OOI AppsNote - Performing Photometric & Colorimetric Measurements of Light Sources

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How to Make Photometric & Colorimetric Measurements of Light Sourcesusing an Ocean Optics Spectrometer and SpectraSuite Software

This is a concise guide to setting up and calibrating your Ocean Optics spectrometer for makingmeasurements of the illuminance and colour of a light source. Ocean Optics SpectraSuite software isa very powerful and versatile application, but light source measurements only require that you use asmall proportion of its capabilities. The aim of this guide is to acquaint the operator with only thatinformation necessary to perform light source measurements. For a more complete review ofSpectraSuite, please refer to the operator manual that comes as a PDF file on the softwareinstallation CD.

In terms of hardware, you will have an Ocean Optics spectrometer (e.g. USB2000+) with optical fibreand cosine diffuser (CC-3). You will also need a calibrated reference light source (e.g. LS-1-CAL).

This guide assumes that you have previously installed SpectraSuite on your computer and copied the

calibration files for the reference light source onto the computer (e.g. save them on the Desktop).

A. Hardware Preparations

1. Connect the spectrometer to a USB port on your PC using the supplied USB cable.2. Launch the SpectraSuite software application.3. Remove the dust cap from the input to the spectrometer and attach one end of the optical

fibre.4. Attach the CC-3 cosine diffuser to the other end of the optical fibre.5. Insert the CC-3 into the opening on the end of your reference light source. On the LS-1-CAL,

you may first need to remove the SMA connector using the Allen key supplied (the SMAconnector is left in place if the CC-3 diffuser is not fitted to the fibre, but we stronglyrecommend that you do not use a bare fibre for photometric or colorimetric measurements).

6. Insert the power jack into the rear of the calibrated light source and turn it on. (N.B. werecommend that you keep a log book to record how long you operate the calibrated lightsource for each time you use it. The calibration of the LS-1-CAL is valid for 50 hours ofoperation).




B. Performing a New Calibration

The first time that you use the spectrometer (and at regular intervals thereafter), you need to create anew absolute irradiance calibration as follows. Please note that when you launch SpectraSuite, aspectrum graph opens automatically. This is called “Graph (A)”. Ocean Optics refer to this chart asdisplaying spectral data in “Scope Mode”. Be very careful not to use Scope Mode data; scope modeis useful only for gauging the level of light relative to the spectrometer’s dynamic range.

Scope Mode simply charts the spectral power distribution in units of counts (Y axis) versuswavelength (x-axis). It is important to understand that the spectrum displayed in Scope Mode neitherallows you to infer quantitative nor qualitative information about a light source. In other words, theshape of the spectrum will be displayed incorrectly in scope mode. This is because the raw data doesnot take account of the spectral sensitivity of the spectrometer and sampling optics. Even if you don’tneed to determine absolute light levels, and simply wish to see the spectral shape of a light source,you must perform a calibration against a reference light source. This is called a Relative Irradiancecalibration, but this guide will explain how to perform an Absolute Irradiance calibration. An absoluteirradiance calibration not only reports the absolute value of the light (spectral irradiance is reported in

μW/cm2.nm), it also correctly displays the spectral power distribution (the spectral shape).




1. Select File, New, New Absolute Irradiance Measurement to open the Absolute IrradianceCalibration Wizard.

2. Select New Spectral Acquisition, then click on Next.




3. The spectral source (i.e. spectrometer) that you wish to calibrate will be highlighted as shownbelow. Click on Next.

4. Select New Calibration then click on Next.




5. Click on the Set Automatically button to set the optimum integration time. The software willchoose a measurement integration time that corresponds to about 85% of full scale. For theUSB2000+, the full scale is about 65,000 counts, so the optimum integration time will bewhen the Last Peak Value is about 55,000 counts.

6. Click on the Advanced Settings button and click in the check box next to Electric Dark

Correction.

7. Set Scans to Average to 3, then click on Next.

8. Store the Reference Spectrum by clicking on the yellow light bulb icon then press Next.Remember to switch your reference light source off at this stage to help maintain thecalibration life of your lamp.




9. Store a Dark Spectrum. First you must block off all light going into the spectrometer. The bestway of doing this is to remove the fibre from the input to the spectrometer and replace thedust cap. Once you have blocked the light, click on the black light bulb and press Next.




10. Load the calibration data file for your reference lamp into SpectraSuite. To do this, click onBrowse and navigate to the location that you saved the calibration files to (e.g. the Desktop).Select the file with the “_CC” suffix in the file name – this is the correct file for the case of theCC-3 cosine diffuser fitted to the input of the optical fibre. An example of the file name isLSC2105_CC.LMP. Now press Next.

11. Select Fibre Diameter and enter the value 3900 (this is the diameter of the CC-3 diffuser).




12. Click on the Save to File button to save your new calibration. The default file location chosenby SpectraSuite is C:/Program Files/Ocean Optics/SpectraSuite/spectrasuite/bin. Use anaming convention that helps you to remember what the calibration relates to, for exampleyou could use the file name “CC-3 Irradiance Cal 23 Sept 09.IrradCal”.

13. When you press Finish, the calibration wizard closes and a new chart will appear inSpectraSuite called “Absolute Irradiance (B)”.

Note that the other chart “Graph (A)” continues to display the raw counts versus wavelength datafrom the spectrometer. This is termed “scope mode”. Please note that in scope mode, the shape ofthe spectrum for any light source will be displayed erroneously – scope mode should not be used foranything other than judging whether the light level is sufficient.




C. Performing a Photometric Measurement

Once you have created and saved an absolute irradiance calibration, you can now start to makephotometric measurements on one of your own light sources. With the spectrometer equipped with anoptical fibre with cosine diffuser at its input, you are able to measure the illuminance (in lux) from alight source. Remember that illuminance varies with the distance between the receiver and the lightsource, so be sure to record your working distance when measuring illuminance. Point the opticalfibre towards your light source and hold it in place.

1. Click on the “I” icon in the tool bar at the top right of the main SpectraSuite tool bar to performa new absolute irradiance measurement. This is a similar process to the calibration that youhave just performed, but now you will load the calibration you just created rather thancreating a new calibration.

2. Select New Spectral Acquisition, then click on Next.




3. The spectral source (i.e. spectrometer) that you wish to use for the measurement will behighlighted as shown below. Click on Next.

4. Select the calibration that you just created from the list of saved calibrations. Press Next.




5. Select Fibre Diameter. The collection diameter of 3900 should already be showing. PressNext.

6. Click on the Set Automatically button to set the optimum integration time for your test light

source. The software will choose a measurement integration time that corresponds to about85% of full scale. For the USB2000+, the full scale is about 65,000 counts, so the optimumintegration time will be when the Last Peak Value is about 55,000 counts. Please note thatthe USB2000+ has a minimum integration time of 1ms; if the light level is too high (i.e. above55,000 counts) at 1ms, you will need to move the fibre further away from the light source.Press Next.




7. Click on the Advanced Settings button and click in the check box next to Electric DarkCorrection.


9. Store a Dark Spectrum. First you must block off all light going into the spectrometer. The bestway of doing this is to remove the fibre from the input to the spectrometer and replace thedust cap. Once you have blocked the light, click on the black light bulb and press Finish.




10. Select Show Data in New Graph and then press Accept.

11. Click in the new chart area to activate the controls. Click on the candle icon at the far right inthe tool bar directly above the chart area.




12. Click the Accept button when the Configure Photometry dialogue box opens.

13. The Photometry Report window will appear as shown below. Note that we have not definedthe steradian field of view of the spectrometer so the values displayed for candelas will beerroneous. Similarly, you are not using an integrating sphere so the value for lumens will alsobe wrong. The illuminance (lux) value on the other hand is correct! Note the use ofengineering format in the reported values. 2.0000E2 means 2 x 10

2 which equals 200, while

2.0000E3 equals 2 x 103 or 2,000.




D. Performing a Colour Measurement

Once you have created and saved an absolute irradiance calibration, you can also start to makecolour measurements on your own light sources. The colour parameters which SpectraSuitecomputes include CIE chromaticity xy (2º observer), u’v’ (10º observer), CCT (correlated colourtemperature), dominant wavelength and colour rendering. Point the optical fibre towards your lightsource and hold it in place.

1. Click on the chromaticity diagram symbol (multicoloured triangle) in the tool bar at the topright of the main SpectraSuite tool bar.

2. In the New Colour Measurement dialogue box, select New Absolute Irradiance Processing

and press Accept.




3. The spectral source (i.e. spectrometer) that you wish to use for the measurement will behighlighted as shown below. Click on Next.

4. Select Get Irradiance Calibration from file and click on Next.




5. Select the calibration that you just created from the list of saved calibrations. Press Next.

6. Select Fibre Diameter. The collection diameter of 3900 should already be showing. PressNext.




7. Click on the Set Automatically button to set the optimum integration time for your test lightsource. The software will choose a measurement integration time that corresponds to about85% of full scale. For the USB2000+, the full scale is about 65,000 counts, so the optimumintegration time will be when the Last Peak Value is about 55,000 counts. Please note thatthe USB2000+ has a minimum integration time of 1ms; if the light level is too high (i.e. above55,000 counts) at 1ms, you will need to move the fibre further away from the light source.

8. Click on the Advanced Settings button and click in the check box next to Electric DarkCorrection.





10. Store a Dark Spectrum. First you must block off all light going into the spectrometer. The bestway of doing this is to remove the fibre from the input to the spectrometer and replace thedust cap. Once you have blocked the light, click on the black light bulb and press Finish.

11. Select the Emissive Mode (this is always the selection when measuring light sources). Also

select reference illuminant E for normalisation (this affects the calculation of dominantwavelength) then press Accept.



12. The Colour Report window and CIE 1931 2 Degree Observer Chromaticity Diagram willappear as shown below.

Prepared by:

Robert Yeo

Pro-Lite Technologywww.pro-lite.co.uk

24h October 2009

OOI AppsNote - Performing Photometric & Colorimetric Measurements of Light Sources

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