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Helsinki University of Technology, Lighting Laboratory Report 30Espoo, Finland 2004

AB HELSINKI UNIVERSITY OF TECHNOLOGY

Lighting Laboratory

LUMINANCES AND VISIBILITY IN ROAD LIGHTING - CONDITIONS, MEASUREMENTS AND ANALYSIS

Marjukka Eloholma Jaakko Ketomäki Liisa Halonen

Helsinki University of Technology, Lighting Laboratory Report 30Espoo, Finland 2004

LUMINANCES AND VISIBILITY IN ROAD LIGHTING - CONDITIONS, MEASUREMENTS AND ANALYSIS

Helsinki University of TechnologyDepartment of Electrical and Communications EngineeringLighting Laboratory

Marjukka Eloholma Jaakko Ketomäki Liisa Halonen

Helsinki University of TechnologyLighting LaboratoryP.O. Box 3000FIN-02015 HUTTel: +385 9 451 4971Fax: +385 9 451 4982E-mail: [email protected]://www.lightinglab.fi

ISBN 951-22-7232-6ISSN 1455-7541

Espoo, Finland 2004

3

Abstract

The mesopic luminance region lies between the photopic and scotopic. Themesopic region covers luminances between about 0.001 ... 3 cd/m2. It is knownthat neither V(λ) or V’(λ) alone are representative of the eye’s spectral sensitivity inthe mesopic region. Still today, all lighting dimensioning is based on the photopicV(λ), which was established in 1924. V(λ) describes the sensitivity of the retinalcone receptors and it is applicable to bright light conditions i.e. daytime vision. Inthe mesopic range both the rods and cones on the retina are active and this isbelieved to cause changes in the spectral sensitivity of the eye.

The mesopic luminance region covers a wide range of lighting applications e.gnight-time traffic conditions. It is especially the higher end of the mesopicluminance region that is of prime importance in practical applications. Theobjective of this work was to find out the effects of light spectrum on visibility inroad and street lighting conditions. The work consists of road and street luminancemeasurements and analysis and of visibility tests in road lighting and laboratoryconditions.

The work introduces a new method for road luminance measurements. A CCDbased luminance photometer has several advantages compared to conventionalspot luminance meters. Luminance measurements on different types of road andstreet lighting and viewing conditions were made with two different CCD basedphotometers.

In night-time driving the visual and luminous environment of the driver is verycomplex. The road luminance measurements show that road surface luminancesin road and street lighting are mostly in the mesopic region, i.e. below 3 cd/m2.The weather conditions have a major impact on the luminance distribution of theroad. In rainy and wet weather the visibility conditions may be significantlydecreased due to varied luminance levels and distributions on the road.

In this work the effects of light spectrum on visibility was studied at luminancelevels encountered in road and street lighting. Pedestrian visibility tests wereconducted in road lighting installations built in an underground tunnel. Thesetests were made with light spectra, luminance level and target eccentricity asparameters. Contrast threshold measurements with different light spectra andluminance level were also carried out in laboratory conditions with a modifiedGoldman perimeter.

The results show that light level has a strong effect on visibility of movingtargets. The effect of light level is not, however, linear in different parts of the visualfield. Light spectra did not affect visibility when the target was viewed foveally. Atlower mesopic levels differences in peripheral visibility were found between lightspectra. The results suggest that lamp spectrum has an effect on visibility in roadlighting conditions in peripheral viewing. Lamps with high content in the bluewavelength region seem to be more efficient than the conventionally used HPSlamps.

4 LUMINANCES AND VISIBILITY IN ROAD LIGHTING

Table of contents

Abstract .......................................................................................................... 3

1 Table of contents ........................................................................................ 4

2 Introduction ............................................................................................... 52.1 Mesopic light levels ............................................................................. 62.2 The spectral sensitivity of the eye ......................................................... 62.3 The mesopic luminance region ............................................................. 62.4 S/P -ratio of light sources .................................................................... 7

3 Road and street lighting measurements ....................................................... 93.1 Methods for road lighting measurements .............................................. 93.2 Results of measurements ................................................................... 103.3 Luminance levels and adaptation luminance in driving situations ......... 11

4 Pedestrian visibility tests in the test room ................................................. 134.1 Experimental set-up .......................................................................... 134.2 Pedestrian visibility test 1 ................................................................. 144.3 Pedestrian visibility test 2 ................................................................. 164.4 Pedestrian visibility test 3 ................................................................ 174.5 Conclusions of pedestrian visibility tests ............................................. 19

5 Contrast threshold measurements ............................................................. 225.1 Experimental set-up .......................................................................... 225.2 Results and conclusions of contrast threshold measurements ............... 23

6 Conclusions .............................................................................................. 25

7 References ............................................................................................... 27

Introduction 5

1 Introduction

This research has been carried out in the Lighting Laboratory at HelsinkiUniversity of Technology during 2000-2003. The research has been financed bythe National Technology Agency of Finland (Tekes), The Finnish RoadAdministration, Helsinki Energy, Idman Oy, Oy Philips Ab, Finnish ConsultingEngineers Ltd (SITO) and Helsinki University of Technology.

The present basis of lighting dimensioning has existed since 1924, when thephotopic spectral sensitivity function V(λ) was established. At the present alllighting measurements and dimensioning are based on that function. Thephotopic V(λ) describes the spectral sensitivity of the foveal cones in bright lighti.e. daytime conditions. In the mesopic region both the rods and cones on theretina are active and their mutual interaction is believed to determine the spectralsensitivity. The mesopic luminance region covers luminances between about0.001...3 cd/m2 and includes e.g. road and street lighting.

It is believed, that the light perceived by the eye at low light levels cannot becorrectly defined with the photopic weighting function. Consequently, with thepresent practice the luminous outputs of lamps at low light levels are believed to beimprecisely defined.

The present work focuses on lighting dimensioning and lighting quality of roadand street lighting. The work set out to investigate the effects of light spectrum onvisibility in road lighting conditions. The work is based on measuring andanalysing road and street lighting and on studying visual performance in variedroad lighting and laboratory conditions.

In this work experimental road lighting installations with high pressure sodiumlamps and daylight metal halide lamps were built in an underground tunnel. Thisis a 200 m long underground tunnel in Otaniemi Espoo. Vision experiments withobservers were carried out in the tunnel using the visibility of pedestrian as thevisual test. Light spectra of the two lamp types were further modified with colouredfilters. We set out to investigate the effect of light spectrum on central and off-axisvision at low light levels. Contrast threshold measurements as a function of lightspectra were also carried out in laboratory conditions using a modified Goldmanperimeter.


2 Mesopic light levels

2.1 The spectral sensitivity of the eye

The perception of light by theeye is dependent on the wavelengthof the incident radiation. Thischaracteristic is called the spectralsensitivity of the eye. In daytimevision, called photopic vision(above about 3 cd/m2), the relativespectral sensitivity is described bythe V(λ) function, which wasestablished in 1924. The photopicV( λ ) wa s pub l i s he d by t heInternational Commission onIlluminat ion (CIE), and i t i sinternationally accepted and used.

In very dark conditions (belowabout 0.001 cd/m2), the scotopicvision, the spectral sensitivity isdescribed by the V’(λ) functiondating from 1951. In photopic visionthe eye is most sensitive to light at wavelength of 555 nm and in scotopic vision thepeak of the spectral sensitivity curve shifts to 507 nm. In addition to these twosensitivity curves, there is the photopic spectral sensitivity function for centrallyfixated large target, V10(λ). These functions are shown in Figure 1. At present alllighting dimensioning and measurement are based on the photopic V(λ).

2.2 The mesopic luminance region

Between the photopic and scotopic regions there is the mesopic region (lowlight levels ranging from about 0.001...3 cd/m2), where both the cones and rods areactive, Figure 2. In the mesopic luminance region the spectral sensitivity isbelieved to be dependent on the visual task (e.g. target location and size) andluminance level.

The upper and lower limits of the mesopic luminance region are not exactlydefined. According to some definitions, the upper limit of the mesopic region is10 cd/m2 [Kokoschka 1997]. The recommended average luminances for road andstreet lighting in Finland are between 0.5…2 cd/m2, which are in the mesopicluminance region. Because all the lighting quantities (e.g. luminous flux,illuminance, luminance) are based on the V(λ), the luminances actuallyperceived by the eye in the mesopic region are unknown at present .

Figure 1. Spectral sensitivity functions of the eye.In photopic vision, when cones are active,the sensitivity follows the function V(λ). At very lowlight levels only rods are active, and spectralsensitivity follows V’(λ)-function. The Vmes(λ) isone example of the possible mesopic spectralsensitivity functions.

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Mesopic light levels 7

Even though the spectral sensitivity of the eye in the mesopic luminance regionis still unknown, a lot of research work has been conducted in the mesopic area[Vision at low light levels 1999]. At present there are several proposed models fordescribing the mesopic spectral sensitivity. Part of the models are to some extentbased on the fact that the mesopic spectral sensitivity function is a combination ofthe photopic and scotopic functions. In the He et al [He et al. 1997, He et al. 1998]

models the combination is done by linearly multiplying the values of one functionby a factor x and the values of the other function by a factor (1-x). The He et almodels are based on reaction times [He et al. 1997] and binocular simultaneitymethod [He et al. 1998]. In these models the values for x are calculated by the meansof iteration, starting from the photopic luminance value. The luminous efficacyfactors for the different functions are accounted for in the same relation.

2.3 S/P -ratio of light sources

At present the photopic spectral luminous efficiency function V(λ) forms thebasis of all lighting calculations and photometry. The luminous flux (lumen)values and luminous efficacy (lm/W) values of lamps are based on V(λ), as wellas recommendations of luminance (cd/m2) and illuminance (lx) values. TheV(λ) is valid for daylight conditions, but as luminance levels decrease, thecalculations are no more necessarily accurate.

One way to consider the potential differences between photopic and mesopicefficacy of light sources is the S/P-ratio. The S/P-ratio is a metric of the scotopic-to-photopic luminous flux of a light source. This ratio describes the changes inthe lamp's luminous efficacy, when the calculations are made either withscotopic V’(λ) or photopic V(λ) weighting.

Figure 3 shows the spectra and S/P-ratios of a high pressure sodium (HPS) lampand a daylight metal halide (MH) lamp, that were used in the road lightinginstallations of this work. The S/P-ratio of the HPS lamp is S/P=0.6. Becausethe radiation of the HPS lamp is more concentrated on the longer wavelengths,the luminous efficacy of the lamp decreases when the calculations are made withscotopic weighting. The daylight metal halide lamp has considerable radiation in theshort wavelenght region of the spectrum. Thus the efficacy of the lamp increaseswhen the weighting is made with the scotopic function and the S/P-ratio is S/P = 2.4.

Figure 2. Photopic, mesopic and scotopic luminance regions. [Rinalducci et al. 1999]

Photopic(cones )

Mesopic(rods + cones)

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L (cd/m2)

SunlightMoonlightStarlightOutdoor lighting


Figure 3. The spectral power distributions and S/P-ratios of a HPS lamp (Osram NAV TS 70)and of a daylight MH lamp (Osram HQI TS 70/D). The spectral sensitivity functions V(λ) (solidcurve) and V’(λ) (dashed curve) are also shown.

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Road and street lighting measurements 9

3 Road and street lighting measurements

3.1 Methods for road lighting measurements

The quality of road lighting installations can be controlled and secured withluminance measurements. Measurements of road surface luminances alsoallow mutual comparison of different road lighting installations. Luminancemeasurements from the field are needed to gather data and analyse theluminous environments from the driver’s point of view. Largely, the availabledata on road lighting luminances has been restricted to the recommendedluminance values for road surfaces.

Conventionally, road lighting measurements have been done with spotluminance meters. With spot meters the luminances are taken from severaldiscrete points on the road surface and the average luminance as well as theoverall and longitudinal luminance uniformities are calculated from themeasured values. Road luminance measurements with conventional spotmeters are however troublesome and time consuming to conduct, because,after all, there are hundreds of discrete luminance points to measure. A newmethod for road and street luminance measurements is introduced. This is aluminance meter based on a camera with a CCD detector array. With this typeof photometer, the image consists of several pixels and the luminances ofindividual pixels are captured simultaneously. The measurement is fast. Inaddition to the road surface luminances, the measurements also includesimultaneous luminance data from the other parts of the visual field of thedriver. Thus the effect of the surrounding areas, road luminaires, oncomingcar headlights etc. can also be analysed.

In HUT Lighting Laboratory road and street luminance measurements arebeing conducted with two photometers based on a CCD camera. The IQCam(left in the Figure 4) is the older system and its capabilities for measuringluminances below 0.3 cd/m2 are limited. The new meter (right in the Figure4) ProMetric 1400 has a Peltier cooled CCD cell, which enables themeasurement of luminance values down do 0.01 cd/m2.

Figure 4. HUT Lighting Laboratory’s luminance photometers, that were used in the road lightingmeasurements. The IQCam photometer (left) and the ProMetric 1400 photometer (right).


3.2 Results of measurements

Luminance measurements were made in different road and street lightingcondi t ions and in d i f fe rent v iewing and weather condi t ions . Themeasurements were made during night-time hours in the Helsinki-Espoo area.Part of the measurements were taken by positioning the camera in the middleof the driving lane at a height of 1.5 m according to the CIE recommendations[CIE 1982]. In part of the measurements the camera was inside the car in thedriver’s position at 1.15 m height from the road surface, while the car wasparked in the middle of the driving lane. The captured luminance scenesinclude simultaneous luminance data from the road surface, areas surroundingthe road and any obstacles in the visual field of the driver. In the measurementplaces the lighting was provided by fixed road lighting and in some cases thecar headlights were also included.

The results of road and street lighting measurements are in detail presentedin our former report [Eloholma et al 2001]. In the following a few examples ofthe measurements are presented.

In Figure 5 the measured luminance scenes of two streets are presented.The left measurement is taken with the IQCam and the right with the ProMetr ic luminance photometer . The road sur face was dry in bothmeasurements. The average luminance of the street surface is x cd/m2 on theleft and x cd/m2 on the right.

Figure 6 shows a luminance distribution of a street in Helsinki residentialarea. The luminance measurements were taken from inside a car parked on theright lane of the street, the street luminaires being on the left side of the street.In dry conditions the average street surface luminance is 2.0 cd/m2 (Figure 6left). In wet conditions the luminances of the bright area of the street surfaceare between 10…30 cd/m2. The luminances of the darker areas of the wetstreet are decreased in comparison to the dry surface values. This all results invery low luminance uniformity of the wet street surface and, consequently, inworse visibility conditions.

Figure 5. Luminance distribution of two streets. Measurements made with the IQCam (left) andthe ProMetric (right) luminance photometers.

10 . . . 1005,0 . . .103,0 . . . 5,02,0 . . . 3,01,0 . . . 2,00,5 . . . 1,00,3 . . . 0,5< 0,3 >100 cd/m210 . . . 1005,0 . . .103,0 . . . 5,02,0 . . . 3,01,0 . . . 2,00,5 . . . 1,00,3 . . . 0,5< 0,3 >100 cd/m2

Road and street lighting measurements 11

3.3 Luminance levels and adaptation luminance in driving situations

The function of fixed road and street lighting is to illuminate the roadsurface and objects on the road and its surrounding areas. Car headlights intheir turn illuminate the road surface, roadsides and in addition, any non-horizontal objects in these areas. The purpose of fixed road lighting is toenhance the visibility of temporary and steady elements on the road and itssurrounding areas outside the reach of the car headlights.

Night-time driving is a very complex situation for the adaptation of the eye.The luminances in the visual field change constantly while the car is movingand the direction of view is changing. The luminances of the visual objectssurrounding the road (traffic signs, guiding systems, buildings etc.) may vary alot depending on the surrounding lighting. Especially in urban areas theluminances in the visual field of the driver may vary significantly.

Despite a lot of research work in defining the adaptation luminance, it isstill unknown which part of the visual field determines the adaptationluminance. The adaptation process is even more difficult to model at mesopicluminance levels, where both the rods and cones are contributing to vision.The distribution of rods in the retina is different from the distribution of cones.Furthermore, the contribution of rods and cones to the visual performance atmesopic levels changes with luminance level. This all leads to a verycomplicated adaptation process to model in the mesopic region.

The measurements indicate that road surface luminances in road and streetlighting in dry conditions are largely in the mesopic region, i.e. below 3 cd/m2

even on well illuminated roads. The luminances can be very low in theadjacent and surrounding areas of the road. There are also higher luminancesin the visual field of the driver. These comprise, for example, traffic signs whenilluminated, road luminaires and the headlights of oncoming cars. The effectof these relatively small areas with higher luminances to the adaptationluminance level is unknown. Traffic signs and the headlights of other carsusually remain only temporarily in the visual field. Fixed street luminaires are

10 . . . 1005,0 . . .103,0 . . . 5,02,0 . . . 3,01,0 . . . 2,00,5 . . . 1,00,3 . . . 0,5< 0,3 >100 cd/m2

Figure 6. Road luminances of the same street in dry (left) and wet (right) conditions.

10 . . . 1005,0 . . .103,0 . . . 5,02,0 . . . 3,01,0 . . . 2,00,5 . . . 1,00,3 . . . 0,5< 0,3 >100 cd/m2


mostly located in the peripheral parts of the visual field, and their effect on theadaptation level may be quite small . Thus in night-t ime driving theluminances of the visual field mostly remain in the mesopic luminance region.It still remains unsolved which part of the visual field determines theadaptation luminance in these mesopic conditions.

In wet conditions the luminances and luminance distributions of roadsurfaces change significantly in comparison to dry conditions. In areas withspecular reflection towards the observation point the luminances of the roadsurface increase substantially and form very bright areas. On the other hand,the darker areas of the road surface increase in size and decrease in luminance.This results in a very low luminance uniformity of the road surface. The roadsurface luminance measurements indicate remarkable changes caused bywetness to road surface luminances and luminance uniformity. These in turnresult in bad visual conditions and decreased visibility.

Pedestrian visibility tests in the test room 13

4 Pedestrian visibility tests in the test room

4.1 Experimental set-up

The effects of light spectrum and luminance level on visibility in road lightingconditions were studied in experimental road lighting installations. The test roomused in these measurements was a long tunnel in an underground air-raid shelter.The length of the tunnel is 200 m, height 3.5 m and width 5 m. No daylight orother external light enters the tunnel.

In the tunnel it is possible to build different road lighting installations usingmovable luminaires. Metal halide (MH) and high pressure sodium (HPS) lampswere used in the installations and the lamp spectra were further modified withspecial coloured filters. The luminance levels of the installations were adjustedwith neutral grey filters attached to the luminaires.

The objective of the vision tests was to study the effects of light spectrum onpedestrian visibility in road lighting conditions. The task of the subject was toindicate the detection threshold of the pedestrian. The pedestrian was walkingtowards the dark end of the tunnel and thereafter approaching the illuminated areaof the tunnel from the dark. In the first and second test series, the pedestrian wasdressed in grey clothing and wore a grey cap to cover his face. The reflectioncoefficient of his grey clothes was 0.20. In the third test series the pedestrian hadwhite pants and white cap to ease the detection. By employing a pedestrian insteadof e.g. a flat cardboard as a visual target, the test becomes much more realistic, as thepedestrian includes threedimensionality. Furthermore, the movement of thewalking pedestrian is an important component of the visual task in night-timeconditions.

The luminaires were positioned in five luminaires group with 8 m spacing. Thesubject sat in the front of the first luminaire. In foveal viewing conditions the

Figure 7. Pedestrian visibility experiments in the road lighting conditions. In foveal viewing thesubject was fixating to the back of the pedestrian and in peripheral viewing to a black area on theside wall.


subject was fixating to the walking pedestrian. In peripheral viewing conditions thesubject was fixating to the a black area on the left side wall of the tunnel, Figures 7and 8. The pedestrian was walking in the dark area behind the fifth luminaire. Thelength of the lighting installation was 40 m, so the detection distance was alwayslonger than 40 m.

The eccentricity of vision in the peripheral test was 15° in the first tests and 20°in the succeeding tests. The pedestrian subtended a visual angle of 2° from 40 mdistance.

Before starting the first actual experiment the subject had adapted to the tunnellighting for 30 minutes. A 5 minutes adaptation time preceded the tests in eachlighting condition. Young subjects of 22-30 age participated in the experiments.Their vision was checked to be normal (colour vision, refraction, visual acuity,visual field) at the Helsinki University Central Hospital.

The statistical analysis of the results was conducted using analysis of variancebased on the Bonferroni and Friedmann tests. The probability level 0.05 was used.

4.2 Pedestrian visibility test 1

The task of the subject was to indicate the detection threshold of the pedestrian.The pedestrian was walking towards the dark end of the tunnel and thereafterapproaching the illuminated area of the tunnel from the dark. The tests were madein foveal and peripheral vision (15° eccentricity). The tests were made at twoluminance levels, the average road surface luminance levels were 0.1 and 1.5cd/m2. The tests were made for each subject in four different lighting conditions(two light spectra HPS/MH, and two luminance levels 0.1 and 1.5 cd/m2). Thespectra of the HPS and daylight MH lamp are shown in Figure 9.

Figure 8. Experimental set up of the pedestrian visibility tests.

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In the experiments the detection distance (m) was recorded as the result. Thepedestrian luminance (at 1.3 m height) at the detection distance refers to thelowest detectable luminance in each lighting condition. The pedestrianluminance decreased with increasing distance from the luminaire at the tunnelend. The luminance distribution at the tunnel end is dependent on theinstallation i.e luminaire optics and lamp outputs. As the luminaire and theiroptics remain the same, the luminance distributions in the different lightingconditions are dependent on the lighting level of each installation. Thus the resultsare presented as a ratio of the pedestrian luminance at the detection distance andthe average road surface luminance level (Lped/Lroad). This is referred to as’Relative Luminance Threshold’.

The tests were made for six subjects (aged 22-25 years) with normal vision. Theresults are shown in Figure 10. The statistically significant differences between twolighting conditions are marked with red arrows and green arrows indicate no

Figure 9. The spectrum of a) high pressure sodium lamp NAV TS-70 SUPER (S/P-ratio 0.56)and b) daylight metal halide lamp HQI-TS 70/D (S/P -ratio 2.0).

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Figure 10. Results of the first test series with six subjects. Relative luminance threshold fordetecting the pedestrian at two road surface luminance levels (1.5/0.1 cd/m2), with two lightspectra (HPS/MH) and for foveal (0°) and peripheral (15°) viewing.


statistical differences between the two observed conditions. The results show thatluminance level has a clear effect on visibility. The relative luminance threshold islower at 1.5 cd/m2 luminance level compared to 0.1 cd/m2 luminance level. Thusthe ability to detect low contrasts is higher at the higher road luminance level.

At the higher luminance level (1.5 cd/m2) visibility is relatively better for targetsin foveal vision compared to peripheral vision. Thus visibility decreases when thetarget is moved from central vision towards the periphery. When the road surfaceluminance is decreased to 0.1 cd/m2, the differences between central andperipheral vision disappear. Thus at the lower light level the detection distanceand relative luminance threshold are the same in foveal and peripheral viewing.This means that the importance of peripheral vision increases in comparison tocentral vision when the light level decreases in the mesopic region. In the first testsno differences were found at either luminance level between the two light spectra(HPS/MH).

In these first tests it was noticed that the visual test was quite difficult to performif the subject was not carefully trained for the task. It is very difficult for a non-trained subject to maintain her/his eye fixation point in a given position. This wastrue especially in peripheral viewing. In peripheral viewing it is also difficult forthe observer to exactly define whether the pedestrian is visible or not.


The second tests employed only one subject, but the same tests were repeatedeight times for him. The subject had normal vision and his age was 28 years. Thesubject was trained and motivated for this task and thus the test process could bevery carefully controlled. A trained subject’s fixation point is more reliablymaintained in the given point and the criterion of visibility is more accurate.

In the second tests the visual test was slightly modified. A new component wasincluded in the visual task: movement of the arms. The detection of movement isan important component of visual tasks in traffic. In driving situations a lot ofinformation is gained from the detection of movement in off-axis vision.

In the experimental conditions the walking speed of the pedestrian has to bevery slow (1 step/2 sec) in order to record the detection distance accurately. At thisspeed the movement of the walking pedestrian seen from over 40 m distance is nomore a critical component of the visibility task. In the second test series, thepedestrian was constantly swinging his hands between downwards and horizontalplane while walking. As in the first test series, the walking speed of the pedestrianwas constant (0.2 m/s) and the length of one footstep was 40 cm. In second testseries the movement of the arms made the detection of the pedestrian easier for thesubject and the detection distance could be more accurately defined. Thepedestrian arm movement also enlarged the horizontal size of the visual target.

Eight test sessions were carried out on subsequent days for the trained subject.One session consisted of four different lighting conditions (two light spectraHPS/MH, and two luminance levels 0.1/1.5 cd/m2). In peripheral viewing the


eccentricity of the target was 20°. Because of the modifications in the visual taskand the use of the trained subject, the detection distances could be moreaccurately defined. This was also shown by the smaller deviations in the recordedresults. The results of the second tests are shown in Figure 11.

Again, the results show that luminance level has a clear effect on visibility. Thisis shown by the lower relative luminance threshold at luminance level 1.5 cd/m2

compared to 0.1 cd/m2 luminance level. Also, as in the first test series, visibility isbetter in foveal viewing than in peripheral viewing at the higher luminance level(1.5 cd/m2). No spectral effects were again found at the higher luminance level.

However, differences between the two light spectra at the lower luminancelevel (0.1 cd/m2) were found. The trained subject could define the detectiondistance more accurately. In addition, the inclusion of movement in the visual taskincreased the sensitivity of the test. It is possibly because of the more accuraterecording of the detection distances, that differences between the two light spectrawere found.

At the lower luminance level (0.1 cd/m2) visibility became relatively better fortargets in peripheral vision compared to foveal vision. When the target was infoveal vision, the light spectrum did not affect visibility. However, in peripheral(20°) vision, the relative luminance threshold was lower with MH lamps comparedto HPS lamps. Thus light with higher content in the blue wavelength regionyielded to better visibility in peripheral vision.


In the third test series the visual task was again modified and a differentapproach was used to define the detection threshold. The arm movements of thepedestrian were again used to stress the movement of the target. The pedestrian

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Figure 11. Results of the second test series with one trained subject and eight measurementsessions. Relative luminance threshold for detecting the pedestrian at two road surface luminancelevels (1.5/0.1 cd/m2), at two light spectra(HPS/MH) and for foveal (0°) and peripheral (20°)viewing.


started to walk towards the dark end of the tunnel, and at after each footstep (40 cmapart), he once swung his arms from downwards to the horizontal plane and back.The task of the subject was to indicate whether the arm movements occured afterthe first or second signal given by the pedestrian. Thus a ‘forced choice’ methodwas used as a detection criterion for the subject. After the subject had indicated thetiming of the arm movements, the pedestrian walked onwards. The detectiondistance was defined through several repetitions around the threshold distance.

The light spectra were different from the first and second test series. Twodifferent light spectra were used and they were achieved using the daylight MHlamps and coloured filters. The “blue” spectra was little more bluish than the MHdaylight lamp in the preceding tests and the “yellow” light was quite similar to theHPS lamp spectra. The spectra of the third tests are shown in Figure 12.

The average road surface luminance levels were 0.5 and 2.0 cd/m2. The testswere made in four different lighting conditions (two spectra and two luminancelevels) and in foveal and peripheral (200) viewing. In this test series the visibilitytests were made for four subjects with normal vision (aged 22-30 years) and eachtest session (four lighting conditions) was repeated four times for each subject.

The results are shown in Figure 13. Again, the luminance level had a significanteffect on relative luminance threshold. The recorded relative luminance thresholdwas lower at the luminance level 2.0 cd/m2 compared to 0.5 cd/m2. Thus visibilitywas increased with increasing luminance level.

Again it was found that visibility was relatively better in central vision compared toperipheral vision (20°) at the higher road surface luminance level 2.0 cd/m2. Nodifferences between light spectra were found at luminance level 2.0 cd/m2. Spectraleffects were found at the lower luminance level of 0.5 cd/m2. At luminance level0.5 cd/m2, the differences in luminance threshold between foveal and peripheralvision disappeared in blue light but not in yellow light. In peripheral vision at the lowerluminance level, visibility was better with blue light compared to the yellow light.Again, for foveal vision no spectral effects were found at either of the luminance levels.

Figure 12. The spectra of the light in the third test series. a) Yellow light (S/P-ratio 0.79) isgained by filtering daylight metal halide light with yellow filters . b) Bluish light (S/P-ratio 2.58)is gained by filtering MH lamp with blue filter .

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4.5 Conclusions of pedestrian visibility tests

The results of the pedestrian visibility experiments show that light level hasa strong effect on visibility of moving targets at mesopic luminance levels. It isinteresting that the effect of light level is not linear in different parts of the visualfield. At the higher luminance levels (1.5 and 2.0 cd/m2), visibility was better incentral than in peripheral vision (150 and 200). When light level was decreased(0.1/0.5 cd/m2), the differences in detection distances between central andperipheral vision disappeared and were partly reversed. This is explained by thestructure of the retina.

In the fovea there are only cones and no rods. The number of cones decreaseswith eccentricity from the fovea, whereas the number of rods increases witheccentricity from the fovea. At high light levels cones are more active than rods.When light levels decrease to the mesopic region, more rods become active, whilethe contribution of cones to the visual process becomes smaller. Thusthe importance of peripheral vision increases when light level is decreased inthe mesopic luminance region.

In the central viewing conditions the target (pedestrian) was viewed with thefovea. The luminance levels in the HPS and MH lamp installations were equal interms of photopically weighted, i.e. V(λ), luminance values. As V(λ) is based onthe spectral sensitivity of the cones and as there are merely cones in the fovea, it isin prospect that light spectrum does not affect visibility in central viewing even atlow light levels. This was actually the result of the experiments at both the higher(1.5 cd/m2) and lower (0.1 cd/m2) luminance levels. Light spectra did not affectvisibility at either luminance level when the target was viewed with central vision.This is similar to our earlier findings, where no spectral effects were found onvisual acuity at mesopic luminance levels [Eloholma et al. 1999].

Figure 13. . Results of the third test series with four subjects and four measurement sessions foreach. Relative luminance threshold for detecting the pedestrian at two road surface luminancelevels (2.0/0.5 cd/m2), at two light spectra (yellow and blue) and for foveal (0°) and peripheral(20°) viewing.

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In peripheral vision both cones and rods are active in the visual process atmesopic levels. As the V’(λ) indicates, the spectral sensitivity of rods is shifted tothe shorter wavelength region, as compared to cones. Thus the spectral sensitivityof the eye in peripheral vision is assumed to shift towards the blue part of thespectrum when light levels decrease. It is in prospect that when luminance levelsdecrease to the mesopic region, the blue part of the spectrum becomes moreeffective for off-axis vision. The results of the second and third test series with armmovements support this hypothesis. Visibility in terms of contrast threshold wasbetter under MH lamps than under HPS lamps in peripheral vision. The effect washowever found only at the low light levels (0.1 cd/m2 and 0.5 cd/m2), where thecontribution of rods is expected to be higher. In the third test series the luminancelevels were slightly higher (2.0 and 0.5 cd/m2) than in the first and second series.In the third test series the spectral effects were not as clear as in the second testseries.

The results of the pedestrian visibility measurements suggest that lampspectrum has an effect on visibility in road lighting conditions in peripheral vision.The lamps with high content in the blue wavelength region seem to be moreefficient than the conventionally used high pressure sodium lamps. However, thespectral effects were not found at the higher mesopic levels (1.5 ... 2.0 cd/m2). Theblue light becomes relatively more efficient when light levels are decreased to0.1 ... 0.5 cd/m2.

Based on mesopic models developed by He et al [He et al 1997, He et al 1998],predictions for mesopic luminances can be calculated. With these modelsmesopic sensitivity functions and luminances are derived from reaction time andbinocular simultaneity method experiments. Based on the He et al models, figure14 shows a calculation of mesopic/photopic luminance ratio for the roadluminances used in our pedestrian visibility experiments. The higher the bluecontent of the lamp, the higher the ratio of mesopic to photopic luminance. Withdecreasing light levels the mesopic luminance gets relatively higher with the MH

lamp. The situation is reversed with the HPS lamp with less content in the bluewavelength region. The He et al models give predictions of mesopic luminances.However, as can also be seen from Figure 14, the calculations of mesopicluminances are highly dependent on the chosen model and the visual criteria onwhich the model is based. Thus the present calculations of mesopic luminancesshould be regarded as predictive. It is evident that a comprehensive set of visualdata must be gathered before final calculations of mesopic luminances can bemade.

The visual task in the experiments was the detection threshold of a pedestrian.This corresponds to the minimum luminance contrast of a target against itssurroundings that is necessary for drivers to become aware of objects in their visualfield. Visual identification or recognition was not included in the task. In theseexperiments the surroundings of the target were relatively uniform and the targetwas always in the expected place. In many driving situations, however, the visualfield is very complex and the conspicuity of a visual stimulus from the background


becomes important. Conspicuity is a term for how the presence of a visual stimulusis capable of attracting the observer’s attention when the background isnonuniform [CIE 1992]. Visual performance in night-time driving is a very complexobject of study because it comprises several visual elements with variousparameters. Therefore the visual task of night-time driving cannot becomprenensively described with a single target and task.

Figure 14. Mesopic/photopic luminance ratios for the photopic luminances of the pedestrianvisibility experiments. Calculations are based on the He et al models [He et al 1997, 1998]. TheS/P ratios are the scotopic (V’(λ) weighted) to photopic (V(λ) weighted) luminous outputs of thespectral lights in the pedestrian visibility experiments.

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5 Contrast threshold measurements

5.1 Experimental set-up

The effect of light spectrum onvisibility at low light levels was alsostudied in with another method inl abo ra t o r y co nd i t i o n s . Theobjective was to compelement thefindings from the road lightinginstallations with visibility testsmade with a different visual taskand using different experimentalmethod.

A modified Goldman perimeterwas used as the measurementequipment. It is a 60 cm diameterhemisphere shown in Figure 15, onthe surface of which a light spot(20) can be reflected as a visualtarget . The luminance of thehemisphere surface is uniform. Thehemisphere surface luminance and the target luminance can be individuallycontrolled. The spectra of hemisphere background and the visual target can bevaried. In the experiments the target and background spectra are the same.

Contrast threshold was measured at different background luminance levels andusing different light spectra. The subject controlled the luminance of the visualtarget first so, that it could just be seen (increment threshold) and then so, that thetarget couldn’t be seen anymore (decrement threshold). This was repeated twice ateach light condition. The average of the results (decrement/increment) was

Figure 15. The Goldman perimeter

Figure 16. The spectra of the contrast threshold measurements. a) Yellow b) Blue

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Contrast threshold measurements 23

recorded as contrast threshold value. A moving visual target was employed. Thetarget was moved by the experimenter on an area of 30 x 40 with a speed of 1circle/second. A static target was used in the preliminary experiments, butthe method of a moving target was found to be more precise.

The measurements were made using blue and yellow light spectra shown inFigure 16 and at background (hemisphere surface) luminance levels 0.1, 0.5 and2.0 cd/m2. The measurements were made for foveal and peripheral (100) viewing.Measurements were made for one trained subject (age 28 years) with normalvision and the tests were repeated four times for him.

5.2 Results and conclusions of contrast threshold measurements

The results of the threshold contrast measurements are shown in Figure 17. Ineach lighting condition the lowest detectable target contrast was recorded as theresult. Contrast sensitivity is the reciprocal of contrast threshold, thus the lowerthe threshold contrast, the higher the contrast sensitivity.

The results show, that the background luminance level did not affect thethreshold contrast between light levels 2 and 0.5 cd/m2. The effects of backgroundluminance level became evident when the luminance level was decreased to 0.1cd/m2. The measured contrast threshold was higher and consequently contrastsensitivity was lower at this lowest luminance level, when compared to the higherluminance levels 0.5 and 2 cd/m2.

At background luminance level of 2.0 cd/m2, there were no differences inthreshold contrast between light spectra or the eccentricity of vision (0° comparedto 10°). When the luminance level was decreased to 0.5 cd/m2, the blue lightyielded to better contrast sensitivity than the yellow light both in foveal andperipheral vision. These differences between light spectra became more

Figure 17. Contrast threshold as a function of background luminance for foveal and periphera(100) viewing. Two light spectra: yellow and blue.

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pronounced at the lowest luminance level of 0.1 cd/m2. A further finding at thislowest luminance level was, that the contrast sensitivity with blue light was higherin peripheral vision compared to central vision.

The results suggest, that light spectrum has an effect on contrast sensitivity whenluminance levels are decreased to 0.5 cd/m2 and below this. The light with highcontent in the blue wavelength region yields to higher contrast sensitivity. Thedifferences between blue and yellow light became more evident when luminancelevels were decreased to 0.1 cd/m2. At this luminance level of 0.1 cd/m2 peripheralvision becomes relatively better than foveal vision. The results are consistent with thefindings from the pedestrian visibility experiments. Both support the hypothesis thatat lower light levels the rods become more active and this causes changes in thespectral sensitivity

Conclusions 25

6 Conclusions

The primary function of road and street lighting is to keep the safety of thepeople. Road and street lighting should provide good visibility conditions andreduce potential hazards by illuminating objects on the roadway and on itsimmediate surroundings. Luminance measurements of road and street lighting areneeded to get data from the field and to analyse the luminous environments fromthe drivers point of view. The luminance measurements are also a good way tocontrol and secure the quality of road lighting. The work introduces a new methodfor road luminance measurements. The advantages of a CCD based luminancephotometer are the speed of measurement and the possibility to gathersimultaneous luminance information from the a large visual scene.

The work set out to investigate the effects of light spectrum and luminance levelon visual performance at mesopic light levels. Pedestrian visibility measurementswere carried out in experimental road lighting installations and contrast thresholdmeasurements were made using a modified Goldman perimeter. Bothmeasurements show, that luminance level has a clear effect on visibility inthe mesopic luminance region. Visibility increased with increasing luminancelevels between 0.1...2 cd/m2. The visual experiments show that movement plays animportant role in detecting targets at low light levels.

At the higher mesopic luminance levels of the study, 1.5...2 cd/m2, foveal visionwas relatively better than peripheral vision. When light levels were decreased to0.1...0.5 cd/m2, the differences in central and peripheral vision disappeared andwere partly reversed. This is explained by the structure of the retina and theincreasing contribution of rods to visual processing with decreasing light levels.

In foveal vision, light spectrum did not affect visibility. This is explained by thefact, that foveal vision is determined by cones. Thus V(λ) is applicable to fovealvision also at mesopic light levels. Spectral effects were found in peripheral visionand especially at low luminance levels. Much of the visual information in trafficsituations is gained with peripheral vision and peripheral vision becomes more andmore important when light levels are decreased. Thus all means to optimiseperipheral vision are important to improve the visual conditions in night-time traffic.

The spectral effects were more pronouncedly observed in the Goldman perimetermeasurements, where the short wavelength content of the blue light was higher. Alsothe adaptation luminance in the Goldman perimeter was uniform over the wholevisual field in contrast to the adaptation luminance of the road lighting installations.

The results support the hypothesis that at lower light levels the retinal rodsbecome more active and this causes changes in spectral sensitivity. The findingssuggest, that high pressure sodium lamp is not necessarily the most optimal lightsource for low luminance levels of street and road lighting. Light sources withhigher content in the blue wavelength region may be more optimal for visualperformance especially in street lighting applications, where the dimensioninglevels are low. The use of e.g. metal halide lamps in street lighting of city areas mayalso be justifiable because of the better colour properties of this lamp type.


The luminous and visual environment in night-time traffic is very complex. Inaddition to mesopic luminances of the road surfaces, the visual field may atthe same time include very low scotopic luminances of the adjacent areas ofthe road and also high photopic luminances of oncoming cars and fixedluminaires. Visual performance in night-time driving is very complex as itcomprises several visual elements with various parameters. Thus a comprehensiveset of visual data must be gathered before final conclusions of mesopic spectralsensitivity and consequently mesopic luminance calculations can be made.

References 27

7 References

CIE 1982. Calculation and measurement of luminance and illuminance in roadlighting. Publication CIE No 30-2. ISBN 92-9034-030-4.

CIE 1992. Fundamentals of the visual task of night driving. PublicationCIE No 100. ISBN 3-900-734-37-2.

Eloholma M, Halonen L. 1999. Vision in Mesopic Lighting Levels - The Effectsof Light Spectrum and Luminance Level. CIE Proceedings, PublicationCIE No 133.

Eloholma M, Ketomäki J., Halonen L. Road Lighting - Luminance and VisibilityMeasurements. HUT Lighting Laboratory Report 29, 2001.

He, Y. & Bierman, A. & Bullough, J. 1997. Evaluating Light Source EfficacyUnder Mesopic Conditions Using Reaction Times. Journal of IlluminatingEngineering Society. 26 (1) pp. 125-138.

He, Y. & Bierman, A. & Rea, M. 1998. A system of Mesopic Photometry.LightingResearch and Technology. 30 (4) s. 175-181.

Kokoschka, S. 1997. Das V(λ)-dilemma in der Photometrie. In 3. InternationalesForum für den Lichttechnischen Nachwuchs. Technishe Universität Ilmenau.

Rinalducci, E. & Higgins, K. & Zavod, M. & Wallace, S. 1999. Mechanisms ofPhotopic, Mesopic and Scotopic Vision. In : Proceedings of Vision at LowLight Levels. EPRI/LRO. TR-110738. pp. 13-24.

Vision at low light levels. 1999. Gough, A ed. EPRI Lighting Research Office.

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