Methods for simulating the color temperature, hue and brightness of daylight illumination for indoor lighting simply by adjusting the intensity of red, green, and blue light emitting diodes are proposed. We obtain uniform color mixing with a light box by adjusting the ratios between the intensities of red, green and blue LEDs. The intensity can be found by measuring the CIE chromaticity coordinates (x, y) and the luminance Y of the daylight with a chroma meter. After the chromaticity coordinates (x, y) and the luminance Y are found, the tristimulus values can be calculated and then transferred to red, green, and blue primaries by linear transformation. With the correct ratio of red, green, and blue intensities, the color temperature, hues and brightness of daylight can be rebuilt by red, green, and blue light emitting diodes.
© 2011 OSA
ErrataChih-Hsuan Tsuei and Wen-Shing Sun, "Momentary adjusting methods for simulating the color temperature, hues and brightness of daylight illumination with RGB LEDs for indoor lighting: errata," Opt. Express 19, 18318-18318 (2011)
Light emitting diode (LED) technology has come to be widely applied in recent years in vehicle, architecture, signal lighting, street [1–4], and indoor lighting [5,6]. Most of these applications, especially in indoor lighting, require a uniform beam illuminance profile and comfortable visual performance. Designs for the utilization of hybrid sunlight illumination and LED lighting systems were proposed in 2010 . As described in that reference, daylight and LED light can be mixed using a light box design to ultimately provide uniform illumination. Lighting can also be controlled by mixing the colors of red (R), green (G), and blue (B) LEDs inside the light box. The performance of color mixing and daylight lighting with RGB LEDs will become the next important topic in the field of indoor lighting.
Figure 1 shows the layout of the cubic light box used in this study . There are six daylight concentrators on the top of the building to collect sunlight and focus on the light guide. The plate beam splitter between the concentrator and the light guide is designed to separate the daylight into visible and non-visible rays. The visible rays go through the beam splitter to be focused by the light guide, and then introduced into the light box; the non-visible rays are collected by solar cells. The visible daylight passes through the light guide into the light box and mixed with LED light uniformly in it, and finally provide more uniform illumination by a diffuser. There are six conical reflectors placed below the light-guide exits at the surface of the ceiling and LEDs arranged around the inside surface of the light box, where we place four sensors in each corner on the top surface of it to detect the daylight. BaSO4 paint, which is often used for integrating spheres and has 95% reflectivity, is applied on the inside surface of the light box, to detect, collect and make uniform the daylight and LED light. Some or all of the LEDs arranged around the inside surface of the light box can be turned on to ensure that illuminance on the table plane exceeds the standard levels, even when the daylight is not sufficient to illuminate the table plane. The light box not only provides uniform lambertian illumination but also uniform color mixing. RGB LEDs were chosen in this study for color mixing in order to simulate the color temperature, hues and brightness of daylight lighting. We are able to provide a sky type lighting environment in indoor spaces, especially for those that do not have any windows to receive daylight.
Any two or more colors can be mixed inside the light box to form different colors. Only three basic colors are needed to create just about all the colors in daylight lighting. These are the primary colors. The three basic colors used for light mixing or illumination are red, green and blue. They are called the additive primary colors. By means of mixing the additive primary colors in the light box and adjusting the ratio of RGB LEDs, we can simulate the color temperature, hues and brightness of daylight.
3.1 Simulated daylight lighting with RGB LEDs
When there is no daylight introduced into the light box, indoor lighting can be produced by relying totally on LEDs. The method for simulating daylight lighting will be discussed below:
To accurately express the correlated color of natural daylight, it is necessary to utilize not only the chromaticity diagram (x, y) but also the tristimulus value Y of the daylight. The value Y provides a brightness function which is also the luminance of daylight. First, the CIE chromaticity coordinates (x, y) and the luminance Y of daylight are measured by a chroma meter. After finding the chromaticity coordinates (x, y) and luminance Y, the tristimulus values X and Z can be calculated by8,9], i.e.,tristimulus values X, Y, and Z. Next, the ratio between the RGB primaries and tristimulus values XYZ in relation to each other is determined by a linear transformation [8,9]:
For each measurement process, the colorimetric calculating method is used to map the color temperature, hues and brightness of the daylight, instead of calculating them directly from x, y, and Y. Suppose the measured chromaticity coordinates as obtained from the chroma meter in early morning are (0.4362, 0.4061); then the correlated color temperature of this pair of measured chromaticity coordinates is about 3000K, as shown in Fig. 2 . From the measured chromaticity coordinates (x, y) and luminance Y of the daylight we can calculate the tristimulus values X and Z. When the tristimulus values XYZ are found, the RGB primary intensities can be calculated with Eq. (4), and the intensity ratio of RGB LEDs can be calculated.
After finding the intensity ratio of RGB LEDs, we can simulate sky-like indoor lighting by suitably arranging and adjusting the intensity ratios of the RGB LEDs inside the light box. Take the measured chromaticity coordinates (0.4362, 0.4061) and luminance 6 k⋅cd/m2 for example. The daylight data can be calculated and simulated by the Matlab, LightTools and DIALux software. The simulated daylight lighting is shown in Fig. 3 .
Similarly, using the same calculating and transferring process, we can simulate the chromaticity coordinates (0.3457, 0.3578) (the color temperature is about 5,000K) with the luminance 10 k⋅cd/m2 at 10 a.m., and (0.3128, 0.3292) (the color temperature is about 6,500K) with the luminance 14 k⋅cd/m2 at noon. The simulated daylight illumination for indoor lighting at 10 a.m. and at noon is shown in Fig. 4 and Fig. 5 , respectively. Figure 3 to Fig. 5 show different examples of daylight lighting successfully rebuilt and simulated. Different color temperature, hues, and brightness can be observed in these simulations.
3.2 Invariable color temperature, hues and brightness lighting with sunlight and RGB LEDs
As mentioned in reference , the light box can combine and blend the daylight and RGB LEDs light to provide uniform lighting, to finally provide the invariable color temperature, hues and brightness needed for lighting. For example, if we want to illuminate the indoor space with early morning daylight and RGB LEDs, maintained in the D65 constant lighting condition , which is the CIE Standard Illuminant that represents a color temperature of 6500 K, we must use the additive color mixing method to mix the early morning daylight and RGB LED light inside the light box.
For additive mixing of the daylight and RGB LED light, the chromaticity coordinates of these two light sources are x1, y1 with brightness Y1, and x2, y2 with brightness Y2, respectively. The additive mixture color coordinates x3, and y3 for lighting condition D65 areEq. (1) to Eq. (4).
In this study we develop a method for simulating the color temperature, hues and brightness of daylight illumination by adjusting the intensity ratio of RGB LEDs. The ratio of RGB intensity can be found and calculated by measuring the CIE chromaticity coordinates (x, y) and illumianance Y of the daylight with a chroma meter. After the chromaticity coordinates (x, y) and the illuminance of daylight are found, the tristimulus values XZ can be calculated, and then transferred to RGB primaries by a linear transformation. Finding the ratio of RGB intensity, the color temperature, hues and brightness of daylight can be rebuilt and simulated by adjusting the ratio of RGB LEDs. The additive color mixing method is used to mix the daylight and RGB LED light inside the light box when we want to illuminate our indoor spaces with constant lighting condition. These procedures provide methods for adjusting and simulating daylight-like illumination in an indoor lighting environment.
This study was sponsored by the National Science Council under contract numbers NSC 98-2221-E-008-021-MY3 and NSC 100-2623-E-008-002-ET.
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