Ambient CCT Light Sensor Nowadays, automation penetrates into almost every area. Lighting industry is not an exception. Possibility of instant reaction to the changes in environment conditions gives great comfort to a user. In case of lighting installation it means that a customer is no more bothered by switching the light on and off, or by adjusting their dimming level according to the intensity of daylight
Fig. 1: The structure of the sensor [1].
coming from outside. Moreover automatic luminance control brings significant energy savings, which is the main reason why light and presence sensors are widely used in modern lighting installation. But the intensity of light is just one side of the coin. The quality of light becomes more and more discussed during the design of a lighting installation. The spectrum of
Fig. 2: Typical (relative) sensitivity (XYZ) of the colour sensor scanned by width broadband light [1].
the light can significantly influence human activity and mood. Since the introduction of Tunable White luminaires – the luminaires that have adjustable correlated colour temperature (CCT), it has been only a matter of time until there will be demand also for sensing and automatic adjustment of CCT.
Fig. 3: The CIE 1931 x,y chromaticity space, also showing the chromaticities of black-body light sources of various temperatures (Planckian locus) [2].
CCT is one the basics parameters of white light that describes chromaticity of light. The CCT of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Measurement of CCT can be performed in various ways, depending of the colorimetric system preferred. Ambient Light Sensor uses MTCSiCS true colour sensor by MAZeT. The sensor is made of 19 x 3 photo diodes (special PIN silicon technology with extended sensibility) integrated on chip. The diodes are carried out as
segments of a multiple-element hexagonal matrix structure with the diameter of 2.0 mm. Each of these photodiodes is sensitized with dielectric spectral filter (named True Colour Filter) for its colour range, preferably for the primary colour standard CIE (Commission Internationale de l'Eclairage or International Commission on Illumination) colour space (Fig. 1). Thanks to the special sensor design tristimulus CIE values (X, Y, Z) are measured directly at the sensor’s output (Fig. 2). Output of the sensor is variable current that is converted to
voltage by trans-impedance amplifier with switchable gain. Afterwards the signals are sampled and digitalised (16-bit resolution). Measured tristimulus values are used for calculation of coordinates in colour space (Fig. 3) and subsequently CCT is calculated using McCamy’s formula. Precision of the measurement is 50 K with refresh rate of 500 ms. The sensor can be also used for light intensity measurement.
Fig. 4: Sensor placed on the PCB.
Fig. 5: Design concept for sensor box.
Fig. 6: Measuring electronic.
Sensing of the CCT is just the first part of the automation process. Next step is the feedback channel. There are several concepts of feedback channels that would regulate CCT in order to reach desired value. The most spread interface for lighting installation control is DALI. Therefore DALI bus is the natural choice for the feedback channel. Moreover DALI bus can also provide power for the sensor. The sensor can work in two different modes: as Master –
configured for direct control of Tunable White luminaire, or as Slave – only providing measured CCT value for application controller, that manages the regulation part. DALI powered CCT sensor is just the beginning (Fig. 4, Fig. 5, Fig. 6). The goal is to develop a sensor powered by energy harvesting from ambient light and communicating wirelessly with application controller – in order to read measured values and set parameters of the sensor.
Annex: 1. Datasheet of MAZeT colour sensor MTCSiCS, online: https://www.distrelec.sk/ishop/Datasheets/mgMAZET_Farbsensor-MTCSICS_EN.pdf 2. Colour temperature, online: http://en.wikipedia.org/wiki/Color_temperature
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