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Spectroradiometry

Spectroradiometry. Hans Baumgartner 2013. Content. Introduction Units Terms Components used in spectroradiometry Gratings Monochromator Detectors Spectroradiometers, devices Calibration Primary standards Secondary standards Calibrations at Aalto university. Reference material.

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Spectroradiometry

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  1. Spectroradiometry Hans Baumgartner 2013

  2. Content • Introduction • Units • Terms • Components used in spectroradiometry • Gratings • Monochromator • Detectors • Spectroradiometers, devices • Calibration • Primary standards • Secondary standards • Calibrations at Aalto university

  3. Reference material • Henry J. Kostkowski, Reliable Spectroradiometry, 1997. • Wikipedia • Google

  4. What is spectroradiometry? • The measurement of absolute optical radiations in narrow wavelength intervals. • Electromagnetic waves or photons with wavelengths between x rays and microwaves, 1 nm – 1 mm. • Usually measured units are spectral irradiance and spectral radiance.

  5. Spectral irradiance • Irradiance is the power of electromagnetic radiation Φ per unit area A. • Spectral irradiance considers each wavelength in the spectrum separately.

  6. Solid angle Ω • Two-dimensional angle in three-dimensional space that an object subtends at a point. • A dimensionless unit called steridian [sr]. • An object’s solid angle is equal to the area of the segment of a unit sphere. • Maximum solid angle is 4π sr.

  7. Radiance • Radiance L measures the radiation Φ that passes through a surface A and falls within a given solid angle Ω in a specific direction cosθ.

  8. Spectral radiance • Spectral radiance is the radiance per wavelength.

  9. Importance • Less demand in spectroradiometry for spectral radiance. • Radiance is more like an analytical tool. • All other radiometric quantities can be derived from spectral radiance, for example

  10. Full width at half maximum (FWHM) • FWHM is given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value. • Gives the width of apulse, spectral width ofa source or theresolution of aspectroradiometer.

  11. Diffraction • Diffractionoccurs with allwaves, including sound, water, and electromagneticwaves. • Occurs when a wave encounters an obstacle. • For monochromatic light (single wavelength), diffraction causes minima and maxima. • A light coming from a point source travels different distance at different angles, which leads to phase difference.

  12. Diffraction

  13. Diffraction grating • Splits and diffracts light into several beams travelling in different directions. • Diffraction grating diffracts non-monochromatic light into different colours.

  14. Spectroradiometer • An instrument designed to measure the spectral power distribution (spectrum). • Can be used to measure absolute of relative spectra. • Spectrograph is a spectrometer that imagesa range of wavelengthssimultaneously.

  15. Monochromator • An optical device that transmits a mechanically selectable narrow band of wavelengths of light. • Uses a diffraction grating to split light into several beams.

  16. Monochromator • Monochromator moves the grating, so only one color comes out from the output slit. • Slit width and grating constant define the bandwidth, grating movement the wavelength step.

  17. Double monochromator • Two monochromators mounted in series form a double monochromator. • The exit slit of the first monochromator usually serves as the entrance slit for the second monochromator. • Much lower stray light than in asingle monochromator.

  18. Filterwheel • Filters the lightbeforeitgoes into the spectroradiometer. • Blocks higher order light.

  19. Photomultiplier tube (PMT) • A vacuum phototube. • A sensitive detector of light in the ultraviolet, visible and near-infrared ranges.

  20. Semiconductor detectors • Used to detect the absorption of photons. • Silicon and InGaAs detectors used at MRI spectroradiometers.

  21. Spectroradiometers at MRI • Spectroradiometer is one of the most used measurement device at MRI. • Two monochromator based spectroradiometers. • Many CCD-based spectroradiometers. • Prices from 3000 to 40 000 euros.

  22. KonicaMinolta CS-1000 • Portablephotodiodecellbasedspectroradiometer. • 0.9 nmresolution. • Wavelengthrangefrom 380 to 780 nm.

  23. Konica Minolta CS-2000 • Successor to Konica Minolta CS-1000. • 380 – 780 nm, 0.9 nm resolution. • Is able to measure low luminance levels down to 0.003 cd/m2. • Diffuser available.

  24. Konica Minolta CS-S10w • Data management software for both Konica Minolta spectroradiometers. • Requires an USB-key to connect to the spectroradiometer. • Also NI LabVIEWdrivers available.

  25. Ocean Optics QE65 Pro / USB4000 • QE65 Pro • Hamamatsu FFT-CCD detector • ~0.14 nm resolution • Not tested yet! • USB4000 • Small and easy to use. • Not for precision measurements.

  26. Ocean Optics USB4000 1 – SMA 905 Connector, 2 – Slit, 3 – Filter, 4 – Collimator Mirror, 5 – Grating, 6 – Focusing Mirror, 7 – Detector Collection Lens, 8 – Detector (UV or VIS), 9 – OFLV Filters, 10 – UV4 Detector Upgrade

  27. Bentham DM150

  28. Bentham DM150 • The old monochromator based spectroradiometer. • Photomultiplier tube as a detector. • Wavelength range from 250 nm to 830 nm.

  29. Bethman DTMc300 • The new double monochromatro based spectroradiometer. • PMT for 250 – 850 nm (UV + visble). • InGaAs-detector for 851 – 2500 nm (infrared). • Si-detector will beadded, 300 – 1300 nm(visible + near IR).

  30. Calibration

  31. Irradiance calibration of spectroradiometer

  32. Irradiance calibration of spectroradiometer • In Aalto university, spectroradiometers are calibrated against 1-kW FEL-lamp. • Spectral irradiance of the FEL-lamp is known. • Calibration distance is 500 mm. • Spectroradiometer response is assumed to be linear. => If the distance doubles, the irradiance drops to one fourth.

  33. Irradiance calibration of spectroradiometer

  34. Primary standards • Black-body radiator • Synchrotron • Filter radiometer • Cryogenic radiometer

  35. Secondary standards • Also known as transfer or working standards. • Calibrated against the primary standard. • 1-kW FEL-lamps at Aalto university. • Also other types of lamps used. • 100-W FEL • 200-W FEL • Deuterium arc lamp (UV)

  36. Filter radiometer • Consists of an optical detector (trap), a wavelength selection device (filter + temperature controlled filter holder) and a high-precision aperture to define the geometry over which light is collected.

  37. Trap traceability • Traps are calibrated at four wavelengths: • HeNe 633 nm and 543 nm • Ar 488 nm and 458 nm • Trap model is used to derive the wavelength response.

  38. Transmittance • Transmittance is the fraction of incidentlight at a specifiedwavelengththatpassesthrough a sample.

  39. Filtertransmittancemeasurement • Filtertransmittancesaremeasured with PerkinElmerLambda 900 spectrometer. • Samelightsource and detector is used for both the insident and the passedthroughlight. • Transmittancevaluesbetween 0–1 (0–100 %).

  40. Emissivity • The emissivity ε of a material is the relative ability of its surface to emit energy by radiation. • It is the ratio of energy radiated by a particular material to energy radiated by a black-body at the same temperature. • A true black-body have an ε= 1 while any real object would have ε< 1.

  41. Emissivity • Emissivity of a “real” black-body • Emissivity of tungsten

  42. Calibration of the FEL-lamp • The spectral irradiance measurement of the FEL-lamp is based on the known responsivity of the filter radiometer.

  43. Calibration of the FEL-lamp • When exposed to optical radiation, the filer radiometer will produce a photocurrentwhere A is the area of the aperture, λis the wavelength, E(λ) is the spectral irradiance to be determined at the aperture plane, τ(λ) is the known spectral transmittance of the filter and R(λ) is the know spectral responsivity of the trap detector.

  44. Calibration of the FEL-lamp • The spectral irradiance of the tungsten (volframi W) filament lamp can be modelled aswhere B is an auxiliary multiplication factor, ε’(λ) represents the effective spectral emissivity of the lamp, c2=1.4388∙102m∙K is the second radiation constant, and T is the temperature of the lamp filament.

  45. Calibration of the FEL-lamp • The effective emissivity ε’(λ) is modelled with an Nth degree polynomial as • where bi’s are free parameters, and maximum N is 7. • Equations on previous slides are solved with a recursive iteration method so, that the spectral irradiance values at the effective wavelengthsof the filters match.

  46. Wavelength calibration of a spectroradiometer

  47. Gas discharge lamp • Exited atom falls back to a lower energy state and emits a photon of a characteristic energy. • Lamp envelopes usually made of quartz to pass the UV. • Can be used to calibrate the wavelength of the spectroradiometer.

  48. Uncertainty • Some error sources for calibrations: • Alignment • Distance • Trap responsivity • Lamp burning hours • Filters • Temperature • Linearity of the spectroradiometer

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