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July, 2020

Solving the Challenges of Assessing Road Luminance

By Abdel H. Naji, International Director of Sales and Marketing, GL Optic T

he use of modern LED luminaires in road lighting can offer significant energy savings and the opportunity to improve lighting con-

ditions on illuminated road sections. One of the methods described in the current EN 13201 standard “Road Lighting” is to measure the distribution of luminance produced on the road. This article pres- ents guidelines for the design of road lighting, methods and instruments of measurement, and experiences and problems related to the performance of lighting measurements.

Verifying the Quality of Road Lighting

The new EN 13201:2016 road light-

ing standard consists of five parts: “Guidelines on Selection of Lighting Classes,” “Performance Requirements,” “Calculation of Perfor mance,” “Methods of Measuring Lighting Performance,” and “Energy Performance Indicators.” The basic lighting requirements for roads

lated as the mean of the luminance points of the

designated primarily for motor traffic at high and medium speeds (M class roads) are based on crite- ria related to the level and uniformity of lumi- nance on the road itself, its direct surrounding and glare limitation. Properly designed road light- ing enhances the safety of both drivers and pedes- trians. The average luminance (LAV) should be calcu-

measuring field grid. The overall uniformity (UO) should be calculated as the ratio of the lowest lumi- nance at each point of the measuring field grid to the

average luminance. The longitudinal uniformity (Ul) should be calculated as the ratio of the least lumi- nance to the highest luminance in the longitudinal direction of the axis of each lane. The luminance distributions are measured in

a field that comprises two successive luminaires in the same row. The meter is positioned 197 ft (60m) from the first luminaire and the measuring points are evenly spaced. The luminance meter should be at a height of

5 ft (1.5m) above the road level. In the transverse direction, the meter should be positioned succes- sively in the center of each lane of the road. The average luminance and the overall uniformity of luminance should be measured for the whole road- way and for each position of the meter. From the driver’s point of view, the road area

between 197 and 525 ft (60 and 160m) in front of the vehicle is important. With the driver’s level of vision set at a height of 5 ft (1.5m) above the road

Properly measuring road illuminance can protect both drivers and pedestrians.

precise system. It must allow the changing of the meter’s inclination with high enough resolution to aim the field of measurement at the distance range described.

Calculation Challenges and Solutions EN 13021-3 recommends that the

measurement points and the observer’s position be the same as the positions used in the calculations. However, these requirements lead to some problems. A large number of points is created, which makes the process of measuring lumi- nance distributions more difficult. A pre- cise and technically advanced luminance meter is required, which should limit the total angle of the measurement cone to 2 arcminutes on the vertical surface and to 20 arcminutes on the horizontal sur- face.

There is the possibility of overlap-

ping measuring fields from neighboring points, and problems with the precise targeting of the meter’s observation field on the measurement field can throw off calculations. Experience shows that measure-

ments made with a spot luminance meter make it nearly impossible to per- form a reliable measurement, especially the distri- bution of luminance. These problems can be solved with an imaging luminance measuring device (ILMD). The operation of such a meter is similar to

surface, the angle between the optical axis of the measuring device and the road surface changes between 0.5 and 1.5°. This type of measurement requires a very

that of a digital camera. Professional versions of such meters do not

have an optical viewfinder, but the analyzed image can be displayed on a PC screen. Measurements are made by: setting the meter in an appropriate place; marking the meas- urement field, including its dimen- sions; single recording of the analyzed surface image; and processing the recorded image apart from where the measurements were taken. Many road lighting experts have

several years of experience in perform- ing luminance measurements on roads with the use of both a spot luminance meter and an imaging luminance measuring device (ILMD). Using an imaging luminance meter requires a connection to a PC, which runs the soft- ware.

An EN 13201 addon can make this

process simpler by enabling calculation of luminance on the road to meet the

standard. However, in most cases, the currently available luminance meters are not sufficient to perform measurements of luminance distributions on the road. The use of an ILMD meter makes it much easier to perform measurements and helps to

Diagram showing the measurement of a road illumination system.

obtain more reliable results, allowing for unam- biguous assessment of the state of road lighting installation. These meters though, are basically

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