A light source with a S/P-ratio of 3 scatters only 10% more light in a standard atmosphere than a warm white light source (S/P-ratio = 1.23). For conditions with a lower meteorological visibility (haze, fog) there is no difference in scatter between these light sources.
• The effective S/P-ratio of light sources declines with the age of the observer due to the yellowing of the eye media. For an observer at an age of 80 years the S/P-ratio of light sources for public lighting is 10% to 25% lower than for a young observer of 30 years old.
• Some studies on face recognition indicate that a higher colour rendering index results in larger face recognition distances. Other studies state that colour does not play an important role; only in situations of degraded pictures and at threshold levels the colour may have an effect.
Experiment : In the experiment no effect was found of the spectrum of the six light sources on the face recognition distance. The face recognition distance strongly depends on the vertical illuminance at the face of the target persons and can be described with a square root function in the form y=a.x0.5. The mesopic and semi-cylindrical illuminance do not give a better prediction of the face recognition distance than the common vertical photopic illuminance. On average, the young observers show a factor 1.7 larger recognition distance than the old observers.
Conclusions : From the literature review we can conclude that there are useful adaptation models, which should be refined in order to take eye movements and luminance distributions in real traffic situations into account. For the time being, it is advised to use the sum of the average scene luminance and the veiling luminance due to disability glare as adaptation luminance. It also appears that the increase of light pollution due to light sources with S/P-ratios up to 3 can be neglected, but we have to account for effect of the age on the S/P-ratio when light for mesopic vision is applied. It is still under debate whether the colour of the lighting plays an important role in face recognition.
From the face recognition experiment we can conclude that the spectra of the lamps do not play a role in face recognition. Thus, light sources with a high S/P-ratio do not perform worse or better than the conventional light sources (fluorescent, HPS) and white led lamps with S/P-ratios between 0.5 and 1.6. Face recognition is a foveal visual task where only the central part of the visual field is involved and where the mesopic effect does not play a role.
The light spectrum optimized for mesopic vision contains a relatively high amount of bluish light (high S/P-ratio) and is therefore effective for peripheral visual tasks at mesopic light levels. Since the current scientific knowledge concerning mesopic vision is insufficient for changing standards for public lighting, we investigated a number of issues concerning mesopic vision and public lighting are investigated. We performed a literature review on light adaptation, effect of age, atmospheric scatter and face recognition and a face recognition experiment at various light spectra.
Methods : In the literature review we focused on the modelling of adaptation luminance, effect of observers age on S/P-ratio, atmospheric scatter and face recognition for light sources optimized for mesopic vision.
In the face recognition experiment in a simulated residential area we measured the face recognition distance of target persons for various positions and light levels. Six different light sources were used for illumination: a warm white fluorescent lamp (S/P=1.26),
a high pressure sodium (HPS; S/P=0.52), two white led lamps (3000 K, S/P=1.16 and 4500 K, S/P=1.61), and two led lamps with a high S/P-ratios of 2.73 and 3.16.
Two groups (young, mean: 16.5 years; old, mean: 60.2 years) of in total 45 subjects assessed the recognition of the faces of the target persons. We measured the vertical illuminance and semi-cylindrical illuminances at the faces of the target persons. The mesopic illuminances were calculated with the CIE-model.
Results : Literature review
• The visual adaptation system is complex, but there are models available for the calculation of the time course of the adaptation process, which can last for 3 minutes for luminance levels in public lighting (<2 cd/m2). The adaptation is affected by eye movements and disability glare and is never optimal in practical situations. There is no reliable model that takes into account eye movements and luminance distributions in real traffic situations.
• A light source with a S/P-ratio of 3 scatters only 10% more light in a standard atmosphere than a warm white light source (S/P-ratio = 1.23). For conditions with a lower meteorological visibility (haze, fog) there is no difference in scatter between these light sources.
• The effective S/P-ratio of light sources declines with the age of the observer due to the yellowing of the eye media. For an observer at an age of 80 years the S/P-ratio of light sources for public lighting is 10% to 25% lower than for a young observer of 30 years old.
• Some studies on face recognition indicate that a higher colour rendering index results in larger face recognition distances. Other studies state that colour does not play an important role; only in situations of degraded pictures and at threshold levels the colour may have an effect.
Experiment : In the experiment no effect was found of the spectrum of the six light sources on the face recognition distance. The face recognition distance strongly depends on the vertical illuminance at the face of the target persons and can be described with a square root function in the form y=a.x0.5. The mesopic and semi-cylindrical illuminance do not give a better prediction of the face recognition distance than the common vertical photopic illuminance. On average, the young observers show a factor 1.7 larger recognition distance than the old observers.
Conclusions : From the literature review we can conclude that there are useful adaptation models, which should be refined in order to take eye movements and luminance distributions in real traffic situations into account. For the time being, it is advised to use the sum of the average scene luminance and the veiling luminance due to disability glare as adaptation luminance. It also appears that the increase of light pollution due to light sources with S/P-ratios up to 3 can be neglected, but we have to account for effect of the age on the S/P-ratio when light for mesopic vision is applied. It is still under debate whether the colour of the lighting plays an important role in face recognition.
From the face recognition experiment we can conclude that the spectra of the lamps do not play a role in face recognition. Thus, light sources with a high S/P-ratio do not perform worse or better than the conventional light sources (fluorescent, HPS) and white led lamps with S/P-ratios between 0.5 and 1.6. Face recognition is a foveal visual task where only the central part of the visual field is involved and where the mesopic effect does not play a role.
Topics
TNO Identifier
765814
Publisher
TNO
Collation
89 p. (incl. bijlagen)
Place of publication
Soesterberg