Dirty LED: effect of dust, fat, fingerprints, water, oil and coal on light output

Transcription

1 Dirty LED: effect of dust, fat, fingerprints, water, oil and coal on light output Ivan Moreno Unidad Academica de Fisica, Universidad Autonoma de Zacatecas, 98060, Zacatecas, Mexico. ABSTRACT The output power of a light-emitting diode (LED) not only is affected by aging but also by dirt buildup. Environment and surroundings are typically characterized by the presence of substances, dust, liquids or vapors that may stick to the LED, reducing its light output. Knowing the effect of dirt on light output, manufacturers and users can efficiently design a cleaning or maintenance program. In this work, both 5-mm LEDs and high-power LEDs were subjected to output power tests for different degrees and types of dirt. In particular, I measure the light flux changes due to deposition of dust (sand), drops of water, coal dust, oil drops, fat (soldering paste), and fingerprints. Key words: Light-Emitting Diode, Dirt, Output Flux, Light Level, Light extraction. 1. INTRODUCTION Now, LEDs are everywhere and cover a wide range of applications from indicator lights to solid-state lighting. They are gradually taking over traditional radiation sources because of their attractive characteristics. 1-3 However, any type of light source is sensitive to the dirt that surrounds the environment (Fig. 1). The light level depreciation based upon the accumulation of dirt upon the LED surface is a key component in the determination of the minimum light flux. And then LED systems must be properly designed to assure they will efficiently operate in real environments. 4,5 Light losses due to dirt accumulation may have a major impact on optical efficiency. To account for this degradation in lighting levels, the LED output power should in most cases be significantly over-specified. This excess of lighting power could be wasted energy that can be saved with an appropriate maintenance program. Fig. 1. Many environmental factors may facilitate the dirt accumulation. * Please do not hesitate to contact me for any question at: imoreno@fisica.uaz.edu.mx Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XIV, Klaus P. Streubel; Heonsu Jeon; Li-Wei Tu; Norbert Linder, Editors. Proceedings of SPIE Vol. 7617, 76171S (2010) 2010 SPIE ISSN: (print) $ 18.00

2 The output power of an LED not only is affected by aging. As with any light source technology, there are many circumstances that may alter or attenuate the effective luminous flux of an LED. 3,6-8 The main factors could be: LED aging (lumen depreciation) LED dirt Ambient Temperature and LED junction temperature Permanent damages of the encapsulating package Burnouts Some of these items are irreversible, meaning that conventional maintenance does not improve or correct these shortcomings. And then individual or group LED replacement is the only way. Dirt however is correctible, because cleaning maintenance can eradicate the negative effects of dirt accumulation. Knowing the effect of dirt on light output, manufacturers and users can efficiently design a cleaning or maintenance program. Therefore, the objective of the study here presented was to investigate the relationship between dirt build up and light output of LEDs. Two types of LEDs (5- mm type and high-power type) were under test for different degrees and types of dirt. In particular, I measured the light flux changes due to accumulation of dust (sand), drops of water, coal dust, oil drops, fat (soldering paste), and fingerprints. 2. EXPERIMENTAL PROCEDURE To measure the relationship between dirt deposition and light flux output, other factors of light loss were minimized by controlling ambient temperature and electric current. Therefore it is assumed to be no other changes in the light emission of LEDs under test, only those based upon the deposition of dirt. All LEDs are driven with a very low direct current (DC) to minimize the thermal effects associated with the driving current. The room temperature is maintained constant. LEDs under test are fixed to a board that can accommodate one LED, which is driven by a constant current source. The test setup consists of a computer-controlled spectrophotometer, integrating sphere, current-source meter, and a characterizer digital camera. In order to avoid the contamination of the inner surface of the integrating sphere, the LED was positioned underneath the integrating sphere (Fig. 2). The LED was driven with constant current. And the radiant flux was measured. After each test, the degree of dirt was increased for each additional measurement. The integrating sphere was free to move up and down according to experimental needs for serial measurements. Fig. 2. Measurement set up.

3 To perform the measurements I selected two representative types of LED: the traditional 5 mm package and a high power LED with hemispherical dome. Three LEDs were characterized: one blue high power LED, one 5 mm blue LED, and one 5 mm white LED. Light depreciation of LEDs may be quantified in several ways. 3,8 We measured the radiant flux (Watts) for monochromatic LEDs and the luminous flux (Lumens) for the white LED. One difficult task was figuring out how to quantify the degree of dirt. We have not found a convenient way yet, so this remains an open problem for forthcoming research. This and other experimental difficulties will be carefully described in a forthcoming publication. Mean-while we chose to take a picture of the most dirty LED for each measurement, which is included in almost all figures in the next section. 3. EXPERIMENTAL RESULTS The measurement results are graphically presented in Figures (3)-(10). They show how light output flux decreases as dirt accumulates. The values of relative light output in Figs. (3)-(8) are displayed for blue LEDs. The measurement with white light LEDs is shown in Figs. (9) and (10). The relative light output is strongly dependent upon the cleanliness of the LED for certain types of dirt. In particular, attention must be paid for sand and coal (Figs 3, 4, 9 and 10), which show a drop below the standard 70% light level. 6 It calls our attention that the effect of water drops and fingerprints (Figs. 5 and 6) is not appreciable, this fact clarifies a myth (for some people) about fingerprints and water drops as light loss factors. Even more surprisingly is that the light extraction may be slightly increased with water drops (5 mm LEDs). Because dirt is deposited over the encapsulating lens, the differences between the 5 mm LED and the high-power LED lead us to conclude that the effect of dirt is quite dependent on encapsulating lens shape. 9 It should be mentioned that cleaning a dirty LED practically restores its initial optical flux. Additionally, it could be interesting to determine the effects on the angular radiation pattern (see Fig. 11). However, a suitable method for performing the required measurements must be designed in further studies. Fig. 3. Light output change from both a high-power LED and a 5 mm LED as dust of coal increases.

4 Fig. 4. Light output change from both a high-power LED and a 5 mm LED as dust of sand increases. Fig. 5. Light output change from both a high-power LED and a 5 mm LED as water drops wet its surface. Fig. 6. Light output change from both a high-power LED and a 5 mm LED as the number of fingerprints increases.

5 Fig. 7. Light output change from both a high-power LED and a 5 mm LED as fat (soldering paste) increases. Fig. 8. Light output change from both a high-power LED and a 5 mm LED as the number of oil drops increases. Fig. 9. Light output change from both a white LED and a blue LED (both are 5 mm LEDs) as dust of coal increases.

6 Fig. 10. Light output change from both a white LED and a blue LED (both are 5 mm LEDs) as dust of sand increases. Fig. 11. This picture of a dirty LED allows us to anticipate a strong distortion of the radiation pattern due to dirt.

7 4. SUMMARY In summary, we analyzed how the output power of an LED is affected by dirt buildup. This is important because real environments are characterized by the presence of substances, dust, liquids or vapors that may stick to the LED, reducing its light output. And knowing the effects of dirt, manufacturers and users can effectively design a cleaning or maintenance program. Both types 5-mm LEDs and high-power LEDs were tested for four different degrees of dirt. The measurements included several types of dirt. In particular, I deposited dust (sand), drops of water, coal dust, oil drops, fat (soldering paste), and fingerprints. It was shown that the light depreciation strongly depends of dust of sand and coal, dropping below the standard 70% light level. In the other hand, the effect of water drops and fingerprints seems not to be significant, which shows that fingerprints and water drops are not important light loss factors. Many issues remain open for further investigations. Just consider the wide variety of types of LEDs and the many kinds of dirt. For example, further work may include other type of LED packages, other colors, other types of dirt (for example snow). Also, future studies must address dirt build up in real environments. In addition, for many applications it could be very useful to characterize the effects on the angular radiation pattern due to dirt. And a complex problem remains: how to assess the degree of dirt? REFERENCES 1. M. H. Crawford, LEDs for solid state lighting: Performance challenges and recent advances, IEEE J. Sel. Top. Quantum Electron. 15(4), (2009). 2. R. D. Dupuis, M. R. Krames, History, Development, and Applications of High-Brightness Visible Light-Emitting Diodes, J. Lightwave Technol. 26, (2008). 3. M. Bürmen, F. Pernuš, and B. Likar, LED light sources: a survey of quality-affecting factors and methods for their assessment, Meas. Sci. Technol. 19, (2008). 4. L. Svilainis, V. Dumbrava, Estimation of the louvers influence on LED video display image visibility IEEE Proceedings of the ITI 2009, (2009). 5. M. Fullerton, E. Peli, DEVELOPMENT OF A SYSTEM TO STUDY THE IMPACT OF HEADLIGHT GLARE IN A DRIVING SIMULATOR, Proceedings of the Fifth International Driving Symposium on Human Factors in Driver Assessment, (2009). 6. N. Narendran, Y. Gu, Life of LED-based white light sources, Journal of Display Technology, Vol. 1, (2005). 7. M. Meneghini, L. R. Trevisanello, G. Meneghesso, E. Zanoni, A Review on the Reliability of GaN-Based LEDs, IEEE Transactions on Device and Materials Reliability, Vol. 8, 323 (2008). 8. C-C Tsai, M-H Chen, Y-C Huang, Y-C Hsu, Y-T Lo, Y-J Lin, J-H Kuang, S-B Huang, H-L Hu, Y-I Su, W-H Cheng, Decay Mechanisms of Radiation Pattern and Optical Spectrum of High-Power LED Modules in Aging Test IEEE Journal of Selected Topics in Quantum Electronics, Vol. 15, 1156 (2009). 9. I. Moreno, D. Bermúdez, M. Avendaño-Alejo, "Light-emitting diode spherical packages: an equation for the light transmission efficiency," Appl. Opt. 49, (2010).