Recommended
More Related Content
What's hot
What's hot (20)
Similar to Are you confident with your LUX Meter?
Similar to Are you confident with your LUX Meter? (20)
Recently uploaded
Recently uploaded (20)
Are you confident with your LUX Meter?
- 1. August 10, 2017 1 Is Measuring LEDs with a lux meter accurate? Ivan Perre Electrical Engineer ivanperre@tfl.gov.uk
- 2. • Problem with measuring LED light • How Lux Meters Work • Practical Advice Agenda
- 5. So what’s the problem? 87 Lux 123 Lux 106 Lux
- 6. 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!
- 7. • 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 Recommendations Some ‘suggested’ ways to overcome this problem
- 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 LED Retrofit 600x600 LED Fitting What do we do here? LED Floodlight LED Pelmet Light
- 10. • 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
- 11. Correction Factor LED Retrofit 600x600 LED Fitting We still have the same problem LED Floodlight LED Pelmet Light
- 12. Judge by eye
- 13. • Works for any type of light • Extra goodies – Colour Temperature – Colour Rendering – Spectral Power Distribution • Expensive!!!!! Get an illuminance spectrometer
- 14. • 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!
- 15. • Problem with measuring LED light • How Lux Meters Work • Practical Advice Agenda
- 16. • 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
- 17. • 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
- 18. 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 Cone Sensitivity ROYGBIV
- 19. 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 Cone Sensitivity 380 – 750 nm
- 20. 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
- 21. • Optical filters approximate the luminosity function • Poor approximation Poor Lux Meter Conventional Lux Meters Light Source Optical Filter Photo Diode Lux Reading SCF Spectral Correction Factor
- 22. • 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 Software Processing Lux Reading Handheld Illuminance Spectrometer
- 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. • LED replacement tube • Kings Cross Thameslink corridor • Significant blue light component Example - LED Lighting
- 28. 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
- 29. • Traditional T12 Tube • Deep Tube Station • Wide Band Spectrum Example - Fluorescent
- 30. 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
- 31. • High Pressure Sodium Lamp • Surface Platforms • Narrow Band Spectrum Example - SoN
- 32. 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
- 33. • Incandescent Lamps • Not used on the Underground Example - Incandescent
- 34. 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
- 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 Photo Diode Lux Reading SCF Spectral Correction Factor 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 • 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,
- 41. Illuminate A Spectral Power Distribution is consistent throughout incandescent family
- 42. LEDs LEDs are not consistent
- 43. LED Correction Factors Good quality lux meter – steady correction factorPoor quality lux meter – unsteady correction factor 1.15 1.17 1.23 1.16 1.06 1.22 1.18 1.23 Lux Meters
- 44. • 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?
- 45. • Problem with measuring LED light • How Lux Meters Work • Practical Advice Agenda
- 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. August 10, 2017 52 Thank you