Characterization of LED luminous flux and photometer spectral responsivity

Tuomas Poikonen Metrology Research Institute

Research Seminar on Measurement Science and Technology MIKES, 20.10.2010

Licentiate thesis

Publication 1 T. Poikonen, P. Manninen, P. Kärhä, and E. Ikonen, “Multifunctional integrating sphere setup for luminous flux measurements of light emitting diodes,” Rev. Sci. Instrum. 81, 023102 (2010).

Publication 2 T. Poikonen, P. Kärhä, P. Manninen, F. Manoocheri, and E. Ikonen, “Uncertainty analysis of photometer quality factor f1’,” Metrologia 46, 75–80 (2009).

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Characterization of LED luminous flux and photometer spectral Responsivity

Table of Contents

•  Characterization of LED Luminous Flux -LEDs, luminous flux, CIE 127 -Challenges in flux measurements -Multifunctional measurement setup -Measurement uncertainty

•  Photometer Spectral Quality -Relative Spectral Responsivity -Spectral quality factor f1’ -Monte Carlo simulation -Simulation results

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Characterization of LED Luminous Flux

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Light Emitting Diodes (LEDs)

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  LEDs are changing the lighting industry -Energy efficiency, flexibility, long lifetime -White LEDs are replacing traditional light sources -Measurement methods need to be modified to be applicable with LEDs

Luminous Flux

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Total power of visible light emitted into 4π sr solid angle •  Photometric quantity, V(λ)-weighting, unit: Lumen, lm •  Luminous flux of LEDs is needed for

-Determining luminous efficacy, lm/W (high-brightness LEDs) -Product design (luminaires, car headlights, mobile phones etc.)

•  CIE 127-2007 Technical Report, Measurement of LEDs

CIE 127-2007, Total Luminous Flux

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

CIE 127-2007, Partial LED Flux

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Challenges in Flux Measurements

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Size of the sphere vs. signal level •  Spectral responsivity (large corrections) •  Spatial uniformity (ports, baffles, holder) •  Optical properties of LEDs

-Angular spread (directional <-> side-emitting) -Absorption of backward emission (holder design) -Temperature control / cooling of high-power LEDs

•  Calibration of the measurement setup -Standard LEDs (color & angular spread) -Standard incandescent lamp (flexible)

Multifunctional Measurement Setup

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Multifunctional Measurement Setup

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Luminous flux measurement of test LED -Luminous flux signal (photometer) -Spectral irradiance (spectroradiometer) -Self-absorption or self-reflection (photometer + aux LED) -Angular intensity distribution (optional, data sheets)

•  Correction factors needed in the analysis -Spectral mismatch correction -Self-absorption or self-reflection correction -Spatial correction (not needed for directional LEDs)

Multifunctional Measurement Setup

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Multifunctional Measurement Setup

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Luminous flux responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Relative spectral responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Spatial responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Spatial responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Spatial responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

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IV = cosn (ϕ)

Measurement uncertainty

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Expanded uncertainty (k = 2) is 1.2 – 4.6 % -Depends on measurement mode, LED color and angular spread

•  Largest uncertainty components -Calibration of luminous flux responsivity (0.3 – 0.5 %) -Spectral mismatch correction (0.4 – 2.1 %) -Spatial nonuniformity correction (0.1 – 0.6 %)

•  LEDs have large variation in optical properties -Uncertainty of each LED measurement is studied individually

Photometer Spectral Quality

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Photometer spectral quality factor f1’

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Photometer spectral responsivity should be close to V(λ) •  Photometer spectral quality factor f1’, CIE 53

-Gives information of typical measurement errors (broadband) -Cannot be used as correction factor (still useful) -Ideally f1’ = 0 %. For good photometers f1’< 3.0 %

•  Standardized method for analyzing the uncertainty of f1’ not yet agreed

Relative Spectral Responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Two standard photometers were measured •  Reference spectrometer

-Monochromator-based light source -Reference detector with known responsivity (Si-trap) -Linear translator for switching detectors

•  Uncertainty budget -Consists of random and biased uncertainties -One of the largest components is the wavelength scale uncertainty of the monochromator (±0.06 nm)

Relative Spectral Responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Relative Spectral Responsivity

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Monte Carlo analysis of f1’

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Statistical method for uncertainty analysis -Large number of values are calculated (n = 100 000) -Data is perturbed within measurement uncertainties -Distribution of values is obtained

•  Random error model is often used -Each wavelength is perturbed using a different random number -May lead to underestimated uncertainty of f1’

-Not applicable for biased uncertainties

Random error model

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Biased error model

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

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sbias(λ) = srel (λ) +δrnd ⋅ εwl (λ)

Result of f1’ MC-simulations

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Result of f1’ MC-simulations

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

Conclusions

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity

•  Multifunctional setup was constructed for luminous flux measurement of low- and high-power LEDs

•  All geometries of CIE-127 can be used •  Expanded uncertainty (k = 2) is 1.2 – 4.6 % •  Wavelength uncertainty of spectral responsivity

measurement may cause large biased uncertainty •  A biased error model was introduced and should

be used as the basis for the uncertainty analysis •  The simulation methods are applicable also for

other spectral integrals

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Characterization of LED Luminous Flux and Photometer Spectral Responsivity