Evaluating sensors for tunable lighting systems

In a tunable lighting system, emitted light is detected by a sensor that then sends information to a system controller so appropriate modifications can be made to the correlated color temperature (CCT).

White-tunable lighting, sometimes called hybrid white or variable white, is a lighting technology that uses a combination of LED sources to generate a range of different CCTs at different times, determined by manual settings or a pre-programmed schedule. Systems using this technology are generally deployed in environments where changing the color temperature of the ambient light contributes to ambience (in restaurants, galleries, etc.) or is necessary to facilitate specific activities (examinations in hospital rooms, for example). Changing CCT is also used to promote circadian response in settings like offices and other indoor spaces without access to daylighting.

In 1931, the Commission Internationale de l’Eclairage (CIE) released the first specification of a color space that was based on experimental research into human visual perception. The human eye contains three types of “cone” cells that are each stimulated by light in separate wavelength ranges roughly corresponding to red, green, and blue, so the CIE 1931 color space was structured around these “tristimulus values.” The CIE 1931 RGB color space formed the foundation of the subsequent XYZ color space. The latter color space is constructed so that the Y function corresponds to luminance, the Z function is approximately equal to RGB blue, and the X function is an amalgamation of red, green, and blue.

To facilitate a graphical depiction, the tristimulus values were then mathematically converted to planar coordinates. Figure 1 shows the CIE 1931 color space expressed in terms of x-y coordinates.

chart of the CIE 1931 color space in terms of x-y coordinates

Figure 1 This chart depicts the CIE 1931 color space in terms of x-y coordinates. Source: Wikipedia

The CIE 1960 color space (u,v coordinates) and superseding CIE 1976 color space (u’,v’ coordinates) each made improvements to the uniformity of the space based on human visual perception. Both the x,y space and u’,v’ space are in common use today.

The CCT of a white color light source is designated as the closest point on the black body locus to a given set of color space coordinates. Figure 1 shows the curved locus traces a path from cool white at higher temperatures through to warm white at lower temperatures. The isothermal lines through the locus indicate the range of coordinates that correspond to the given color temperature.

The diagram in Figure 2 shows a typical strategy for creating a white-tunable-lighting product, which involves using two sets of phosphor-coated LEDs with differing CCTs, one around 2700K and one around 5000-6500K. An algorithm stored in the lighting system controller varies the output of each set, enabling both a range of CCTs and intensity.

block diagram of a white tunable lighting system

Figure 2 A typical strategy for creating a white-tunable-lighting product involves using two sets of phosphor-coated LEDs with differing CCTs.

A sensor sends chromaticity information to the system controller so CCT changes can be made as indicated by a pre-determined schedule or due to the chromaticity shift of the light sources themselves, which inevitably occurs over time.

Chromaticity shift of LED sources continues to be investigated, but the question of the stability of the sensor response is just beginning to be examined. A recent U.S. Department of Energy report, conducted by RTI International in association with KeyLogic Systems, evaluated initial performance and reliability of a six-channel sensor that can be used to provide information for chromaticity adjustment of a tunable white lighting product.

The sensor tested in the DoE study consisted of six CMOS photodiodes, five of which were covered with a Gaussian interference filter designed for a specific response to differing input wavelengths. The five filters incorporated into the sensor included a dark filter (all light absorbed), the tristimulus X function, the tristimulus Y function, the tristimulus Z function, and near-infrared. The sixth photodiode was left uncovered. For purposes of the study, only the responses of the X, Y, and Z channels were used to calculate chromaticity.

Because there are currently no standards for evaluation of long-term sensor performance, the test protocol was developed internally and resembled typical test protocols for electronics equipment. The units were operated continuously for 5,000 hours at three different environments: room temperature, an elevated ambient temperature of 75°C, and a temperature-humidity environment of 75°C/75% relative humidity (RH). A total of 20 sensors were tested with only one anomaly: one sensor displayed a parametric failure following 5,000 hours of operation in the 75°C/75% RH environment. Based on these findings, the study concluded that the sensor design was “highly reliable.”

As lighting systems evolve from simply providing light to more sophisticated implementations that include dynamic functionality, it will be important to ensure that the sensors that provide control system feedback operate reliably over the lifetime of the system. Follow-on studies of additional sensor products would help to provide confidence in long-term sensor reliability.

Yoelit Hiebert has worked in the field of LED lighting for over 10 years and has experience in both the manufacturing and end-user sides of the industry.

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