DESCRIPTION

The present invention relates to an explosion-proof case for a video camera according to the general part of claim 1.

As is well known, in industrial appl ications and/or for process control, in addition to the standard video surveillance cameras (CCTV) working in the visible spectrum (part of the electromagnetic spectrum that falls between red and v iolet, including all colours perceptible to the human eye) special video cameras are used, known as "thermographic cameras", working in the zone called "infrared", where the electromagnetic spectrum has a greater wavelength than visible light.

As an example, we may cite thermographic cameras working in the long infrared wav e zone ( LW!R ).

Thermographic cameras, when employed in environments having a potentially explosive atmosphere, must be placed inside explosion-proof cases.

One of the conditions to be met by such explosion-proof cases is the one that dictates that the viewing window , located at the front of the case, must be constructed with a special material, transparent to infrared radiation, rather than the ordinary float glass, which blocks infrared radiation.

Normally, the w indows able to work in the electromagnetic spectrum of infrared are made of high purity germanium for optical use, particularly pol ished and coated with a layer of special materials (coating) in order to increase the transmission of infrared radiation.

It should be noted that a viewing window for a video camera made of germanium costs approx imately twenty times more than a corresponding viewing window made of glass. As an alternative to germanium with a wavelength 2-15 um, sapphire (synthetic sapphire), or zinc-selenium (ZnSe) are used, albeit less frequently.

In order to be used in environments with a potentially explosive atmosphere, video cameras must get the explosion proof certification; in particular, they must be subjected to the tests defined by the I EC EN 60079-0:201 0 § 26.4.2 Standard of December 2010, which aims to test the impact/shock resistance of the viewing window.

Operationally, the test consists in dropping a weight of 1 kg on the viewing window from a predetermined height; the test is passed if the w indow is not damaged in any way.

In cases for video cameras that operate in the electromagnetic field of the visible l ight, the viewing window is made of a tempered glass of considerable thickness, which allows passing the impact test; usually, with a 1 2 mm and larger thickness, tempered glass is sufficiently resistant to impact and is not damaged.

In contrast, v iewing windows which employ material transparent to infrared radiation (germanium and other similar materials) do not pass the test, since this material is considerably fragile and does not resist the impact, is easily broken or, at least, the surface of said window is scratched as a result of the impact with the test sample. According to the prior art, in order to obviate this draw back and thus allow cases made of material transparent to infrared radiation (germanium and other similar materials) to pass the impact test and obtain the certification, it is provided to place, at a predetermined distance from the v iewing w indow, a metallic protective grid sufficiently resistant to stop the weight sample during the impact test. Operationally, the presence of the grid ensures that the viewing window is not touched by the weight, since the impact is totally absorbed by the protective grid. In practice, if the application of the grid allows obtaining the ex plosion -proof case certification, it exhibits the drawback that, being positioned in front of the viewing window to be protected, it hinders the visual field of the video camera installed inside of the case.

In essence, although the manufacturers of cases design grids that are specially shaped, so that the impact on the visual field is minimized, the presence of the grating that makes up the grid is always visible whereby, for certain objectives and/or video cameras, it hinders their visual field.

By way of example, we may cite the patent document CN201909672 U, which describes a viewing window consisting of tw o w indows, made of germanium, mutually spaced apart, betw een w hich a metal grid is inserted.

Upon an initial examination, this constructive solution advantageously allows using two windows of reduced thickness made of germanium ( fig. 1 , ref. 4 and 6, the references relate to the cited document ) and therefore with a considerable economic advantage, leaving to the intermediate metal grid (fig. 1 , ref. 5) the function of protecting the view ing window, the innermost one (fig. 1 , ref. 6) if the additional window, the outermost one (fig. 1 , ref. 4) undergoes damage.

In contrast, upon a more thorough examination, this constructive solution has the drawback deriving from the fact that a metal grid is always used (fig. 1 , ref. 5) which, even if it has a grating shaped differently (see figs. 2, 4 and 6) still gives rise to a remarkable v isual hindrance which, in particular uses of the video camera, is not acceptable. By way of example, the following additional patent documents are cited: CN 201 515

436 U, US 2012/154521 A l and US 2012/008930 A l .

There is also document DE 10 2008 029 407 A 1 which, apparently, is a prior art document since it describes an explosion-proof case containing a v ideo camera adapted to operate in hazardous areas and which has in its interior an irradiator element of infrared rays, such radiation having the purpose of illuminating the scene that the video camera must shoot in low light conditions (darkness or night). This irradiator emits infrared at a wavelength of about 0.84 to 0.85 urn. Therefore, a viewing window may be used which uses standard glass; such glass could not be absolutely used in the technical video cameras, since standard glass blocks the passage of electromagnetic radiation in the spectrum between 2 and 1 5 μιτι, that is the aid of the present invention. Thus in our case, windows made of germanium or special materials such as sapphire and zinc-selenium transparent to such radiation spectrum must be used. These materials have a behavior against deflagration that is absolutely different from standard glass and therefore it is clear that the teachings of this document cannot be applied to the technical problem addressed by the present invention.

The object of the present invention is to provide a case for a video camera, in particular explosion-proof, prov ided w ith a protection of the v iewing window which is free of the drawbacks of similar products of known type.

Specifically, the object of the inv ention is to provide a protection of the viewing window that does not absolutely require the use of a metal grid.

This object is achieved by overcoming a so-called "technical prejudice", according to which it is not possible to ensure the protection of the v iewing window of a video camera case by means of the application only and exclusively of a second window, made of the same material as the viewing window, since currently it is believed that the thickness of the second window, necessary to ensure the protection of the viewing window, must necessarily be significant; hence a high economic cost for the purchase of the infrared transparent material ( germanium and other materials suitable for the purpose) used to make the additional window, which would be l ikely to result in a considerable increase in the cost of production of the entire case, which would not be acceptable by the market.

Such a technical prejudice has been overcome by applying, in front of the viewing window, with commercially regular thickness, a further window, hereinafter referred to as "sacrificial" window, since it has the function of absorbing the impact energy, breaking during the impact test or in use, but without damaging and thus totally protecting the v iewing window from the aforesaid accident.

In fact, this additional or "sacrificial " window has the same function of the protective metal grid but, being formed by a wall of a material able to work in the electromagnetic infrared spectrum (germanium and others), is not visible from the v ideo camera, so that it is transparent for its entire extension.

Operationally, the design of the optimal "sacrificial" window was carried out based, primarily, on the following parameters:

a) Diameter thickness ratio of the "sacrificial" window: a detailed study has been carried out on the sizing of sacrificial "window", in order to find the ideal measures of diameter and thickness of the window.

The thickness/diameter ratio is in fact particularly critical : the larger the diameter of the window, the more fragile becomes the same when it is subjected to a shock/impact; accordingly, as the diameter is increased, the thickness of the window must also be increased to make it more resistant to the destructive impact, with the result that too thick windows would be too expensive.

After performing the necessary calculations, the necessary practical technical tests (impact break test) and the estimates of the costs of production of windows with different sizes, it was found that the ideal value of the "s/d" ratio between thickness "s" and diameter "d" of the window is between the following values: 1/12 > s/d > 1/8. By way of example, we may cite the manufacture of a "sacrificial" window of germanium having a thickness of 5 mm and diameter of 60 mm.

b) Ratio of width to diameter of the gap (partition space forming a sealed chamber between the two windows, the viewing and the "sacrificial"): this distance is of fundamental importance: a too little space between the two windows would cause the breakage of the viewing window during a shock impact, since the incident mass would not be sufficiently slowed down by the "sacrificial" window and the shock wave would be transferred to the above viewing window, causing it to break; in contrast, a too large distance would allow the incident mass dropping to regain speed after the first impact with the "sacrificial" window, therefore making the damage of the aforesaid viewing window more likely.

In essence, the "gap" between the two windows must therefore be neither too small nor too large; concretely, after the necessary practical technical tests (impact breakage test), it was found that the ideal value of the "H/D" ratio, between width "H" and diameter "D" of the gap or partition space is between the following values: 1/20 > H/D > 1/14, so as to allow for the impact energy to be dissipated before the debris and the blunt mass contact the viewing window, thus damaging it.

By way of example, we may cite the manufacture of a gap having a thickness of 3 mm and diameter of 50 mm.

c) Debris during the breaking the "sacrificial" window: the values defined in the above two parameters were defined taking account of debris that are produced at the time of breaking of the "sacrificial" window, which are in the form of smal l pieces having a maximum size of a few mill imeters and which occupy the volume of the chamber defined betw een the two windows, being arranged randomly on the surface of the viewing window. In practice, therefore, a sort of buffer of debris is formed which cushions the stroke of the falling mass and protects the viewing window, d ) Elastic support points of the "sacrificial" window: particular attention has been paid in the design phase to the elastic support on which the "sacrificial" window rests, which consist of three rubber gaskets (O-ring type) having a suitable degree of elasticity; in particular, the "sacrificial" window leans at both its sides on two "O- rings", while the third "O-ring" is arranged over the entire outer circumference of said window.

In practice, the two "O-rings" resting on the two flat surfaces of the "sacrificial" window are meant to also ensure a watertight seal, to prevent the infiltration of dust and liquids in the gap between the two windows, while the third "O-ring" has been added as a "circumferential" seal, w hich mitigates the impact in stopping the run of the fall ing mass. The invention will be better defined by the description of a possible embodiment thereof, given only by way of non-l imiting example, with the aid of the accompanying drawings, in which :

- figs. 1 - 2 (dwg. I ) show two views, respectively perspectiv e and sectional, of a case according to the invention;

- figs. 3 - 4 (dwg. I I ) show two views, respectiv ely exploded and sectional, of the front window prov ided with the "sacrificial" window of the invention;

- figs. 5 - 6 (dwg. I l l ) show exploded views, respectiv ely, of the assembly of the protective "sacrificial" window and the assembly of the v iewing window.

As is seen in the figures, the video camera case 100, in particular of the explosion- proof type, comprises a viewing window, indicated as a whole with reference numeral 1 , consisting of a flange 2, fixed to body 3 of the case by means of screws 4, where the transparent viewing disc 5 is inserted, retained by a ring nut 6 with an intermediate "O-ring" 7, ail as known per se.

The protection of the viewing window 1 is achiev ed by the appl ication, on the front of the viewing window 1 , of a second window, indicated as a whole with reference numeral 10, consisting of a shaped ring 1 1 , which retains a second transparent disc 12, which is positioned parallel to and spaced apart from the previous transparent disc 5, so as to form an air chamber 13.

The second window 1 0 is referred to as "sacrificial" since when it is hit by a blunt object, said transparent disc 12 cracks until it breaks, but protects the transparent disc 5, located more internally.

Moreover, the transparent disc 1 2 is mounted leaning against an "O-ring" 14, which acts as a sealing means of the air chamber 1 3 and is retained by two additional "O- rings", a front ring 1 5 and a circumferential ring 1 6. to prov ide an elastic support of said transparent disc.

The entire second window, or "sacrificial" window 10 is removably placed, by means of screws 1 7, on flange 2 of the front window 1 .

The transparent disc 12 of the "sacrificial" window 10 is made of the same material transparent to infrared radiation, such as germanium or other materials suitable for the purpose, with which the transparent disc 5 of the viewing w indow 1 is made.

To highlight the adv antage of the invention, by way of example, we may cite the operating condition in which the video signal obtained from the video camera is managed by special software, which analyzes the image to extract particular information.

In this condition, the video camera is used specifical ly as a sensor to detect, in particular, the temperature of the filmed objects, for which the use of the grid (known method ) would be particularly harmful, because it would alter the measurement made, in some cases making the images/information provided by the video camera itself unusable.

The invention thus conceived is susceptible of modifications and variations, and its details may be replaced by technically equivalent elements, provided that all falls w ithin the inventive concept defined by the following claims.