Studies on daylight-responsive dynamic lighting system

Abstract

The daylight-integrated artificial lighting system demands a special type of light source of tunable spectral composition and luminous flux in accordance with those of available daylight at the window plane or installed daylighting system. Variation of spectral composition of daylight results in variation of its correlated color temperature (CCT). In this research work a systematic approach is taken up to conceive suitable lighting control algorithm and subsequently to design and develop a daylight-responsive dynamic lighting system which is capable of monitoring dynamic nature of daylight and hence tunes its spectral composition and lumen output in consonant to instantaneous available daylight to ensure visual comfort of occupants in an indoor space illuminated by daylight-integrated artificial lighting system. In first stage of the study an artificial dynamic lighting system (DLS) is designed, developed whose spectral composition and light output vary according to variable CCT and illuminance of available external daylight. In the second stage of the study, the performance is experimentally evaluated to achieve daylight-responsive dynamic lighting system by two testing procedures- laboratory testing and in-situ testing. Attempts are made to design the target lighting system by using two types of LED chips of two different spectral compositions such that one emits yellowish-white (WW) and the other emits bluish-white (CW) light energy. This composite WLED module exhibits the range of CCT from 2840-5750K while the target range is 2700-6000K. On the other hand, it exhibits the range of lumen output from 0-648 lm while the target range is 0-600 lm. To operate the developed composite WLED module as a tunable lighting system, a light controller, named as dynamic light controller (DLC), is designed and developed. The DLS is realized by connecting the DLC with the composite LED module. Light control logic of developed DLC is based on Grassman’s color mixing law and hence the range of CCT variation of the developed composite WLED module lies within the CCT values of the WW and the CW LEDs. A pulse width modulation (PWM) technique is applied to implement the light control logic and embedded in a PIC 18F4550 microcontroller to control the light output of two WLED arrays through enhanced PWM mode with dual PWM output. The developed controller is connected with the composite LED module and controllable by infrared (IR) remote within a distance of 4 m. Performance evaluation of the developed DLC is carried out by driving the DLS for three operational modes viz., (1) variable illuminance at fixed CCT, (2) variable CCT at fixed illuminance and (3) variable CCT and variable illuminance. Under the first mode of operation, the DLC operates as a simple dimmer whereas under the second and third mode of operations, it operates as color-tunable light controller. Performance of the developed DLC is experimentally validated for a set of desired CCT points (2840–5750K) and constant illuminance (300 Lux) measured at a fixed distance. Measured CCT values show maximum deviation of 160K and 194K for warm white and cool white regions, respectively, compared to desired CCT points. On the other hand, the deviation of measured illuminance values lies within -9.7% to +8.3% with respect to set illuminance value of 300 Lux. Variations of measured CCT lie within the acceptable range. Experimental results establish the satisfactory performance of the developed DLC. Furthermore, scope of modification in the embedded control logic is made available to operate under mode (3) to vary both the CCT and illuminance of the DLS either in steps or continuously to follow a pre-set time-varying pattern to generate dynamic light scene in an indoor environment. Step variation performance is tested with 16 possible combinations of four CCT values (range: 2900–5600K) and four illuminance values (range: 100–300 Lux). Experimental result shows that all the 16 set points are realized without any perceived flicker and the observed variations of measured CCT points are within an acceptable range and the variations of the measured illuminance values are also very small (<16%) and not visually perceivable. Thus, the developed DLC successfully generates the PWM signals for the desired duty cycles. On the other hand, continuous variation performance is tested with a typical pattern applicable for office lighting where CCT varies from 3000K to 4700K and illuminance varies from 200–280 Lux (scaled down). Satisfactory performance of the DLC is observed based on the generated PWM signals of the desired duty cycles. But for the dynamic light source, significant variations are observed at two points of lower CCT values. Deviated light output of the CW LED array from the estimated light output at lower duty cycles (<10 𝜇𝑠) causes these variations. Finally the developed DLS is upgraded accordingly to make it daylight responsive. It is achieved by two control schemes, viz., (1) open-loop and (2) closed-loop. Under the open-loop control scheme, the system functions primarily with the response signal of one RGB color sensor - daylight sensor (𝑆𝑒𝑥𝑡) to monitor the CCT and illuminance of available external daylight at window plane. Depending on the measured external light scene, the light controller estimates the required lumen contribution from the individual LED array to follow the instantaneous CCT of daylight and at the same time to respond to the variation of window plane illuminance in reverse pattern. Then corresponding duty cycles, computed from an experimentally evaluated empirical non-linear relationship between duty cycle and lumen output of individual LED array, are estimated for both the LED arrays. The performance of the developed open-loop light controller is experimentally evaluated by laboratory testing and in-situ testing. Under laboratory testing, the performance is tested with the sudden (step) variation of external CCT and illuminance. Here five test conditions are simulated using an external light source and corresponding output of DLS is measured. The maximum deviations in CCT, illuminance and chromaticity from the required values are found as 99K, 7.8 Lux and 0.0018 respectively. Under in-situ testing, the performance is evaluated with available natural daylight by placing the 𝑆𝑒𝑥𝑡 at mid-height of the window plane and maximum deviations in CCT, illuminance and chromaticity from the required values are found as 280K, 9.9 Lux and 0.0053 respectively. Moreover, out of 710 measured data points, 682 data points yield deviation in CCT <100K and 694 data points yield deviation in illuminance <5 Lux. Under the closed-loop control scheme the system functions primarily with the response signals of two RGB color sensors, viz., daylight sensor to monitor daylight scene at window plane and internal light sensor to monitor the light scene created by the test lamp module. The control scheme controls the lumen delivery of individual LED arrays of the test lamp module by adjusting the duty cycles to follow the instantaneous external daylight CCT and at the same time variation of external illuminance in reverse pattern similar to open-loop system. Evaluation is conducted by laboratory testing and in-situ testing. The maximum deviations in CCT, illuminance and chromaticity from the required values under laboratory testing are found as 25K, 0.7 Lux, 0.0032. Under in-situ testing the maximum deviations are found to be 84K, 5 Lux, 0.0032 respectively. Moreover, out of 4658 measured data points, 4621 data points yield deviation in CCT within the tolerance value (30K) and 4608 data points yield deviation in illuminance within the tolerance value (1 Lux) which established the acceptable performance of the iterative closed-loop light control scheme. The limitation of the developed system is few seconds is required in order to converge to a desired point. The convergence time of the order of 10s is observed due to small size step variation of duty cycles with in-between delay which is necessary to avoid perceived flicker during continuous variation of CCT and illuminance. The developed dynamic lighting control system has two prospective applications- energy saving through dimming of artificial light source where the useful daylight illuminance (UDI) is greater than the required light level and simulation of dynamics of daylight where the daylight availability is limited or even absent.