Explosion-proof thermal imaging system

A thermal imaging system is provided. The thermal imaging system includes an explosion-proof housing with an optical window configured to contain an explosive pressure. The optical window allows electromagnetic thermal energy to pass. A thermal imaging sensor is disposed within the explosion-proof housing. Thermal imaging electronics are coupled to the thermal imaging sensor and configured to provide at least one thermal image based on a signal from the thermal imaging sensor. A lens assembly is disposed at least in front of the optical window external to the explosion-proof housing. A composite optical window for thermal imaging is also provided. In another embodiment, a thermal imaging system includes an explosion-proof housing having an optical window configured to contain an explosive pressure. An infrared (IR) camera is disposed within the explosion-proof housing. A reflector reflects electromagnetic thermal energy to the IR camera, but prevent an object from impacting the optical window.

BACKGROUND

Infrared cameras generally form an image using infrared radiation, similar to the way in which a standard camera forms an image using visible light. However, an infrared camera typically operates with longer wavelength illumination, such as 14,000 nanometers. Infrared cameras are highly useful in a number of applications to provide a non-contact indication of heat present in an image field. Moreover, the infrared cameras, in some contexts, can be calibrated such that an indication of surface temperature can be derived directly from the image provided by the infrared camera.

One environment in which infrared cameras are particularly useful is in process control and monitoring. In such environments, process fluids, such as petrochemicals, slurries, pharmaceutical compounds, and the like may be processed and conveyed to various locations within the processing facility. However, process control and monitoring environments represent a challenge for a number of devices in that the environment itself may have highly flammable or explosive gases present therein. Accordingly, in some such environments, it is important for electronic devices used therein to be housed in an explosion-proof enclosure. When so housed, even if the circuitry of the device generates a spark or has an electrical component with a surface temperature high enough to ignite the environment, the resulting ignition will be entirely contained within the enclosure and not able to escape into the ambient environment. This is important in order to ensure safety of the process control installation and workers therein.

One example of an explosion-proof rating is an ATEX certification to Ex-d standards EN60079-0 and EN60079-1 for potentially explosive atmospheres. Generally, explosion-proof housings are relatively bulky in order to be mechanically robust enough to contain an internal explosion without rupturing. Generally, such explosion-proof containers are very robust metal enclosures that are designed to withstand explosive pressures. However, for devices, such as infrared cameras that rely on optical sensing of the environment, the enclosure must accommodate a window of some sort in order to allow the infrared camera to view the environment. However, in order to contain the explosive pressures and impact requirements, the window must be relatively thick. Unfortunately, increasing the thickness of the window such that it is mechanically robust enough to contain the explosive pressures will reduce the transmissivity of the window, increase the cost of the window, and undesirably affect the radiometric temperature measurements.

SUMMARY

A thermal imaging system is provided. The thermal imaging system includes an explosion-proof housing with an optical window configured to contain an explosive pressure. The optical window allows electromagnetic thermal energy to pass. A thermal imaging sensor is disposed within the explosion-proof housing. Thermal imaging electronics are coupled to the thermal imaging sensor and configured to provide at least one thermal image based on a signal from the thermal imaging sensor. A lens assembly is disposed at least in front of the optical window external to the explosion-proof housing. A composite optical window for thermal imaging is also provided.

In another embodiment, a thermal imaging system is provided that has an explosion-proof housing including an optical window configured to contain an explosive pressure. The optical window allows electromagnetic thermal energy to pass through. An infrared (IR) camera is disposed within the explosion-proof housing. A reflector is configured to reflect electromagnetic thermal energy to the IR camera, but prevent an object from impacting the optical window.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention generally improve a thermal imaging system by modifying or providing an explosion-proof window and/or additional modifications prior to the thermal imaging optics. In general, thermal imaging optics of thermal imaging cameras are not designed to withstand the internal pressures that are required to meet explosion-proof approvals. For example, the modulus of rupture (MR) for suitable IR transmitting materials is relatively small.

Embodiments of the present invention generally provide an infrared window that is suitable for acting as a pressure barrier. In some embodiments, the infrared window is formed of zinc selenide (ZNSE). While one possibility for providing an explosion-proof thermal imaging system is simply to place a conventional infrared camera within a robust housing having a thick IR window formed of an IR transmitting material, such as ZNSE, the size of the window required would be of such diameter that the thickness of the window required to contain explosive pressures would impermissibly affect optical performance. Further, such a window would be cost prohibitive. In accordance with some embodiments of the present invention, the IR window is placed within or at least behind at least one optical element of the thermal imaging optics. In this way, the optical elements can reduce the size of the ray bundle that must pass through the IR window. In this way, the diameter of the IR window can be reduced, which can then allow the thickness to be reduced as well while still complying with pressure and impact containment requirements.

FIG. 1is a diagrammatic view of an explosion-prop thermal imaging system in accordance with an embodiment of the present invention. System100includes thermal imaging sensor102and associated electronics104coupled to sensor102. Electronics104are electrically coupled to connector106that passes through housing108to allow system100to be coupled to a suitable source of power and/or communication. A lens assembly110is disposed proximate sensor102and is arranged to focus thermal radiation on thermal imaging sensor102. Lens assembly110includes, in the illustrated embodiment, four distinct lenses112,114,116, and118. Additionally, an explosion-proof window120is disposed between lenses114and116. Window120is formed of a material with high infrared transmissivity, such as ZNSE. An environmental seal, such as elastomeric O-ring122, seals explosion-proof window120to the frame of lens assembly110. Window120is sired, with respect to frame124to provide a frame-quenching path126. Since window120is disposed within lens assembly110, the diameter of window120smaller than would be required if window120were placed in front of lens112. By reducing the diameter of window120, the thickness of window120can be reduced while still being able to comply with pressure containment requirements. This allows the material cost to be reduced while also potentially increasing the optical performance of the overall system.

FIG. 2is a diagrammatic view of window120disposed within lens assembly110in accordance with an embodiment of the present invention, Thermal imaging electromagnetic radiation first passes through the first section (indicated at reference numeral130) before passing through window120. The first section,130, is generally comprised of outer focusing lens112, and secondary focusing lens114. Once the thermal imaging electromagnetic radiation passes through secondary focusing lens114, it passes through window120, which, in one embodiment, is formed of zinc selenide. Then, the thermal imaging electromagnetic radiation passes through focusing lens116, which is arranged to converge the ray bundle. Thermal imaging electromagnetic radiation exiting ions116enters focusing lens118, which further focuses the ray bundle upon camera sensor102for image acquisition.

In order to comply with relevant explosion-proof approvals, a device must pass certain tests. In one test, a one inch diameter sphere is impacted on the optics with four joules of force. This ensures that approved designs will be able to withstand at least some level of impact without unduly affecting the pressure-containing abilities of the device. Unfortunately, IR windows (particularly those formed of ZNSE) are quite susceptible to damage from such impact tests.

Embodiments of the present invention that provide an IR window disposed within a camera lens assembly inherently protect the IR window from the impact of such tests. Providing the IR window within the lens assembly ensures that the impact of the one inch diameter sphere only affects the outer lens and does not affect the pressure-containing ability of the system.

FIG. 3is a diagrammatic view showing an impacting sphere150striking and passing through focusing lens112. However, metal shroud152is sized such that the one inch diameter sphere is unable to reach IR window120or even lens114. Thus, at least some embodiments of the present invention provide an improved explosion-proof thermal imaging system with a shroud152disposed outside of lens112having a diameter that is smaller than one inch. Impacting object150is thus stopped prior to reaching and potentially impacting IR window120. When impacting object150comes into contact with the lens assembly, it will first be slowed by outer lenses112,114and finally stopped by the lens frame (shroud152) before impacting IR window120. Such arrangement facilitates approval compliance for explosion-proof ratings. Once impacted, outer lens112is destroyed, and the system will no longer be able to produce a viable measurement. However, this measurement failure will be easily identifiable by the thermal image itself, and an operator will be alerted to the fault. Accordingly, electronics104, coupled to sensor102(shown inFIG. 1) may be configured, via hardware, software, or a combination thereof, to compare a time sequence of thermal images or a parameter related to transmissivity of the lens system over time in order to detect changes, such as fracture or deterioration of one or more lenses in lens assembly110. In this way, should a lens break or become damaged, electronics104will provide an alert to the operator.

As set forth above, impact tests of zinc selenide windows have indicated that such IR windows are not particularly impact resistant. However, impact resistance is very important in order to obtain and maintain explosion proof-approvals. While embodiments described thus far, generally protect the explosion-proof window by placing it within in the lens assembly of the thermal imaging system, at least some embodiments specifically adapt the IR window for impact.

FIG. 4Ais a diagrammatic view of an impacting object150impacting a zinc selenide window160at location162. Upon impact, a number of fractures164expand from point162in both the x and z directions. Once a fracture travels across the entire thickness (T) of window160, the mechanical integrity of window160is compromised and explosive pressures will no longer be contained.

FIG. 4Bis a diagrammatic view of a composite IR window170in accordance with an embodiment of the present invention. Window170is formed of a pair of thinner zinc selenide layers172,174that sandwich an IR-transparent polyamide layer176. Polyamide layer176is highly impact resistant and does not fracture like zinc selenide. Accordingly, when impacting object150strikes layer172at location178, the fracture propagates as indicated at reference numeral180. The fracture will ultimately propagate across layer172, but will not propagate through polyamide layer176. Accordingly, the mechanical integrity of layer174is unchanged and still able to contain the explosive pressures required for obtaining and maintaining requisite explosion-proof approvals. Most of the energy of the impact is absorbed by first layer172, thereby lowering the impact applied to second layer174to a level that will resist fracture. Furthermore, polyamide layer176can facilitate interrupting the propagation of such fractures. Layer174contains a pressure bearing seal and is required to remain intact for explosion-proof approvals. Utilizing the composite IR window illustrated inFIG. 4B, embodiments of the present invent may be able to provide a simple enclosure for known thermal imaging systems. However, it is expressly contemplated that the composite IR window provided inFIG. 4Bcan be used in embodiments described above, by incorporating the hybrid window design into IR window120.

FIG. 5is a diagrammatic view of an improved explosion-proof thermal imaging system in accordance with an embodiment of the present invention. Thermal imaging system200includes IR camera202disposed within explosion proof housing204. Explosion proof housing204includes, at one end, IR window206. IR window206does not have any grid or solid structures that would otherwise interrupt infrared radiation. In some embodiments, IR window206may include the hybrid IR window design shown inFIG. 4B. However, embodiments also include IR window206as simply being a single monolithic piece of IR-transparent material, such as zinc selenide. In order to protect IR window206from impacts, reflector208is placed substantially in front of IR window206. Accordingly, objects, such as impacting object150are prevented from striking IR window206. In this way, reflector208provides a mechanical shield for IR window206while optically participating in the acquisition of thermal images. In the example shown inFIG. 5, reflector208is a parabolic mirror thus helping to focus the image field upon IR camera202. Additionally, system200also includes emissivity reference210disposed proximate IR window206and arranged to have at least one surface212with a known emissivity. When IR camera202images surface212, the known emissivity in combination with the image of surface212can provide valuable information about the condition of one or both of mirror208and IR window206. For example, should mirror208or IR window206become dirty or damaged, such condition would be ascertainable by IR camera202by viewing emissivity reference surface212. Additionally, in at least some embodiments, the entire structure shown inFIG. 5may rotate about axis214such that a 360 field of view can be thermally imaged using IR camera202. This would allow even relatively low-resolution imagers to be capable of increasing resolution if the IR camera is used in a line scanner mode. Additionally, in at least some embodiments, the mirror208may simply be rotated about the optical axis of IR camera202in order to provide the 360 degree field of view imaging. This would simplify the design in that rotation of IR camera202, and the associated rotational connectors, would not be required.