Optical Measurement Techniques for LED Clusters and LED Safety Hugh Barton OptiConsulting UK A presentation to EuroLEDs conference 16-May-2006 Stor-Lampe 1
OptiConsulting UK Optical metrology, Optical product assessment, Technical investigations, Standards development. Associate Member, Institution of Railway Signal Engineers Member and UK Representative, CIE-UK, Division 4 Lighting for Signalling and Transport. Member, Optical Radiation Measurement Club, National Physical Laboratory. Convenor, CEN TC256 Working Group 9 Railway Applications - Lighting. Expert, CEN TC256 Working Group 2 Railway Applications Structureal Requirements. Chairman, BSI Committee RAE/1/-/7, Railway Applications Lighting Chairman, "LED Clusters" focused interest group, hosted by the National Physical Laboratory. 2
LED Cluster. an array of two or more surface-emitting, visible LEDs within the same assembly and intended to be viewed together Best Practice Guide - Optical Measurement of LED Clusters, NPL, draft 2006 OptiLED Leda-Lite Hella 3
Overview 1. Purpose of Optical Measurements 2. Optical Quantities of Interest 3. Measurement Techniques 4. Best Practice 5. LED Safety 4
1. Purpose of Optical Measurements Design / development Optical characterisation Assessment / Specification compliance Investigations into service performance These demand quality and consistency 5
2. Optical Quantities of Interest Photometric. Luminous Intensity cd Luminance cd.m -2.... As a function of electrical supply conditions, lit duration, projected angle, range of ambient temperatures, etc, etc 6
Photometric considerations Photopic / Mesopic photometry Luxeon Point source? 7
2. Optical Quantities of Interest Spectroradiometric. Spectral power distribution, for the determination of dominant wavelength, calculation of CIE(1931) chromaticity co-ordinates, colour-rendering values, k-values, etc... As a function of electrical supply conditions, lit duration, projected angle, range of ambient temperatures, etc, etc 8
Typical spectral power distribution plots OptiConsulting 9
3. Measurement Techniques Considerations: Measurement requirements Equipment selection Measurement environment Calibration Measurement techniques 10
3. Measurement Techniques Goniophotometry Courtesy Craig Gutteridge 11
3. Measurement Techniques Goniophotometry / Spectroradiometry Courtesy Craig Gutteridge Spectradiometer may be: Scanning CCD Imaging Filter radiometer 12
4. Best Practice The LED Clusters Focused Interest Group (FIG) set up within NPL s ORM Club in December-2002, and terms of reference were established. The FIG has wide representation and aims to promote a better understanding of optical measurement issues in connection with LED Clusters. Round-robin measurements conducted on a Westinghouse LED Signal aspect and Marl lamp replacement modules.. the findings of this exercise were used to identify the measurement issues of interest. These measurement issues were used to develop a Best Practice Guide document, containing references to existing documents (notably CIE 127-1997, IALA E-122 2001 and ZVEI LED Definitions ). 13
4. Best Practice Artefacts used for the round-robin measurements: Westinghouse prototype signal aspect Marl LED Lamp replacement unit 14
4. Best Practice BPG contents: 1. History of LED Cluster sample 2. Environmental conditions 3. Electrical conditions 4. Geometrical considerations 5 Measurement issues 6. Calibration issues 7. Other issues 7.1 LED safety considerations. 8. Reporting 15
4. Best Practice 1.1 Tested as-new or after a specified burn-in 1.2 Thermal history of test sample ( cold-start or conditioned ) 2.1 Ambient temperature. Necessary for testing consistency. Range chosen to fit with existing standardisation: 25 +5-10 ºC. 16
4. Best Practice 2.2 Local Temperature (e.g. on LED chip) Necessary to monitor stabilisation for testing purposes, and to provide an indicator of LED junction temperature, which determines lumen depreciation, and therefore service life. LED Cluster manufacturer to specify location of Tc-point and to include suitable electrical connections. Implications on accelerated testing? 17
4. Best Practice 2.3 Ambient / Stray Light To be minimised and/or factored out. 3.1 Supply Voltage / Current & Stability Appropriate electrical modes: Constant current (as for individual LED, ref. CIE-127) Rated supply voltage Range of supply voltages Supply voltage is particularly relevant for pulse-width modulated operation of LEDs. 18
4. Best Practice 4.1 Definition of axis (geometrical / optical): Mechanical / optical axis of LED Mechanical Axis + Optical Axis Geometric axis of LED Cluster (reference to enclosure or alignment device), Optical axis (peak or mid-point between 50% intensity values) Optical Axis 19
4. Best Practice 4.2 Definition / Interpretation of Lit area of LED Cluster. Necessary to define according to the application 4.3 Determination of the Minimum Measurement Distance. Necessary to avoid calculated photometric result varying with measurement distance. Possible by empirical techniques. Possible by calculation, e.g. IALA E-122 Section 2.4 Zero-length photometric arrangement. 20
4. Best Practice 5.1 Colour as a function of lit duration. Spectral measurements using scanning spectroradiometers require full stabilisation of the test sample In the case of flashing lights, fast measurement devices are required.. Referenced to scanning spectroradiometer, and integration time must allow averaging if flash rate is less than the critical flicker frequency. 5.2 Colour as a function of angle. Goniospectroradiometry! 5.3 Photometric quantity as a function of lit duration. Photometric measurements may require full stabilisation. Monitoring of thermal stabliisation. 21
4. Best Practice 5.4 Photometric quantity as a function of angle. Goniometer, ref. CIE 43 or CIE 70 Quote the convention used for the angular measurements. 5.5 Photometric quantity as a function of angle. As for colour (Section 4.3) 5.6 Definition of measurement area for luminance measurements. Sampling and averaging effects. Example shows difference in luminance caused by measurement area / fill factor 22
4. Best Practice 5.7 Measurement of phantom / swamping effects. LED Cluster designs generally have differnet reflective characteristics conpared to equivalent lamp / filter based systems. Need to be able to measure, in order to predict field performance. Need to control illuminance, spectral properties of external source, and geometrical arrangement. 5.8 Special requirements for flashing and/or pulsed LED Clusters 23
4. Best Practice 6.1 Calibration of illuminance meters. Dynamic range, linearity, display resolution, spectral error, alignment, temperature characteristics, cosine correction. 6.2 Calibration of filter radiometers. Ditto as above, spectral correction where appropriate. 6.3 Calibration of standard lamp / detector / meter. CIE 127 Sections 2.2.1.4, 4.1 and 4.2. Calibration of reference LED of the same type and colour as those in the LED cluster sample. 24
4. Best Practice 6.4 Measurement uncertainties. Under development. 6.5 Traceability of calibration. To establish a valid calibration of a measurement process, with step-by-step comparison up to an accepted or specified standard (usually a national or international standard). UKAS accredited laboratories. For novel measurements, NPL can provide measurement solutions Importance of quoting test conditions with test results. 25
5. LED Safety LED Safety considerations.. Photochemical hazard. Lasers and LEDs (since 2001) covered by BS EN 60825-1:1993 (industrial and consumer products).. AEL - Accessible Emission Limit.. MPE Maximum Permissible Exposure limit Emission Safety Factor BS EN 60825-1 originally authored for industrial laser products, and based on ICNIRP guidelines. Application to LEDs requires some interpretation and has caused confusion. Previous interpretations suggest the risk from naked LEDs present low eye hazards, but LEDs with additional optical elements are potentially hazardous...consider latest LED emitters! BS EN 60825-1 contains tables for continuous wave operation. 26
5. LED Safety LED Safety considerations (continued). For pulsed conditions, a range of approaches must be considered: Pulse train analysis, etc. Wavelength. for single colour LEDs, λp can easily be determined for white LEDs the peak of the UV or blue line is suggested. Beam propagating from LEDs makes analysis difficult. Physical Agents Directive (Artificial Optical Radiation) was published in the Official Journal of the European Union 27-April. includes optical radiation threshold limits, which are expected to become UK law in 2009 / 2010. 27
5. LED Safety Physical Agents (Optical) Directive 2006 Key features. Covers 100nm to 1mm, based on ICNIRP requirements, Requirements apply to workers and exposure to radiation (not products), Employers should assess the likely exposure from artificial optical radiation, Measurements and/or calculations are only required if there is a potential problem, An investigation should be carried out if there is an over-exposure, Information, instruction and training should be provided, There should be hazard demarkation and labelling and, if appropriate, health surveillance. 28 Residual concerns may be addressed in National legislation.
5. LED Safety LED Safety considerations further information. Comparative Optical Radiation Safety Analysis of New LED devices and lamps, Horak, W and Neuhaus, R, 2nd CIE Expert Symposium on LED Measurement. Occupational Exposure to Optical Radiation in the Context of a Possible EU Proposalfor a Directive on Optical Radiation, O Hagan J.B., Driscoll J.M.H, Pearson A.J., NRPB-W35 (2003) Proposed Physical Agents Directive, Smith, N, HSC Paper 04/75 (2004). HSE Proposal for a Physical Agents (Optical) Directive Initial Regulatory Impact Assessment (2004). Directive 2006/25/EC on the minimum health and safety requirements regarding the exposure of workers to risks arising from physical agents (artificial optical radiation). 29
Optical Measurement Techniques for LED Clusters and LED Safety Thank you for your attention Any questions or suggestions on the Best Practice Guide? hugh@opticonsultinguk.com 30