February, 2018
www.us-tech.com
Successfully Measuring LED Modules, Lamps and Luminaires
Continued from page 64
dominant wavelengths are mostly outside of our photopic vision. This has a small impact on mismatch, but still leads to considerable measure- ment errors.
Considerable measurement er -
rors of monochromatic diodes are possible even when a good quality measurement head is used, e.g., when single RBG channels are meas- ured. It is not sufficient to only refer to the mismatch characteristic to select the measurement head to measure diodes with different wave- lengths. The actual spectral curve must be taken into account and the correction of mismatch should be defined on that basis.
Spectrometric Measurement The color of LEDs and the spa-
tial distribution of light intensity change significantly, depending on the viewing angle. Therefore, an integrating sphere is recommended to help determine average values for flux, colorimetric coordinates, color temperature, and color rendering index. CIE recommendations and NIST publications describe spectro- radiometric measurements in an integrating sphere, on the basis of which both photometric and colori- metric values are calculated. There are a few important rea-
sons why spectral measurement of diodes seems more precise than sim- ple photometer measurements. A spectroradiometer measures the diode spectrum on the basis of the signal read for each wavelength, so, theoret- ically, there is no problem of spectral mismatch, as in the case of a single photodiode with optical correction. With spectroradiometers, colori-
metric quantities are measured con- currently with luminous flux. Color is defined as the average for the spatial distribution. A ruler consisting of miniature photo elements (pixels) is the light-sensitive element in the spectrometer. Diffused light with a given wavelength falls on the photo elements, facilitated by proper loca- tion of the diffraction grating. Each photocell changes optical radiation into current. In this way it is possible to determine the amount of radiation energy at each individual wavelength. CIE 127:2007 describes a con-
cept of spectral measurements, according to which a spectroradiome- ter can be used as a detector in an integrating sphere and the total luminous flux can be measured with- out spectral mismatch — if it is adapted to the measurements of radiation energy generated by LEDs.
NIST Coating Recommendations Due to the variables of sphere
geometry, size and location of measur- ing aperture, location of detector, and different non-homogenous shapes of the photometric body, measurements are burdened with errors resulting from the spatial non-uniformity of the integrating sphere. Depending where the luminous flux falls on the surfaces of the integrating sphere, readings in the detector change. The higher the reflection factor of
various coatings, the greater the num- ber of multiple reflections of the lumi- nous flux will appear within the inte- grating sphere. In the case of a coating
with a lower factor, the number of reflections is significantly reduced. There fore, the flux falling on the part of the sphere that is not seen by the detector is gradually smaller and smaller and does not hit the measure- ment window. With a higher reflection factor,
the signal in the measurement win- dow increases and the dependence of measurement accuracy on the shape of the photometric body or place on which the luminous flux falls during measurements in the sphere decreas- es. This is particularly important in
the case of narrow stream sources or luminaires.
Currently, paint mixtures made
with barium sulfate are available on the market. Their spectral reflection factor is relatively high in the entire spectral range, including in the range of short blue and ultraviolet waves. This mixture can be used to make a coating with total reflection factor higher than 97 percent. LEDs require different meas-
urement techniques for several rea- sons. LEDs have a different spectral distribution than halogen lamps or
Page 67
discharge lamps. LED lamps also have different structures and more directional light distribution. CIE recommendations and tests conduct- ed by NIST support that an integrat- ing sphere should be used together with a spectrometer and the coating of the sphere should be made of bari- um sulfate with total reflection factor above 90 percent. This allows accu- rate and repeatable simultaneous capturing of photometric and colori- metric quantities of LEDs. Contact: GL Optic Lichtmesstechnik GmbH, Tobelwasenweg 24, 73235 Weilheim/Teck, Germany % +49-7023-9504-0 E-mail:
office@gloptic.com Web:
www.gloptic.com r
A new era in process control begins
Get the full power of 3D inspection with SIGMA Line.
Meet us
at booth # 3707 at
IPC APEX EXPO
Zero defects. Perfect solder joints. No false calls. With the new SIGMA Line software module from Vi TECHNOLOGY, you get an automated optical inspection gatekeeper that guarantees the highest quality of every PCB. Based on extended functionality, SIGMA Line combines precision real-time analysis of inspection data from the award-winning programming-free PI series 3D SPI and the high-precision 5K3D AOI with its complete defect coverage. In short, one fully integrated solution for automated optical inspection to improve your SMT process and productivity.
Vi TECHNOLOGY is a member of the Mycronic Group.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116