Fluorescent Excitation from White LEDs

Transcription

1 Fluorescent Excitation from White LEDs David R. Wyble Munsell Color Science Laboratory Chester F. Carlson Center for Imaging Science Rochester Institute of Technology

2 The Problem? original images from Wikimedia and NASA

3 The Problem content <400nm 1%? 1% 0% 9% percent of nm content between nm original images from Wikimedia and NASA

4 Motivation Growth of solid state lighting for indoor illumination applications is inevitable Many benefits over conventional illumination Cost Life Energy usage Environmental concerns Some issues: Color rendering Fluorescence

5 Characterizing Fluorescence Bispectral spectrophotometry moving slit selects excitation wavelength light source excitation monochromator Record complete emitted spectrum for each excitation wavelength Any detected light not of the same wavelength as excitation indicates fluorescence Example shows material emitting green and red light when being excited by green sample spectral detector emission monochromator

6 Characterizing Fluorescence Excitation wavelength (μ) Emission wavelength (λ) 380 β R β R 390 : β R β R 780 Donaldson Matrix Here: Excitation wavelength (μ) is nm. Emission wavelength (λ) is measured from nm

7 Characterizing Fluorescence Excitation wavelength (μ) Emission wavelength (λ) 380 β R β R 390 : β R β R 780 Donaldson Matrix Here: Excitation wavelength (μ) is nm. Emission wavelength (λ) is measured from nm

8 Fluorescence calculations Reflected Radiance Factor β R,λ [the corrected diagonal] Donaldson matrix Reflected TSV s Fluorescent Radiance Factor Fluorescent TSV s β F,µ,λ W R = k β F,λ = W F = k λ µ λ β R,λ s λ w λ Δλ s µ β F,µ,λ s λ β F,λ s λ w λ Δλ λ = emission µ = excitation W R = X,Y,Z w λ = x λ, y λ, z λ Total TSV s W T = W R + W F Calculate CIELAB from these tristimulus values using D65, 1931 observer.

9 Paper Radiance Factors radiance factor Reflected Luminescent Total wavelength (nm) Illuminant D65 Epson

10 The Experiment Set of 6 typical white office papers Donaldson matrix Bispectral measurements Process Colorimetric data Normalized source data White LED

11 The Experiment Set of 6 typical white office papers Donaldson matrix Bispectral measurements Process Colorimetric data Normalized source data White LED

12 Light Source Details White LEDs Blue LED + yellow phosphor RGB 405 nm LED + yellow phosphor Source normalization Y tristimulus value = 100 Best compromise for the intended application

13 LEDs with Peak at 405 radiance

14 Synthetic LEDs Synthetic 405+Y radiance Original B+Y Maintain the shape of the yellow emission.

15 Normalized Virtual Sources CIE D65 Cool white CIE A normalized units wavelength (nm)

16 Normalized LED Output normalized units NVLAP-1 SSL-5 RGB2 SSL wavelength (nm)

17 The Experiment Set of 6 typical white office papers Donaldson matrix Bispectral measurements Process Colorimetric data Normalized source data White LED

18 Substrate Details White office paper Standard Epson stock All exhibit fluorescence to some degree radiance factor Reflected Luminescent Total Illuminant D65 Epson wavelength (nm)

19 Substrate Details radiance Q5462A S S S04124 S Excitation spectra

20 What Do We Expect? radiance excitation range sources

21 Results radiance factor Reflected Total wavelength (nm) Calculate a color difference between the reflected and total radiance factors. How visible is the change imposed by the luminescent radiance factor?

22 Results radiance factor Reflected Total wavelength (nm) Calculate a color difference between the reflected and total radiance factors. How visible is the change imposed by the luminescent radiance factor?

23 Results Light Sources papers RGB NVLAP-1 SSL-5 SSL-3 CIE D65 CIE A Cool white Q5462A S S S S ΔE * ab between luminescent and total radiance factors.

24 Results ΔE * ab sources illuminants Q5462A S S04124 S S papers RGB NVLAP-1 SSL-5 SSL-3 D65 A Cool white Interpretation: this shows the color difference that would be expected by illuminating each paper under the given light source if paper OBAs were removed. Put another way, this is as though two papers were viewed side by side, one with and one without fluorescent OBAs.

25 Results ΔE * ab sources illuminants Q5462A S S04124 S S papers RGB NVLAP-1 SSL-5 SSL-3 D65 A Cool white Interpretation: this shows the color difference that would be expected by illuminating each paper under the given light source if paper OBAs were removed. Put another way, this is as though two papers were viewed side by side, one with and one without fluorescent OBAs.

26 excitation radiance wavelength (nm)

27 Conclusions and Recommendations Commercially available white LEDs (RGB, B+Y) do not adequately excite office paper optical brightening agents. An LED configuration including a lower wavelength source, such as the 405nm blue, can provide the necessary excitation to preserve paper appearance. Alternative strategies could include adjusting the paper OBA chemistry. None of the issues are show stoppers compared to the other significant benefits of solid state lighting. Printing, packaging and other graphic arts applications could potentially require process adjustments.

28 Future Work Add substrates and sources Your contributions are encouraged Data or samples Simulation images Consider other aspects of sources CRI, cost, lifetime, etc Luminescence - safety markings Not future work (for this researcher): Engineer a new LED Adjust OBA chemistry

29 Acknowledgments Funding for this work came in part from Munsell Color Science Laboratory RIT s Center for Imaging Science X-rite Incorporated Many thanks for technical discussions and the sharing of data from: Dr Cameron Miller (NIST) Dr Art Springsteen (Avian Technologies) Mr Jim Leland (Gamma Scientific)