Patent ID: 12238449

DETAILED DESCRIPTION

I. Overview of Two Different Embodiments

FIG.1provides a schematic diagram identifying the functional areas of the optical infrared (IR) imager31of the present invention. Optical infrared imager31has at its bottom a light source33and detector35operatively connected to substrates34bottom. In region39imager31has a series of transparent layers of different refractive index (RI) that act as spectral and spatial filters that process the light37passing through to create a defined reference source and beam37. The light37passes through region41which further spatially filters and laterally displaces the direction of the path of the refined beam of light toward the detection region43. In detection region43the refractive index can be set in different ways for different embodiments or functions. One embodiment is to configure of the transparent layers are set such that when no infrared light is reaching detection region43light beam37arrives at an angle that is slightly less than its critical internal angle. The critical angle is the minimum angle at which total internal reflection (TIR) occurs, where all light above the critical angle is 100% reflected. Thus, when light beam37arrives at detection area43it is scattered and transmitted out of imager31into the area47, typically air above imager37A. However, given the makeup of the layers in detection region43, when IR light45is shined or focused by lens49on region43it causes a change in the refractive index (RI) of region43causing the critical angle of the boundary to change and in fact the internal light's angle is closer to the critical angle or higher such that it is reflected beam37B down towards the light detector35. It passes down through various layers that act as displacement and spatial filters in region51and then through various layers that act as spectral, spatial and noise filters to then arrive at light detector35. As will be discussed below in detail, detection region43consists of three different layers. It should be noted that the critical angle is dependent on the refractive index value of high-index core layer and low-index cladding layer.

In an alternative embodiment, cladding layer42in detection region43can be configured such that when no infrared light45is irradiating cladding layer42when light beam37C arrives at detection region43, it arrives at its critical angle and is reflected as beam37B back into imager31. In this embodiment, when infrared light shines or is focused in region43the critical angle of the light is lowered and it is transmitted out37A of imager31into region47.

A. One Variation of the Invention

FIGS.2,2A,2B,3,3A,3B,4,4A and4Bto be discussed in the following paragraphs relate to the variation of the invention where the critical angle is slightly higher than the angle the internally generated light is when it arrives at detection area43when there is no infrared light irradiating the detection region and thus, it will scatter and exit through the top layer of imager31. However, as soon as infrared light shines on detection area43the critical angle shifts lower and some or all of the internal light is internally reflected towards detector35.

FIG.2is a schematic diagram of the embodiment of the imager31where light37generated by light source33impacts the detection region43at an angle slightly less than the critical angle and there is no infrared light incident on the detection region43. In such a situation the light37A exits the top surface of imager31and is not reflected back down to imager detector33.FIG.2Ais a schematic of a graph of the transmission intensity through a single thin-film stack versus internal light incident angle, where the y-axis is transmission intensity and the x-axis is the angle of incidence, and spike55A is the total transmitted internal light. Since no infrared light is incident on detection region43, all of the light37is transmitted into the top layer and is scattered out of the imager. The scattering can be due to nano-particles embedded in the layer, or fabrication of optical structures, such as pyramids, on the top surface. The transmission curve55is smooth as it approaches the critical angle, 0, with a spike55A just prior.FIG.2Bis a schematic diagram of a bar graph of light intensity y-axis of light received at detector35and infrared light incident at detection region43shows that in this situation there is none.

FIG.3is a schematic diagram of the setup inFIG.2with infrared light at 50% dynamic range intensity striking detection region43. The dynamic range of the detection device35is set by its integration time. InFIG.3, some of the light37B generated by light source33remains in imager31and is reflected down towards detection device35, which can be a simple light detector or camera. The transmission curve is smooth as it approaches the critical angle, θC, with a spike55B at the new critical angle59.FIG.3Ais a graph of transmission intensity through the thin-film stack or filters and the internal incident angle of light37generated by light source33in the situation depicted inFIG.3where infrared light45is incident on detection area43. In this instance, there is a shift caused by the boundary between the layers of the critical angle θCof light37that results in reflection of the light37B towards receptor35. Dotted outline57shows the original location of the critical angle when there was no incident infrared light on detection area43as depicted inFIGS.2and2A. Thus, the infrared light45has caused a shift as depicted inFIG.3Ain the critical angle θCfrom57to59.FIG.3Bindicates internal light intensity and incident infrared light intensity, the y-axis indicating the amount of intensity. Internal light intensity61is measured at detector35and infrared light intensity63is measured in detection region43. As depicted in the graph inFIG.3B, they are equal. The bar graphs are meant to be descriptive or illustrative of the concept and not precise. In actuality, the variation tends to be a nonlinear process.

FIG.4is a schematic diagram of an embodiment of the imager of the present invention inFIG.2with infrared light shining in a 100% dynamic range on detection region43. As depicted inFIG.4, the light37generated by light source33and transmitted through the reference region is reflected back down37B toward detector35.FIG.4Ais a graph of transmission intensity through the thin-film stack or filters and the internal incident angle of light37generated by light source33in the situation depicted inFIG.4where infrared light45is incident on detection area43. In this instance, there is a shift of the critical angle θCcaused by the difference in refractive indexes at the boundary of light37that results in reflection of the light37B towards detector35. Dotted outline57shows the original location of the critical angle when there was no incident infrared light on detection area43as depicted inFIGS.2and2A. Thus, the infrared light45has caused a further shift in the location of the critical angle as depicted inFIG.4Afrom57to59A.FIG.4Bindicates internal light intensity and incident infrared light intensity, the y-axis indicating the amount of intensity. Internal light intensity61is measured at detector35and infrared light intensity63is measured in detection region43. As indicated by the bar graphs61A for the internal light and63A for the incident IR light, both have increased. The bar graphs are meant to be descriptive or illustrative of the concept and not precise.

B. A Second Variation of the Invention

FIGS.5,5A,5B,6,6A,6B,7,7A, and7Bto be discussed in the following paragraphs relate to another variation of the invention. In this variation, the angle at which the internally generated light arrives at the detection area73when no IR light is shining on detection area73is at the critical angle.

FIG.5is a schematic diagram of the embodiment of the imager31where light71generated by light source33impacts the detection region73at an angle that is at or more than the critical angle and there is no infrared light incident on the detection region73. In such a situation, all the light71is reflected down to imager detector35.FIG.5Ais a schematic of a graph of the transmission intensity75through a single thin-film stack versus internal light incident angle, where y-axis is transmission intensity and the x-axis is the angle of incidence, and spike75A is the total transmitted internal light reflected towards light source35. Since no infrared light is incident on detection region73, all of the light71is reflected down to imager35. The transmission curve75is smooth as it approached the critical angle, θC, with a spike75A.FIG.5Bis a schematic diagram of a bar graph of light intensity y-axis of light74received at detector35with no infrared light incident at detection region73.

FIG.6is a schematic diagram of the setup inFIG.5with infrared light77at 50% dynamic range intensity striking detection region73. The dynamic range of the detection device35is set by its integration time. InFIG.6, some of the light71A generated by light source33remains in imager31and is reflected down towards detection device35, which can be a simple light detector or camera. Some of the light71B passes out of imager31at detection region73. Referring toFIG.6A, the transmission curve79is smooth as it approaches the critical angle, θC, with a spike81at the new critical angle83.FIG.6Aa graph of transmission intensity through the thin-film stack or filters and the internal incident angle of light71generated by light source33in the situation depicted inFIG.6where infrared light77at 50% intensity is incident on detection area73. In this instance, there is a shift caused by the boundary between the layers of the critical angle θCof light71that results in reflection of the light71B out of imager31as well as some light71A towards detector35. Dotted outline75inFIG.6Ashows the original location of the critical angle when there was no incident infrared light on detection area73as depicted inFIGS.5and5A. Thus, the infrared light77has caused a shift as depicted inFIG.6Ain the critical angle θCfrom75to81.FIG.6Bindicates internal light intensity and incident IR light intensity, the y-axis indicating the amount of intensity. Internal light intensity85is measured at detector35and infrared light intensity87is measured in detection region73. As depicted in the graph inFIG.6B, they are equal. The bar graphs are meant to be descriptive or illustrative of the concept and not precise.

FIG.7is a schematic diagram of an embodiment of the imager of the present invention inFIG.5with infrared light shining in a 100% dynamic range on detection region73. As depicted inFIG.7, the light71generated by light source33and transmitted through the reference region passes out of imager31, none of it being reflected back down toward detector35, all of the light71generated by light source33passes out of imager.FIG.7Ais a graph of transmission intensity91through the thin-film stack or filters and the internal incident angle of light71generated by light source33in the situation depicted inFIG.7where infrared light77is incident on detection area73. In this instance, there is a shift of the critical angle θCcaused by the difference in refractive indexes at the boundary of light71that results in passage of all of the light71C out of imager31and not down to light detector35. Dotted outline75shows the original location of the critical angle when there was no incident infrared light on detection area73as depicted inFIGS.5and5A. Thus, the infrared light77has caused a further shift in the location of the critical angle as depicted inFIG.7Afrom75to82.FIG.7Bindicates internal light intensity and incident infrared light intensity, the y-axis indicating the amount of intensity. Internal light intensity is measured at detector35and infrared light intensity77A is measured in detection region73. As indicated for the internal light intensity is zero since it is all passing out of imager31with the incident infrared light being at a maximum. The bar graphs are meant to be descriptive or illustrative of the concept and not precise.

II. Detailed Description of the Structure of a Preferred Embodiment

FIG.8provides a detailed schematic diagram of one preferred embodiment of the present invention. Infrared imager31has a light source33and light detector35which attach by an optical coupling layer101, to substrate103. An anti-reflection coating105is deposited on substrate103, in turn high refractive index layer107is next with near-critical angle, anti-reflective layer109. In turn next is a low refractive index layer111and in turn another near-critical angle, anti-reflective layer113and then another high refractive index layer115, another anti-reflective layer117, and then as the top layer a specialized low refractive index layer119. In a preferred embodiment, the various layers are deposited in a standard semiconductor fabrication process, such as Plasma Enhanced chemical Vapor Deposition, or PECVD.

Regarding each of the layers in this embodiment depicted inFIG.8, the following are specifics for one variation of this embodiment:

Description of Layers of FIG.8

103Substrate: refractive index n1: transparent, rigid substrate, its primary purpose is as a foundation for layer growth. Typical materials are Borosilicate Glass (BK7), Fused Silica (FS), or Sapphire.

107High Refractive Index Layer: refractive index n2: This is an amorphously grown layer with relatively high refractive index. Potential materials are silicon nitride (SiN3, n=2.0), oxynitride (SiNxO, n=1.46-1.9), or Indium Tin Oxide (ITO, n=1.85).

109Near-Critical Angle, Anti-Reflective Layer: refractive index n3: This is an amorphously grown layer with relatively low refractive index. Its refractive index is approximately 0.001 less than “Low Refractive Index Layer, n4.” This layer's thickness is a critical dimension, and is calculated from thin-film interference theory. (A summary of the applicable equations of thin-film theory is provide at the end of this specification.)

111Low Refractive Index Layer: refractive index n4: This is an amorphously grown layer with relatively low refractive index. Possible materials are silicon dioxide (SiO2, n=1.46) and oxynitride (SiNxO, n=1.46-1.90). It has the lowest refractive index of all layers. This layer's thickness is determined by the distance of lateral displacement, due to refraction, that is needed in the device. The thicker the layer, the greater the displacement. Stated differently, the lateral distance between the “reference,” “detection,” and “measurement” regions is determined by the thickness of this layer.

113Near-Critical Angle, Anti-Reflective Layer: refractive index n3: This layer is identical to109in terms of material, refractive index, and thickness.

115High Refractive Index Layer: refractive index n2: This layer is identical to107in terms of material and refractive index, but not necessarily thickness. Its purpose is to shield the underlying layers from infrared light and act as a thermal buffer between the infrared absorption layer above and the underlying layers. Certain materials, such as ITO, are highly reflective to infrared light, and can serve to reflect infrared light incident that has passed through119and117back through, effectively doubling the light path length and increasing infrared absorption.

117Near-Critical Angle, Anti-Reflective Layer: refractive index n3: This layer is identical to109and113in terms of material, refractive index, and thickness.

119Low Refractive Index Layer: refractive index n4: This layer is typically (but not in all possible designs) identical to layer111in terms of refractive index, but not necessarily material or thickness. Possible materials for this layer are SiO2, SiNx, PMMA, (n=1.51), or SU-8 (n=1.58). The thickness of this layer should be as thin as possible to minimize its thermal mass. This layer should be translucent, scattering both internal visible light and incident infrared light. There are many known methods for achieving this, the most promising being embedded nano-particles, randomly textured surface, or structured patterning of the surface, specifically with a micro-pyramid structure. For the internal visible light, this layer scatters the transmitted light in the “detection region” either out of the thin-film layers or at angles that will be filtered by the “measurement region.” For the infrared light, scattering can increase the light path length in layer and increase the absorption. This layer must also be highly absorbing to infrared light, either natively or by the additional of a blackbody absorbing material.

Regarding layer119(42ofFIG.1), it is the most significant layer of imager31. As depicted inFIGS.1and8, imager31is a stack of layers in the order of n2, n3, and n4 of varying thickness. Each layer is homogenous and planar, except for the final, top layer119, furthest from substrate103in the stack of layers depicted inFIG.8. As noted above, layer119is configured to react to infrared light with a change in the refractive index when infrared light irradiates coating or cladding119it causes internal light37inFIG.1emitted by light source33that arrives at layer119to change course and either be passed out through layer119out of imager31or remain in the imager and be reflected down towards the detector35. Various polymer and dielectric materials can be configured to change the refractive index of layer119when irradiated by infrared light and thus cause a change in the direction of the light emitted by the light source due to a change of the critical angle for the emitted light arriving at coating119. Such change could be at or greater than the critical angle of refraction and direct emitted internal light that was going to pass out of imager31down towards detector35. Alternatively, the change in the refractive index could be such that emitted internal light will arrive at an angle less than the critical angle and thus pass out of layer119into region47, whereas if no infrared light had been irradiating layer119it would have remained as interior light and have been reflected down towards detector35.

105Standard Anti-Reflection Coating: Depending on the refractive index of the substrate103, this layer can be a simple oblique angle, quarter wavelength, or a near-critical angle, anti-reflection coating if the substrate103and the low refractive index layer111have similar refractive indexes.

101Optical Coupling: Below are some methods used to optically couple the internal light source and measurement detector (camera or photo-diode) to the substrate. The incident angle of the light needs to be near the critical angle, to enable this optical coupling technique is required. For the internal light source, ideally this method will also diffuse the internal light to create a uniform intensity across the illumination area. For the measurement detector (camera or photo-diode) this method must not distort the image. Methods include:a. Optical Epoxy: Referring toFIG.9A, a schematic diagram of substrate103and the component, in this case the light source33, the refractive index of the substrate103and component33are matched by use of an index-matching epoxy141.b. Scattering Layer:FIG.9Bis a schematic diagram where random surface texturing of substrate103and light source33creates a diffuser that is optically bonded by use of an index-matching epoxy141.c. Prism:FIG.9Cis a schematic diagram of the incorporation of a prism143that can be used to modify the incident angle of light as depicted to connect both light source33and detector35to substrate103. Prism143is optically bonded to substrate103with an index-matching epoxy141.d. “Fresnel Prism”: or micro-array of prisms can be used to non-contact, optically couple the light source and detector to the substrate with the benefit of reducing size. This is an array of micro-prisms designed so that light enters the prism normal to the input surface and after an internal reflection, exits normal to the output surface. This minimizes reflection loses, and eliminates refraction, at the input and output surface. It also ensures that the entire prism surface area is of clear aperture (no shadowing) for input and output light at the design angle. The array of micro-prisms can be etched into the substrate material, stamped onto the surface with a polymer, or fabricated from a polymer and then optically bonded to the substrate surface.FIG.9Dis a schematic diagram of an arrangement using a Fresnel prism. As can be seen inFIG.9D, an array of Fresnel prisms151are either etched or added to the bottom surface of substrate103. They are positioned between light source33and detector35and the substrate.e. Fluorescent Layer:FIG.9Eis a schematic of diagram of an example of using a fluorescent material145that can be applied to the substrate surface103and irradiated with UV light by light source147, which then fluoresces “internal light” with a higher wavelength. In the example shown, the fluorescent material is applied by embedding fluorescent particles in index-matching epoxy.f. Diffraction:FIG.9Fis a schematic diagram of an example that uses a thin-film diffraction layer149that could be fabricated onto the surface of substrate103or alternatively a diffraction grating could be bonded onto to substrate103with index-matching epoxy to diffract light into substrate103.

By way of example, Table I below provides one variation of an embodiment of the spatial filter of the present invention. It is meant only as one example of a spatial filter made according to the precepts of this disclosure.

TABLE 1LayerDesignLayerRefractiveRefractiveDesignDesignDesignationIndexIndexThicknessMaterial103n11.5200.5mmglass substrate107n22.1001,000nmSi3N4109n31.471110nmSiO2111n41.4705,000nmSiO2113n31.471110nmSiO2115n22.1001,000nmSi3N4119n41.470110nmSiO2117n31.471110nmSiO2105n51.520—epoxy101n61.5-1.9λ/4Oxynitride

The above described spatial filter can be fabricated using standard semiconductor fabrication processes. By way of example, the majority of the deposition process can be performed using plasma enhancement chemical vapor deposition (PEVCD). The above values for the refractive indexes are in part dependent on the materials used and the fabrication process. However, depending on the deposition process and materials chosen, the refractive indexes could have a range 1 to 5.0. Additionally, depending on the fabrication process and materials the difference between the refractive index of the cladding layer could be less than the anti-reflective layer from 0 to 5.0.

The dielectric materials are formed into the thin-film layers and are generally amorphous dielectrics. Plasma-enhanced chemical vapor deposition (PECVD) is used to fabricate most of the layers of the thin-film filter structure. This method provides the precision required for the deposition of each of the layers. The only layer that has the option of being a dielectric or polymer is the outer-most layer. The outer-most layer119ofFIG.8can be made from a dielectric, a polymer, or a combination of the two different materials.

As previously noted, the detection region43ofFIG.1consists of three different layers. The three layers as depicted inFIG.8are layers119,117and115. The three layers make up a basic spatial filter. The term “cladding detection layer” refers to the top-most layer at the air boundary of the device.

Of significance in the cladding detection layer is top coating119inFIG.8, since it is in this layer where the thermally induced refractive index change takes place, by the absorption of infrared light. This top coating layer119can be composed of dielectric material (SiO2, Calcium Fluoride, or Oxynitride) or polymer (polystyrene, PMMA, or other photoresists), and is typically patterned into individual pixels. The refractive index of this layer119must be carefully set so that it is the same or close to the refractive index of layer111, depending on whether a negative or positive thermo-optic coefficient material is used, respectively. The advantage of using a polymer is due to its negative and significantly larger thermo-optic coefficient.

A basic component of the invention is the single spatial filter200as depicted inFIG.10. The basic spatial filter has a bottom high refractive index core layer201and typically composed of Silicon Nitride or similar material that has a refractive index typically in the range of 2.1 by way of example. A near-critical angle, anti-reflective (AR) layer203, which can be composed of Silicon Dioxide or similar material with its refractive index in the range of 1.461. A top layer205with low refractive index and typically composed of Silicon Dioxide which has a refractive index 1.460. As is noted in detail elsewhere, the very top layer which is exposed to air207. Top layer205is referred to herein in some instances as the cladding detection layer. This cladding detection layer or just cladding layer, is unique unto itself. In a preferred embodiment, cladding detection layer has an index slightly less than AR layer203. In the example schematically depicted inFIG.10, it has a refractive index just 0.001 less than the refractive index of anti-reflection layer203. In the embodiment of the invention depicted in the spatial filter200, since the filter structure of the three layers are in the form of a thin film, they have an underlying substrate207to provide a base. The substrate and its makeup are discussed above at length.

The structure inFIG.10as noted is the simplest embodiment of the spatial filter of the present invention, which as noted has a high refractive index (core) layer, a low refractive index (cladding) layer and a near-critical angle, anti-reflection layer in between. This structure filters the internal light by only permitting the efficient transmission of a narrow band of angles very close to the critical angle. Also, due to its proximity to the critical angle, strong refraction occurs which causes a lateral displacement of the transmitted light as it passes through the near-critical angle, anti-reflection coating203and the low refractive index (cladding) layer205. This effect further spatially filters the internal light. This is a unique solution to the near-critical angle spatial filter equations and has not been considered or demonstrated in prior art. This design, transmission from a solid dielectric203into another solid dielectric205, requires that the refractive index of the near-critical angle anti-reflection coating203be greater than, but very close in value to the low refractive index (cladding) layer, typically on the order of 0.001. Prior art has only considered the case of transmission from solid dielectric into gaseous air. For this case the difference between the refractive index of the near-critical angle anti-reflection coating layer is much larger. It is the small refractive index difference between the two layers mentioned above that results in the structure's sensitivity to small refractive index changes, in this case, due to the absorption of infrared radiation.

To create a spatial filter with a high degree of edge steepness as a function of angle of incidence, the reflectance slope is maximized. The highest reflectance slope appears near the critical angle of total internal reflection. This optical effect can be used to design a thin-film filter to cancel the reflected plane-wave front if it is incident at an angle slightly smaller than that of the critical angle at the film-substrate. Textbooks, patents, and scientific literature contain many mathematical methods and formalisms to obtain our solutions. The following are the most basic equations that can be used to find a numerical solution.

Input angle, θ1
θ2=arc sin(n4sin θ1/n3)
θ3=arc sin(n4sin θ1/n2)

Reflectance Amplitude, p polarization:

r1⁢2⁢p=n3⁢cos⁢θ1-n4⁢cos⁢θ2n3⁢cos⁢θ1+n4⁢cos⁢θ2⁢r2⁢3⁢p=n2⁢cos⁢θ2-n3⁢cos⁢θ3n2⁢cos⁢θ2+n2⁢cos⁢θ3

Reflectance Amplitude, s polarization:

r1⁢2⁢s=n4⁢cos⁢θ1-n3⁢cos⁢θ2n4⁢cos⁢θ1+n3⁢cos⁢θ2⁢r2⁢3⁢s=n3⁢cos⁢θ2-n2⁢cos⁢θ3n3⁢cos⁢θ2+n2⁢cos⁢θ3

Reflectance Percentage for the coated surface:

Rp=r1⁢2⁢p2+r2⁢3⁢p2+2⁢r1⁢2⁢p⁢r2⁢3⁢p·cos⁢2⁢β1+r1⁢2⁢p2+r2⁢3⁢p2+2⁢r1⁢2⁢p⁢r2⁢3⁢p·cos⁢2⁢β⁢Rs=r12⁢s2+r23⁢s2+2⁢r12⁢s⁢r23⁢s·cos⁢2⁢β1+r12⁢s2+r23⁢s2+2⁢r12⁢s⁢r23⁢s·cos⁢2⁢β⁢R==12⁢(Rp+Rs)⁢β=2⁢πλ⁢n3⁢h⁢cos⁢θ2whereRis the average reflectance, β (in radians) is the phase difference in the external medium between waves reflected from the first and second surfaces of the coating, h is the thickness of the coating, and λ is the wavelength. The variables are λ, n2, n3, n4, and Δθ, the proximity to the critical angle. Once the variables are set, the equations can be numerically solved for the minimum reflectance by varying h and Δθ. A good solution typically results in the lowest reflectance for s and p polarization light having similar values.

The example of infrared imager31structure inFIG.8has three different spatial filters similar to that depicted inFIG.10. InFIG.8, the three different spatial filters are the following combination of layers starting from substrate103: 1stspatial filter layers107,109and111; 2ndspatial filter layers111,113and115; and 3rdspatial filter layers115,117and119. As can be seen, each filter has in the direction of the injected light that will pass through them a first high refractive index layer, an anti-reflective coating with a refractive index lower than the first layer and a third low refractive index layer. The number of stacked layers depicted inFIG.8is just one example, and the actual number of stacked layers of spatial filters will depend on the intended use.

Referring toFIG.11, a schematic diagram of the infrared imager shows the various uses of the spatial filters of infrared imager300. It essentially is three regions indicated by the numbered circles: a) a Reference Region301that filters the light from internal light source33both spatially and spectrally. The light which is transmitted through this region is the “internal light” or sometimes referred to as the “injected light.” b) Detection Region303is where the internal light is either transmitted out of the “Thin-Film Stack,” the layers309above substrate311or reflected to the measurement region305, depending on the absorption of infrared light at the exposed surface of cladding detection layer307. c) Measurement Region305is an optical noise filter preventing any scattered light from the “Detection Region” from transmitting to camera35. This region also has the camera or photodiode and measures the “Internal Light” intensity or even captures an image from the infrared light irradiating cladding detection layer307.

As noted, cladding detection layer is of unique importance for the present invention. The top coating or “cladding detection layer” has a unique structure and is reactive to irradiation by infrared light which causes a change of the refractive index of the cladding detection layer—the variation in the change of the refractive index is proportional to the intensity of the infrared radiation.

All of the other layers of the thin-film filter structure are made of a dielectric material with the exception of the cladding detection layer, which can be made of either a polymer or a dielectric. The key feature of the top-most cladding detection layer is that its refractive index changes when irradiated by infrared light. The change in refractive index causes the critical angle to shift. The magnitude of the change is dependent of the material of the layer's thermo-optical coefficient, or Δn/ΔT, which is the change in refractive index for a change in temperature by 1 degree Celsius. The temperature change being the result of infrared light impacting the top cladding detection layer. The larger the thermo-optical coefficient of the material, the greater the change of the refractive index and the more sensitive the device. This coefficient also has positive or negative sign (+, −) which indicates the direction of the shift of the change in the refractive index. Typically, dielectrics have thermo-optical coefficients of an order of 10−6and polymers have coefficients of −10−3. The direction the refractive index changes, increasing or decreasing, determines the direction the critical angle changes by, increasing or decreasing. Polymers will have a change in refractive index 1,000× greater per degree of temperature change than dielectrics, representing a significant increase in device sensitivity. A polymer-dielectric “hybrid” layer combination could be used to enhance and fine tune performance. Oxynitride (SiNxO) is an example of a dielectric material that can be used in combination with a polymer as its refractive index can be tuned to be nearly identical to polymers, particularly commercial photoresists such as Poly(methyl methacrylate) (PMMA) and SU-8 photoresist.

This layer must also be designed to enable the efficient exit of visible light from the layer into the air above. If composed of dielectric material, this can be accomplished by fabricating the layer into a pattern or by roughening the surface to induce scattering. If a polymer is used, an additive can be included as well, such as nano-particles, to induce scattering.

FIG.12Ais a schematic diagram of one variation of top coating301of cladding detection layer303. In this variation, the top surface304has a rough surface, as opposed to optically flat, equivalent to a pixelated scattering surface. Roughening can be implemented mechanically or via a chemical etching process.FIG.12Bis a schematic diagram of another variation of the top coating305that has embedded nano-particles306to create a pixelated scattering volume. This is typically done by incorporating nano-particles, such as carbon nano-tubes, into a polymer and spin coating it into said layer.FIG.12Cis another variation in which top coating307has a pixelated scattering geometry due to nano-fabricated pyramid structures308. The pyramid structures308can be two-dimensional or three-dimensional. These pyramid structures can be made using standard photolithography and etched into the dielectric surface. They can be etched or stamped if the surface is a polymer.FIG.12Dis a view of the top of the detection area from the direction that infrared light would irradiate the detection surface. Since the infrared light is in the form of an image, it irradiates the cladding detection area in varying degrees of intensity to thereby cause varying amounts of injected internal light to pass out of the top surface or to be reflected down to the camera. The refractive index of the cladding detection area will vary across the surface to cause this.

It should be noted that the critical angle can vary over the cladding detection layer allowing some of the injected light to escape and some to be reflected back down. Thus, if there is an image in the infrared light irradiating the cladding detection layer it will transfer this image to the injected light by causing some of it to exit the cladding detection layer at an angle less that the critical angle, and some of the injected light to reflect down out of the cladding detection layer towards imaging device or camera.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents.