This makes measuring LED output an important activity but their small size, directionality and narrow spectral emissions that traditional photometric and colorimetric measurement methods are not always the most appropriate. Measurements of major characteristics of a LED that manufacturers and end-users find most important are like total spectral flux, luminous flux, chromaticity, CCT (Correlated Colour Temperature), CRI (Colour Rendering Index), peak wavelength, dominant wavelength, current (I), voltage (V) and luminous efficacy in the field of lighting.
Techniques for measuring EOT
The lamp measurement system will depend on three things that is measurement needed, physical size of the lamp, and magnitude of the lamp output. Lamp physical dimensions must also be considered. The sphere diameter must accommodate the maximum length of the lamp under test. The CIE recommends that for tubular fluorescent lamps, the sphere diameter should be at least twice the longest dimension of the lamp, and that for compact lamps the sphere diameter should be at least ten times the largest dimension of the lamp. There are different methods and considerations for lamp measurement radiometry and photometry. In general, the measurement of a lamp's luminous flux and its spectral radiant flux are identical except in how the radiation is detected. There are two methods for measuring the total flux of a lamp. The first employs a goniometer that scans the whole sphere of radiation from the source. Integration over this scan yields a very accurate measurement of the lamp output, but the process is not always easy or quick. The second method employs an integrating sphere to capture all the radiation from the source at the same time associated colour parameters are measured. A detector is used to measure the output of the sphere. By comparing the output with a test lamp in the sphere to the output of a known standard in the sphere, the total flux emitted from the lamp can be calculated. This procedure can be done easily and quickly.
Integrating Sphere
The integrating sphere integrates the total light output of a tested source to produce a single measurement. It is a hollow sphere coated with Barium Sulphate of fundamental optical technology, commonly used in photometry, radiometry, for applications including the test and measurement of light sources (lamps, LEDs), sensors (spectrometers, camera systems), and material properties (reflectance and transmittance). It has become standard instrument in photometry and radiometry. Luminous flux is the total photometric power emitted in all directions from a light source, measured in lumens. With the proliferation of high brightness, LEDs are used for illumination, and measuring the total flux of LEDs has become widespread. The variation in the spatial and angular response of many detector leads to incorrect results when the flux to be measured is non-uniform or the beam shifts slightly. Non-uniformity can originate from the source or optical path. Beam movement can come from moving components or refractive index variation in the air path. Measurements with fiber optics can be influenced by launch or fiber bend conditions. The recorded output pattern changes as the fiber output pattern changes, and light moves across or off a detector.
The integrating sphere light measurement systems are in sphere sizes ranging from 0.5 meters. The system consist of a spectrometers, source meter, thermal element, DC power supply, Auxiliary lamp (AUX), and calibration lamp (CAL). The integrating sphere must include a lamp socket fixture, baffles, and a viewing port for the photodetector. In high accuracy measurements, an auxiliary lamp may be used to correct for lamp self absorption.
Fig. 1: Integrating Sphere (Courtesy: www.labsphere.com)
A baffle is placed between the lamp and the detector port. Because the total luminous flux of the lamp is directly proportional to the illumination of the sphere wall, the detector must be baffled from direct illumination by the lamp. The CIE recommendation for the placement of this baffle is that it be 1/4 to 1/6 the sphere diameter from the photometer head. The baffle is coated with the same material as the integrating sphere wall. The most common mounting arrangement places the lamp at the centre of the integrating sphere and the baffle at approximately one-third of the radius from the viewing port.
Measuring procedure
At first initialization of integration sphere is very important one because to get accurate measurement. For initialization purpose the CAL lamp can be calibrated for accurate total luminous flux and total spectral flux, proceeding with AUX lamp calibration for self-absorption correction. Each lamp screened, seasoned and calibrated then only integrating sphere ready for measurement.
The LED is placed in the centre of the sphere and connected to the power port. After fixing the LED updating the auxiliary calibration compensates for errors due to size, shape and colour of device under test. Using temperature controller sets the temperature of the LED mounting block. LED power supply panel control the current or voltage to the LED. Then LED total flux is calculated from signals with a standard (known) flux source. But, anything placed in the sphere affects its throughput.
Fig. 1 shows Integrating spheres measurement setup that collect the total flux emitted from a lamp. Because an integrating sphere reflects and integrates all the light entering the sphere, the light received by a small area of the sphere is directly proportional to the total flux from a light source mounted within the sphere. The total flux from a test lamp is determined by comparing it to a calibrated working standard. Then the instrument produced the output screen, it consists of total spectral flux, total luminous flux graph and parameters, CRI block and CIE chromaticity diagram and parameters.
TOC analysis is the thermal transient response of the LEDs resulting from a step function excitation carried a large amount of information about the thermal behaviour of the device. The six types of tests that can be performed are:
- ILV @ constant T: Step and control I, wait for T to stabilize, measure L and V
- VLI @ constant T: Step and control V, wait for T to stabilize, measure L and I
- TLV @ constant I: Step and control T, wait for T to stabilize, measure L and V
- TLI @ constant V: Step and control T, wait for T to stabilize, measure L and I
- ILV/T: Perform an ILV @ constant T, then step T and repeat at each T
- VLI/T: Perform a VLI @ constant T, then step T and repeat at each T
Where, L=Lumens, V=Voltage, I=Current,T =Temperature.
Results and Discussion
The EOT characteristics of LEDs are measured using laboratory Integrating sphere as shown in Fig. 2. The total spectral flux,total luminous flux, CIE-XYZ chromaticity, color rendering index and TOC analysis (case temperature control vs. electrical and optical parameters) of LEDs are measured.
Fig. 2: Laboratory Integrating Sphere
Spectroradiometry is the techniques of how the energy distributed over visible range & the total flux is the measurement types of light emitted in all directions. Spectroradiometry + Total Flux = Total Spectral Flux (Watts/nm).
Photometry involves the physical measurement of visible light energy and attempts to compensate for the psychophysical attributes of the human response and physical units of power. Photometry is just like radiometry except that everything is weighted by the spectral response (luminosity function) of the human eye as defined by the CIE. luminosity function as the transfer function of filters which approximate the sensitivity of the human eye. Photometry + Total Flux = Total Luminous Flux (lumens/ nm).
(a) Total spectral flux output
(b) Total luminous flux output
Fig. 3: Single Green LED with Heatsink
Fig. 3: Single Green LED with Heatsink
Spectroradiometry, photometry measurement result of single green LED with heatsink and single yellow LED with heatsink are shown in Fig. 3 and Fig. 4 respectively. Fig. 3(a) and 4(a), total spectral flux waveform x-axis represents visible wavelength range from 360 to 1000 (nm), y-axis represents spectral power (W), this graph indicates spectral parameters in terms of lumens, Kelvin’s, dominant wave, peak wavelength, purity. CIE-XYZ chromaticity diagram x-axis represents the normalized value of X-coordinates, y-axis represents the normalized value of Y-coordinates, this diagram indicates the value of CIEx, CIEy, DUV prime, u prime, v prime and also measure full width half max (FWHM) and center wavelength, centroid wavelength and temperature. Fig. 3(b) and 4(b), Total luminous flux waveform x-axis represents visible wavelength range from 360 to 1000(nm), y-axis represents total flux (lumen/nm). Luminous Flux is the flow of light from a source per unit of time and is measured in Lumens. Then other waveform & measurement parameters are same as that of total spectral flux waveform.
(a) Total spectral flux output
(b) Total luminous flux output
Fig. 4: Single Yellow LED with Heatsink
Fig. 4: Single Yellow LED with Heatsink
From the Figures 3 & 4, it is proved that LED light sources are much more efficient at converting watts to lumens. Different materials can be used within the LED sources themselves, each of which has its own light extraction efficacy. For these and other reasons, Single green LED with heatsink and Single yellow LED with heatsink can consume the same power but differ widely in lumen output. Because power (Watts) can’t be used as an index of light output, evaluating the “brightness” of LED sources.
In data analysis of redstrip, whitestrip, single LED without heatsink, single green LED with heatsink, single yellow LED with heatsink are display screen in Fig. 5. This LEDs spectral waveform of the total spectral flux (units Watts/nm) over the visible wavelength range from 360 to 1000 (nm).
Fig. 5: Analysis the characteristics of LEDs
Fig. 5 reveals that lumens of the red strip is high compared than others, because array of led but the value of CCT is very low so it come under warm colour. White strip has lumens is low than LED arrays because is just a module of LED but the value of CCT is very high compared than others so it come under the cool colour. Single LED with heat sink is used to get the same lumens of module LEDs and it has its own power supply ratings. Single LED without heat sink its lumens level is very low. With different colour LED measurement results the lumen is same, the CCT and CRI value of each LED can be varied, so come under different category of quality.
Table 1 gives EOT parameters of LED, i.e., CIE chromaticity parameters, CCT in Kelvin’s, lumens, wavelength, purity, integration time, temperature, CRI, voltage, current, power. Measures are characterized according to the measurement value, it is useful for customers to identifying the quality of LED and also this measurement is useful for production level industries to improve quality of LED.
- Chromaticity is an objective specification of the quality of a colour regardless of its luminance. The CIE Chromaticity Diagram showing all visible colours. x and y are the normalized amounts of the X and Y primaries present, and hence z = 1 - x - y gives the amount of the Z primary required. These values used for colour matching purpose.
- The Correlated Colour Temperature (CCT) is a specification of the colour appearance of a light source, relating the colour to a reference source heated to a particular temperature, measured by the thermal unit Kelvin. The measurement can also be described as the warm, cool and neutral of a light source. Generally, sources below 3500K are considered "warm;" while those above 4100K are considered "cool" sources, in between that values are considered neutral.
- Lumen is a unit of light flow or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp. As lamps and fixtures age and become dirty, their lumen output decreases.
- Dominant wavelength is the spectral colour which can be mixed with white light in order to reproduce the desired colour.
- Centre wavelength is the average of two wavelengths determined in the 3 dB width of measurements.
- FWHM describes the spectral width of the half power points of the LED. Half Power points are where power spectral density is one half of the peak amplitude. FWHM in the LEDs are classed as high power level or low power level.
- Purity is defined as the ratio of the distance from the equal energy point E to the color coordinate. All intensity fairly close to the dominant wavelength (spectral colour is 100%).
- The integration time is the main parameter for the user to adapt the modulation of a spectrometric system to the signal level of the measurement. In many applications it is necessary to have a linear relation between the integration time and the signal, especially in radiometric measurements.
Conclusion
The electrical, optical and thermal characteristics of various LEDs like18 Volts red LED strip(redstrip), 12 Volts white LED strip (whitestrip), 3 Volts single LED without heatsink, 3 Volts single green LED with heat sink, 3 Volts single yellow LED with heat sink. From the experimental results it is observed that two different LED sources can consume the same power but differ widely in lumen output. The illumination level of LED with heat sink holds good when compared to other LEDs. From the obtained results best lamp can be chosen for particular lighting purpose and also this measurement is useful for production level industries to improve the quality of LED.
Dr P S Manoharan is working as Associate professor in Thiagarajar College of Engineering, Madurai. N Uma, is pursuing Post-graduate (Control and Instrumentation) in Thiagarajar College of Engineering, Madurai.
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