Light measurement Q&A
Q: Where is the center of my lamp?
A: When measuring a lamp must it be adjusted so the photometric center of the lamp is at the center of rotation of the goniometer. The photometric center of a E27 light bulb is fairly easy to determine, but it is not so easy to determine for other lamps such as recessed panels and spots. Physically should the photometric center be the position where the sum of all light emitted by the lamp meets, but in practice is this not feasible as some light comes from the source and some from the reflector making the calculation too complicated. Instead does the standard EN 13032-1:2004 give a simple set of rules via illustrations of how to define the photometric center of a lamp. Please have a look below.
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Q: How do I calculate UGR values in Light Inspector software?
A: At present, the Light Inspector software does not calculate the UGR values itself. In order to figure out the values you can use another open source software, for example, LDT Editor. UGR values depend on the placement of an observer. The LDT Editor offers a table with multiple positions for an observer as an overview of how the UGR values change relative to position. Therefore the angular intensity distribution needs to be symmetric for their algorithm to work. If your light source doesn’t have a symmetric distribution, you can make an adjustment in the Light Inspector software (Edit -> Photometric -> Corrections -> Correct Asymmetry). Export your measurement then in an LDT format and open it in the LDT Editor. The table with UGR values is shown under Light Distribution tab. Additionally you can calculate the exact UGR values for a specified observer’s placement in Dialux.
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Q: The lumen outcome from measuring LED PCB is much lower/greater than the one from measuring the same LED PCB covered with street lenses. It should be the other way around. Why did I get such a result?
A: When an LED lamp is covered by a lens its lighting distribution changes according to the desired purpose of the lamp. Thus street lenses are designed in a particular way to illuminate some areas more than others, which in technical terms mean that their angular lighting distribution becomes non-uniform. An adequate calculation of the luminous flux from a non-uniform surface requires information from multiple C-planes. Therefore if you were measuring your fixture with LightSpion with only 2 C-planes, the results can be inaccurate. We recommend a full spherical measurement, i.e. all available 36 C-planes on the LabSpion. However, if you only have a LightSpion in your possession and still want to estimate the case, it is possible to measure a total of 4 C-planes with manual rotation of the light source.
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Q: I export the same measurement in LDT and IES formats. When I insert the files into Dialux the same fixture shows different dimensions. Why is that?
A: When you specify the dimensions of your lighting fixture in the Viso Light Inspector software, there are two parameters. Light width/height and physical width/height. The difference between the two parameters are the edges of a fixture. When this fixture file is imported in Dialux, the LDT and IES files are processed differently. Therefore the IES file reflects only the lighting dimensions, whereas the LDT file presents also the physical edges of the lamp.
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Q: Are the measurements done with LabSpion correspond to the EN13032 standard?
A: Yes, the LabSpion is a type-C horizontal goniometer, where the measurements are done when the light source is rotating.
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Q: Can we measure a laser diode with LightSpion or LabSpion?
A: A laser diode is a point source, whose power is concentrated in one spot. Therefore the laser diode alignment shall be extremely precise, with much greater precision than a general alignment would require. If this condition is met, then you can measure such a light source with LightSpion. The shortest integration time of the spectrometer sensor for LabSpion is of the order of miliseconds (whereas that for the LightSpion is of the order of microseconds), so the risk of saturating the sensor with the laser power is quite low.
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Q: What are the type of lamps one can measure with the Viso light measuring systems?
A: All kinds of lamps are easily measured with our systems. Automotive lamps, street lamps, commercial lamps, retail lamps, industrial lamps – all kinds of lamps. Each Viso system has its criteria according to the physical dimensions of measuring lamps. Otherwise, there are no limitations.
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Q: I would like to use the Viso measurements in AGI32, but I face difficulties with loading the IES and LDT files into the software. Otherwise, when loaded, the files are not properly read out and error messages keep popping out. Why is that, and what do I do with it?
A: Indeed, the earlier versions of the Viso Light Inspector software (all versions prior to Version 4.43) produced measurement files, whose formats were incompatible with the AGI32. Therefore every single attempt of loading files (particularly IES files) resulted in crashing of AGI32. We solved the incompatibility issue, so the newer software releases produce an adequate format. If you face the described issue, you can download and install the newest software version.
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Q: Can Viso products measure light sources according to the EN62471 standard?
A: EN62471 is investigating the photo-biological safety of the light radiation. Because of the subject of investigation, the standard requires a very broad spectral range, namely between 200 and 3000 nm. The Viso products are using the visible light spectrometer sensors, which are capturing radiation within 360-830 nm. Therefore it is still possible to measure light according to that standard using the Viso products but with additional IR and UV sensors.
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Q: I have measured the same lamp on an integrating sphere (and/or another goniometer) and the LabSpion. The measured flux from the LabSpion was 15% higher than the one from a different system. Why is that?
A: There can be several explanations to the origin of the discrepancy. Firstly, when measuring lamps with narrow beam angles in an integration sphere, the flux result will be lower than the one from a goniometer system. The light distribution in an integration sphere is not homogeneous, as there can be distinct hot spots at certain C-planes. Secondly, ambient light can interfere with the measurement. Thirdly, if the sensor alignment is not correct (for example, the distance isn’t measured to the centre of rotation, or the distance is too large) – it also influences the resulting intensity. Additionally, if the light distribution from your lamp has a non-uniform shape and you only measured a few planes, we recommend to measure all of the available 36 planes to get the most accurate result. Usually the flux accuracy is +/- 4%, which means the maximum error should not exceed 8%.
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Q: The flux measurement is giving me too little lumen (I am using the LightSpion). Why is that?
A: Most likely you have to check the alignment of the system. Make sure that the arm with the sensor is extended as a straight line (so it is not bent). To check whether the sensor is centred, measure the intensity values in candela: move the lamp up and down to ensure that the maximum value is at the centre position.
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Q: The colour temperature I measured with the LightSpion is slightly different than the one I measured with a different system. Why is that? Also, how can I make sure that the sensor is well-calibrated?
A: Firstly, any deviations within the confidence interval of 35K are considered a good match. Secondly, make sure that during the measurement process there was no ambient light present, i.e. make sure to provide dark-room conditions to obtain the best results. Ambient light (such as an open window or reflections from surrounding surfaces) does influence the spectral characteristics that the detector is registering. Therefore they can influence the calculation of the resulting colour temperature. To test the calibration of the detector, however, you can make a very simple test. Measure the spectrum of a tungsten lamp and compare it to the one you find online. This will show you, how well the sensor is calibrated.
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Q: I am measuring a lighting fixture with a LabSpion and set the sensor to an appropriate distance, which is ten times larger than the fixture length. The resulting lumen counts are lower than what I should get. Why is that?
A: First of all, you have to make sure that the alignment is correct. Check both the horizontal and vertical sensor-fixture alignment. You can do it with the green laser box, which accompanies the system. If you confirmed the perfect alignment and you still continue to register the lower lumen values, then it is quite likely you have a stray light problem. Look in the LabSpion manual to find more information about how to overcome the stray light inside of the narrow corridor problem.
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Q: I wonder, if the spectrometers you use in the Viso products are divided into classes? If yes, which class are you using?
A: Spectrometers are not divided into classes, and comparison is happening through individual specification of stray light sensitivity and other parameters. For the LightSpion we use OceanOptics spectrometer, which is considered a lower-end spectrometer. The LabSpion system has a small high-end Ibsen Photonics Freedom spectrometer. It has a unique transmission grating technology designed and built by Ibsen Photonics in Denmark. The high sensitivity of the device makes the measurement process fast and accurate. You can read more about it at http://ibsen.com.
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Q: Do I need to calibrate the spectrometer with a photometer before I start my measurement? Or can I trust the factory calibration and use the spectrometer right away?
A: The spectrometer sensor is already calibrated when you received the Viso products. That is why you also get a calibration certificate that states the calibration date. We recommend to perform an annual (latest bi-annual) calibration. The sensor can also be accredited by a third party organisation, if desired. Additionally you can use the Viso reference lamp for your own estimation of the calibration of the spectrometer.