10th International Symposium „Topical Problems in the Field of Electrical and Power Engineering“ Pärnu, Estonia, January 10-15, 2011
Illumination measurement stand for artificial light sources Olegs Tetervenoks, Ansis Avotins, Ilja Galkin Riga Technical University
[email protected]
Abstract This article is related to development of evaluation stand to ensure optical and electrical parameter simultaneous measurements. The illumination parameter measurement process is automatic to speed up the analysis for different lighting device calculations. The stand is suitable for preliminary testing of developed luminaries, existing products and analyzing data to evaluate lighting dimming process efficiency at different dimming levels.
In such a situation it would be very convenient when electrical and photometrical parameter measurements can be performed simultaneously using luminary evaluation stand with automatic measurement data acquisition. Table 1. Efficacy of lighting devices and fixtures[4].
Keywords Automatic photometrical measurement, artificial light source evaluation, solid-state lighting, energy efficiency
Introduction Nowadays a demand for more energy efficient devices also in lighting industry is increasing, as there is a great potential for energy savings and CO2 emission reduction all over the world. For artificial light sources a light-emitting diodes (LED) has been become very popular in various applications, due to good color rendering index and dimming possibility, as well as luminous efficacy (lm/W) of solid-state technology is still increasing. When looking at some economic aspects from engineering - economic analysis done in [4], which indicates that white solid-state lighting already has a lower levelized annual cost (LAC) than incandescent bulbs, and will be lower than that of the most efficient fluorescent bulbs by the end of this decade. Different solid-state lighting products are available on global market, with various parameters like ballast efficiency, light source efficacy and fixture efficiency. The Table 1 corresponds to efficacy comparison of different lighting devices and fixtures technology [4]. Also the wide range of each parameter value complicates the overall evaluation, especially when designing new product. Optical properties of the lighting devices are certified in the laboratory of photometry, but also during development of power supply unit for new LED luminary it is necessary to get preliminary photometric readings to evaluate lighting dimming process efficiency at different dimming levels. 250
*
Light source efficacy, lm/W
Ballast efficiency, %
Fixture efficiency, %
Total efficacy, lm/W
Incandescent 4 to 18 lm/W
100
40 - 90
2 -16 lm/W
Halogen 15 to 33 lm/W
100
40 - 90
6 - 30 lm/W
Fluorescent tubes 60 to 105 lm/W
65 - 95
40 - 90
16 - 90 lm/W
CFL 35 to 80 lm/W
65 - 95
40 - 90
9 - 68 lm/W
HID 14 to 140 lm/W
70 - 95
40 - 90
4 - 120 lm/W
White LED 60 to 188* lm/W
75 - 95
40 - 95
18 - 170 lm/W
188 lm/W is a target for white LED at 2015 of US DOE [4]
This article considers the possibility of building lighting evaluation stand, using photo detectors on Avago APDS-9300 ambient light photo sensor base.
1 Illumination measurement stand layout Illumination measurement stand consists of metal frame (Fig.1.a.), light source fixture (Fig.1.b.), photodetector matrix (Fig.2.) and data analysis platform (Fig.5.). The frame construction has a parallelepiped form, and is made from metal square tube material, painted in black color, where the layout is shown in Fig.1. To avoid measurement interference with ambient light, the stand is tapestried with thick black textile with low reflection properties. The total height is 3,0 m, where upper part of 1,0 m can be removed if necessary, and base of the stand is 2,2 x 2,2m. For ease of transportation and assembly, the stand is fastened with screws. At the bottom of the illumination measurement stand a matrix of 25 photodetectors is placed, as shown in Fig.2. The matrix is made as separate construction with adjustable height, thus it is portable and easy for assembly and transportation.
a)
b)
Fig. 1. a- frame of illumination measurement stand; b-fixture of light source
Data analysis platform
Measurement chamber
Fig. 5. Overall layout of illumination measurement stand
Adjustable height
photdetector x 25
Fig.2. Matrix layout with 25 photodetectors
Photodetectors via communication interface are connected to data analysis platform, which consists of PC with special software for data acquisition, saving, analysis and graphical interpretation. Avantes spectrometer with special optical equipment is connected to PC via USB cable (both communication and power supply) for wavelength measurements and spectral analysis. Overall illumination measurement stand layout is shown in Fig.5.
2 Photo sensor and responsivity Lighting measurements uses radiometric (e) or photometric (v) quantities (measured in visible spectrum 380 to 780 nm). The V(λ) curve is used to link the radiometric quantity with the sensitivity function of the human eye. The photometric quantity luminous flux Φv is then obtained by integrating radiant power Φe(λ) as follows:
Fig.3. Overall constructive layout of illumination measurement stand
Fig.4. Sensor matrix placement
The unit of luminous flux Φv is the lumen [lm]. Factor Km = 683 lm/W establishes the relationship between the (physical) radiometric unit watt and the (physiological) photometric unit lumen. All other photometric quantities are also obtained from the integral of their corresponding radiometric quantity weighted with the V(λ) curve [8]. APDS-9300 ambient light photo sensor contains two photodiodes, where one is a broadband photodiode (visible and infrared spectra, channel 0), the other one is an infrared photodiode (channel 1) [5]. Fig.6. shows normalized responsivity of the channel 0 and channel 1 photodiode, depending on the spectral distribution (wave length), comparing to the responsivity of the human eye (the associated standard spectral response is often referred to as the CIE luminous efficiency function, the V(λ) function, or more commonly, as the photopic response curve [2]).
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response is close to the CIE standard photopic observer (see Fig. 7.), it consists of a photodiode and an IC that performs amplification of the photodiode output signal and conversion to a logarithmic output current.
Fig. 6. Responsivity of the APDS-9300 photo sensor and human eye (CIE) APDS-9300 photo sensor responsivity is significantly different from the human eye responsivity. Channel 1 (Ch1) cannot fully cover visible spectrum, but the channel 0 (Ch0) is sensitive to invisible radiation spectrum, which may affect the illuminance measurements. To get correct readings, it is possible to use special colored filter or a multilayer glass filter to correct the inherent detector responsivity [1]. Colored glass filters are doped with materials that selectively absorb light by wavelength, and obey Bouger’s law. By varying filter thickness, you can selectively modify the spectral responsivity of a sensor to match a particular function [3]. A method using filters would be inconvenient to get preliminary photometric readings to evaluate lighting dimming process efficiency at different dimming levels. For sensor lux calculation an empirical formula can be used as specified in APDS9300 application note [5] and shown in Table.2. Table 2. Empirical formula for Lux calculation. CH1/CH0
Sensor Lux Formula
0≤CH1/C H0≤0.52
)
0.52≤ CH1/CH0 ≤0.65 0.65≤ CH1/CH0 ≤0.80 0.80≤ CH1/CH0 ≤1.3 CH1/CH0 ≤1.3
Formula is based on measured Ch0 and Ch1 ADC count values for the solid-state light sources with visible 640nm and 940 nm wavelengths. To get readings in visible spectrum, a secondary sensor can be used like APDS-9007, whose spectral 252
Fig.7. Spectral response of APDS-9007 photo sensor[9]. APDS-9007 is able to produce a high gain photo current that can be converted to an output voltage via a standard value external load resistor, and then it is easily integrated into micro-controller that has an available A/D input.
Fig.8. Application circuit of APDS-9007[9]. A logarithmic current output is advantageous, when measuring low brightness levels, small changes in those levels need to be detected, and at high brightness levels, relatively bigger changes in those levels would be significant, thus logarithmic current output, is able to provide a good relative resolution over the entire ambient light brightness range [9]. For spectral measurements within wavelength ranges of 200 nm to 1100 nm an Avantes AvaSpec-2048 Fiber Optic Spectrometer is used. It is based on the AvaBench-75 symmetrical Czerny-Turner design with 2048 pixel CCD Detector Array, 16-Bit AD converter and equipped with cosine corrector and UV/VIS fiber optic cable.
3 Description of photo detector Each APDS-9300 device consists of two photodiodes and two integrating ADC, which converts photodiode currents to a 16-bit digital output and represents the irradiance measured on each channel. This digital output can be input to a microprocessor (MCU). Device uses I2C protocol for data exchange. It is possible to assign one of three fixed identification addresses for each APDS-9300
device, this mean that it is possible to connect three photo sensors at one I2C bus to MCU, as shown in Fig. 9. [5]
The recommended plastic material is polycarbonate, with smooth surface, without any texture is available from Bayer AG and Bayer Antwerp N. V. (Europe), Makrolon LQ2647 or Makrolon LQ3147 with visible light transmission 87% and refractive index of 1.587, where both are suitable for APDS-9300 and APDS-9007 applications. 3.3 Communication of a photo detector It is planed to use multiple photo detectors for illumination measurements. Information from all detectors will collect in main unit (MCU) that will be connected to the computer (Fig. 10.). Then the collected information will be further processed. It is therefore necessary to provide the opportunity for communication of photo detector.
Fig. 9. Using of I2C bus for connection of photo sensor to a MCU 3.1 Microprocessor selection Since the data exchange APDS-9300 photo sensor used I2C interface, it is recommended that the microcontroller would have I2C interface. APDS-9300 power supply voltage lies in range of 2.4 to 3.8 V. In order to avoid the supply voltage adjustment problems it is necessary to chose MCU with the similar operating voltage. It is possible to make a system that works from the autonomous power supply. In this case energy consumption of devices is also essential. Another consideration would be the price of MCU. Based on the observations described above, one of the options could be Texas Instruments 16-bit MSP430F2112 microcontroller with I2C interface, 1.8 to 3.6 V supply voltage range. 3.2 Optical window selection For outdoor or box-type applications, an optical window in front of the photo light sensor needs to be chosen. As it can affect angular response of photo sensor, the thickness of the window should be kept as minimum as possible because there is a loss of power in every optical window of about 8% due to reflection (4% on each side) and an additional loss of energy in the plastic material [5,9].
Fig.10. Illumination measurement system communication diagram In this application it is possible to use the I2C interface also for the photo detector grouped connection together with the main unit MCU. The interface works on the Master-Slave principle. Each slave device requires it own 8-bit individual address. This sets the maximum number of connected devices - 128 devices on one bus. However, the capacity of the bus must be less than 400 nF, but each device must have input capacity of 5 to 10 nF range. Master device (here the main unit MCU) send requests for slave by it address in the bus, and is read data from slave or record in. [6] The main data collection unit (see Fig.11) collects data from photodetectors via communication interface I2C, realized by MCU with I2C interface, further the gathered data are sent to a computer (PC) via UART interface for further processing. Communication between PC and MCU is galvanically isolated through optocouplers, and further mediation via USB to RS-232 interface converter (FT232BM), which computer sees as a Virtual COM Port (VCP). 253
Fig.11. Data communication of Main data collection unit. External power supply and communications for the photo detectors of the illumination measurement stand are realized by wire connection, but in principle it is possible to create separate system. Such system can send received data over wireless network, and autonomous power supply for each device, could be provided by rechargeable batteries. If necessary there can be installed Step-Up DC-DC Converter such as LM2621 (Fig.12.), which will provide stable power supply during all battery performance time.
References 1. M. Johnson (2003). Photodetection and Measurement. Maximizing Performance in Optical Systems. McGraw-Hill Professional Publishing. (317 p). 2. Dr. Yoshihiro Ohno (1997). „Photometric standarts”. In Casimer DeCusatis. Handbook of Applied Photometry. OSA/AIP. pp.55–100. 3. A. Ryer (1997). The Light Measurement Handbook. International Light. Newburyport. pp. 14, 51-56. 4. I. Lima Azevedo, M. Granger Morgan, F.Morgan The Transition to Solid-State Lighting. Proceedings of the IEEE, Vol. 97, No. 3, 481-510, March 2009
Fig.12. Step-Up DC-DC Converter with LM2621 Such autonomous design would be helpful at outdoor applications, where usage of wires is not convenient.
4 Conclusions To develop illumination measurement stand, a construction and materials are described, for lux measurement a APDS-9300 photodetectors is selected. For data acquisition a Texas Instruments 16-bit MSP430F2112 microcontroller with I2C interface is chosen. The main difficulties or a challenge in stand development could arise from correlated compatibility of three used communication protocols in main data collection unit and data acquisition algorithm and software development.
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5. Avago APDS-9300 datasheet (2008). Available electronically at: www.avagotech.com/docs/AV02-1077EN 6. Philips semiconductors. The I2C bus specification (January 2000). Technical documentation. Available electronically at: www.nxp.com/documents/other/39340011.pdf 7. Labsphere. A Guide to Integrating Sphere Radiometry and Photometry. Technical documentation. Available electronically at: http://www.labsphere.com/tecdocs.aspx 8. INSTRUMENT SYSTEMS GmbH. Handbook of LED Metrology. Available at http://www.instrumentsystems.com/applications/ led-test-measurement/ . 9. Avago APDS-9007 datasheet (2007). Available electronically at: http://www.avagotech.com/docs/AV02-0512EN
