Development of a variable output LED-365 irradiance standard

Transcription

1 Development of a variable output LED-365 irradiance standard G. P. Eppeldauer Contributors: V. B. Podobedov C. C. Cooksey NIST CORM Conference, July 30, 2018 NIST, Gaithersburg, Maryland

2 Outline Technical problems, objectives Integrated irradiance measurement Flat-response UV-VIS pyroelectric standard Broadband UV irradiance scale transfer UV-365 irradiance calibration setup LED-365 source of variable output UV irradiance detector standards Source characteristics UV-A test meter measurement Conclusions

3 Technical problems The UV-365 excitation source for non-destructive fluorescent testing of cracked materials needs accurate calibration against flat-response detector standards The source needs adjustable irradiance levels to evaluate nonlinearity of UV meters To keep the calibrations simple, avoid using UV source standard(s). The integrated irradiance at 40 cm should be between 7 mw/cm 2 and 0.5 mw/cm 2 The required measurement uncertainty is 5 % (k=2) Mercury lamps should not be used.

4 Technical objectives Spatially uniform LED-365 collimated irradiance-source will be developed. UV-damage resistant Si detector will be used to monitor the integrated irradiance levels The Si monitor will be calibrated against a flat-response pyroelectric UV transfer standard The LED current will be adjustable to change irradiance. Integrated irradiance responsivity of UV meters can be calibrated The excitation source and the irradiance-meters will be mounted in a lighttight enclosure that can be used in the field.

5 Integrated irradiance E Using the pyroelectric standard with flat (constant) responsivity s(l) in the UV-VIS, s(l) = s = constant. When measuring its output signal i: The integrated (broadband) irradiance is: E = i s where i is output current [A], s is responsivity [A cm 2 /W], E and is integrated irradiance [W/cm 2 ]. The responsivity s must be determined and standard source will not be needed!

6 Determination of the constant irradiance responsivity s of the pyroelectric standard for the UV-VIS substitution in the same beam

7 Absorptance Pyroelectric irradiance standard of flat UV-VIS responsivity The relative response was obtained form spectral reflectance measurement Uncertainty of irradiance responsivity is 0.5 % (k=2) 916 Tie point Responsivity, V W -1 cm /- 0.22% responsivity change for the spectral range of the LED Wavelenghth, µm Constant responsivity of 910 V/W/cm 2, with an uncertainty of 0.5 % (k=2)

8 Picture of 5-mm temperature-controlled organic-black coated pyroelectric hybrid standard

9 Broadband UV irradiance scale transfer - Determine the integrated irradiance at user selected levels using a UV-damage-resistant Si working standard - Calibrate the Si working standard against the constant irradiance responsivity of the pyroelectric standard in broadband mode measuring the same source - Distance measurement is not needed because of the same front-to-detector separations in the two standards - Substitute test UV meters to pyroelectric standard - Check nonlinearity of UV detector standards - Check stray radiation using the Si working standard at different locations inside light-tight box - Determine uncertainty budget of UV integrated irradiance measurements - Test other existing UV-365 sources using the new irradiance calibration setup

10 Variable output UV-365 irradiance calibration setup Reference plane Covered opening for heat dissipation Collimated LED-365 source UV beam Shutter Iris Chopper UV uniform beam Ref.block Cables cover Baffle LED temperature controller LED currentcontroller Shutter control Chopper controller DVM Lock-in amplifier ELECTRONIC RACK Test UV irr. meter Si working irr. stand. Flat UV irr. ref. stand. Detector-based scales derived using detector substitution LIGHT TIGHT BOX REMOVABLE LID Magnetic breadboard 2 x 3 feet

11 Variable output (0.5-7 mw/cm 2 ) LED-365 excitation source LED temperature stabilized to 25.0 o C. Constant current control 0 to 1A (1W radiant flux). Beam is collimated with a 2 UV lens.

12 Pyroelectric irradiance meter standard Switch is added to bottom of housing for selection gains of 10 GW or 1 GW to measure integrated irradiance up to 50 mw/cm 2 Detector surface Front reference-surface

13 UV damage-free Nitrided-Si irradiance working standard Distance measurement in calibration setup is not needed! Detector-to-front distance is equalized to that of the pyroelectric standard with a distance difference < 0.01 mm Front surface is moved and fixed

14 Pyroelectric and nitrided-si standards with the same front-to-detector separation Nitrided Si working standard Temperature monitored flatresponse pyroel. transfer standard

15 Reference block for positioning of standard detectors and test UV meters Reference surface Both the detector standard(s) and the test UV meter touch the reference surface for irradiance transfer 6 inch travel for the Ref. block to move detector std and UV meter into beam in same plane

16 Experimental LED-365 irradiance calibration setup (large baffle is not shown for better illustration) Collimated LED-365 Chopper Shutter Iris Si Reference block Pyroel. std

17 Spatial uniformity of LED-365 irradiance in detector-plane X Y Distance from center [mm] Spatial nonuniformity of target spot of -12 mm to +12 mm (for both X and Y) at 40 cm: +/- 5 %; for -5 to +5 mm spot: +/- 0.4 %

18 Flux and peak-wavelength change versus LED forward current LED current [A] For forward current change between 50 ma and 1 A, the LED peak changes only 1.5 nm LED current [A] Flat pyroelectric standard measured integrated irradiance: 48.6 mw/cm 2 at 1.0 A LED forward current and source - detector distance of 40 cm

19 Test UV-A meter DM-365X measures LED-365 irradiance at 40 cm distance Nonlinearity: 3 % lower at 90 ma Fit equation: y = x R= LED-365 current [ma] Linear fit made to measured LED-365 irradiance between 0.5 mw/cm 2 and 7 mw/cm Large error in the measured integrated irradiance (spectral product of LED SPD and meter-response) Test UV meter response DM-365X measured 20 mw/cm 2 at 700 ma and 40 cm. Test meters will be calibrated against the flat-response pyroelectric standard Wavelength [nm] Wrong non-flat meter-response LED SPD

20 Conclusions Standard source is not needed for integrated irradiance measurement Hg source is not used. Spatially uniform tunable-output-level LED-365 irradiance source has been developed Pyroelectric detector standard with constant UV-VIS responsivity has been developed based on spectral reflectance measurement. First time use of a low-nep pyroelectric detector for irradiance measurement. The pyroelectric standard is used for simple and low-uncertainty integrated irradiance calibrations. UV damage-free Si working-standard is used to set the user required irradiance levels. Distance measurement is not needed. Test UV meters can be calibrated against the flat-response pyroelectric irradiance standard Non-linearity tests can be performed using current-tuning to change the LED irradiance Integrated irradiance can be measured from <0.5 to 20 mw/cm 2 Planned integrated irradiance calibration uncertainty: 5 % (k=2)