Imaging. Imaging is the process of acquiring images, the process of sensing our surroundings and then representing the measurements that are made in the form of an image. Topics: Passive and Active Imaging The electronic Camera Image Formation by a Converging Lens Charge Coupled Devices
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Imaging is the process of acquiring images, the process of sensing our surroundings and then representing the measurements that are made in the form of an image.
Passive and Active Imaging
The electronic Camera
Image Formation by a Converging Lens
Charge Coupled Devices
The Human Eye
Energy in the form of electromagnetic radiation reacts with earth materials in several ways as illustrated below. In most situations, incident energy from the sun interacts with a specific material and it is either reflected, scattered, emitted, transmitted or absorbed completely.
In remote sensing, we are predominately interested in energy that has been reflected, scattered, or emitted.
All regions of the EM spectrum are suited to imaging.
This reflected/scattered/emitted energy can then be measured using various kinds of remote sensing instruments. Thankfully, many earth materials have very unique spectral signatures, almost like fingerprints.
Shown above are the reflected spectral signatures of two important alteration minerals, kaolinite in blue and alunite in red. Wavelength is along the x-axis and is given in microns from 2.0-2.5 um. Reflectance is reported in percent from 0-1.0 on the y-axis.
Passive imaging employs energy sources that are already present in the scene, whereas active imaging involves the use of artificial energy sources to probe our surroundings.
Passive imaging is subject to the limitation of existing energy sources: The Sun, for example , is a convenient source of illumination. Active imaging is not restricted in this way, but it is more complicated and expensive procedure, since we must supply and control a source of radiation in addition to an imaging instrument.
Day or night, passive optical imaging systems can detect light reflected or emmited from any external illumination source.
Unlike passive systems, active optical imaging systems detect light reflected from an internal illumination source. This internal illumination usually comes from one or more white light sources (arc lamp, etc.) or lasers. The internal illumination source may also be pulsed, allowing for depth- or distance-profiling.
Passive sensors detect naturally reflected or radiated energy. Active sensors supply, or send out, their own electromagnetic energy and then record what comes back to them. An example of a passive remote sensing satellite is Space Imaging's IKONOS. A common type of active remote sensing is radar.
Data: SRTM Image credit: NASACharacteristics: C and X band, 30m pixel, 16m absolute vertical height accuracyProcessing Shown: Interferometric techniques were used to create a Digital Elevation Model (DEM) which was then color coded for elevation, where browns are the highest points.Notes: These radar data were taken from the space shuttle using a large antenna that sent out EM energy to the surface of the Earth from the shuttle, and then received it back. Mt. Fuji figures prominently in the image with Tokyo in the foreground.
Data: IKONOSImage credit: Space ImagingCharacteristics: Five bands, 1m sharpened pixel resolution, satellite basedProcessing Shown: True color image, georeferencedNotes: Light reflecting off of the shallow waters and reef complexes was received "passively" by IKONOS. The sun is the natural illumination source.
Active: RADARSAT,SRTM,ERS,SIR-CPassive: Landsat,IKONOS,HyMap,
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What is it?
Hyperspectral remote sensing is the science of acquiring digital imagery of earth materials in many narrow contiguous spectral bands. Hyperspectral sensors or imaging spectrometers measure earth materials and produce complete spectral signatures with no wavelength omissions. Such instruments are flown aboard space and air-based platforms. Handheld versions also exist and are used for accuracy assessment missions and small scale investigations.
The samples and lines of a hyperspectral image cube simply represent the x and y directions of the image collection. The number of bands in the z direction for the cube varies depending on the instrument, but is on the order of 100 or greater. Each pixel has one spectral signature associated with it, relating degree of radiance or reflectance with respect to each individual wavelength chunk. The bands summed together create one continuous spectra.
The above Figure shows: two arrows, a converging lens, and rays of light being emitted by the red arrow. The red arrow is the object, while the green arrow is the image that results after the rays have passed through the lens. The Figure also displays two focus shown as blue dots.
The image formed by a converging lens can be made using only three principal rays.
· Ray 1 is the ray which travels parallel to the axis and after going through the lens it passes through the focal point.
· Ray 2 passes through the center of the lens.
· Ray 3 goes through the focal point and then travels parallel to the axis after passing through the lens. Thus any point on the object can be mapped, using the rays above, into a corresponding point on the image. This point is located on the intersection of the rays.
Magnification factor is
By similar triangles , we can also say that
where u is the distance from an object to the lens and v is the distance from the lens to the image plane. Hence
It is usual to express the magnifying power of a lens in terms of its focal length, f, the distance from the lens to the point at which parallel incident rays converge.
Focal length is given by the lens equation
From (1) and (2)
Example: We need to form an image of a 10-cm wide object, 50cm away, on a sensor measuring 10mm across.
The magnification factor we require is
hence the focal length should be
we need a lens with a focal length of approximately 45mm.
Aperture can be no larger than the diameter of the lens itself, and it is usually made smaller this by means of a diaphragm – a circular hole of adjustable size, incorporated into the lens. It is normal to express the aperture of a lens as an “f-number”- focal length is divided by aperture diameter
It is equal to the ratio of the focal length of the lens divided by the diameter of its limiting opening (aperture): f-number = focal length/iris diameter. Note that the number becomes smaller as the aperture grows larger and that it must be squared to directly measure the area which is the light gathering capacity.
Most lenses offer a sequence of fixed apertures (f2.8, f4, f8, f11)
All lenses suffer from defects or aberrations, which can affect image quality
As a result we have BLURRED IMAGES
Any spherical mirror or lens will bring to a focus only those light rays that emanate from the radius of curvature.
Light from a distance source, which is essentially parallel, will not come to a precise focus.
The effects of aberrations can be reduced by making the lens aperture as small as possible.
A charge coupled device (CCD) is a detector that provides digital images.
- The digital format allows the images to be manipulated by a computer, which can electronically sharpen, modify, or copy them.
- A CCD relies on semiconductor properties for its operation. Basically, photons of certain energies strike a layer of silicon, promoting electrons into the conduction band. A CCD collects and counts these electrons to determine how many photons were absorbed by the silicon.
- The CCD is a solid-state chip that turns light into electric signals. -
- CCD’s have become the sensor of choice in imaging applications because they do not suffer from geometric distortion.
Every CCD starts with a backing of some sort, usually glass. The backing is then covered with metal electrodes, as shown in Figure 1 below. The bottom row of electrodes is designated as the readout register.
A thin layer of silicon dioxide is placed on top of the electrodes. Above the silicon dioxide is a layer of n-type silicon. Finally, a thin layer of p-type silicon lies a top the n-type silicon. This creates a p-n junction that covers the electrodes. The purpose of the silicon dioxide is to separate electrically the silicon from the electrodes. This helps to isolate the electrons in the silicon.
Between each column of electrodes there is a channel stop, indicated by dashed lines in Figure 1. Channel stops prevent electrons from flowing horizontally across the CCD array. There are no channel stops in the readout register; in this row, charges are free to move horizontally to the detection device. A side view of a CCD is shown in Figure 2.
Each CCD is divided into many groups of electrodes called pixels. The exact number of pixels depends on the individual pixel size and the cost of the array. Typically, a low-cost commercial array has dimensions of 600 x 300 pixels, while high cost arrays for scientific applications have dimensions closer to 3000 x 3000 pixels. Each pixel has two, three, or four electrodes in it with three being the most common.
As photons of light enter the CCD, they are absorbed by the silicon layer if they have sufficient energy. Each absorbed photon causes an electron to be promoted into the conduction band. During CCD photography, for every one hundred incident photons, between forty and eighty are absorbed by the silicon. (Conventional photographic film absorbs approximately 2% of all incident photons.) The electrons then collect above the center electrode, which has been positively charged.
After the exposure is completed, all of the collected electrons are sequentially moved to the detection device where they are counted. This is accomplished by changing the charge on the electrodes in a timed, sequential manner.
To illustrate, electrodes 2 and 5 are initially positively charged and so collect all the electrons photogenerated around them, Figure 4a. Next, electrodes 3 and 6 gradually begin to acquire positive charge while 2 and 5 gradually become negatively charged. This causes the electrons to move to electrodes 3 and 6, Figure 4b. Then, electrodes 1, 4, and 7 gain positive charge while 3 and 6 become negatively charged. This causes the electrons to move once more, Figure 4c. The electrons from the bottom most-pixel (pixel #2) are now in the readout register, electrode 7.
The readout register now begins to deliver the electrons to the detection device by the same process of varying the voltage applied to the electrodes, but now across the bottom row. The detection device counts the number of electrons by applying a known voltage across the final electrode. (The final electrode is located inside the detection device, not on the CCD array.) The voltage will increase slightly as it picks up electrons from the CCD. By subtracting the background voltage, the voltage from the CCD electrons can be found. The number of electrons can then be calculated using Faraday's constant and this voltage increase.
Electrons from pixel #1 would follow the same procedure. They would be the next group to be moved into the readout register and to the detection device.
Finally a computer takes the numbers from the detection device and uses them as relative intensities to display the image on the screen. It takes as little as 100 ms to completely move all electrons from a 2000 x 2000 pixel CCD to the detection device. Exposure time may be much longer for low level light sources.
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1. Flexible lens can adjust focal center to image objects near or far .
- lens flattens for distant objects
-lens thickens for near objects
2. Retina contains photoreceptive cells that form the image.
· 3. Cones(bright-light ) vision
- - highly sensitive to color
- - 6-8 million of them centered in the fovea
· 4. Rods ( dim-light) vision
-- 75-150 million distributed across the retina
-sensitive to low level of illumination
- - non sensitive to color
·Perceived brightness is not a simple function of intensity
·Although there are nearly 130 million photoreceptors in the retina, the optical nerve contains only a million fibres.
Retina performs excellent spatial decorrelation.
Suppression of noise
Receptive fields of retinal ganglion cell are well described with Center-surround type whitening filters (Difference of Gausians).
Linear predictive coding
Lateral Geniculate Nucleus
The main purpose of the PVC processing is to complete the temporal decorrelation of the retinal signal.
Primary Visual Cortex
Receptive fields of simple – cells are well described with a Gabor function.
Higher Visual Area
The basis functions that result from contrast sensitivity evaluative procedure are similar to the receptive field shape of cells in the retina.
Lateral Geniculate Nucleus
The basis functions that result from sparse codes learning are localized, oriented, and banpass, similar to the receptive field shape of cells in the cortex.
Primary Visual Cortex
Higher Visual Area
To assist you, I have placed a yellow dot in each picture. The idea is to make yourself go cross-eyed so that the two dots join and form a third dot in between them which is closer to you. Below is a diagram of what you should make your eyes do
The separation of the points is named the disparity.
There is an inverse relationship between disparity and depth in the scene, disparity will be relatively large for points in the scene that are near to us and relatively small for points that are far away.
There are numerous different type of machines that show a stereo pair of images to the viewer. The most popular kind is probably the "View Master" which most of you have probably seen in toy stores
These machines are basically an assisted version of the parallel viewing method. They contain lenses to magnify the image and make sure that each eye only looks at the image it was meant to see.
Consider a point (x,y,z), in three-dimensional world coordinates, on an object.
Let f be the focal length of both cameras, the perpendicular distance between the lens center and the image plane. Then by similar triangles:
Solving for (x,y,z) gives:
In order to measure the depth of a point it must be visible to both cameras and we must also be able to identify this point in both images.
As the camera separation increases so do the differences in the scene as recorded by each camera.
Thus it becomes increasingly difficult to match corresponding points in the images.
This problem is known as the stereo correspondence problem.
Imaginis - Computed Tomography Imaging (CT Scan, CAT Scan)
Computed Tomography (CT) imaging, also known as "CAT scanning" (Computed Axial Tomography), was developed in the early to mid 1970s and is now available at over 30,000 locations throughout the world
Computed Tomography (CT): Questions and Answers, Cancer Facts 5.2