1. Field of the Invention
The present invention relates generally to image sensors and, more particularly to, an image sensor and method for measuring refractive index of a material.
2. Description of the Related Art
It is known to provide a device for measuring a refractive index of a material. One such conventional device uses light transmitted through an optical fiber in contact with a liquid material for measuring a refractive index of the liquid material. Current measurement techniques for measuring chemical concentration of the liquid material using refractive index require laser light injected into the optical fiber. When this occurs, a portion of the optical fiber is in contact with the liquid material to be tested and the light injected by a laser into the optical fiber and into the liquid material. Injected light comes into contact with the surface of the liquid material and is reflected off the surface. A detector separate from the optical fiber is used to detect the reflected light for measuring the refractive index of the liquid material.
One disadvantage of conventional devices is that they require a separate light source and a separate detector. Another disadvantage of conventional devices is that they require lasers or fiber optics. Yet another disadvantage of conventional devices is that changes in the material effect the transmission of light through the optical fiber. Therefore, it is desirable to provide an image sensor that integrates the light source and detector into one component. It is also desirable to provide an image sensor that eliminates the use of lasers or fiber optics. Thus, there is a need in the art to provide an image sensor that meets at least one of these desires.
The present invention provides an image sensor for measuring a refractive index of a material. The image sensor includes a semiconductor substrate having an exposed surface facing the material and an array of pixels on the semiconductor substrate spaced from the exposed surface. The image sensor also includes a light source on the semiconductor substrate configured to emit light into the semiconductor substrate toward the exposed surface to reflect the light off the exposed surface toward the array of pixels, wherein the array of pixels detect the light reflected by the exposed surface for calculating the refractive index of the material.
In addition, the present invention provides a method for measuring a refractive index of a material with the use of an image sensor including a semiconductor substrate having an exposed surface for facing the material, an array of pixels on the semiconductor substrate, and a light source on the semiconductor substrate. The method includes the steps of emitting light into the semiconductor substrate from the light source toward the exposed surface, reflecting the light off the exposed surface and toward the array of pixels, and detecting the light reflected from the exposed surface with the array of pixels. The method also includes the steps of calculating the refractive index of the material based on the detected light.
One advantage of the present invention is that a new image sensor and method is provided for measuring a refractive index of a material. Another advantage of the present invention is that the image sensor includes an integrated light source and detector. Yet another advantage of the present invention is that the image sensor has a relatively compact integrated light source and detector and does not require separate components. Still another advantage of the present invention is that the image sensor and method does not require lasers, optical fibers, or light modification. A further advantage of the present invention is that the image sensor and method uses a single silicon sensor as both the light source and the detector for the purpose of measuring refractive index of a material. Yet a further advantage of the present invention is that the image sensor has the light source present thereon, making for a very compact sensing unit. Still a further advantage of the present invention is that the image sensor and method can be used to measure a chemical composition of liquid materials.
Other features and advantages of the present invention will be readily appreciated, as the same becomes better understood, after reading the subsequent description taken in conjunction with the accompanying drawings.
With reference to the Figures, wherein like numerals indicate like parts throughout the several views, one embodiment of an image sensor 10, according to the present invention, is shown for measuring refraction of a material 12. The material 12 is, for example, of a liquid type. In one embodiment, the image sensor 10 is used to measure a chemical concentration of the liquid material 12 such as chlorinated water. By measuring the change of refractive index of the liquid material 12, the chemical concentration of the liquid material 12 can be measured. It should be appreciated that the image sensor 10 may be used to measure the refractive index of other types of materials.
Referring to
The image sensor 10 also includes an array 20 of pixels 22 on the semiconductor substrate 14. The pixels 22 are of a photo-sensitive type. The array 20 of pixels 22 is disposed in or on the substrate surface 18. It should be appreciated that the array 20 of pixels 22 is generally rectangular in shape, but may be any suitable shape. It should also be appreciated that the pixels 22 detect light and produce a charge packet corresponding to the light detected as is known in the art.
The image sensor 10 also includes a light source, generally indicated at 24, on the semiconductor substrate 14. The light source 24 is disposed on the substrate surface 18 adjacent the array 20 of pixels 22. In one embodiment, the light source 24 is a transistor 26 such as a MOSFET transistor. The transistor 26 includes a source 28 and a drain 30. The source 28 and drain 30 are of an n+ dopant on or in the semiconductor substrate 14. The transistor 26 also includes a gate 32 disposed between the source 28 and drain 30 and separated from the substrate surface 18 by an insulating layer 34. It should be appreciated that a voltage from the image sensor 10 on the gate 32 controls the amount of current flow from the source 28 to the drain 30. It should also be appreciated that the drain voltage is high enough such that electrons flowing under the gate 32 experience a large potential drop from under the gate 32 to the drain 30. It should further be appreciated that the large potential drop creates hot electrons that can emit a photon, generally indicated at 36, as is well known in the art.
The majority of the photons 36 have a wavelength near the energy gap of the semiconductor substrate 14, for example, silicon at 1.12 μm (at room temperature). These photons 36 are not easily absorbed by the semiconductor substrate 14. For example, the absorption length in silicon is approximately 5 mm at room temperature. The long absorption length means the photons 36 can reflect off the exposed surface 16 of the semiconductor substrate 14 and be detected by the array 20 of pixels 22.
As illustrated in
then the photon 36 will totally be reflected by the exposed surface 16 thereby creating a reflected photon 40. The intensity of a transmitted photon 38 will be zero. The reflected photon 40 will be detected by the array 20 of pixels 22.
Referring to
The critical angle θC of the photon 36 will be equal to:
Combining equations (1) and (2) gives a refractive index η of the material 12 in contact with the exposed surface 16 of the semiconductor substrate 14 in the following equation:
Referring to
Referring to
In the embodiment illustrated in
Referring to
As illustrated in
For operation of the image sensor 110, the data acquisition process would begin by clearing all signals from all pixels 122. Then, the power to the transistor 126 would be turned ON and the power to all peripheral circuits in the column read out circuitry 150, row select circuitry 152, and signal processing and timing generator 154 would be turned OFF. After the image of light reflected off the exposed surface 116 of the semiconductor substrate 114 has been collected, the transistor 126 is turned OFF and the power is applied to the peripheral circuits in the column read out circuitry 150, row select circuitry 152, and signal processing and timing generator 154 to enable image readout.
Referring to
In addition, the image sensor 110 may include a transition layer 160 added between the semiconductor substrate 114 and the material 12 being measured to increase the accuracy. For example, the transition layer 160 may be a layer of silicon nitride SiN, silicon dioxide SiO2, or a graded index of refraction from approximately n=3.5 for silicon to an index of refraction slightly larger than the material 12 being measured to increase the accuracy. It should be appreciated that having the graded index of refraction increases the critical angle θC for total internal reflection which, in turn, increases the distance, d, traveled by the light in the semiconductor substrate 114. It should also be appreciated that the transition layer 160 would also serve the purpose of protecting the exposed surface 116 of the semiconductor substrate 114 from oxidation or chemical attack. It should further be appreciated that the transition layer 160 may be used for the image sensor 10.
The CCD image sensor 10 has the advantages of noiselessly sum pixel rows together to maximize signal strength and a CCD does not have any transistors that can corrupt the signal near the transistor 26. The CMOS image sensor 110 has the advantage of providing the light illumination source and detector and processing circuitry all on one silicon substrate. Furthermore, the CMOS image sensor 110 can be powered by a single low voltage supply and be placed in a package having less than eight (8) pins.
Moreover, a method for measuring a refractive index of the material 12 with the use of the image sensor 10, 110 is disclosed. The method includes the steps of emitting light into the semiconductor substrate 14, 114 from the light source toward the exposed surface 16, 116. The method also includes the steps of reflecting the light off the exposed surface 16, 116 and toward the array 20, 120 of pixels 22, 122, detecting the light reflected from the exposed surface 16, 116 with the array 20, 120 of pixels 22, 122, and calculating the refractive index of the material 12 based on the detected light.
The method also includes the steps of measuring the distance, d, between the light source and a column of the array 20, 120 of pixels 22, 122 that detect the greatest intensity of reflected light and calculating the refractive index of the material 12 based on the measured distance. The method includes the steps of generating charge packets associated with each pixel 22, 122 of the array 20, 120 of pixels 22, 122 based on the intensity of light detected by the array 20, 120 of pixels 22, 122 and transferring the charge packets to a horizontal charge coupled device (HCCD) shift register 42 of the image sensor 10, 110. The method includes the steps of summing the charge packets from columns of the pixels 22, 122 in the horizontal charge coupled device.
Accordingly, the image sensor 10, 110 of the present invention does not require a laser, optical fibers, or light modulation. The image sensor 10, 110 of the present invention has the light source 24 present on the semiconductor substrate 14, 114, making for a very compact sensing unit.
The present invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation.
Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the present invention may be practiced other than as specifically described.