Three dimensional imaging

Details Three dimensional imaging Three dimensional imaging

2002-06-24

2004-12-07

Lee, John R. (Department: 2881)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S341800, C250S358100

Reexamination Certificate

active

06828558

ABSTRACT:

The present invention relates to the field of imaging samples with radiation in the infra-red (IR) and Terahertz frequency range. More specifically, the present invention relates to apparatus and methods for imaging samples in three dimensions using electromagnetic radiation in the higher Gigahertz (GHz) and the Terahertz (THz) frequency ranges. However, in this type of imaging technology, all such radiation is colloquially referred to as THz radiation, particularly that in the range from 25 GHz to 100 THz, more particularly that in the range of 50 GHz to 84 THz, especially that in the range from 100 GHz to 50 THz.
Recently, there has been much interest in using THz radiation to look at a wide variety of samples using a range of methods. THz radiation has been used for both imaging samples and obtaining spectra. Work by Mittleman et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 3, September 1996, page 679 to 692 illustrates the use of using THz radiation to image various objects such as a flame, a leaf, a moulded piece of plastic and semiconductors.
THz radiation penetrates most dry, non metallic and non polar objects like plastics, paper, cardboard and non polar organic substances. Therefore, THz radiation can be used instead of X-rays to look inside boxes, cases etc. THz has lower energy, non-ionising photons than X-rays, hence, the health risks of using THz radiation are expected to be vastly reduced compared to those using conventional X-rays.
There is considerable interest in both medical and non-medical fields in the production of 3D images. For example, in dentistry the ability to produce 3D images of a tooth would enable dentists to locate exactly where caries (tooth erosion) or other abnormalities occur in the tooth. Most of the conventional imaging modalities—X-Ray, MRI, etc.—are handicapped by the fact that they can intrinsically only produce 2D images, with 3D images possible only by translating the patient or body part through the X-Ray beam or through the magnetic field in the case of MRI.
The use of THz for imaging the internal structure of a flat object (a floppy disc) has been described in EP 0 864 857. Here, the inventors measured reflection of a beam of THz radiation to produce an image of the internal structure of the sample.
However, this method is not suitable for obtaining 3D images of objects where the front and back surfaces are curved. Most objects have interfaces and/or external surfaces which are non-planar, i.e. have substantial radii of curvature. If a beam is reflected from a curved surface, it is reflected at an angle to the incident beam. The method of EP 0 864 857 does not show how to obtain an image when the radiation is reflected from a curved surface.
Also, partially absorbing objects give rise to weak reflections from buried layers resulting in long absorption lengths for certain reflected pulses. This limits the thickness of objects which can be accurately imaged in 3D using THz reflection data alone.
The present invention addresses the above problems in a first aspect provides a method of imaging a sample, the method comprising the steps of:
(a) irradiating the sample to be imaged with an irradiating beam of pulsed electro magnetic radiation with a plurality of frequencies in the range from 25 GHz to 100 THz,
(b) detecting both the radiation transmitted through the sample and the radiation reflected by the sample;
(c) generating an image of the sample from the radiation detected in step (b).
Collecting both the reflected and transmitted radiation allows a greater range of curved surfaces to be measured. Hence, the method of the present invention is capable of imaging a sample of virtually any shape. The collection of both the transmitted and reflected radiation also allows a compositional image of the sample to be obtained.
Radiation transmitted through the sample is primarily used to determine the sample shape and the composition. Radiation which is reflected from the sample is primarily used to measure the positions of dielectric surfaces within the sample in addition to giving shape information. This technique allows the curvature of both internal and external surfaces to be measured. Thus, using both reflected and transmitted radiation is an extremely powerful tool to determine the three dimensional compositional structure of the object.
Taking a uniform sphere as a simplified example. In such an example there are no internal interfaces. Therefore, the pulse is at either transmitted through the sphere or will be reflected on entering or exiting the sphere. Subdividing the sample into a 2D array of pixels and measuring the time of flight of the transmitted pulse through the sample will allow the thickness of the sample to be determined at each pixel. However, this will not determine the shape of the sample as the position and shape of the front interface is not known. The shape of the front interface can be determined from the time of flight of the pulse which is reflected on entering the sphere. Thus, it is possible to obtain information about the shape of a sample by plotting the difference between the time of flight of the transmitted and reflected pulses relative to the time of flight of the reflected pulse.
Therefore, the step of generating the image preferably comprises the steps of calculating the time of flight of the pulse transmitted through the sample; calculating the time of flight of a pulse reflected from an interface or surface of the sample; and plotting the difference or a function of the difference of the time of flight of the transmitted pulse and the reflected pulse relative to the time of flight of the reflected pulse.
A function of the difference can be plotted in order to correct for variations in the refractive index of the sample.
In the case of a sphere, theoretically, the pulse reflected on exiting the sphere could be used to determine the shape of the sample in conjunction with the pulse reflected on entering the sphere. However, it is not desirable to use the pulse reflected on exiting the sample as it will be scattered through a fairly large angle, possibly outside the range of the detector. Further, as the reflected pulse has passed twice through the sample, it is likely to be very weak.
The present invention can be used to image far more complex objects than the above uniform sphere. As mentioned above, it is difficult to detect a reflected pulse from the interfaces which are deepest within the sample. Such a complex sample is measured using a reflected radiation detector (or detectors) which is located at the same side on the sample as the incident THz pulse and a transmitted radiation detector (or detectors) which is located on substantially the opposite side of the sample to the incident THz pulse. In a sample with many interfaces, some of the radiation detected by the transmitted radiation detector will have been transmitted through the whole of the sample. However, some of the radiation collected will have undergone multiple reflections. For example, radiation can be reflected back into the sphere from the sphere's external surface onto an internal interface. The pulse is then reflected for a second time at the internal interface out of the sphere. This reflected pulse will be collected by the transmitted radiation detector. The position of interfaces deep within the sample can be determined by looking at the signal due to such doubly reflected pulses or pulses which have undergone an even number of reflections.
Therefore, preferably, the step of generating the image comprises the step of extracting the parts of the detected transmitted pulse which are due to an even number of reflections within the sample, and determining the position of an interface using the said signal caused by said even number of reflections.
In order to be able to directly compare the reflected and transmitted signals, it is preferable if a reference signal is provided. Said reference signal is preferably provided by a reflection off an object which is a known distance with respect to either t

Affiliated with

Also associated with

Rating

Three dimensional imaging does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Three dimensional imaging, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Three dimensional imaging will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3281265