Pre-Launch Absolute Spectral Flux Calibration

Introduction

A common approach to pre-launch absolute spectral flux calibration of an instrument is to 

break the problem into two parts [1]. In the first part, the relative spectral responsivity (RSR) is 

determined in the laboratory, often by tuning a quasi-monochromatic source across the bandpass 

of the instrument. The result from this first part is a spectrum (for each pixel and each band)

normalized at some convenient wavelength. In the second part, the absolute flux calibration is 

measured in the laboratory, often by stimulating the instrument using a broadband calibration 

source of known absolute spectral flux (termed spectral irradiance, having units, for example: 

Wm-2nm-1). The result from this second part is a number (for each pixel and each band) having 

irradiance responsivity units, for example DN W-1m2 (where DN stands for Digital Number and 

represents the counts from the detector). These two parts are then combined to determine the 

absolute spectral responsivity (ASR) of the instrument, resulting in a spectrum (for each pixel and 

each band) having irradiance responsivity units, for example, DN W-1m2.

The RSR is typically determined by either a multiplying the measured component RSR’s 

or using a system-level measurement approach. As an example of a component approach (for a 

case where the complete instrument consists of a detector, bandpass filter, and telescope), the 

quantum efficiency (QE) spectrum of the detector pixels is measured separately from the 

transmittance of the bandpass filters and the transmittance of the telescope. Then the measured 

spectra are simply multiplied to determine the RSR. All optics should be included, and so for a 

realistic instrument the component approach may involve many optical elements. Note that the QE 

spectrum measurement requires a standard reference detector having a spectrally-known response, 

whereas the optical component transmittance measurements, being relative measurements 

(component in / component out), do not. 

In the system-level approach for RSR determination, the entire instrument (including 

detector, bandpass filter, and telescope as in the example considered above) is illuminated during 

the pre-flight calibration. The quasi-monochromatic source is typically either a lampmonochromator system or, in the case of the Spectral Irradiance and Radiance Responsivity 

Calibrations using Uniform Sources (SIRCUS) technique described below, a broadly tunable laser 

system [2]. A key point is that lamp-monochromator systems generally do not fill the pupil (both

entrance aperture area and field of view at the same time) of the instrument in the exact way that 

it is filled on orbit, whereas the SIRCUS technique generally does provide for proper filling of the 

instrument pupil. An example of the typical difference in RSR determined using the two 

approaches (component vs. system-level) on the same instrument is discussed later. 

As a practical matter, for WFIRST it may be best to use a hybrid approach – call it a 

subsystem level approach. Here the WFC instrument, for example, involving everything except 

the 2.4 meter telescope, would be measured at system level. The transmittance of the telescope 

would then be determined (measured?) separately. Then the RSR of the WFC instrument would 

be multiplied by the transmittance of the telescope to determine the full RSR.

An example of a broadband calibration source is a type FEL spectral irradiance lamp, 

calibrated for spectral flux (at 50 cm from the lamp) with traceability to the SI [3]. It can be used 
with a diffuser, and that diffuser can be at the focus of a collimator for illumination of instruments 

coupled to a telescope. Alternatively, the FEL/diffuser combination is often replaced by a lampilluminated integrating sphere. Usually, rather than attempting to calibrate the throughput of the 

integrating sphere, diffuser, or collimator, the spectral radiance and irradiance (and hence spectral 

flux) exiting from the sphere or collimator is measured with a calibrated spectroradiometer, and 

the SI traceability is then through the spectroradiometer, calibrated for spectral irradiance 

separately using an FEL lamp. The FEL lamps issued by NIST measurement services are, in turn, 

calibrated traceable to the NIST Primary Optical Watt Radiometer (POWR), an electrical 

substitution radiometer [4,5]. POWR also currently serves to provide the scale to SIRCUS, 

described in detail below. Note that the SIRCUS technique is, in principle, capable of providing 

an ASR calibration, not just RSR, of the instrument. This is because, as will become clear later, 

the SIRCUS technique uses standard detectors that carry an absolute scale. This may seem to 

obviate the need for an absolutely calibrated broadband source. However, most pre-flight 

calibration programs still follow the prudent practice of including a broadband calibration source 

in the calibration plan for a variety of good reasons.

SIRCUS Technique

In the SIRCUS facility at NIST, shown in Fig. 1, relatively high-power (typically 100 mW 

to 1 W), tunable lasers are introduced into an integrating sphere, producing a uniform, quasiLambertian, radiant flux source [2]. At each wavelength, both the device under test (representing, 

for example, the WFC instrument) and a standard detector are exposed, one after the other, to the 

same radiance emerging from the integrating sphere, and the response of each is recorded on a 

computer. The purpose is to transfer the flux calibration scale from the standard detector to the

device under test. To effect this in an automated manner, either the detectors are moved using a 

motorized translation stage (as depicted in Fig. 1) or the integrating sphere is moved between fixed 

detectors (as would be for large detectors). The distance between the source and the detectors can 

be varied as well along the optical axis, and the entrance apertures of the instruments are generally 

placed at the same reference plane for flux calibration transfer. A wavemeter uses a Michelson 

interferometer to measure the wavelength of the tunable-laser radiation, referenced to a precision 

HeNe laser, with an uncertainty of 0.01 nm or less. 

In SIRCUS, the laser radiation illuminates an integrating sphere using an optical fiber, as 

shown in Fig. 1. Speckle in the image from the source, originating from the coherent nature of the 

laser radiation, is effectively removed by placing a short length of optical fiber in an ultrasonic 

bath. A monitor detector is located on the sphere to correct for any radiant flux changes in the 

sphere output between measurements with the reference instrument and the device under test. A 

laser intensity stabilizer, consisting of a feedback loop between a laser beam attenuator and a 

monitor detector, holds the light level emerging from the sphere stable to typically within a part in 

104 or better. A number of different lasers are used to cover the spectral range from about 350 nm 

to 2500 nm, including continuous-wave (cw) dye lasers, solid-state Ti:sapphire lasers, as well as 

quasi-cw primary, doubled, tripled, quadrupled systems and optical parametric oscillator systems. 

Different integrating spheres are used, depending on the radiometric calibration and the 

wavelength of calibration. Small diameter integrating spheres – typically diameters of 25 mm to 

50 mm – equipped with precision apertures with diameters ranging from 3 mm to 8 mm are 

typically used for irradiance responsivity (i.e. flux) calibrations. For astronomical instruments, a 
collimator can be used between the integrating sphere and the detectors to limit the field of view 

and effectively place the uniform source image at infinity. Larger diameter spheres – 30 cm 

diameter – with 5 cm to 10 cm diameter exit ports are used for extended-area radiance 

measurements, typically useful for Earth-observing instruments. The spheres are made of sintered 

polytetrafluoroethylene-based coating [6] that has high diffuse reflectance from about 250 nm to 

2500 nm. Typical flux levels at 1 m from the sphere, using a 25 mm diameter integrating sphere 

with a 5 mm diameter aperture, range from approximately 1 μW/cm2 to 10 μW/cm2. Radiance 

levels between 1 mW/cm2/sr and 5 mW/cm2/sr are standard for a 300 mm diameter sphere with a 

75 mm output port. These radiance and irradiance levels can be continuously adjusted down to 

zero output, allowing for linearity measurements over many orders of magnitude. Note that the 

exit apertures are normally calibrated at the NIST facility for aperture area measurement [7].

FIG. 1. Schematic diagram of the NIST SIRCUS technique. From Ref. [1].

Ultimately, lasers determine the spectral coverage available in laser-based calibration 

facilities while the quality and characteristics of the reference standard detectors determine the 

achievable uncertainty. The standard detectors used at SIRCUS are trapped silicon photodiodes 

for the spectral range 350 nm to 950 nm, and InGaAs and Extended InGaAs detectors for the 

spectral range 950 nm to 2500 nm [1]. They are fitted with precision apertures calibrated by the 

NIST aperture area measurement facility [7] and are calibrated for spectral power responsivity 

directly against the NIST Primary Optical Watt Radiometer (POWR) [5]. The latter is an absolute

cryogenic radiometer that operates on the principle of electrical substitution, whereby optical 

power is measured in terms of the equivalent electrical power required to hold a black absorbing 

cavity at a fixed temperature difference above a heat sink near 5 kelvin in vacuum. This technique 
is in common use to establish radiometric scales in the visible-near infrared at national 

measurement institutes [4], and is a cryogenic version of the technique used to measure solar 

irradiance in what are commonly called active cavity radiometers [8].

Calibration scientists at NASA Goddard Space Flight Center have recently developed a 

version of SIRCUS called the Goddard Laser for Absolute Measurement of Radiance (GLAMR)

that has been used for pre-flight calibration of Earth-observing instruments [9]. The radiance 

reference standards in the GLAMR facility operate across the 320 nm to 2500 nm spectral range. 

They have been periodically calibrated for spectral radiance responsivity against the NIST 

SIRCUS facility over parts of this spectral range. For example, within the 400 nm to 870 nm region 

a standard uncertainty below 0.3 % has been achieved, and in the 870 to 1100 nm region a standard 

uncertainty ranging from 1 % to 1.5 % has been achieved. With a dedicated effort at NIST, the 

uncertainty could be improved and the spectral range increased. Plans are in place during 2017-

2019 to improve the SIRCUS transfer calibration techniques to enable standard uncertainty of the 

GLAMR radiance standards to drop below 0.3 % from 320 nm to 1620 nm and below 1 % from 

1600 nm to 2300 nm. With further dedicated effort and the use of newer transfer detector 

technologies, it is conceivable that uncertainties below 0.3 % in the 1620 nm to 2300 nm range 

could be achieved.

Component vs. System-level RSR

Differences between the component approach and system-level measurements can occur 

because of different measurement geometries – if optical elements and detectors are illuminated 

differently during component measurement than when in the system - and also because the former 

doesn’t take into account inter-reflections between optical elements that are present in the complete 

system. For example, Fig. 2 shows the calculated (by component) and measured relative spectral 

responsivity of 3 channels of a telescope, the Robotic Lunar Observatory (ROLO), that was a 

ground based system used for lunar irradiance measurements [1]. Spectral shifts, changes in the 

spectral shape and width are readily observable. The out-of-band response (not shown here) also 

differs significantly between the two approaches.

FIG. 2. Comparison between component (open diamonds) and system-level (closed diamonds) 

relative spectral responsivity of three ROLO filter channels. From Ref. [1].
References

1. G.P. Eppeldauer, S.W. Brown, and K.R. Lykke, “Transfer standard filter radiometers: 

applications to fundamental scales,” Experimental Methods in the Physical Sciences, 

Volume 41: Optical Radiometry, Edited by A.C. Parr, R.U. Datla, and J.L. Gardner 

(Elsevier, Amsterdam, 2005) Ch. 4.

2. S.W. Brown, G.P. Eppeldauer, and K.R. Lykke, “Facility for spectral irradiance and 

radiance responsivity calibrations using uniform sources,” Appl. Opt. 45 (32) 8218-8237, 

(2006). 

3. H.W. Yoon, C.E. Gibson, and P.Y. Barnes, “Realization of the National Institute of 

Standards and Technology detector-based spectral irradiance scale,” Appl. Opt. 41 (28), 

5879-5890 (2002).

4. N.P. Fox and J.P. Rice, “Absolute Radiometers,” Experimental Methods in the Physical 

Sciences, Volume 41: Optical Radiometry, Edited by A.C. Parr, R.U. Datla and J.L. 

Gardner (Elsevier, Amsterdam, 2005) Ch. 2.

5. J.M. Houston and J.P. Rice, “NIST reference cryogenic radiometer designed for versatile 

performance,” Metrologia 43, S31-S35 (2006).

6. Spectralon, Labsphere, Inc., North Sutton, NH.

7. J. Fowler and M. Litorja, Geometric area measurements of circular apertures for 

radiometry at NIST, Metrologia 40, S9-S12 (2003).

8. G. Kopp and G. Lawrence, “The Total Irradiance Monitor (TIM): instrument design,” 

Solar Physics 230, 91-109 (2005).

9. https://neptune.gsfc.nasa.gov/uploads/files/618_2015_04_rh_mccorkel.pdf