VARIATIONS OF COLOR CORRELATED TEMPERATURE OF WHITE LED LIGHT Mitja Prelovšek, Grega Bizjak Univerza v Ljubljani, Fakulteta za elektrotehniko, Ljubljana, Slovenia Keywords: Light-emitting diodes, white LED, correlated color temperature CCT, CCT consistency, color rendering index, illumination, solid state lighting sources Abstract: Correlated color temperature (CCT) variations and low color rendering index (CRI) present many reasons for not using white light-emitting diodes (LEDs) in lighting. In this research color characteristics of light emitted by LEDs were measured on 29 (In)GaN LEDs of 6 different kinds.We found that color characteristics, notably CCT, are significantly dependent on distance from geometrical axis of the LED, of which the ones that do not use light-diffusion layer were more susceptible to CCT variations. Furthermore, variations of color characteristics of LEDs belonging to the same type were significantly higher than the minimum color difference that human eye can notice. The obtained data suggest, that in orderto achieve sufficient light quality and color consistency between different white LEDs, diffusion layer use is highly recommended, due to its smoothing effect. Razlike v najpodobnejši barvni temperaturi svetlobe belih LED Kjučne besede: svetleče diode, bele LED, najpodobnejša barvna temperatura CCT, konsistentnost CCT, indeks barvnega videza, razsvetljava, pol-prevodniški svetlobni viri Izvleček: Razlike v najpodobnejši barvni temperaturi in indeksu barvnega videza predstavljajo danes glavno oviro pri uporabi belih svetlečih diod (LED) v razsvetljavi. V opisani raziskavi smo izmerili osnovne značilnosti barve svetlobe pri skupaj 29 (In)GaN belih svetlečih diodah šestih različnih tipov. Ugotovili smo, daje najpodobnejša barvna temperatura (CCT) svetlobe LED zelo odvisna od kota opazovanja glede na geometrično os LED. Pojav je najbolj izražen pri LED, ki nimajo nanesenega difuzijskega sloja za razprševanje svetlobe. Ugotovili smo tudi, da so razlike v najpodobnejši barvni temperaturi svetlobe pri LED istega tipa večje, kot je meja zaznavnosti pri človeku. Pridobljeni podatki kažejo, da je za uporabo v razsvetljavi priporočljiva edino uporaba belih LED z difuzijskim slojem, če želimo doseči ustrezno kakovost barve svetlobe. 1 Introduction Color temperature is a way of describing the hue of white light. The term color temperature originates from the correlation between temperature of planckian black body and its emission spectrum. For example, planckian body heated to 3000 K radiates white light with reddish hue, while at 6000 K the light adopts a bluish hue. The hues and corresponding temperatures are shown on Fig 1, which presents a CIExy color space, while the Table 1 presents the exact CCT values with corresponding x,y coordinates. The term correlated color temperature (CCT) is used with light sources, that do not function as black body radiators and therefore, the color of the emitted light does not follow the planckian curve plotted on Fig 1. CCT can be obtained by an algorythm, which moves the color of light source on the planckian curve with a minimum visual change in hue. Obviously this procedure can lead to significant mistakes in color representation, especially if there exists a significant offset of the color of the emitted light from the planckian curve. The CIExy color space, which is shown on Fig 1, is a much more accurate way of specifying color of light. This method of color representation is therefore the prefered one Fig 1: CIExy diagram with piancl 380 480 580 wavelength [nm] 680 Fig 3: Spectrum of GaN white LED However, low CRI is not the only weakness of this type of white LEDs. The other major disadvantage originates from the fact, that the amount of ydlow, which is represented in the spectrum, is directly proportionate to the distance through yellow phosphor, that has to be traversed by the emitted light. Furthermore, since that distance is biggerat larger angles of observation, CCT depends not only on the thickness of the phosphor coating, but also on the angle of observation as shown on the following figure. Here di and d2 label distances through phosphor at an arbitrary angle and on the geometrical axis. phosphor granules d2>di. Die-atach epoxy Fig 4: GaN white LED structure CCT variations and dependancies of GaN white LEDs, that are mentioned above, were measured on 3 types of through-hole LEDs, 2 types of high-output LEDs with diffusion layer and 1 type of high-output LED without diffusion layer. The corresponding viewing angles or maximum radiation angles are presented in Table 2, where diodes have been labeled according to their type with TH, HOD or HO, meaning through-hole, high-output diffusion and high-output respectively. Tabie 2: measured LEDs Label Viewing angle Number of LEDs THl 10° 5 TH2 15° 5 TH3 15° 5 HODl 110° 5 H0D2 110° 4 HO >90° 5 Fig 6: iiigh output LED with diffusion iayer positioned on the measurement holder Atypical through-hole, high-output diffusion and high-output LED can be seen on Fig 5, Fig 6 and Fig 7 respectively. Fig 5: through hole LED 136 Fig 7: high output LED 3 Experimental procedures CCT was measured by rotating the LEDs around their vertical and geometrical axis as shown on Fig 8. LEDs were rotated on two custom-made holders, while the CCT data were measured by a spectrometer, which was positioned 10cm from the LED. Position of the instruments is shown on the following figure. The rotation around vertical axis was in our experiment limited to 70 degrees, because 70 degrees present the angle where luminous flux is usually cut off In normal white LED applications. That is achieved either by a reflector of some sort or the LED itself includes lenses that limit the viewing angle, which is a standard practice with indicator LEDs. Therefore, we have set the angle (p either to the detector vertical axis 135°/ 457/A^' 1 ll(\ geometrical axis Fig 8: rotation around geometrical and vertical axes m Fig 9: position of the instruments viewing angle of the LED or to 70 degrees, if the viewing angle of the LED exceeded 70 degrees. Measurement angle 5 was approximately one half of (p. Measurement angles for corresponding LEDs are shown in Table 3. Table 3: Measurement angles Label Ö 9 THl 5° 10° TH2 7° 15° TH3 T 15° HODl 35° 70° H0D2 35° 70° HO 35° 70° ..... \ 8 /angla_Ü n 1 (S 14 \ yi9 \ X' 14 \ ,1^20 15 .............. \io With this procedure we obtained 17 different measurement points, which are shown as black dots in the following figure: Fig 10: Measurement points Since all of the measured LEDs showed significant CCT dependance on the measurement angle, it was decided, that we will calculate 3 CCT averages for every type of LED - one for every measurement angle (0, 5, (p). Thus we could evaluate the dependance of CCT on the angle of observation. As for CCT variance calculations, we wanted to be able to evaluate CCT variations, which can be noticed, when an observer looks at a luminaire which employs a cluster of LEDs. The observer is in this very typical LED application able to notice the maximum and minimum CCT variations of separate LEDs compared to an average CCT of the whole cluster at that specific angle of observation. Therefore we calculated the variances separately for every measurement point (Fig 10). The average used for variance calculation was the average of measurements of the same measurement angle, that the measurement point belonged to. For example: to calculate a variance in point 4 (Fig 10), we used 5 CCT measurements of the point 4 (one for every LED in the group) along with the average of 40 CCT measurements (5-times 8, for 5 LEDs in a group and 8 measurement points at the measurement angle 5). 4 Results and discussion As mentioned above, CCT data of all of the measured LEDs have shown significant dependance on the measurement angle, that is whether the measurement point belonged to 0, 5 or (p. All of the LEDs without light diffusion layer have shown significant decrease in CCT with increasing measurement angle, while the CCT of LEDs with diffusion layer was slightly higher at angle (p compared to values at angles 0 or 5, as can be seen from Table 4 and Fig 11 Table 4: dependance of CCT from measurement angle 0.07 CIExy distance from geometrical axis average CCT[K1 angle 0 angle 5 angle cp THl 7427 7174 6487 TH2 15840 11358 7137 TH3 16100 8062 7756 HODl 5787 5805 5850 HOD2 6118 6132 6140 HO 5509 5408 5226 1 1.6-1.4 O o 1 CCT dependancy on measurement angle 0.8 0.6; 0.4 - TH1 — TH2 —~ TH3 ^ HODI ^ H0D2 ^ HO 0.2 0.4 0.6 0.8 measurement angle/maximum angle Fig 11: CCT variation These data suggest that unevenly spread diffusion layer on LEDs H0D1 and H0D2 caused the reverse dependance of CCT, which can be seen in Fig 11 To better evaluate the perceived differences in color of light, hovi/ever, it is more precise to use the distance betw/een tw/o points in CIExy space as a representation. Table 5 and Fig 12 show distances of CCT averages at 5 and cp to CCT average on geometrical axis. Minimum difference in color, that human visual system can detect, is approximately 50Kto 100K [2] in range of daylight variations, which span from 2000K to 6000K. 10OK equals approximately 0.007, which is also shown on Fig 12. For an observer it presents a boundary, when he/she notices the change of color of the emitted light. Table 5: Distances from central average in CIExy space Distance in CIExy space angle 5 angle cp THl 4.98-10"' 2.3-10""' TH2 2.59 •10"' 6.38-10"' TH3 6.87-10"' 6.5-10"' HODl 2.79-10"' H0D2 2.95-10^ 4.75-10"^ HO 5.96-10"' 1.68-10"' 007 0 0.2 0.4 0.6 0.8 1 measurement angle/maximum angle Fig 12: CCT variation shown as distance In CIExy space It can be observed, that LEDs, which use diffusion layer show the best performance in terms of angular stability of CCT. Furthermore, these types of LEDs are the only ones, that remain below the minimum noticeable distance in CIExy space, regardless of the measurement angle. It is thought, that use of diffusion layer with GaN+yellow phosphor white LEDs has positive effect on angular stability of CCT. There is, however, another obstacle, which must be dealt with, when using white LEDs and these are differences in light color between different LEDs. We calculated 17 variances, one for every measurement point, for each type of LEDs, as explained in chapter 3. The maximum variances at each angle are shown in Table 6 and Fig 13. Table 6: Maximum variances at measurement angles Maximum variances angle 0 angles angle (p THl 9,5-10"' 1.26-1Ö"' 1,99-10"' TH2 2,35 ■ 10"' 2,7-10"' MO"' TH3 1.73-10 2,8-10"' 2,28-10"' HODl 5-10"' 5,8-10"' H0D2 2.4-10"' 2.99-10"' 9-10"' HO 7.9-10"' 8.18-10"' Again it can be seen, that HODI and H0D2 show the best performance, as they are in general always below the noticeable distance in CIExy space. This can be explained with luminance distribution of GaN white LEDs. The point of the highest luminance, which is called the optical axis of the LED, corresponds with the point of the highest CCT. This is because yellow phosphor can down-shift only a limited amount of light. However, optical axis does not, in general, correspond with geometrical axis of the LED and therefore the point of highest CCT is very rarely positioned on the geometrical axis of the LED itself. As the positioning of the LEDs in our experiment did not reflect the direc- Variance 0.03 0.025 8- 0.02 0.015 5 0.01 0.005: ----- TH1 — TH2 —^ ^ TH3 \ H0D1 ^ H0D2 r \ ^ HO L __— ____-o -----cv-^ 0.007 0.2 0.4 0.6 0.8 measurement angle/maximum angle Fig 13: Maximum variances shown as distance in CIExy space 6 References /1./ M. Prelovšek, Barvne značilnosti svetlobe belih svetlečih diod, FERU, 2006 /2./ D. A. Steigerwald, Illumination Vifith solid state lighting technology, IEEE, Vol. 8, No. 2, str. 310-320, Marec/April 2002 /3./ D. A. Kerr, Color temperature, PE, Vol. 4, November 2005 /4./ http://www.fho-emden.de/~hoffmann/ooltemp18102003.pdf /5./ J. Furlan, Osnove polprevodniških elementov. Tehniška založba Slovenije, 2002 /6./ F. Smole, Fotonski polprevodniški elementi, Založba FE in FRI, 2001 /7./ E. F Schubert, Light-emitting diodes, Cambridge, 2003 /8./ J.K.Sheu, White-light emission from near UV InGaN-GaN LED chip precoated with blue/green/red phosphors, IEEE Photonics technology letters. Vol. 15, No. 1 , str 18-20, januar 2003 /9./ CIE technical report 127-1997, Measurement of LEDs, 1997 tion of the optical axes in any way, it is suggested, that these position variations of the optical axes are the origin of the variances of LEDs without diffusion layer shown in Fig 13. The advantage of using diffusion layers regarding the optical axes problem mentioned above is, that it smooths out the CCT gradient of the LED and consequently makes position variations of the optical axes less noticeable. 5 Conclusion Considering the fact, that LEDs without diffusion layer showed poor performance in terms of CCT variations, it is thought, that the prevailent technology of manufacturing white LEDs today is not capable of enabling a widespread use of white LEDs in illumination. However, use of diffusion layers on LEDs can greatly improve their characteristics, especially in terms of angular stability of CCT. /W/fya Prelovšel< u.d.i.e. Zarnil