毫无疑问，颜色在我们对周围世界的看法中起着重要作用。但是，当一个物体用不同的光源照亮时，它通常似乎会改变颜色。可以将照明光源的这种特性量化为CRI（颜色渲染索引）。CRI定义了样品光源的准确程度，与可比色温的参考光源相比，照明物体的颜色重现了一个光源。Figure 1shows an example of how the color of an object appears to change when illuminated with light sources of different CRI.
1. When the object in the figure is illuminated with light sources of different CRI values, the color of the object appears to change.
当然,白光是最常用的light source, be it in home, industrial, or architectural lighting. It would be nice if a user had the option to finely tune the white light source until the object under observation looked its best. For the most part, this is not an option, but sometimes the tuning of the light source is critical, such as in medical lighting applications. Here, we will analyze the characteristics of white light and the ways and means to tune it.
Characteristics of White Light
在解释如何调整白光之前，我们需要充分理解色彩的概念。有趣的是，颜色不能像其他物理属性一样容易量化，例如温度或密度。这是因为对象的颜色取决于对象，环境和感知眼睛的特征等因素。
What humans perceive as color is actually electromagnetic spectrum in the wavelength range of 390-750 nm. The human eye, in general, observes each element of the image on the retina with three different ‘cone’ types that serve as photo detectors. What makes things interesting is that the spectral response of each of these cones is different. This means that when a portion of the retina is exposed to a visible spectrum, the response of each of the cones will be different. Actually, each ‘cone’ multiplies the whole spectrum with its own spectral response. This means that at each wavelength of the incident spectrum, the spectrum strength at that wavelength is multiplied by the spectral response of the cone at the same wavelength and all the products are added to give the resultant response of the cone. In the end, what the brain perceives is a combination of the results of the three cone types. The three cones are generally termed L, M, and S for Large, Medium, and Small, referring to the fact that the peaks of their spectral responses are at different wavelengths.
白光是各种波长的组合。如您所知，如果您通过棱镜将白光传递给，您可以看到各种组件波长（Fig. 2). The component wavelengths range can be given by VIBGYOR (recall this is Roy G. Biv backwards), with V-violet having the shortest wavelength and R-red having the longest.
2. A prism splits white light neatly into its component wavelengths.
CIE和白光
通常，可以使用红色，绿色和蓝色的三种基本颜色生产白光。开发人员的问题是如何确定实现白色的基本颜色的正确比例。即使我们得出正确的比例，我们将以什么单位表示它，因此可以被广泛理解和接受？CIE（国际照明委员会，源自其原始法语名称）XYY色彩空间是最常见的答案。XYY颜色空间建立在这样的概念上，即任何颜色都可以通过三维色彩空间中的色调，饱和度和亮度/亮度充分表示。尽管亮度和亮度略有不同，但我们暂时会忽略这种差异。在XyColor空间中，X表示色调，Y表示饱和，Y表示亮度。
Before we further analyze the CIE xyY color space, let’s briefly look at how it was arrived at. In the 1920s, extensive experiments were done on quantifying color. One such approach was to quantify and express color with three coordinates –R, GandB。Here, each value referred to the proportion of the three primary colors [R, G, and B] required for generating the specific color ‘C’:
C =R。R+G.g+B。B (1)
R, G and B represent the primary colors andR, G and Bsignify the required mixing proportion of the primary colors to get color ‘C’. However, this format had an issue where the coordinate ‘R’was negative for a few colors. To overcome this, a new color space XYZ was defined, where X, Y and Z are imaginary primaries such that for all the possible colors, none of the co-ordinates are negative. In this new representation, color ’C’ isX.x+Y.y+Z.z，x，y，z和X,Y,Zare similar to R,G,B andR，G，Bas defined earlier. In this system, ‘Y’ represents both the luminance and one of the coordinates.
To make things easier, these three coordinates (X, Y, Z) were further reduced to a two-dimensional space, with the definition of chromaticity co-ordinates (x,y) as
x=X/X+Y+Z(2)
y=y/x+y+z(3)
So, the complete visible spectrum was represented in a two-dimensional space, named the CIE chart.
‘Y’, which is equivalent to luminance, can be imagined as being vertical to the (x,y) graph. Any color can be described by the (x,y) chromaticity coordinates and luminance ‘Y’。
One critical property of the CIE chart is that any color on a straight line joining two colors can be produced by appropriately mixing the two colors at the endpoints of the straight line. For example, any color on line MN can be produced by mixing M and N in the right proportion(Fig. 3)。将相同的逻辑扩展到三角形RGB（带有红色，绿色和蓝色颜色的顶点），可以通过正确比例混合R，G和B来产生三角形中的任何颜色。考虑到这一点，让我们将R，G和B颜色源视为LED光源。让这些LED的色度坐标为（xred,yred），（Xgreen,ygreen）和（xblue,yblue).
3. CIE图表示完整的可见频谱。
Producing White Light With R G B LEDs
如果我们希望与坐标获得颜色“ c”（xmix,ymix) and with luminance ‘Ymix’，那么第一个任务是检查是否（x）是否mix，ymix) falls in the triangle covered by R,G and B. Once we determine this, we need to calculate the mixing proportions of R, G, and B to achieve the desired color. By mixing proportion, we mean the dimming level required for the R, G, and B LEDs. There are many algorithms for these calculations. One such algorithm is the matrix approach, with the below steps:
- Calculate matrix A as:
- Take the inverse of matrix A
- Calculate the required lumen level for each of the R,G, and B LEDs as:
Here,Yred,Ygreen， 和Ybluegive the required lumen output for each of the basic R, G, and B LEDs to achieve the desired color. This method has two advantages. If any of theYred,Ygreen, orYblueis negative, then it means that the requested color is outside of the gamut of the RGB triangle. Second, if any ofYred,Ygreen， 和Yblueis greater than the maximum lumens of the respective LEDs, then the lumens of the final color,Ymix, is too large and needs to be scaled down.
Assume that we are using a PWM of resolution ‘N’ for dimming, then the signal density for the PWM driving the Red LED can be expressed as below, where Ymax,redis the maximum lumens of the Red LED:
DimValuered= (Yred/Ymax,red) * ((2^N)-1)
In summary, the requirement for generating any color within the triangle RGB is basic Red, Green, and Blue LEDs with known (x,y) chromaticity coordinates and maximum lumens. Using suitable algorithms, we would have to calculate the dimming values for the R ,G, and B LEDs to achieve the desired color. One such algorithm is the matrix approach we just discussed.
Logically, white light used for general illumination also falls within the triangle RGB and any shade of white light can be obtained by mixing R, G, and B sources in the right proportion. It is time to introduce another important parameter that is used to measure the chromaticity of white light, the CCT parameter. CCT (Correlated Color Temperature) is based on the concept on Color Temperature (CT).
普朗克定律给出了黑体辐射的能量的强度，这是波长和温度的函数。因此，每个CT都有与之相关的独特颜色，由其色度坐标（x，y）定义。CCT被定义为黑体散热器的温度，其色度最接近感知均匀的颜色空间上的光源。Figure 4gives the black body locus on the CIE chart.
4. This figure shows theBlack Body Locus superimposed on CIE 1931.
Note that a particular CCT refers to a line of chromaticity on the color space instead of a single color. For example, points A and B have the same CCT but different (x,y) coordinates. White lighting for general illumination usually falls in the range of 2,000 to 10,000K. So, the input parameters to a tunable LED white light system can be either the (xwhite，ywhite,Ywhite）坐标或输出白光的预期CCT。基于此，系统必须计算组件R，G和B LED所需的调光级别。
Tunable LED White Light Systems
Figure 5显示了一个典型的led照明电气系统。A rectifier is used to convert line voltage (AC) into DC. In general, the LED fixtures in home lighting/architectural lighting are controlled from a common master and the fixtures are slaves. The common communication interfaces used in lighting networks are the DMX and DALI interfaces. In a tunable white light system, the master can communicate the expected (xwhite，ywhite,Ywhite) output to each of the slave fixtures through these communication protocols. Another option for the master is to communicate the desired CCT of the white light. It is then the job of the LED driver to calculate theYred,Yblue， 和Ygreen值并以计算出的调光值驱动LED。
5. This circuit shows a typical LED electrical system.
LEDs show variation in spectral properties with temperature. Effectively, this means that their (x,y) coordinates change with temperature. This can be critical in tunable white light applications, where the user would have set the dimming level for the R, G, and B LEDs to get a desired white color at a specific temperature. But, with variation in temperature, as the (x,y) coordinates of the basic LEDs change, the output deviates from the desired color. As such, temperature compensation techniques have to be implemented in firmware with a real-time temperature feedback system, along with efficient heat sinks.
在LED中，另一个设计挑战是它们进入了各种垃圾箱，其中LED属于一个特定的垃圾箱，具有相似的光谱特性。当我们从供应商那里采购许多LED的特定颜色时，它们可能不属于同一垃圾箱，因此产生略有不同的输出。设计人员的选择是在同一垃圾箱下采购LED，这可能非常昂贵，要么可以补偿这种固件差异。这意味着在固件方面，LED驱动器上的负载包括实际的颜色混合算法，并具有额外的温度补偿和安装补偿技术。此外，如果LED驱动程序支持DMX/DALI接口，则可能是许多应用程序的额外优势，因为这可以实现单芯片，具有成本效益的插入。
There are many controllers well-suited for driving LEDs, with integrated MOSFETs, CSAs (Current Sense Amplifiers), hysteretic controllers, and other peripherals. Mixed-signal controllers also offer multiple modulation blocks like PWM, SSDM, and PrISM for controlling the dimming level of LEDs. A hysteretic controller, for example, takes the feedback from the CSA, Modulator block, and the Trip input. The trip input can be used to shut off the LED channel in case of any extreme current or voltage conditions. The calculatedY-red,Y-blue andY-green dimming values can be fed as the signal density values for the modulation blocks controlling the Red, Green, and Blue LED channels. The CSA is used to monitor the current through the LED. The hysteretic controller output goes to the gate of the FET, which acts as the controlling switch in a standard buck or boost converter. These controllers can support multiple LED channels.Figure 6shows the top-level set up for driving a single LED channel in buck configuration.
6. This is an example of a LED driver circuit using the Cypress PowerPSoC.
The firmware for the mixing algorithm, as well the firmware for temperature compensation and binning compensation can be run directly on the controller. Furthermore, it can support the DMX/DALI interfaces.
总而言之，给用户一个选择精细调整白光源的选项越来越多，而不是复杂的增值功能。考虑到在照明设备中，空间是一个主要限制，必须选择一个可以使用单个芯片解决方案实现系统的LED驱动器，同时还具有支持复杂混合算法并将其无缝集成到现有照明网络中的计算能力。
