L i n o v a

ScanSpec

Technical details of ScanSpec

The ScanSpec system is here described in some detail. The main parts of the system is shown in the figure below.


The parts of the ScanSpec system.

The hardware of ScanSpec consist of the ABB-Bomem spectrometer (A) with the telescope (B), the support construction (C), the mirror (D), and the mirror-turning system (E). It also consists of a PC, a motion controller, and wiring. The support construction is dimensioned so that bending and vibrations will be negligibly small. Aluminum U-beams is used to reduce the weight.

The ABB-Bomem spectrometer MR254

Since the ScanSpec system is built on the commercial spectrometer MR254 from ABB-Bomem, we will here present some data about the spectrometer. The spectrometer is a Fourier transform spectrometer for the infrared region (an FTIR spectrometer). The principle behind an FTIR spectrometer is similar to that of a Michelsson interferometer.


The MR254 spectrometer from ABB-Bomem.

The MR254 at FOI has one InSb and one MCT detector that can be used simultaneously. A built-in black-body source is used to subtract internal IR radiation. The black-body source and the detectors are cooled with liquid nitrogen. The detectors can be replaced. The spectral range of the spectrometer itself is 0.7 to 20 micrometers. A narrow-angle telescope (also from ABB-Bomem) is used together with the spectrometer. When the largest apertures are used in the system, the field of view of the telescope is around 0.20 degrees.

The spectrometer is controlled by a PC and a program developed by ABB-Bomem called Acquire. While the spectrometer is collecting interferograms, the data is sent from the spectrometer to an acquisition board in the PC via an optical fiber. The memory of the acquisition board is limited to 64 MB which is the maximum amount of data that can be collected by the spectrometer. Each point of each interferogram is stored with 2 bytes, so a maximum of approximately 32 million interferogram points can be acquired. When the acquisition of the interferograms is finished, the data is sent from the acquisition board to the RAM-memory of the PC. With Acquire then produce calibrated spectra from the acquired data and calibration files. The calibrated data is stored in a file format that is read by the ScanSpec software.

For more information on the spectrometer, contact ABB-Bomem.

The mirror

The mirror is a 6 mm float glass mirror with aluminum coating. It weighs 1.9 kg. The flatness of the mirror is enough for the application, since only one "pixel" is measured at a time with the system. The mirror is not designed to be used for an imaging application with the narrow-angle telescope.

The mirror is mounted in three points to the mirror turning system by two-component epoxy adhesive. The mountings are constructed so that they will not introduce any tension in the mirror.

The mirror turning system

The mirror turning system is based on two high accuracy rotary tables (RV120PP and RV160PP) from Newport (www.newport.com). The stepper motors of the tables are controlled by a motion controller (MM4005) also from Newport. The motion controller is programmed by ScanSpec software in the PC. The angular resolution of the tables is 0.001 degrees and the maximum speed is 40 degrees/s. The mirror turning system has two emergency power inhibit switches to protect the tables from rotating too far which could cause serious damage to the construction.

Communication between the parts

The figure below shows the communication between the parts of ScanSpec.


The communication between the parts of ScanSpec.

The following happens during an normal acquisition of a spectral image with ScanSpec. The letters refer to the figure above.
  1. The user sets the parameters for the acquisition by an input GUI (Graphical User Interface) or by loading a parameter file [A]. The user may set the start position by moving the mirror with a joystick [E, I, G, F].
  2. When the parameters are set the ScanSpec software writes a motion control program and sends it to the PC over a serial (RS-232) cable [G]. The ScanSpec software also writes a script for the Acquire software. This script sets all the necessary parameters for the acquisition.
  3. Acquire uses the script file to set parameters on the spectrometer [C] and make it ready for acquisition.
  4. The PC sends a start command to the motion controller (MM) [G] and the motion program is started as an independent program running on the MM. The motion controller sends a synchronization trigger signal to the PC [F]. A trigger is also sent to the spectrometer [H] which starts the actual acquisition of spectra.
  5. During the acquisition the spectrometer sends data to an acquisition board on the PC [D] over an optical fiber. Meanwhile the motion controller (MM) controls the motion of the rotary tables [K] and records their positions with a certain sample rate. If something is wrong, the hardware switches could stop the motion of the rotary tables [J].
  6. After the acquisition is completed, the produced files of the spectral data are read by ScanSpec software. Also, the positions of the rotary tables are read from the motion controller over the serial cable [F]. Some signal processing is done and the final spectral image is produced and stored in a file (with a file format called "sim").
  7. The spectral image is displayed by the "look" software that reads a sim-file and lets the user explore the data [B].

Software

The ScanSpec software is written mainly in Matlab (version 6.0). A small part of the program is written in C/C++ because of performance reasons and the need to access the parallel port of the PC.

The software is divided into four modules with well defined tasks and interfaces.

Below are two links to screenshots of the two graphical user interfaces in the ScanSpec software. The first image ("In GUI") shows the input GUI used by software in Module 1 (set_paramters). The second image ("Look GUI") shows a screenshot of the software used in Module 4 (look) to view the acquired spectral image.

The details of the ScanSpec software is available in the Software Manual for ScanSpec.

Specifications

Spectral coverage

The wavelength band is limited by the MR254 spectrometer and depends on the detectors that are used. The InSb- and MCT-detectors used at FOI gives a total spectral range of 0.7 to 18 mm.

Spectral resolution

The maximum spectral resolution is 1 cm-1 which corresponds to 0.1, 2.5, and 10 nm at 1.0, 5.0, and 10 micrometers respectively.

Field of view (FOV)

The field of view depends on the image size that is chosen. If a 64 by 64 image is acquired the field of view is 6.4 degrees. The FOV of the instrument is 0.10 degrees (1.75 mrad) per pixel. The telescope used has a FOV of 0.20 degrees.

Focusing range

The minimum focus range is 37 m according to measurements on the equipment at FOI. The maximum focus range is infinity. These parameter are determined by the narrow angle telescope from ABB-Bomem.

Transmission efficiency

The total system transmission of the ABB-Bomem system is 30 per cent not including the narrow-angle telescope. There is no specification from ABB-Bomem of the transmission of that telescope and it has not been measured by FOI. The ScanSpec mirror has a transmission better than 0.90 for most wavelengths. All together the total ScanSpec system transmission is around 25 per cent.

Dimensions

The dimensions of the instrument are: 2380 x 610 x 740 mm including the ABB-Bomem spectrometer, but not the PC and the motion controller. See figure below. The length of the device could be decreased.


The dimensions of the ScanSpec system.

Weight

ScanSpec weighs 89 kg not counting the ABB-Bomem parts or the motion controller from Newport. The spectrometer MR254 weighs 45 kg and the narrow-angle telescope 7 kg. All together the equipment shown in the figure above ("The dimensions of ScanSpec.") weighs 141 kg. This does not include the motion controller from Newport (12 kg) or the PC. Weight was not an important issue when ScanSpec was built. The weight could be reduced.

Energy consumption

All together the power consumption of ScanSpec is less then 650 W when the rotary tables are moving and less then 200 W when they are not. This includes the power consumption of the spectrometer, but not the PC.

Speed

The speed of acquisition of data is limited by the spectrometer from ABB-Bomem. The maximum number of spectra per second, the number of points per spectrum, and the maximum number of data points collected per second for different resolution settings (in cm-1) are shown in the table below.

-----------------------------------------------------------------
Resolution  Spectra/s   Data points/spectrum   Data points/sec
-----------------------------------------------------------------
    1         10.03            16384               160,000
    4         34.36             4096               140,000
   16         83.33             1024                85,000
  128        112.36              128                14,000
-----------------------------------------------------------------

The acquisition time for a whole spectral image obviously depends on the image size. See the section below ("Standard scan programs") for examples of acquisition times.

Standard scan programs

In the last version of ScanSpec (version 01_1) there are five so called "standard scan programs" which are used to test the system since they cover a broad range of possible scan parameter settings. Note that ScanSpec is not at all limited to these programs. The parameters can be set freely within the possible limits. Also note that the image sizes is not restricted to square sizes even though all standard scan programs use square image sizes. The table below shows the details of these standard scan programs.

Program Resolution Size Scans Time
p1_NORMAL 16 32 2 1:03
p2_LOW_NOISE 16 32 16 7:10
p3_FAST 128 16 1/2 0:08
p4_HIGH_RES 4 16 2 0:38
p5_LARGE 128 64 1/2 1:10

Program The name of the scan program.
Resolution The spectral resolution in inverse centimeters (cm-1).
Size The image size. All the spectral images above are square, but this is not a requirement of ScanSpec.
Scans The number of whole scans per measured spectrum. "1/2" means that only a half scan (a one way scan) is used to acquire a spectrum.
Time The acquisition time.