A PORTABLE SPECTRO-POLARIMETRIC IMAGER:

POTENTIAL MINE SAFETY AND GEOLOGIC APPLICATIONS 

Charles Sabine 

Geopix, 1280 Bodega Drive Sparks, NV 89436 

Louis J. Denes, M. Gottlieb, B. Kaminsky, P. Metes 

Carnegie Mellon Research Institute, 

700 Technology Drive, Pittsburgh, PA 15230 

Ronald T. Mayerle, Jami M. Girard 

National Institute for Occupational Safety and Health, 

315 E. Montgomery Ave., Spokane, WA 99207 

ABSTRACT 

A significant risk factor in assessing and modeling potential catastrophic slope 

movement in open-pit mines is the presence of argillic alteration in host rocks. Highresolution hyperspectral imagery operating in the visible to shortwave infrared can be 

a useful tool for identifying and mapping potentially dangerous argillic zones. We 

have conducted preliminary experiments with a new type of hyperspectral imager 

that employs an acousto-optic tunable filter, a variable retardation plate and a 

standard digital camera. The instrument we tested operates in the .5-1.0 micrometer 

range. A shortwave infrared instrument is planned, and mid-infrared instruments 

have been designed. An AOTF consists of an acoustic transducer that generates an 

acoustic wave that propagates through an attached crystalline substance. The wave 

sinusoidally modulates the refractive index of the crystal, selectively diffracting 

incident light according to its wavelength. The instrument also measures polarization 

signatures of reflecting surfaces as well as their spectral properties, opening up new 

areas of possible research. Images can be acquired and processed at 30 

frames/second. The AOTF camera system offers numerous advantages to geologists. 

It can be used as an airborne imager or as a portable field device to image pit walls, 

outcrops, rock specimens or drill core. It can be adapted to image polished sections 

under a microscope, and it could be used in underground workings. Algorithms can 

produce processed images in real time (e.g., ratio images), and, because it is a camera 

system, aerial images can be orthorectified. 

1.0 INTRODUCTION 

The slope stability team at the National Institute for Occupational Safety and Health Spokane 

Research Laboratory has been investigating ways to detect and mitigate hazardous conditions that 

could lead to catastrophic slope failure in open pit mines. A contributing factor in many highwall failures that we have investigated is the presence of mechanically incompetent argillically 

altered rock in pit walls. We are investigating imaging spectroscopy as a means of mapping the 

geology of active mines, especially argillic zones in open pit highwalls. Imaging spectrometers in 

use today operate exclusively as airborne instruments and lack the flexibility and cost 

effectiveness for operational use in a mine. Our investigations have led us on a quest for a fieldportable imaging spectrometer capable of obtaining high-resolution imagery from a variety of 

platforms at any scale and in a wide range of environments. 

 
The system that we describe in this paper is based on an acousto-optic tunable filter (AOTF), a 

device capable of obtaining spectral measurements for an entire scene at 30 microseconds per 

spectral band. The system uses an off-the-shelf CCD camera, which allows image rectification 

through standard photogrammetric techniques. It also includes an optional liquid-crystal phase 

retarder which allows independent filtering based on polarization signatures in a scene. Designed 

for operation as a field-portable device in demanding outdoor environments, the device is rugged, 

compact, and modular. Its design offers unprecedented flexibility and permits microscopic to 

telescopic imaging in a wide range of configurations under open sky or artificial lighting 

conditions. 

2.0 AOTF SPECTRO-POLARIMETRIC IMAGER 

The spectro-polarimetric imager (SPI) consists of three parts: the camera enclosure, the control 

electronics, and the host computer. It operates much like an ordinary CCD camera, except that the 

spectral and polarimetric content of light to be viewed is electronically controlled by a computer. 

During operation, the host computer sends commands to the controller to select the desired 

spectral and polarization parameters. The controller interprets the commands, generates the 

required analog signals, and passes them to the camera enclosure. Finally, the incoming image is 

optically filtered and sampled by the camera, and the resulting video signal feeds back to the host 

computer where it is digitized by a frame grabber. The resulting data cube can then be further 

processed by standard multispectral and hyperspectral techniques. 

2.1 AOTF CAMERA 

The hyperspectral capabilities of the imaging system rely on the AOTF located in the camera 

enclosure (Figure 1). The AOTF itself is a tellurium dioxide (TeO2) crystal with acoustic 

transducers bonded to the surface on one side. A radio-frequency (RF) signal applied to the 

transducers creates a traveling acoustic wave inside the crystal. As light passes through the 

crystal, it is optically filtered and spatially separated into three components: two are bandpass 

filtered according to the frequency spectrum of the applied RF signal, and one is notch filtered 

using the complement of the RF spectrum. The bandpass filtered components are plane polarized 

orthogonally to one another. The AOTF can switch between two bands in 30 microseconds and 

operates with a 10-nm spectral resolution over a range from 450 to 850 nm, corresponding to the 

visible spectrum and near infrared. The operating range of the AOTF is 

Figure 1. The AOTF camera in its housing (left), and the camera components within the housing 

(right). 
chiefly determined by the choice of crystal material. We are currently investigating other 

materials that will extend the AOTF’s range further into the infrared. 

Following the optical path through the camera enclosure, incoming light first passes through the 

acceptance optics, which provide active zoom control and collimate the light before it passes 

through the phase retarder and AOTF. Using the active zoom on the acceptance optics, the 

camera’s field of view is adjustable from 1.6 to 16 degrees. After passing through the phase 

retarder, light enters the AOTF, where it is decomposed into its three components. We align the 

imaging optics to focus the vertically polarized bandpass filtered image onto the CCD camera 

image plane while excluding the other two images. 

2.1.1 Phase Retarder 

The liquid crystal variable phase retarder provides optional polarimetric control for the imaging 

system. Conceptually, a retarder operates by resolving incident light into two orthogonal 

components with electric vectors oriented along what are called the fast and slow axes of the 

device. By varying the voltage applied to the retarder, we control the amount of phase delay 

between the components, achieving the overall effect of changing the polarization state of the 

incoming light. The AOTF acts as an analyzer by transmitting only vertically polarized light, and 

by varying the retardation, we change the intensity of light reaching the CCD camera. 

For natural scenes in the visible spectrum, polarized light is predominantly linearly polarized, and 

under direct illumination from the Sun, Moon, or under overcast skies, the polarization of light is 

primarily horizontal. To observe the amount of horizontally linearly polarized light in a scene, we 

mount the retarder with the fast axis oriented 45 degrees from horizontal. If the input voltage to 

the retarder is set to achieve λ/2 phase delay, horizontally polarized light is rotated 90 degrees 

and becomes vertically polarized. In this case, the light would be transmitted by the AOTF. With 

no phase delay, the light is blocked by the AOTF. By switching between these two extremes, 

objects can be discriminated based on their polarization. By sweeping the retardation from 0 to 

λ/2, we can obtain polarization signatures of points in the scene including the polarization angle 

and magnitude. 

2.2 CONTROLLER AND HOST COMPUTER 

The controller acts as an intermediary between the host computer and the camera. Inside the 

controller, a Motorola 6816 microcontroller interprets the serial commands from the host 

computer and generates the necessary control signals for the retarder and the AOTF. The retarder 

voltage is controlled digitally using the manufacturer-supplied controller. However, generation 

of the RF signal for the AOTF is more complicated. The microcontroller programs up to four 

voltage-controlled oscillators (VCO), the outputs of which are combined and amplified to 

produce the RF signal. We control the frequency of each VCO independently, and amplitude 

control is achieved with a digital step attenuator. 

Any computer with a serial port and a frame grabber can control the camera with no hardware 

redesign. Our goal is to use a laptop PC, making the system truly portable, but current bus 

bandwidth limitations on PCMCIA frame grabbers prevent video frame rate digitization. At this 

time, we are using a 200 MHz Pentium-Pro PC with an Image Nation PX500 frame grabber 

capable of capturing 30 frames per second. 
2.3 OPERATIONAL MODES 

The SPI offers unprecedented operational flexibility limited only by the imagination of the 

operator. Two control strategies lie at the extreme ends of this range of choices: sweep mode and 

switching mode. In sweep mode, we use a single VCO to perform a full spectral sweep over the 

functional range of the camera (450 to 850 nm), collecting 41 10-nm images. This method gathers 

the maximum amount of information in a scene. In switching mode, the camera alternates 

between a small number of parameter settings. The parameters for each image can use one VCO 

to acquire a single band in the scene, or they can be chosen to create more complex filters by 

combining multiple VCO’s and the retarder. In the extreme, an arbitrary waveform generator 

could be used to generate arbitrarily complex filters. In switching mode, the system can be 

programmed to obtain real-time band ratio images. 

3.0 ROCK SPECTRA 

We obtained SPI data cubes at 10-nm resolution from a suite of rock specimens with spectral 

features or spectral contrast in the visible and near infrared. Various types of reflective surfaces 

were imaged as well, including metallic, glassy and granular surfaces. Foliated rocks, a slate and 

a mica schist, were also imaged to find out if rock fabric influences polarization signatures. 

Figure 2 shows reflectance spectra from four of these specimens: bastnaesite from Mountain Pass, 

California; azurite; dunite; and hematite stained tuff. Solid lines represent spectra extracted from 

SPI images, and dotted lines represent spectra measured with an ASD full-range spectrometer at 

one nm resolution and resampled to 10 nm resolution. We find good agreement between the SPI 

and ASD spectra, particularly in the 600 to 800 nm range, with a slight tendency of the two 

measurements to diverge below 600 nm and above 800 nm. 

Figure 2. SPI and ASD spectra of bastnaesite from Mountain Pass, California (upper left); azurite 

(upper right); dunite (lower left); and hematite-stained tuff (lower right). 
We did not find significant polarization differences in any of the specimens except from a 

metallic surface of pyrite where a slight difference was observed. Foliated specimens showed no 

polarization difference. This might be because the phase angle (the angle between line of sight of 

the camera and line of the illumination) was too small. We are planning further experiments to 

investigate possible relationships between polarization signatures and phase angle. 

4.0 CONCLUSIONS 

The current and planned versions of the SPI offer unprecedented imaging flexibility. The 

instrument can be adapted to operate as an aerial camera or as a field instrument where it could be 

used to image pit walls, outcrops, hand specimens and drill core. The device could also be used 

to map underground workings under artificial lighting, and it can be fitted to a microscope to 

image polished sections and drill cuttings. It is conceivable that a fiber-optic device could be 

adapted to be inserted into drill holes. Because it operates as a digital camera, standard 

photogrammetric techniques can be applied to the imagery, and because data are digital, 

conventional image processing and spectral matching techniques can be employed. Because the 

instrument is computer driven, spectral resolution can be pre-programmed and complex filters 

and specialized algorithms can be written to obtain band ratio or principal component images, for 

example, in near real time. 

The SPI was designed to image dynamic scenes from moving battle field vehicles, so speed of 

data acquisition was paramount over noise and image fidelity. In most geologic applications 

other than aerial imaging, static scenes would be acquired. Noise and image contrast would 

therefore be more important necessitating longer exposure time or averaging multiple exposures. 

We are developing designs to extend the spectral range of AOTF further into the infrared, 

particularly the all important 2-2.5 μm short wave infrared and 8-12 μm mid-infrared ranges 

where clays, carbonates and silicates can be mapped. This will require different crystalline 

substances to make the AOTF and different, and more expensive, focal plane arrays sensitive to 

these wavelength ranges.