Detailed explanation of spectral tuning of LED light sources

If consumers were asked to describe the characteristics of a perfect light source, you would probably hear a variety of desirable characteristics in their descriptions, such as the need for minimum energy, adjustable light output and color, and long service life. Minimum input energy refers to the efficient conversion of input power into lumen output, known as efficacy; adjusting light output is related to dimming, which reduces the brightness of the light, or color adjustment of the lighting device can be increased to simulate daytime relative to nighttime conditions; and light output can be maintained over a long service life by adjusting the bias current flowing through the LED.

Incandescent lamps have low efficiency and short life; sodium lamps offer few color options and short life; fluorescent lamps have few dimming options and short life. High-brightness LEDs claim good efficiency, long life, easy color selection and dimming control, and no ultraviolet (UV) radiation. Thanks to the intelligent design of control gear or LED driver electronics, these claimed advantages are now realized. Intelligent LED drivers can adjust for brightness decay during life, provide drive characteristics to adjust color, and replace LED binning to obtain the desired color and brightness: by using spectral tuning for different LEDs in the system to achieve the desired color and brightness.

Spectral Tuning Contrast Sorting

Spectral tuning mixes the spectral energy distribution of several LEDs, for example, a proper mix of red, green, and blue LEDs can produce white light. This RGB combination is also used to produce almost any color of light. If the LED driver is not designed to tune a group of different LEDs, then the designer must choose from sorted LEDs to produce a specific color. Sorting is the process by which manufacturers sort LEDs according to luminous lumens and color. The example in Figure 1 shows a group of industry-standard LED "sorting standards."

Figure 1: LED classification standards on a chromaticity diagram.

The classification is shown by the rectangular areas plotted on the chromaticity diagram. A group of LEDs contained within the same class will be similar in color and brightness characteristics. However, in a large office or factory environment containing many luminaires, the different classes may still result in uneven light color, which is noticeable in a large group of luminaires. A sorted LED design does not provide a method to change the color of a luminaire; however, a group of different colored LEDs uses feedback to tune the spectral characteristics of the different LEDs in the system, making it possible to create a compensating lighting system in an office environment, thereby making the light uniform throughout the space. Spectral tuning can also compensate for other effects, such as natural light on the side of a room with windows facing outside or hallway lighting reflected into the room.

Another effect of LEDs is color shift, which comes from changes in the forward current of the LED. Figure 2 shows a set of curves of LED color change with forward current in the industry.

Figure 2: LED light units classified by application.

LED drivers can be designed with tight constant current (CC) output tolerances, however, tightening CC tolerances will increase the cost of the LED driver. Since the forward current passes through a group of LEDs, a lower-cost solution is for designers to use a feedback system to adjust the LED color shift, compensating for color changes through feedback.

Sorting LEDs often brings manufacturing impacts, resulting in increased LED procurement costs. Because the LED binning settings are specific, some LED drivers may still not be able to match the forward bias current settings of many classified LED applications. In addition, temperature effects and lifetime degradation effects can cause changes in the color of lighting equipment.

Application of feedback in spectral tuning

A lighting device that automatically adjusts color and brightness is proposed below to illustrate a feedback control scheme that can offset the effects of system changes. A color sensor and a microcontroller are used to process the sensor input. For example, the color sensor uses a photodiode and adopts a non-organic three-way color filter to provide excellent stability and very low drift in terms of temperature and aging changes, and the color filter can be designed to implement the spectral sensitivity curve of the human eye (CIE1931).

The schematic diagram of the closed-loop spectrally tuned light source is shown in FIG3 .

Figure 3: Spectrally tunable light source.

The control loop uses a microcontroller as shown. The control loop measures brightness and color through sensors and uses a PWM signal to adjust the current in the LED string. Using a PWM input signal, the FAN7346 can control the current in a single LED string. The power supply can be a power factor correction pre-stage followed by an LLC DC/DC secondary to power multiple LED strings, as shown in Figure 4. The power supply can also be an existing design with the FAN7346 controlling the feedback to the power supply. Alternative designs can use three power converters (30W/10W/10W) to control each set of LED strings, using white, green and amber to create a white-based tuning system, or use three strings with the same power supply to "mix" three LED strings of red, green and blue to achieve a wider color tuning range. LED colors do not need to be sorted; low-cost LEDs with the required performance for the lighting application can be selected.

Figure 4: Power supply for spectrally tunable light source.

System Example

Here, a system example of white light tuning in an office environment is established, with three flyback PFC power supplies running in parallel, with the main power supply with an output power of up to 30W driving the main white LED string, and two additional power supplies each providing up to 10W for the LED strings containing amber and green LEDs, providing a total power of 50W. Figure 5 shows the light source design at full power. The color sensor is located in the middle of the light source array, facing downward, to achieve correct measurement of light color and light intensity.

Figure 5: Spectrally tunable light source at full power.

The testers could not see a difference in color or brightness. Figure 6 shows the light sources being dimmed. Note that due to high ambient light conditions, such as sunlight from a window, some of the lights were actually turned off.

Figure 6: Spectrally tuned light source brightness reduction.

Conclusion

As can be seen in the examples, a system using spectrally tuned LEDs provides consistent color characteristics in an office or factory environment. Spectral tuning allows for color compensation for sunlight or other light sources in a space that may affect ideal brightness and color control. The feedback system can also compensate for the effects of aging or drift related to LED life and color shift. Because spectrally tuned feedback can be used to control color, it also eliminates the cost impact of strictly sorting LEDs. Other advantages of the examples mentioned above include calibration, implementing pre-protection features, and setting safe lighting conditions to balance light output with the power from backup batteries to illuminate the required escape routes. Targeting specific equipment versus controlling the entire system, dimming can be remotely controlled through a wireless interface.