Guidance Note 3/16

Measurement of thephotometric performanceof LED lighting

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Background

The ILP Technical Board have been madeaware of a number of instances where it isbeing claimed that the application of theperformance measurement procedures asdescribed within BS EN 13201-4:2003Methods of measuring lighting

performance and ILP Technical ReportTR28 Measurement of road lighting

performance on site are not suitable for:• Installations where LED lighting has

been installed;• Measurement within the mesopic range.

The purpose of this guidance note is toclarify the requirements for photometricperformance measurement and specificallyaddress these two points.

Requirements for undertaking

photometric performance

measurements

Measurement of photometric lightingperformance should only be undertaken bypersons who are professionally qualifiedand competent in the discipline ofilluminating engineering. Thoseundertaking such work should be fullyaware of:• Measurement standards and guidance;• The properties and calibration of the

instrument(s) being used;• The lighting installation

design/measurement grids;• Health and safety requirements for the

site.

Where the task relates to highway lightinginstallations those undertakingmeasurements should be registered underthe National Highways Sector Scheme(HERS)1 for Quality Management inHighway Works. There are two keycompetency requirements for the role:• Found 01 Basic Health & Safety (ECS

Highway Electrical Test);• Test 04 Photometric performance

testing.

Measuring instrument

Technical Report TR28 discusses therequirements for measuring newinstallations and on-going performanceparameters. It advises with regard to thephotometers to be used and what thepurchaser should look for in aninstrument. These relate not only to thequality of the instrument but also therequirements for the sensor – ensuringthat it has, for example, the correctdetection field and that the sensor’sresponse closely matches the CIE V(λ)photonic eye response curve. It alsoadvises on the requirements for thecorrect calibration of the instrument,especially where readings are to beundertaken for contractual purposes, andthe need to maintain an operational log.

Instruments used for measurement ofphotometric parameters shall be Type F(for field use) as characterized accordingto BS 667:2005 for illuminance metersand BS 7920:2005 for luminance meters.

Photometers are generally calibratedagainst a standardized tungsten lightsource (i.e. one calibrated by anaccredited laboratory by reference tonational measurement standards), and inthe past photometric performancemeasurements have generally beenadjusted to correct to the spectralperformance of the light source beingmeasured by the application of a colourcorrection factor (CCF). This factor canvary from less than one per cent toseveral tens of per cent, depending on thelight source and the spectral match of thephotometer to the V(λ) function. Thisapproach has been replaced by arequirement to use a photometer with aspecified colour match performance, ascharacterised by the so-called f’1mismatch index defined in CIE 69:1987. Ifyou need to calculate a particularcorrection factor for a given light spectrumand photometer combination, then theprocedure given in annex C.9 of BS667should be used.

It is this aspect that is being referencedwith the comment that special

Guidance Note 3/16 Measurement of the photometric performance of LED lighting

Institution of Lighting Professionals 3

1 http://www.thehea.org.uk/HERS/

considerations need to be made whenmeasuring the performance of LEDinstallations. This is a consideration for alllight sources and not specific to LEDs.

The type of light source under which themeasurement is taken makes nodifference to the procedures and does notrequire any change in the type ofilluminance meter used. For allmeasurements, whether of an LEDinstallation or any other light source, thespectral mismatch of the photometer is apotential source of measurement error,which is minimized by using an instrumentwith a small f’1 mismatch index.

Measuring highway illuminance

The standards are based upon photopiclighting levels, which is what is generallymeasured, and these are the values usedin all design guidance. Scotopic lightingperformance can also measured but thesevalues are rarely, if at all, mentioned indesign guidance because the light levelsare extremely low.

Outdoor lighting installations generally fallwithin the range of 2 to 50 lux and aretherefore within the mesopic range. Theexact thresholds of the mesopic rangedepend on the amount of light that asurface reflects to a person's eye and sofor a typical road surface (with an averageluminance coefficient of around 0.1) themesopic range would run from about 50lux down to about 0.05 lux. The mesopicilluminance will fall somewhere betweenthe photopic and scotopic illuminances,but the position between these limitschanges with adaptation level so it is not astraightforward correlation. Additionally,although CIE 191:2010 defines the systemto be used for calculating mesopicquantities according to the photopicadaptation luminance and the S/P ratio ofthe adaptation field, there is currently noagreed method for evaluating theseadaptation conditions. This means it iscurrently not possible to make photometricmeasurements in the mesopic range foractual installations. CIE technicalcommittee TC2-65 Photometric

measurements in the mesopic range2 isawaiting research relating to the size,shape and location of the adaptation fieldwithin the entire visual scene, which willallow the appropriate visual adaptioncoefficient to be evaluated so thatpractical, on-site measurements can bemade.

The approach used within BS5489-1:2013is one of first choosing the lighting classand the light source. Where the task beinglit permits, the S/P ratio of the light sourcemay be used to determine the allowedreduction in the specified photopicilluminance. This reduction is scaled usingthe CIE system for mesopic photometry3

and is the method described in PLG03(Lighting for subsidiary roads: Using whitelight sources to balance energy efficiencyand visual amenity).

When considering the requirements ofTR28, the measurement procedure is setagainst the design calculations and istherefore based upon the performance ofthe luminaire and light sources asprovided by manufacturers in their datafiles.

TR28 requires the production oftest/survey grids based on the designcalculations which are generally takenalong the kerb and centre lines of theroad. Those undertaking the surveys thenmeasure the actual illuminance levels atthese points and compare them to thecalculated levels to determine theperformance of the installed installationagainst that calculated. This can all beundertaken with a standard photometerand there is no requirement to measurethe S/P ratio on site.

As an example:1. Choose lighting class and lamp type.2. Run design programme. Where the

lamp Ra and S/P ratio permit, theprogramme will target lower

Measurement of the photometric performance of LED lighting Guidance Note 3/16

4 Institution of Lighting Professionals

2 Chair Teresa Goodman National PhysicalLaboratory, email teresa.goodman@npl.o.uk

3 Commission International De L’Éclairage.Recommended system for visual performancebased mesopic photometry. CIE 191:2010.Vienna: CIE

illuminances than that stated for thatclass.

3. These lower illuminances are the valuesthat should be found in on-sitemeasurements.

For example, assume class P2 and a lamphaving Ra>60 and S/P = 2.4 (for example,a cool white metal halide lamp). For thislamp the design programme will target anaverage illuminance of 7.3 lux rather than10.0 lux (see Table 1 in PLG03). On-sitemeasurements using a standardphotometer will be expected todemonstrate an average illuminance of 7.3lux.

When considering an existing lightinginstallation where there is no design dataand the standards permit the applicationof an S/P ratio to reduce the targetphotopic illuminance, then the measuredresults could be ‘reverse assessed’ basedupon the light source S/P ratio and themeasured results, using PLG03 to assignthe appropriate lighting class.

In the example above, if the full measuredgrid provides an average illuminance of7.3 lux, then, through the application ofPLG03, this can be determined ascomplying with the requirements of a P2class.

Uncertainties

As discussed in TR28 and CIE 194 therewill be uncertainties within themeasurement process and these should beconsidered.

References• CIE 194:2011 On site measurement of

the photometric properties of road andtunnel lighting

• BS EN13201-4:2003 Part 4: Methods ofmeasuring lighting performance

• CEN/TC 169 N 1246 prEN 13201-4:2013 Road Lighting, Methods ofmeasuring lighting performance

• ILP TR28 Measurement of road lightingperformance on site

• BS 667:2005 Illuminance meters.Requirements and test methods

• BS 7920:2005 Luminance meters.Requirements and test methods

• ILP PLG03 Lighting for subsidiary roads:using white light sources to balanceenergy efficiency and visual amenity

• CIE 191:2010 Recommended systemfor visual performance based mesopicphotometry

Acknowledgements

Allan Howard WSP|Parsons Brinckerhoff

Consultation:

Teresa Goodman National PhysicalLaboratory/Chair CIETC2-65

Peter Raynham University College London

Steve Fotios Sheffield University

The ILP runs a Practical Skills Course:Light meters, photometers and

measurement.

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