October 8, 2014 1

Is Measuring LEDs with a lux meter accurate?

Ivan Perre

Electrical Engineer

ivanperre@tfl.gov.uk

• Problem with measuring LED light

• How Lux Meters Work

• Practical Advice

Agenda

Benefits of LEDs

Benefits of LEDs

So what’s the problem?

87 Lux 123 Lux 106 Lux

So what’s the problem?

We lack confidence in the results were are getting

It’s a new technology and anything new is under scrutiny

We have a responsibility to our customers and staff to make sure there is sufficient light

Confidence!

• Calibrate the meter to suit that light fitting

• Multiply by a correction factor

• Multiply by the manufacturers SP Ratio

• Don’t worry about illuminance… judge it by eye

• Get an illuminance spectrometer

RecommendationsSome ‘suggested’ ways to overcome this problem

Calibrate the meter

• Send the meter away to test house– Raise a purchase order– Send the meter away with a sample luminaire– Get the meter calibrated– Send the meter back

• Requires ongoing costs and delays to projects

• Gets more complicated when more types of LEDs are used

Calibrate the meter

LED Retrofit

600x600 LED Fitting

What do we do here?

LED Floodlight

LED Pelmet Light

• Colour Correction Factor (CCF) or Spectral Mismatch Correction Factor

• A single lux meter can be used for different light sources, but getting the CCF is difficult

Correction Factor

Single Lux Meter Many LEDs

Correction Factor

LED Retrofit

600x600 LED Fitting

We still have the same problem

LED Floodlight

LED Pelmet Light

Judge by eye

• Works for any type of light

• Extra goodies– Colour Temperature– Colour Rendering– Spectral Power Distribution

• Expensive!!!!!

Get an illuminance spectrometer

• We could find out which lux meters give accurate readings for LEDs…if any

• Could we use them to validate LED installations?

• How would we do this?

But what if…?

Lets find out!

• Problem with measuring LED light

• How Lux Meters Work

• Practical Advice

Agenda

• How do humans see light?

• How do lux meters work?

• Will they work the same way for LEDs?

• How do you tell how good your lux meter is?

Back to Basics

• Light goes into the eye and hits one of two sensors which contribute to sight– Cones– Rods

• Lux meters are modelled against cones

Human Eye

Cones Rods

Colour Wavelength (nm) Luminance (lm)

Violet 400 1

Indigo 430 8

Blue 470 62

Green 520 485

Yellow 580 594

Orange 620 260

Red 650 73

1 Watt of red light detected as 73 lumens1 Watt of green light detected as 485 lumens

Cone SensitivityROYGBIV

Colour Wavelength (nm) Luminance (lm)

Violet 400 1

Indigo 430 8

Blue 470 62

Green 520 485

Yellow 580 594

Orange 620 260

Red 650 73

1 Watt of red light detected as 73 lumens1 Watt of green light detected as 485 lumens

Cone Sensitivity380 – 750 nm

This is how humans see light in most conditions

Lux meters use this V-Lambda curve to replicate the responsiveness of the human eye

V-Lambda Curve

• Optical filters approximate the luminosity function

• Poor approximation Poor Lux Meter

Conventional Lux Meters

Light Source OpticalFilter

Photo Diode Lux Reading

SCF

Spectral Correction

Factor

• The spectrometer is able to accurately measure the lux from any light source

• V-Lambda curve is calculated in software, no optical filters

Light Source Multiple Photosensors

SoftwareProcessing

LuxReading

Handheld Illuminance Spectrometer

£ 1000

Konica Minolta T10

Wavelength (nm)

£ 100

Extech LT300

Wavelength (nm)

£ 100

Extech 401025

Wavelength (nm)

£10 (eBay)

ISOTECH ILM350

Wavelength (nm)

• LED replacement tube

• Kings Cross Thameslink corridor

• Significant blue light component

Example - LED Lighting

Lux Meter Lux Error

Actual 252 -KM T10 250 1%Extech LT300 257 2%ISO Tech 350 205 21%Extech 401025 228 10%

LED - Results

• Traditional T12 Tube

• Deep Tube Station

• Wide Band Spectrum

Example - Fluorescent

Lux Meter Lux Error

Actual 149 -

KM T10 147 1%Extech LT300 151 2%ISO Tech 350 121 21%Extech 401025 135 10%

Fluorescent – Results

• High Pressure Sodium Lamp

• Surface Platforms

• Narrow Band Spectrum

Example - SoN

Lux Meter Lux Error

Actual 127 -

KM T10 122 4%Extech LT300 127 0%ISO Tech 350 102 22%Extech 401025 114 10%

SoN - Results

• Incandescent Lamps

• Not used on the Underground

Example - Incandescent

Lux Meter Lux Error

Actual 360 0%KM T10 359 0%Extech LT300 359 0%ISO Tech 350 363 1%Extech 401025 360 0%

Incandescent - Results

Summary - Results

• Lux meters are typically calibrated to Illuminate A, i.e. incandescent

• Measuring incandescent – Always accurate

• Measuring other light – Not always accurate

Calibrating the meterOne way to calibrate a lux meter is with a Standard Illuminant, a

known source of light

Illuminant A Illuminant D Illuminant F

Calibrating the meter

Standard Illuminant

OpticalFilter

Photo Diode Lux Reading

SCF

Spectral Correction

Factor

Laboratory Conditions

Spectral Correction Factor

Baseline x SCF = Calibrated to Illuminant A

Lux Meter Can be used to measure incandescent light

Colour Correction Factor

Illuminance x CCF = Correct reading for non Illuminant A source

CCF is dependant on the lux meter and the target light source

LED Correction Factors

• Why can’t you just get the lux meter calibrated to a standard LED?

• Why can’t you just use some Colour Correction Factor for your lux meter?

• What makes LEDs so special anyway?

It is worth asking the question,

Illuminate ASpectral Power Distribution is consistent throughout

incandescent family

LEDsLEDs are not consistent

LED Correction Factors

Good quality lux meter – steady correction factorPoor quality lux meter – unsteady correction factor

1.151.17

1.23

1.16

1.06

1.22 1.18

1.23 Lux Meters

• Good quality lux meters

• Do not need to be very expensive

• Have consistent levels of accuracy

• Can measure LEDs with confidence

Lux Meter Verdict

What can you do now to assess your lux meter?

• Problem with measuring LED light

• How Lux Meters Work

• Practical Advice

Agenda

Pro Tip #1

Always ask for the datasheet

You will be surprised how many datasheets do not have the V-Lambda graph

Pro Tip #2

F1’ doesn’t work with LEDsMeasures “Goodness of Fit” against the CIE Curve

Narrow band lights sources still produce significant error, i.e. LEDs

Pro Tip #3

Look at the V-Lamba graph

Which one looks better?

Pro Tip #4

Compare the V-Lambda graph against the CIE curve

Pro Tip #5

Do your own Analysis

The same lux meter!

Useful ResourcesConvert Graphs into Data

WebplotDigitizer

http://arohatgi.info/WebPlotDigitizer/

Linear Interpolation

http://en.wikipedia.org/wiki/Linear_interpolation

CIE Datasets

Colour and Vision Research Laboratory (CVRL)

http://www.cvrl.org/

Correction FactorsBS667:2004 Illuminance meters - Requirements and testmethods

October 8, 2014 52

Thank you