You are here:   Home


The scale of spectral irradiance maintained at Biospherical Instruments

Fig. 1: FEL lamp V-020 mounted in kinematic lamp socket.Fig. 1: FEL lamp V-020 mounted in kinematic lamp socket.Irradiance and radiance calibrations performed at Biospherical Instruments are traceable to the scale of spectral irradiance established in 2000 by the Facility for Spectroradiometric Calibrations (FASCAL) of the National Institute of Standards and Technology (NIST) [Yoon et al., 2002]. NIST/FASCAL distributes this scale in the form of 1000 W tungsten-halogen lamps of type FEL (Fig. 1). Biospherical Instruments maintains several NIST/FASCAL-calibrated FEL standards. Our current calibrations of customer instruments are traceable to lamp F‑616, which was calibrated by NIST/FASCAL on 11 August 2008. As of 2019, the lamp has only seven hours of run-time. According to Yoon and Gibson [2011], “any lamp calibrated by NIST must be stable to better than 0.5% in spectral irradiance at 654.6 nm over a 24 hour period.” This implies that the spectral irradiance of the reference lamp F-616 should not have changed by more than 0.15% since its calibration at NIST.

In order to ensure that the scale of spectral irradiance maintained at Biospherical Instruments is stable over time, the primary standard F-616 is used as infrequently as possible. For this reason, customer instruments are calibrated against "working standards", which are also lamps of type FEL. The scale of spectral irradiance of lamp F-616 is transferred to these working standards in Biospherical Instruments’ calibration laboratory using the procedure outlined below. The scales of these working standards are regularly (e.g., monthly or when inconsistencies are suspected) compared with each other, and if discrepancies of larger than 1% are observed, the lamps in question are either recalibrated or retired. Every two years, our primary reference lamp F-616 is also compared with other NIST/FASCAL calibrated standards.

Calibration of working standards

Working standards are calibrated in Biospherical Instruments’ calibration laboratory, which includes facilities to operate lamps under closely controlled conditions as described in Section 6.3 of Hooker et al. [2012]. For example, the system to power the lamps allows to set and maintain the lamp current to within a precision of ±50 mA (±0.0006% for a target current of 8.2 A). The scale of irradiance is transferred from lamp F-616 to the working standards using two transfer radiometers, called the OXE and the XGUV [Hooker et al., 2012]. Both instruments are temperature stabilized to ±0.1 °C and use an array of microradiometers. The OXE covers the wavelength range of 320 to 1,640 nm while the XGUV measures from 291 to 875 nm. The large overlap of the wavelength ranges of the two instruments allows to verify that transfers are performed consistently.

Fig.2: Ratio X(λ) of the net signal of working standard V-048 and the net signal of the reference lamp F-616 for measurements of the OXE (red symbols) and XGUV (blue symbols). The green line is a spline fit to the measurements of both instruments.Fig.2: Ratio X(λ) of the net signal of working standard V-048 and the net signal of the reference lamp F-616 for measurements of the OXE (red symbols) and XGUV (blue symbols). The green line is a spline fit to the measurements of both instruments.The transfer of the irradiance scale is implemented by measuring the dark and light signals of the reference and the workding standard with the OXE and XGUV. Lamp and instrument alignment, as well as lamp operation are described in Sect. 6.3. of Hooker et al. [2012]. Because the set up uses kinematic mounts for the two transfer radiometers, the OXE and XGUV can be swapped while the lamp is energized, which significantly reduces the time required for the transfer. Dark signals are measured with an occulting device positioned between lamp and radiometer. Both dark and light signals are integrated over 60 seconds, and net signals are calculated by subtracting the dark from the light signal. The resulting net signals are denoted Sr(λ) for the reference standard and Sw(λ) for the working standard to be calibrated, and λ indicates the wavelength.

Fig.3: Ratio of X(λ) / Xs(λ) for OXE (red symbols) and XGUV (blue symbols) measurements. Fig.3: Ratio of X(λ) / Xs(λ) for OXE (red symbols) and XGUV (blue symbols) measurements. The ratio X(λ) ≡ Sw(λ) / Sr(λ) is shown in Fig. 2 for measurements of the OXE (red symbols) and XGUV (blue symbols). Because the spectrum of FEL lamps resembles the spectrum of a Black Body (e.g., https://en.wikipedia.org/wiki/Black_body), the ratio is a smooth function of wavelength. An approximating spline is fitted to X(λ) and denoted XS(λ). This spline fit is indicated by the green line in Fig. 2.

According to Fig. 3, The difference between X(λ) and XS(λ) is smaller than ±0.02% for wavelengths larger than 400 nm and smaller than ±0.10% at 300 nm with little systematic difference between measurements of the OXR (red symbols) and XGUV (blue symbols). This excellent agreement indicates that the scale of spectral irradiance can be transferred from the reference to the working standard with an uncertainty of less than 0.1%.

Finally, the spectral irradiance of the working standard, Ew(λ), is calculated from the spectral irradiance of the reference lamp, Er(λ): Ew(λ) = X(λ) × Er(λ). NIST/FASCAL provide values of spectral irradiance in steps of 10 nm in the UV to 150 nm in the infrared. Er(λ) is the spline-interpolated spectral irradiance of the values provided by NIST/FASCAL.

Uncertainty of scale of spectral irradiance of working standards

The uncertainty of the scale of spectral irradiance of working standards include the uncertainty of the scale of the primary standard (currently lamp F-616) and the uncertainty of the transfer of this scale to working standards. The uncertainty budget was established according to Taylor and Kuyatt [1994].

The expanded uncertainty (coverage factor k = 2, equivalent to a confidence level of 95%) of standards issued by NIST/FASCAL is 1.56% at 250 nm, 1.12% at 350, 0.63% at 655 nm, 0.47% at 900, and 0.33% at 1,600 nm [Yoon et al 2002]. As discussed above, the low run-time of this lamp suggests that the scale has drifted by less than 0.15% since its calibration at NIST. The uncertainty of the method to interpolate values of spectral irradiance tabulated in the NIST certificates is estimated to be 0.1%.

Uncertainties in the transfer of the FASCAL scale represented by standard F-616 to working standards include uncertainties of the calibration method, stray light in the laboratory, and uncertainties in lamp alignment and lamp current. The method of the scale transfer described above has an uncertainty of 0.10%. Uncertainties arising from stray light in the laboratory are considered insignificant (< 0.05%). First, the contribution of stray light to the total spectral irradiance at the diffuser of the transfer radiometers has been measured to be less than 0.2% at all wavelengths. Second, the light paths for the reference standard and the working standard are virtually identical. Hence, any stray light contribution will cancel when performing the transfer.

The reference and working standards, as well as the two transfer radiometers, are held in place with kinematic mounts (e.g., Fig. 1). Even though the distance between lamp and transfer radiometer may be in error by 0.5 mm, the effect on the transfer is negligible because both standards are affected in the same way and the associated errors will cancel. The lamp power system can set the lamp current to within a precision of ±0.006%. Any systematic errors in setting the lamp current (e.g., a systematic error in the shunt value used to calculate the current) will also cancel.

By combining the various uncertainty components, the expanded (k = 2) uncertainty of the scale of spectral irradiance of working standards was calculated and is 1.4% at 300 nm, 1.1% at 400, 0.9% at 500 nm, 0.8% at 700 nm, 0.7% at 900 nm, and 0.6% at 1,600 nm.

References

Hooker, S. B., G. Bernhard, J. H. Morrow, C. R, Booth, T. Comer R. N. Lind, and V. Quang, 2012: Optical Sensors for Planetary Radiant Energy (OSPREy): calibration and Validation of Current and Next-Generation NASA Missions., NASA/TM–2011–215872, NASA Goddard Space Flight Center, Maryland.

Taylor, B. N, and C. E. Kuyatt, 1994: Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, NIST technical note 1297, National Institute of Standards and Technology, Gaithersburg, Maryland.

Yoon H. W., C .E. Gibson, and P. Y. Barnes, 2002: Realization of the National Institute of Standards and Technology detector-based spectral irradiance scale, Appl. Opt., 41, 5879 - 5890.

Yoon, H. W., and C. E. Gibson, 2011: Spectral Irradiance Calibrations, NIST Special Publication 250-89, National Institute of Standards and Technology, Gaithersburg, Maryland.

 

Last Updated on Wednesday, 21 August 2019 10:49