The document discusses the challenges of accurately measuring LED light with lux meters, emphasizing the need for calibration and correction factors due to the variability of LED lighting. It outlines methods for improving measurement accuracy, such as using illuminance spectrometers and the importance of understanding how lux meters work in relation to human vision. Additionally, practical advice is provided for evaluating and selecting high-quality lux meters suitable for LED applications.

1. Is Measuring LEDs with a lux meter
accurate?
Ivan Perre
Electrical Engineer
ivanperre@tfl.gov.uk
October 8, 2014 1

2. Agenda
• Problem with measuring LED light
• How Lux Meters Work
• Practical Advice

3. Benefits of LEDs

4. Benefits of LEDs

5. So what’s the problem?
87 Lux 123 Lux 106 Lux

6. So what’s the problem?
Confidence!
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

7. Recommendations
Some ‘suggested’ ways to overcome this problem
• 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

8. 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

9. Calibrate the meter
What do we do here?
LED Retrofit
600x600 LED Fitting
LED
Floodlight
LED
Pelmet
Light

10. Correction Factor
• 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
Single Lux Meter Many LEDs

11. Correction Factor
We still have the same problem
LED Retrofit
600x600 LED Fitting
LED
Floodlight
LED
Pelmet
Light

12. Judge by eye

13. Get an illuminance
spectrometer
• Works for any type of light
• Extra goodies
– Colour Temperature
– Colour Rendering
– Spectral Power Distribution
• Expensive!!!!!

14. But what if…?
• 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?
Lets find out!

15. Agenda
• Problem with measuring LED light
• How Lux Meters Work
• Practical Advice

16. Back to Basics
• 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?

17. Human Eye
• Light goes into the eye and hits one of two sensors which
contribute to sight
– Cones
– Rods
• Lux meters are modelled against cones
Cones Rods

18. Cone Sensitivity
ROYGBIV
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 lumens
1 Watt of green light detected as 485 lumens

19. Cone Sensitivity
380 – 750 nm
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 lumens
1 Watt of green light detected as 485 lumens

20. V-Lambda Curve
This is how humans see light in most conditions
Lux meters use this V-Lambda curve to replicate the
responsiveness of the human eye

21. Conventional Lux Meters
SCF
• Optical filters approximate the
luminosity function
• Poor approximation Poor Lux Meter
Light Source Optical
Filter
Photo Diode Lux Reading
Spectral
Correction
Factor

22. Handheld Illuminance
Spectrometer
Light Source Multiple
Photosensors
Software
Processing
Lux
Reading
• The spectrometer is able to accurately measure the lux
from any light source
• V-Lambda curve is calculated in software, no optical filters

23. £ 1000
Konica Minolta T10
Wavelength (nm)

24. £ 100
Extech LT300
Wavelength (nm)

25. £ 100
Extech 401025
Wavelength (nm)

26. £10 (eBay)
ISOTECH ILM350
Wavelength (nm)

27. Example - LED Lighting
• LED replacement tube
• Kings Cross Thameslink
corridor
• Significant blue light
component

28. LED - Results
Lux Meter Lux Error
Actual 252 -
KM T10 250 1%
Extech LT300 257 2%
ISO Tech 350 205 21%
Extech 401025 228 10%

29. Example - Fluorescent
• Traditional T12 Tube
• Deep Tube Station
• Wide Band Spectrum

30. Fluorescent – Results
Lux Meter Lux Error
Actual 149 -
KM T10 147 1%
Extech LT300 151 2%
ISO Tech 350 121 21%
Extech 401025 135 10%

31. Example - SoN
• High Pressure Sodium Lamp
• Surface Platforms
• Narrow Band Spectrum

32. SoN - Results
Lux Meter Lux Error
Actual 127 -
KM T10 122 4%
Extech LT300 127 0%
ISO Tech 350 102 22%
Extech 401025 114 10%

33. Example - Incandescent
• Incandescent Lamps
• Not used on the
Underground

34. Incandescent - Results
Lux Meter Lux Error
Actual 360 0%
KM T10 359 0%
Extech LT300 359 0%
ISO Tech 350 363 1%
Extech 401025 360 0%

35. Summary - Results
• Lux meters are typically calibrated to Illuminate A, i.e.
incandescent
• Measuring incandescent – Always accurate
• Measuring other light – Not always accurate

36. Calibrating the meter
One way to calibrate a lux meter is with a Standard Illuminant, a
known source of light
Illuminant A Illuminant D Illuminant F

37. Calibrating the meter
Standard
Illuminant
Optical
Filter
SCF
Spectral
Correction
Factor
Photo Diode Lux Reading
Laboratory Conditions

38. Spectral Correction Factor
Baseline x SCF = Calibrated to Illuminant A
Lux Meter Can be used to measure incandescent light

39. 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

40. LED Correction Factors
It is worth asking the question,
• 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?

41. Illuminate A
Spectral Power Distribution is consistent throughout
incandescent family

42. LEDs
LEDs are not consistent

43. LED Correction Factors
1.15 1.17
1.23
1.16
1.06
1.22 1.18
1.23 Lux Meters
Good quality lux meter – Poor quality lux meter – u nssteteaaddyy c coorrrerecctitoionn f afacctotor r

44. Lux Meter Verdict
• Good quality lux meters
• Do not need to be very expensive
• Have consistent levels of accuracy
• Can measure LEDs with confidence
What can you do now to assess your lux meter?

45. Agenda
• Problem with measuring LED light
• How Lux Meters Work
• Practical Advice

46. Pro Tip #1
Always ask for the datasheet
You will be surprised how many datasheets do not
have the V-Lambda graph

47. Pro Tip #2
F1’ doesn’t work with LEDs
Measures “Goodness of Fit” against the CIE Curve
Narrow band lights sources still produce significant error, i.e. LEDs

48. Pro Tip #3
Look at the V-Lamba graph
Which one looks better?

49. Pro Tip #4
Compare the V-Lambda graph against the
CIE curve

50. Pro Tip #5
Do your own Analysis
The same lux meter!

51. Useful Resources
Convert 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 Factors
BS667:2004 Illuminance meters - Requirements and test
methods

52. Thank you
October 8, 2014 52