SR-5000N design: spectroradiometer's new performance improvements in FOV response uniformity (flatness) scan speed and other important features

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1 SR-5000N design: spectroradiometer's new performance improvements in FOV response uniformity (flatness) scan speed and other important features Dario Cabib *, Shmuel Shapira, Moshe Lavi, Amir Gil and Uri Milman CI Systems (Israel) Ltd., Ramat Gavriel, Migdal Haemek 10551, Israel, ABSTRACT As far as we know, CI has been the only manufacturer in the world of commercial visible/infrared spectroradiometers for remote sensing applications for many years. In this paper we describe the new design and some performance improvements that we are developing to renew and modernize the system. Simultaneous visible and infrared spectroradiometry, field of view flatness of response and scan speed are only some aspects of the system which have undergone significant improvement. The challenge is to achieve these functional improvements without losing any of the advantages of the traditional system as far as ruggedness for field use, interchangeability of spectral range and field of view and aiming and image recording facility. Important factors in the success of this endeavor are: i) the development of a new electronic signal processing package, ii) a modular optical concept that mechanically separates modules for small, medium and large field of view ranges, and iii) a compact overall shape for convenience of use. Actual instrument performance results will be reported in a future paper. Keywords: Spectroradiometer, spectral radiometers, infrared spectroradiometer 1. INTRODUCTION The first product developed and sold by CI Systems when the company was founded in the late 70 s was an infrared spectroradiometer, the SR 1000, a completely analog system, since at the time there were no personal computers and the measured spectra were plotted on millimetric paper. The company s know-how in the field of infrared spectroradiometry became during the years the basis for much of the later products in other fields of infrared applications. In the mid 80 s the spectroradiometric capability was extended to the visible, Near and Short Infrared (VIS/NIR and SWIR) wavelength ranges and the instrument was turned into PC based: spectra were since plotted on the computer screen and saved in digital form in its hard disk. Operated with DOS it was called SR A short historical review of CI s involvement in spectroradiometry is given in reference 1: in this paper, published in 2004, we described the transition of the system s software from DOS to Windows. Additional work was done in 2006 and described in reference 2, when CI developed a high sensitivity version of the SR 5000 for special applications of infrared equipment testing. Recently, CI decided to renew and restructure this product: motivated mostly by the obsolescence of many electronic components in the system, CI is taking the opportunity to introduce many opto-mechanical and user interface improvements, which will hopefully make the instrument more useful and easier to use. The instrument is called SR 5000N (SR for Spectroradiometer and N for New ), a prototype of which is now being built. In this paper we mention the improvement considerations, the new design, and the expected performance values of the important parameters. In a future paper we will report actual performance results based on system testing. 2. SYSTEM IMPROVEMENTS In order to understand the motivations for this work, let us summarize the main characteristics of the SR 5000, the traditional instrument being used now for more than two decades, first under DOS and then Windows operating systems. First a definition: A spectroradiometer is an instrument that measures the total spectral emission (self or reflected) of the objects within its field of view in absolute units of Watts/(steradian.cm 2.μ) as function of wavelength. * dario.cabib@ci-systems.com Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XXII, edited by Gerald C. Holst, Keith A. Krapels, Proc. of SPIE Vol. 8014, SPIE CCC code: X/11/$18 doi: / Proc. of SPIE Vol

2 High performance in a list of critical features is most important and useful for most field applications. Most of the parameter values and the design details on the engineering solutions of the SR 5000 can be found in the cited references 1 and Measurement speed in both radiometric (chopped) and transient (unchopped) modes; the former will be improved by almost a factor of two from 30 to 50 scans/second, the latter by a factor five from 20 to 100Ksamples/second. 2. Uniformity of response as function of position of a point source (in short called FOV flatness ) within the field of view of the instrument (partially achieved in the SR 5000 with focal point blurring on the detector); it will be significantly improved from a deviation from flatness of about 11% of the SR 5000 down to a worst case of 5%, where the optics contribution is designed to be only 1%. 3. The ability to measure spectra in as wide wavelength range as possible in one scan (this is done in the SR 5000 with two pairs of stacked detectors, called sandwich detectors, with wide range reflective optics and with two interchangeable Circular Variable Filters or CVF s, one covering the visible and near infrared and one the mid and long wave infrared ranges; however, the visible to far infrared ranges cannot be covered simultaneously in one measurement); the additional visible/near infrared optical channel equipped with a visible spectrometer in the SR 5000N allows the two wide spectral ranges to be covered simultaneously in one measurement. 4. The total dynamic range of the SR 5000 is 4x10 6, covered with four x10 gain ranges and 14 bits digitization in each range (of which 12 significant). In order to get this full dynamic range in one measurement automatic gain control (AGC) must be used, but this acquisition mode has significant gain switch noise and limits the scan rate to 0.5 scan/second. In the SR 5000N the signal is sampled simultaneously at two gain ranges (automatically selected for best signal to noise ratio) with 16 bits digitization and a gain ratio of 100. The final spectrum is recorded and displayed by joining spectral sections obtained from the gain range giving the better signal to noise ratio of the two. As a result, taking into account that only 14 are significant out of the 16 bits, and the factor 100 between the two ranges, the final dynamic range of each measurement is 1.6x10 6 with lower noise and usable up to 20 scans/second. 5. A facility to accurately point the instrument on the target or object to be measured (done in the SR 5000 with a co-aligned optical channel for viewing the measured field of view and its background without parallax and a boresighted telescope); in the SR 5000N pointing, focusing and image recording is implemented with a boresighted CCD without parallax. 6. A user friendly software package that allows: i) automatic recognition of the instrument configuration, ii) measurement parameter settings, iii) data acquisition and simultaneous visualization, iv) calibration and other algorithms for spectral handling, v) spectral display, vi) spectral data saving and exporting to universally readable formats and vii) data files management. 7. Connectivity will be improved from IEEE-488 bus to single 1G LAN. Figure 1 shows the optical diagram of the instrument. 3. OPTICAL DESIGN OF THE SR 5000N Proc. of SPIE Vol

3 Focusing collecting mirror Flat 45 0 folding mirror Spectrometer for visible region See figure 2 CCD for viewing Chopper Incoming rays from field of view Dichroic filter Figure 1: General view of the optics layout of the SR 5000N for the 7.5 millirad. field. Region in dashed zone is enlarged in figure 2. The incoming radiation from the field of view is shown for convenience as if coming from the bottom of the figure but in practice the 45 0 folding mirror is rotated 90 0 around a horizontal axis passing from its center and parallel to the plane of the paper, soo that the radiation comes perpendicularly into the plane of the paper. Chopping and internal blackbody reference for high performance NET is maintained in this new design. However, a significant improvement in field of view uniformity of response as function of point source position is achieved by the detection optics concept adopted here. Referring to figure 1, the collection optics is an F#3 on-axis Newtonian system with 120 mm. clear aperture. The radiation to be measured from the field of view of the instrument (FOV) is first reflected by a large 45 0 flat folding mirror with a central aperture. The reflected rays travel towards the focusing collecting mirror for the infrared spectral measurement while the ones through the central aperture of the 45 0 flat mirror are collected by the spectrometer for the visible range measurement. This design allows spectra in the infrared and visible range (up to 1.1 microns) to be measured simultaneously. Before being focused on the field stop (see figure 2) the beam encounters a dichroic filter at 45 0 which reflects the infrared radiation to be measured toward the field stop and transmits the visible radiation towards a CCD for viewing and recording images of the measured FOV and its surroundings. The CCD channel is boresighted with the measurement channel. Proc. of SPIE Vol

4 Infrared detector window Detector Detector lens Focal plane with a number of different size interchangeable field stops Circular Variable Filter (CVF) Internal reference blackbody Dichroic filter Chopper CCD Figure 2: Detection optics: enlargement of dashed zone of figure 1. Figure 2 shows the enlarged detection system in the dashed zone of figure 1. After being chopped at high frequency (several thousand Hertz depending on the CVF scanning speed selected) by a chopper placed at 45 0, the beam is focused on the field stop, which is a motorized computer controlled plate with different size apertures: this allows the selection of the FOV size from 0.5 to 7.5 milliradians. The 45 0 chopper configuration allows the radiation from an internal floating temperature blackbody to be reflected towards the detector: this is the baseline blackbody radiation which will be used in the calibration procedure to provide the measured spectral radiance of the FOV in units of Watts/(sr.cm 2.μ). Figure 3 shows the exterior look of the instrument mounted on a tripod. The view of figure 3 shows the back side of the instrument: the measured radiation is incident horizontally from the opposite side of the figure. Laptop for functional control and data storage and calibration Interchangeable detector module CCD image of the scene on a built-in screen: central black circle is the measured FOV Interchangeable collection optics module (shown for 7.5 mrad. FOV) Electrical connectors to computer and mains Proc. of SPIE Vol

5 Referring to figure 3, both the detector and collection optics modules are interchangeable units without need of optical realignment. The detector can be a liquid nitrogen cooled sandwich InSb/MCT for medium and long infrared ranges, or an InGaAs for short infrared or a silicon detector for visible spectroradiometry. The collection optics module of figure 3 can be swapped with two different wider angle collection modules, one for a maximum of 5 degrees and one for 10 degrees. Figure 4 shows the optical elements used for the wider angles operation. All three optical collection modules (7.5 millirad., 5 0 and 10 0 ) are suitable for all wavelength ranges (visible, short wave infrared, medium and long wave infrared from 0.4 to 15 microns). However, whereas in each case the infrared channel has a selectable FOV, the visible spectrometer channel FOV is fixed to the nominal value. Spectrometer for visible region Detector Folding 45 0 mirror Interchangeable field stops CVF CCD for viewing Incoming radiation from FOV Wide angle collection lens for 5 0 or 10 0 CCD for viewing Figure 4: Optical configuration for the wide angle operation of 5 degrees and 10 degrees. The collection lenses have different focal lengths in the two situations. The folding mirror and lens make up an interchangeable module for each of the 5 0 and 10 0 situations. The dichroic filter, CCD, chopper, field stop, CVF and detector with its own lens are the same as the ones in figure 1 except that the CVF and detectors are interchanged with the ones suitable for the different spectral ranges. The FOV uniformity of response to signals from point sources is made significantly more constant than today s SR This is achieved by designing the detector lens to image the instrument entrance pupil on the detector element instead of the field stop. The result is that variations of response on the detector surface and variations of direction dependent optical transmission are averaged out and the calculated instrument response as function of point source position in the FOV for the 7.5 milliradians optics is as shown in figure 5. Proc. of SPIE Vol

6 Normalized response to point source at infinity Angle (milliradians) Figure 5: Response versus angle. The calculation is done by the Zemax program, as an energy diagram through all the optical elements in the optics train. Variations from constant behavior are in the order of 1% instead of 10% in the opresent SR Detector module CVF module being replaced Collection optics module for 7.5 millirad. FOV Optics back panel Figure 6: The instrument is shown here being opened for CVF interchange. 4. DESIGNED PARAMETER VALUES The design specifications of the important parameters of the system are as in the following table. In a future paper we will present the actually measured values. Specification Value Notes Narrow FOV (NFOV) 7.5 to 0.5 mrad Manual focus 3m to infinity. f/ 3.3, All reflective ±5% FOV uniformity (max). 5 diameter aperture Optics and FOV Spectral range and resolution Medium FOV (MFOV) 5 to for the infrared channel. Fixed to 5 0 for the spectrometer channel. Wide FOV (WFOV) 9.4 to for the infrared channel. Fixed to 10 0 for the spectrometer channel. 0.2 to 25 µm Fixed focus to infinity. Minimal working distance 10m. ±5% FOV uniformity (max). Fixed focus to infinity. Minimal working distance 5m. ±8% FOV uniformity (max). Integrated Spectrometer µm; spectral resolution < 3nm Proc. of SPIE Vol

7 Specification Value Notes CVF/Detector combinations µm spectral resolution < 2% of wavelength Bandpass Filter/Detector combinations throughout the entire spectral range Sampling and scan rates Aiming, focusing and FOV image recording Detector types Up to 20 scans/sec High sensitivity spectral mode Up to 50 scans/sec High speed spectral mode >4 Ksamples/sec High sensitivity radiometric mode >100 Ksamples/sec High speed radiometric mode Non parallax integrated CCD camera Si, InGaAs, InSb, MCT, Pyro With FOV designation overlay LN2 or Stirling Cycle cooling, TEC cooling Internal Blackbody reference Floating ambient temperature reference Monitoring accuracy > 0.1 C Chopping frequency Noise performance >4000 Hz Dynamic range 1:1000 Signal sampling digitization 16-bits. Wavelength and detector dependant. Internal or external (capable of locking onto a remote chopped source) Example: < 5 mdeg NET with InSb detector and CVF at 5µm, 1 Hz bandwidth, 100 C target filling 7.5 mrad FOV Simultaneous sampling of two detectors in two gain levels. 5. SPECTRAL RADIANCE CALIBRATION The calibration algorithm and procedure is as in section 3 of reference 1. The measured radiance spectrum is obtained by scaling the detector signal at each wavelength (which is linear in the incident radiance difference between the source and the internal blackbody) with the ratio of the signal received from a known temperature blackbody source filling the field of view of the instrument and the Planck function value at that wavelength and temperature; the result is then added wavelength by wavelength to the Planck function at the internal blackbody temperature. 6. CONCLUSIONS In this paper we have described the optical design, main characteristics and performance of a new visible/infrared spectroradiometer (SR 5000N) being developed at present. Actual performance values according to system testing will be presented in a future paper. REFERENCES [1] Cabib, Dario et al., New user interface and features of the SR 5000: revival of infrared CVF based spectroradiometry, Proc. SPIE 5431, p. 46, (2004). [2] Cabib, Dario et al., High performance spectroradiometer for very accurate radiometric calibrations and testing of blackbody sources and EO test equipment, Proc. SPIE 6207, 62070L, (2006). Proc. of SPIE Vol