Characterization of LED luminous flux and photometer...

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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Light Emitting Diodes (LEDs)

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•  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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•  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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CIE 127-2007, Partial LED Flux

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Challenges in Flux Measurements

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•  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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•  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)


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Luminous flux responsivity

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Relative spectral responsivity

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Spatial responsivity

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€

IV = cosn (ϕ)

Measurement uncertainty

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•  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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Photometer spectral quality factor f1’

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•  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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•  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)


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Monte Carlo analysis of f1’

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•  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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Biased error model

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€

sbias(λ) = srel (λ) +δrnd ⋅ εwl (λ)

Result of f1’ MC-simulations

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Result of f1’ MC-simulations

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Conclusions

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•  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...

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