Example design (1): The Spextube III for imaging
of static objects:
A low cost portable instrument has been designed to test out the above principle. A picture of the assembled system is shown in figure 3. The systems optical diagram is identical to figure 2. The camera head made by the company Electrim contains a 753x242 pixel CCD detector. The size of the CCD is 6.38x4.78 mm with 8.5x19.75 mm sized pixels. The dynamic range is 8 bits per pixel and the spectral range is 400-1100 nm. The pixel well capacity is 80,000 electrons and the R.M.S. noise is 35 electrons per pixel. Gain, bias and exposure time are controlled by a PC through an ISA bus interface capture card.
Figure 3. Picture of the assembled Spextube Imager III. M is rotary table, T front surface mirror, L1 front lens, A 35mm camera lens adapter, O laser pointer, B barrel contains spectroscope, L3 camera lens, CCD camera head, I lift table, and E two steel bars.
A standard C-mount 50 mm focal length lens (L3) is used to focus the diffracted and dispersed light from the grating/prism (GP) onto the CCD. The lens has manual iris control used to detect the background level of the CCD. The front of the lens is connected to a 100 mm long barrel, which is 48 mm in diameter with threads on both ends to accept components.
A handheld spectroscope made by Paton Hawksley Electronics Ltd. is fitted into the barrel. It features a 30 degree Littrow dispersive prism (P), a 600 grooves/mm transmitting holographic grating (G), a 42 mm focal length lens (L2), and a fixed size slit (S1). These components are housed in a stainless steel tube. The lens is 12 mm in diameter. The slit is 8.8 mm high and 25 mm wide. The grating is fixed parallel to the prism. The prism and grating normal coincide.
The field lens (L1) or the front lens of the spectroscope is connected to a 35 mm camera bayonet base adapter. The adapter has standard C-threads on the other end to fit the barrel with the spectroscope's entrance slit positioned in the focal plane of the lens. This type of adapter enables us to select from a wide range of 35 mm camera lenses manufactured by Nikon. The lens in figure 3 is a 50 mm focal length normal objective including variable focus and manual iris control. All mounts and adapters are found in the Mix and Match assemblies of the company Edmund Scientific (ES).
The above described spectrograph is named Spextube. It is mounted to a lift table, which slides on two 450 mm long steel bars. In addition, a rotary table is installed at the end of the steel bars (See figure 3). Light from the target object is reflected by a front surface aluminized 70 mm square glass mirror into the field of view of the field lens (L1). The mirror is attached to a magnetic disk which fit disk of the rotary table. The hardware of the rotary table consists basically of a unipolar stepper motor connected to a 1:512 gear drive. The stepper motor was disconnected from an old floppy disk drive. It has 200 steps per revolution. The stepping sequence is generated by software via the digital output of the PC's parallel port. An array of Darlington transistor pairs, Motorola's ULN2003 chip, switches and amplifies the current to the coils in the motor. The ULN2003 is also used to switch ON/OFF a relay to control a laser pointer mounted on top of the spectrograph. The laser delivered by ES is used to keep track of the position of the mirror relative to the location of the target object. It is also used in wavelength calibration.
Example design (2): The Spextube III airborne;
Figure 4 shows an example of the spextube design used on an airborne carrier. There is no need for a rotary element since movement of the airplane makes the image from the front lens slide across the slit. The instrument is tilted at an angle of about 45 degrees relative to the vertical in order to view the ground through a dome mounted on the side of the airplane.
Figure 4. (a) Airspex Imager, (b) video camera, (c) tripod and (d) dome.
The main trick is to make sure that you sample spectrograms fast enough in order to not miss any target regions at ground level. The altitude was 10.00 feet, speed 63 m/s, slit width 250 um, and the exposure time was 60 msec.
Example design (3): The Spextube IV for lab.
An experimental instrument has been constructed to be used in lab. work. Especially, on targets located close to the instrument using light sources of known intensity. 3 imagers have sofar been constructed to carry out measurements of solar albedo in Lhasa, Tibet, to study light properties of marine species at Fiskeriforskning in Tromsų, and one to support our firm. In this design we decided to use a stepper motor controlled plane reflecting grating instead of the transmitting grating prism element described above. This more traditional design is shown in Figure 5.
Figure 5. Picture of the assembled Spextube IV Imager at Fiskeriforskning in Tromsų, Norway. (1) is detector (CCD), (2) camera lens, (3) instrument house containing grating and fixed front surface mirror, (4) grating stepper motor, (5) Stamp II microcomputer, (6) adjustable iris, (7) variable length tube / barrel with collector lens, fixed slit and camera bayonet adapter, (8) field lens, (9) front surface mirror, (10) gear box, (11) stepper motor, (12) aluminium mount bars, and (13) steel rods.
For these instruments speed is not an issue. The detector is a 16-bit TE cooled CCD from the company StarXpress (MX516) and the readout time for one frame is in the order of seconds. Nevertheless, the images produced by these instruments have a high dynamic range. The detector is also very sensitive in the blue part of the spectrum. Exposure times can be in order of minutes!
Example design (4): The FishTube I spectrograph:
Figure 6. The FishTube I spectrograph. A is ring mount fiber source, B spectrograph housing, C camera lens, and D detector (StarXpress MX516).
This instrument was assembled 03.11-05.11.99. It's set up to measure diffuse reflected or transmitted light of any sample in the visible wavelength region. The use of spectroscopy has proven promising for assessing aspects of fish quality, e.g. fat, water, protein and salt contents. Measurements have been conducted on samples of cod muscle. A general correlation between attenuated light and storage time of cod fillets on ice has been found, indicating that the technique is a good candidate to assess freshness of cod, measured as days on ice.
Example design (5): The Airspex spectrograph / system:
Figure 7. The Airspex spectrograph
This instrument was assembled 18.05-22.05.2000. It's design to be used onboard an airborne carrier measuring reflected solar irradiance from ground targets in the VIS/NIR spectral region. Wavelength and sensitivity calibration were carried out from Longyearbyen by ARCo. The design of the instrument is equal to the above design (1), but the f/value of the instrument is increased from 3.5 to 2.8. The initial tests shows us that this is a good candidate for airborne campaigns.
Example design (6): The Drone Spectrograph:
Figure 8. The real time imaging spectrograph (Dronespex)
Example design (6) uses a grism (30 deg. Littrow prism with a 600 lines per groove grating) to disperse the light. The optical design is identical to figure 2. It uses a rotating table instead of a front surface mirror. It is small in size (approximately 22 x 3.5 cm 2 ) and it uses a highly sensitive video camera, 0.00015 lux). Bandpass is close to 1 nm throughout the visible wavelengths (400 - 700 nm). A video frame rate of 25 images per seconds (PAL) is possible, depending on the recording medium. The instrument was assembled in August 2004.