Testing of Highly Accurate Blackbodies Harri Latvakoski Mike Watson Shane Topham Mike Wojcik SDL/10-087

Outline High-accuracy blackbodies CORSAIR blackbody Blackbody testing needs and testing types Blackbody testing examples Conclusions

High accuracy blackbody needs There is increasing need for high accuracy blackbodies emissivity to Temperature uncertainties ~100 mK or less Driven by increasing accuracy needs of modern Earth-observing infrared instruments (e.g. FIRST and CLARREO) FIRST accuracy goals: K (1σ) cm K (1σ) cm -1 CLARREO accuracy requirements: ~ K (3σ) ~200 to 2000 cm -1 Maintain accuracy over 5 years on orbit CLARREO has strictest requirements but other instruments are close Both FIRST and CLARREO require long wave performance (unusual)

Blackbodies SDL has built several high accuracy blackbodies in the past E.g. for FIRST SDL has built a prototype (the CORSAIR blackbody) to show ability to meet CLARREO requirements As part of the NASA Langley IIP program CORSAIR Design is based on the FIRST blackbodies Easily scalable for different aperture sizes or requirements Simple to build LWIRCS

CORSAIR Blackbody Specifications Blackbody is designed to be a useful ground unit Wavelength range2 to 50  m Controlled temperature range:200 to 350 K Operating environment:Ambient or inside larger vacuum chamber Coolant:Liquid nitrogen Environment temperature:77 K to ambient Aperture:1.75 inches Beam divergence:6° full angle Emissivity  Temperature uncertainties30 mK under CLARREO like conditions Large operating range adds significant complexity

CORSAIR Blackbody Overall Design Vacuum shell, LN 2 and heated shroud are needed for wide range of environments

CORSAIR Blackbody Cavity Design Aluminum, steel, and fiberglass Specular trap design, works to long wavelength Numerous features keep gradients low

Blackbody testing CORSAIR emissivity  , temperature uncertainty 30 mK Can we believe it? …want proof for these high performance levels In general, blackbody performance is calculated from physical models and temperature sensor calibration. These results are quoted as performance Blackbodies usually NOT tested for performance or compared to other blackbodies Blackbody end-to-end testing is a significant expense Earth observing mission, such as CLARREO, attempting to contribute to a long record of observation need documented SI traceability. This requires testing Historically, when equipment or experimental results have been compared, they are often found to disagree by more then the claimed uncertainties

Types of blackbody testing Blackbody testing usual considered to mean end-to-end performance testing Compare blackbody to another blackbody Requires stable, highly accurate radiometer to observe both blackbodies Spectral test Can’t prove new design claiming higher accuracy than previous designs meets its requirement Observe with absolute radiometer based on electrical substitution standard Broadband test Requires very low background, otherwise similar to above It is also possible to test various blackbody characteristics one at a time 3 blackbody error sources; each can be tested to some extent Temperature gradients Temperature sensor errors Non-zero reflectivity Disadvantage: not a complete performance test Advantages: Increases confidence in model results Simpler then end-to-end test Blackbody can be designed for simple testing in some cases

Blackbody testing CORSAIR blackbody was designed for simple testing of some characteristics SDL has performed several type of blackbody testing with several blackbodies Preliminary results of testing performed on the CORSAIR blackbody follow Temperature gradient test Temperature sensor test Emissivity test End-to-end test

Testing examples – Temperature gradients Gradients across surfaces viewed in blackbody introduce error Can test for temperature gradients with sufficiently accurate temperature sensors at critical locations in blackbody Plot of sensors on CORSAIR blackbody cylinder and cone shows no gradients at -39 C No gradients to few mK level at other temps

Testing examples – sensor uncertainties Blackbody temperature sensors can be calibrated to 10 mK or better. Need to ensure sensors maintain calibration Can drift over time Can change reading from mounting or handling (mechanical stress) 1. In CORSAIR blackbody, multiple sensors agree when in blackbody so calibration is unlikely to have changed 2. CORSAIR blackbody contains phase change cells Cell with water, gallium, and mercury Cell is independently temperature controlled from blackbody Allows for calibration of temperature sensors: Take cell through melt point Calibrate sensor in cell Turn off cell, it equilibrates with blackbody Transfer calibration to blackbody sensors Blackbody TEC Container with temperature sensor Hg, H 2 O, Ga

Testing examples – sensor uncertainties Take cell to melt point and hold Absolute standard allows check of temperature sensor calibration Early results show CORSAIR cell temperature sensors consistent with known melt points to within ~20 mK. Temperature sensors were calibrated to ~15 mK (1σ) prior to placement in blackbody Can identify melt point by going through melt point and observing melt curve Absolute accuracy limited: melt curve is never completely flat, by definition melting requires gradients across cell Works well for trending Current cell design monitors material expansion to unambiguously identify melt point

Testing examples – Emissivity Emissivity must be high or at least well-known for an accurate blackbody The CORSAIR blackbody includes a emissivity monitor based on a quantum cascade laser (QCL) CLARREO will require emissivity monitor, CORSAIR includes one for demonstration Single 9.6  m 150 mW QCL Instrument viewing blackbody is detector Broad-beam Laser shines into blackbody with no optics

Testing examples – Emissivity Simple monitor design mimics observation geometry, so it allows a measure of blackbody emissivity Monitor used during test Spectrometer did not detect laser Emissivity  0.999,992 This is consistent with model and paint data at 9.6  m

Testing examples – End-to-End test We used the SDL Transfer Radiometer (SXR) to compare the CORSAIR blackbody with our existing Long-wave Infrared Calibration Source (LWIRCS) SXR (left) CORSAIR blackbody (right) The SXR is a combination spectrometer and radiometer in a single vacuum shell

SXR SXR basic operating specifications Wavelength range2 to 30  m DetectorsSi:As (11 K, single element used at a time) CoolantLiquid Helium (20 K optical bench) Filters28 (4 wheels, 8 slots) singles Window (removable)KBr, heated to 310 K ModesSpectrometer, radiometer Spectrometer resolution0.5, 1, 2, 4 cm -1 Spectrometer samplingOnce per HeNe laser fringe, Nyquist 7900 cm -1 Aperture70 mm (2.76”) Beam divergence1 mrad All SXR options software selectable while chamber is cold

CORSAIR SXR Test For testing on SXR, CORSAIR blackbody aperture was increased to 3” LWIRCS observed at 8 temps ~100, 179, 233, 249, 273, 290, 310, 335 K 8 filters in radiometer mode, 6 in spectrometer mode CORSAIR observed at 5 temps 233, 249, 273, 290, 310 K Same mode, filter set as for LWIRCS Data not yet analyzed Quick look analysis of 8.4 and 12  m narrow bands show LWIRCS, CORSAIR at same temperature consistent to better than 1%

Conclusions / Future Work Blackbody testing is useful We have performed several types of blackbody tests Test data will be processed May need to consider how to improve tests if accuracy is not at desired level