Instruction Manual of Diffuse Reflectance …

1 Aalto UniversitySchool of Electrical EngineeringMetrology Research InstituteVersion ManoocheriSaulius NevasInstruction Manual ofDiffuse Reflectance MeasurementsInstruction Manual of DiffuseReflectance MeasurementsPage 2 (17)Version: : October 9, 2015 Last edited by: of of contents .. and field of application .. Diffuse Reflectance .. used in measurements of spectral Diffuse Reflectance .. of the equipment .. traceability and calibration .. chain of spectral Diffuse Reflectance .. components .. 8 Reference standard uncertainty .. 8 Aging of reference standards .. 8 Geometric non-equivalence .. 8 Dark uncertainty .. 9 Wavelength scale uncertainty .. 9 Detector non-linearity .. ranges and best measurement capabilities .. methods and procedures .. method .. procedures .. accommodation and environment .. 17 Instruction Manual of DiffuseReflectance MeasurementsPage 3 (17)Version: : October 9, 2015 Last edited by: Instruction Manual describes the principle and the operation of the equipment usedto realize the national scale of spectral Diffuse and field of applicationReference standards: Diffuse Reflectance standards directly traceable to absolute scalesof spectral Diffuse spectrometer and integrating sphere-based detection unit: Used for traceabil-ity transfer from reference to working standards and for calibrating customer s measurement facility consists of the reference spectrometer of MRI [1,2] and anintegrating-sphere collection unit.

2 The system is designed to perform automated meas-urements of spectral Diffuse Reflectance by alternately irradiating a reference standardand a sample under test and comparing the reflected radiation [3]. Possible measure-ment geometry of irradiation/view can be either 8 / Diffuse (with specular componentincluded and excluded) or 0 / Diffuse . Diffuse Reflectance factor from the range of can be measured over the wavelength region of 360 830 Diffuse reflectanceSpectral Reflectance of a surface is defined as the ratio of the radiant flux reflected bythe surface to the flux incident upon the surface at a given wavelength ,)(/)()( ir =.(1) Reflectance of radiation from a surface is perfectly diffused if the incident radiant flux isreflected in all directions in accordance with Lambert s cosine , direct measurements of the two fluxes, reflected from a sample and inci-dent on it, are not easily realized. Therefore, by an international agreement reflectanceis characterized byreflectance factor, which is defined as the ratio of the radiant fluxreflected in the direction delimited by a given cone with apex at a point of the surface[1] Quality Manual of reference spectrometer laboratory.

3 [2] F. Manoochehri, Licentiate Thesis, ''High Precision Spectrometer for Reflectance Measurements'',Helsinki University of Technology, 1993.[3] Saulius Nevas, Facility for measurements of spectral Diffuse Reflectance , Master s Thesis for thedegree of Master of Science in Technology (Helsinki University of Technology, Metrology ResearchInstitute, Espoo, Finland 2000). Instruction Manual of DiffuseReflectance MeasurementsPage 4 (17)Version: : October 9, 2015 Last edited by: FMunder test to that reflected in the same directions by a perfect reflecting diffuser iden-tically irradiated. Theperfect diffuser is an ideal uniform Lambertian diffuser with a re-flectance equal to 1. If the solid angle of the cone approaches 0, or 2 steradian, thereflectance factor approachesradiance factor or Reflectance , radiance factors and Reflectance values are dependent not only on the materialitself but also on other parameters such as the geometrical conditions for the incidentand measured radiation.

4 The geometry of irradiation and collection is usually denotedas i/ m in subscript, where i indicates geometry of the incident radiation, and m indi-cates that of the measured radiation. The first term in the subscript may be eitherd fordiffuse or hemispherical incident radiation or a value in degrees indicating the angle ofa narrow incident beam. The second term similarly refers either to hemispherically col-lected reflected radiation with the specular component excluded (d) or included (t) orto the angle of observation, all angles being measured from the surface normal. Hemi-spherical irradiation or collection of the reflected radiation is usually accomplished byan integrating sphere. For example, 0/45 and 0/d denote radiance factor for0/45 geom-etry and Reflectance for0/d(normal/ Diffuse ) geometry, respectively (Figure 1).IrradiationViewingViewingIrradiationS ampleSampleDetector0 / 45 geometry0 / d geometryIntegratingSphereBaffle(a)(b)Fig ure 1. Determination of the (a) radiance factor for 0 / 45 geometry and (b) reflectancefor 0 / d Manual of DiffuseReflectance MeasurementsPage 5 (17)Version: : October 9, 2015 Last edited by: used in measurements of spectral Diffuse reflectanceEquipment used in Diffuse Reflectance measurements consists of a reference / workingstandard, the reference spectrometer, a six-port integrating sphere, a computer-con-trolled rotation stage, a detector unit, a digital voltage meter (DVM), a beam shutter,and a control computer.

5 A schematic of the measurement setup is shown in Figure stageOPMA djustable irisM1 CSM1 CSM2 GratingM2 PCDVMI ntegrating sphere Light sourceCylindricalMirrorSphericalMirrorOS FMCC ontrolUnitControllerboardInterfacecardIE EE-488 Monochromator (MC)Source and inputoptics enclosureLight tight enclosureFigure 2. Schematic structure of the measurement system for the measurements of spectraldiffuse Reflectance . Abbreviations: OSF, order sorting filter; M1, M2, flat mirrors; CSM1,CSM2, collimating mirrors, OPM, off-axis parabolic reference spectrometer including light source, monochromator, input and outputoptics is described elsewhere [1, 2]. The other components utilized in the measurementsare listed in Table 1. Equipment used in Diffuse Reflectance sphere unit1. Integrating sphere1 Labsphere RT-060-SF2. 0-deg. sample holder2 Labsphere3. 8-deg. sample holder2 LabsphereInstruction Manual of DiffuseReflectance MeasurementsPage 6 (17)Version: : October 9, 2015 Last edited by: FM4.

6 Detector unit, built at MRI1 Hamamatsu S5591,black anodized and data acquisition1. Rotation stage1PI Controller board1PI Shutter (filter wheel)1 CVI AB3014. Shutter control unit1 CVI5. Interface unit (Pascal) betweenAB301 controller and Source code of the measure-ment Digital voltmeter1HP standards1. White opal glass (8/d refl. factortraceable to NRC)1 HUTDR-W12. White opal glass (d/0 rad. factortraceable to PTB)1 HUTDR-W23. Grey spectralon (d/0 and d/8rad. factors traceable to PTB)1 HUTDR-G14. Black opal glass (8/d refl. factortraceable to NRC) standards1. Diffuse Reflectance standardscalibrated against the referencestandards and used in routinemeasurementsAt least 1 HUTDR-W3,SRS-99,SRS-75, of the equipment Information on the maintenance of the reference spectrometer and details on itscontrol electronics can be found in [1] and [2]. The integrating-sphere coating is susceptible to contamination and shock.

7 Beforestarting calibration measurements the sphere is visually inspected for its used, the sphere should be handled with an extreme care to prevent dust orother particles from getting into the sphere. The accessory is stored in a closet in theInstruction Manual of DiffuseReflectance MeasurementsPage 7 (17)Version: : October 9, 2015 Last edited by: FMreference spectrometer lab, properly covered from possible sources of contamina-tion. Th e rotati on stage is ch ecked for proper op eration b efore being used and a repairarranged if necessary. The photodetector is tested for linearity every second year. This can be done by reference ND gray filters The multimeter is autocalibrated every time before the start of the calibration meas-urements. Calibration of the multimeter is performed according to the calibrationschedule of Calsched. The reference standards are stored with the protective caps closed whenever theyare not in use. This is to keep the surfaces clean and free of physical damage.

8 Beforecalibration the condition of the surface is visually checked. If there is minor visibledust, it may be carefully cleaned with a soft brush or flow of pressurized gas (air ornitrogen). If a container of pressurized air is used the operator has to follow guide-lines for its safe usage in order to avoid damaging the reference standard. This in-cludes precautions such as: to make sure that the air flow does not leave any resi-dues on a surface before directing it onto the sample; not to shake the containerwhen using it; keep it in upright position when the nozzle is opened. The referencestandards are not to be washed. The surface can be touched neither with bare- norwith glove-protected Manual of DiffuseReflectance MeasurementsPage 8 (17)Version: : October 9, 2015 Last edited by: traceability and chain of spectral Diffuse reflectanceThe relative scale of spectral Diffuse Reflectance at MRI takes its traceability of diffusereflectance in0:d and8:d geometries from the absolute scale maintained by the Goni-oreflectometer facility at MRI.

9 The traceability has been established by calibrating ref-erence standards. The traceability of the reference standards is renewed every componentsThe uncertainty components in measurements of spectral Diffuse Reflectance are listedas standard uncertaintyReference standard uncertainty is the uncertainty associated with the reference stand-ards at MRI that are directly traceable to absolute scales. The standards have their spec-tral Reflectance values declared in certificates of calibration. The certificates also statecorresponding uncertainties of the values of reference standardsThis uncertainty component originates due to the finite time interval between the re-newals of traceability. Depending on the reference standard material, its exposure tothe measurement beam and time elapsed since it was calibrated by an absolute scale,the Reflectance properties of the standard may have been degraded. This degradationmight be especially pronounced in the UV-short visible spectral an example, the aging of a white Spectralon sample has been estimated based onmonitoring its Reflectance throughout the period of two years.

10 Reflectance decrease % was observed within 360 440-nm wavelength range. At the longer wave-lengths, however, no significant changes in Reflectance were repeatability of the measurements has been estimated by making several measure-ments with the same sample and same reference, the collection unit of the facility beingremoved and placed back into the setup before each of the non-equivalenceGeometric non-equivalence can be estimated by making two sample-to-reference re-flectance ratio measurements. The second measurement is made with the test samplebeing interchanged with the reference standard. The difference between the measuredreflectance ratios provides an estimate of the geometric non-equivalence between thetwo measurement Manual of DiffuseReflectance MeasurementsPage 9 (17)Version: : October 9, 2015 Last edited by: FMDark uncertaintyDark uncertainty is due to the uncorrected optical offset error. Its magnitude dependson the Reflectance level of a sample being measured [3].