Practical Optical Measurements of OLED Panels for Lighting Applications TOKI KAWABATA KONICA MINOLTA, INC. JAPAN YOSHI OHNO National Institute of Standards and Technology U.S.A. * This work was conducted when TOKI was a guest researcher at NIST from January 2012 to January 2013.
1. Background OLED is thin and diffuse area light source and has other unique features. OLED will be cost effective energy efficient lighting. The present problems of OLED panels are cost, lifetime, and efficacy. 2
1. Background Some manufactures provide OLED panel samples and some OLED luminaires. Standardized test method for OLED will be needed. CIE and IES has started to standardize, but enough measurement data has not been available. 3
2. Outline of presentation Optical characteristics of OLED panels were measured and evaluated including basic characteristics, operating position, angular distributions, spatial non-uniformities and life time. 4
2. Outline of presentation Finally, suitable measurement methods and conditions for testing OLED panels are discussed. 5
3. Specifications of OLED panels 6 OLED panel samples from 5 manufactures Specifications (given by manufactures) Size: 50 x 50 mm to 80 x 80 mm Thickness: 1 mm to 3 mm Current : 70 ma to 400mA Power: 0.03 W to 4 W Efficacy: 10 lm/w to 45 lm/w 6
3. Specifications of OLED panels Figure 1. The chromaticity coordinates (u'v') of test samples 7
3. Specifications of OLED panels Figure 2. Examples of spectral power distributions of OLED panels tested 8
4. Measuring equipment and tested characteristics Figure 3. Sphere geometries for total luminous flux measurement using a spectroradiometer. 9
4. Measuring equipment and tested characteristics Spectroradiometer with a diffuser for cosine correction Wavelength range : 380 nm to 780 nm at 1 nm intervals Spectral bandwidth : 5 nm or less Figure 4. Pictures of spectroradiometer and sphere. 10
5. Measurement results 5.1. Electrical characteristics Figure 5. Current-to-voltage characteristics. 11
5.2. Total luminous flux vs. electrical parameters Figure 6. Total luminous flux vs. forward current. 12
5.2. Total luminous flux vs. electrical parameters Figure 7. Total luminous flux vs. forward voltage. 13
5.3. Time for stabilization (a) After 0 min from power-on (b) After 10 min from power-on Figure 8. Stabilization characteristics; total luminous flux vs. operating time. 14
5.3. Time for stabilization (a) Chromaticity x (b) Chromaticity y Figure 9. Stabilization characteristics; chromaticity (x, y) vs. operating time, after 10 min from power-on. 15
5.4. Temperature characteristics Figure 10. (a)total luminous flux vs. temperature 16
5.4. Temperature characteristics (b) (c) Figure 10. (b)chromaticity x vs. temperature (c) Chromaticity y vs. temperature 17
5.4. Temperature characteristics Figure 11. Forward voltage vs. temperature. The ambient temperature should be kept stable for accurate measurements. 18
5.5. Position characteristic Figure 12. Sphere geometry for measurement of operating position sensitivity. 19
5.5. Position characteristic (a)δφ without stabilization (b) ΔΦ with stabilization Figure 13. Total luminous flux vs. operating position, with and without stabilization. 20
5.5. Position characteristic (a) Δx without stabilization (b) Δx with stabilization Figure 14. Chromaticity (x,y) vs. operating position, with and without stabilization. 21
5.5. Position characteristic (c) Δy without stabilization (d) Δy with stabilization Figure 15. Chromaticity (x,y) vs. operating position, with and without stabilization. 22
5.6. Luminance spatial uniformity (a) Luminance (b) Temperature Figure 16. An example of spatial distribution of luminance and temperature 23
5.7. Angular characteristics (a) Angular luminous intensity (b) Chromaticity xy shifts Figure 17. Angular intensity distribution and color shifts at observation angles from 0 to 60. 24
5.10. Life Time (ongoing) Figure 18. Life time of Total luminous flux 25
Δx Δy 5.10. Life Time (ongoing) (a) Δx (b) Δy Figure 19. Life time of chromaticity (x, y) 26
6. Discussions * Constant current should be supplied to the OLED panel for testing. * The ambient temperature should be kept in a small tolerance range (e.g. 25 ±1 ). * The operating position of OLED panel affects the luminous flux and chromaticity of the OLED panel and must be kept consistent or always reported. 27
6. Discussions * There are significant drift in light output from OLED panels. 15 min stabilization time may be sufficient. * Color of OLED panels changes with different emission angles. 28
6. Discussions * Angular intensity distributions of OLED panels are near Lambertian, but the degrees of deviation from Lambertian characteristics largely differ by different OLED panels. * The spatial nonuniformity of luminance and color can also be an issue for light quality of the OLED panel products. 29
7. Conclusions Various optical characteristics of OLED panels have been investigated. Critical measurement conditions have been identified and some recommendations have been given. These conditions should be used for accurate and reproducible measurements of OLED panels. 30
7. Conclusions The measured results reported here may be used for developing a standard test method for OLED panels. The aging characteristics of these panels are in progress and test method for lifetime characteristics is under investigation. 31
Acknowledgements 32
Thank you for your attention tokihisa.kawabata@konicaminolta.jp 33