Field of invention

The invention relates to safety alarm devices such as fire alarms, smoke alarms, heat detectors and carbon monoxide sensors.

Background

Safety alarm devices such as fire alarms, smoke alarms, heat detectors, and carbon monoxide sensors provide a warning when smoke, fire, carbon monoxide or other harmful atmospheric environmental conditions are detected. It is critical for building safety that such alarm devices are maintained in good working order. Safety alarms must often be periodically inspected to determine that there is adequate ambient air flow through the alarm in order that a condition, e.g. smoke from a fire within the building, is detected. Maintenance and inspection requirements for safety alarms are burdensome for the occupants of a building. This problem is particularly acute given that safety alarms are often placed in difficult-to-reach areas such as on a ceiling.

An aim of this disclosure is to provide a fire alarm with less burdensome maintenance requirements.

Summary

The scope of the invention is defined by the appended claims.

According to a first aspect of this disclosure there is provided a safety alarm device comprising an outer casing comprising a rear surface configured for attachment to an external wall surface, and an opposing front surface. The safety alarm device further comprises three or fewer optical detectors each comprising a light emitter for emitting a light signal along an optical axis extending outwardly from a perimeter region of the outer casing and a light receiver for receiving a reflection of the light signal from an object, the perimeter region located between the front and rear surfaces. The safety alarm device further comprises a controller configured to interrogate each of the three or fewer detectors to determine the presence and/or distance of the object from the safety alarm device.

Optionally, the perimeter region is integral with the front surface. Optionally, the perimeter region comprises an intermediate surface in between the front and rear surfaces, the three or fewer optical detectors being mounted on the intermediate surface, the intermediate surface being at an acute angle greater than 0 degrees, which is preferably between 30 and 60 degrees, with respect to the external wall surface.

Optionally, each optical axis is at an acute angle greater than 0 degrees, which is preferably between 30 and 60 degrees, with respect to the external wall surface.

Optionally, each light emitter is configured to emit a light signal via a light beam having a beam angle of 30 to 90 degrees, preferably, the beam angle is 45 to 75 degrees, and more preferably the beam angle is 60 degrees.

Optionally, the safety alarm device comprises two or three detectors, and wherein each optical axis is angularly spaced from the optical axis of each other optical detector by an equal amount when viewed from the external wall surface.

Optionally, the safety alarm device comprises three optical detectors, and wherein each optical detector is configured to emit light along an optical axis that has an angle of 120 degrees from the optical axis of each other optical detector when viewed from the external wall surface.

Optionally, each optical detector is configured to provide a grid array output, the grid array output comprising a plurality of grid values each corresponding to a portion of a field of view of the optical detector, each grid value indicating the presence and/or distance of the object detected within the corresponding portion of the field of view. Optionally, the controller is configured to determine a size of the object based on the number of grid values indicating the presence and/or distance of the object.

Optionally, the controller is further configured to determine the size of the object based on the number of contiguous grid values indicating the presence and/or distance of the object.

Optionally, the safety alarm device comprises a plurality of optical detectors; the controller is configured to process the grid array output from two or more of the plurality of optical detectors; and when grid values corresponding to an edge of each field of view indicate the presence and/or distance of the object, the controller is configured to determine the size of the object when the object spans across the field of view of the two or more optical detectors.

Optionally, the controller is further configured to identify an object spanning across the field of view of the two or more optical detectors by: determining that at least one grid value at a first edge of the field of view of a first detector of two or more optical detectors indicates the presence and/or distance of the object; and determining that at least one grid value at a second edge of the field of view of a second detector of the two or more optical detectors indicates the presence and/or distance of the object, the first detector being adjacent the second detector and the first edge being adjacent the second edge.

Optionally, determining that at least one grid value at the first and/or second edge indicates the presence and/or distance of the object comprises determining that a plurality of contiguous grid values at the first and/or second edge indicates the presence and/or distance of the object.

Optionally, the controller is further configured to determine that an object is not present when the determined size of the object is below a threshold.

Optionally, the safety alarm device comprises a means for detecting the presence of smoke within a cavity defined by or within the outer casing and wherein at least one of the optical detectors is configured to emit light in the path of a flow of smoke into one or more apertures in the outer casing.

Optionally, the safety alarm device comprises: a cover in front of each optical detector and an opaque barrier located in between the light emitter and the light receiver of each optical detector for preventing photons emitted by the emitter from being reflected by the cover to the receiver.

Optionally, the cover and/or the barrier is integral to the outer casing.

Optionally, the light emitter is a laser.

Optionally, the laser is a vertical-cavity surface-emitting laser. Optionally, each optical detector comprises a processor for determining the presence and/or distance of the object based on a time of flight measurement of the reflection of the light signal.

Optionally, the controller is configured to transmit a proximity warning signal to a remote server when the determined presence and/or distance of the object from the safety alarm corresponds to a predetermined proximity criteria.

According to a second aspect of this disclosure, there is a method for determining the presence and/or distance of an object from a safety alarm device having an outer casing comprising a rear surface configured for attachment to an external wall surface, and an opposing front surface. The method comprises the steps of: for each of three or fewer optical detectors, emitting, from a light emitter, a light signal along an optical axis extending outwardly from a perimeter region of the outer casing, the perimeter region located between the front and rear surfaces; receiving, from a light receiver, a reflection of the light signal from the object; and interrogating, using a controller, each of the three or fewer detectors to determine the presence of the object and the distance of the object from the safety alarm device.

Optionally, the light signal is emitted via a light beam having a beam angle of 30 to 90 degrees, preferably, the beam angle is 45 to 75 degrees, and more preferably the beam angle is 60 degrees.

Optionally, the method further comprises providing, by each optical detector, a grid array output, the grid array output comprising a plurality of grid values each corresponding to a portion of a field of view of the optical detector, each grid value indicating whether or not the presence of the object is detected within the corresponding portion of the field of view; and determining a size of the object based the number of grid values indicating the presence of the object. Optionally, the method further comprises determining the size of the object based on the number of contiguous grid values indicating the presence and/or distance of the object.

Optionally, the safety alarm device comprises a plurality of optical detectors and the method further comprises processing, by the controller, the grid array output from two or more of the plurality of optical detectors; and when grid values corresponding to an edge of each field of view indicate the presence of the object, determining the size of the object when the object spans across the field of view of the two or more optical detectors.

Optionally, the method further comprises identifying an object spanning across the field of view of the two or more optical detectors by: determining that at least one grid value at a first edge of the field of view of a first detector of two or more optical detectors indicates the presence and/or distance of the object; and determining that at least one grid value at a second edge of the field of view of a second detector of the two or more optical detectors indicates the presence and/or distance of the object, the first detector being adjacent the second detector and the first edge being adjacent the second edge. Optionally, determining that at least one grid value at the first and/or second edge indicates the presence and/or distance of the object comprises determining that a plurality of contiguous grid values at the first and/or second edge indicates the presence and/or distance of the object.

Optionally, the method further comprises determining that an object is not present when the determined size of the object is below a threshold.

Optionally, the method further comprises emitting light in the path of a flow of smoke into one or more apertures in the outer casing.

Optionally, the method further comprises determining, by a processor of each optical detector, the presence and/or distance of the object based on a time of flight measurement of the reflection of the light signal.

Optionally, the method further comprises transmitting, by the controller, an alert signal to a remote server when the determined presence and/or distance of the object from the safety alarm corresponds to a predetermined proximity criteria.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the aspects, examples or embodiments described herein may be applied to any other aspect, example, embodiment or feature. Further, the description of any aspect, example or feature may form part of or the entirety of an embodiment of the invention as defined by the claims. Any of the examples described herein may be an example which embodies the invention defined by the claims and thus an embodiment of the invention.

Brief Overview of Figures

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a perspective view diagram of a safety alarm device.

Figure 2 shows a side view diagram of the alarm device of Fig. 1 .

Figure 3 shows a top-down view diagram of the alarm device of Fig. 1

Figure 4 shows a bottom-up view diagram of the alarm device of Fig. 1 with additional visualisation of light beams.

Figure 5 shows a perspective view of the alarm device of Fig. 1 with additional visualisation of light beams.

Figure 6 shows a grid array output of a controller of a safety alarm device of this disclosure.

Figure 7 shows a bottom-up diagram of an alarm device of this disclosure, with visualisation of light beams and external objects.

Figure 8 shows a close-up cross-sectional side view of an alarm device according to this disclosure.

Figure 9 shows a close-up perspective view of the alarm device of Fig. 8.

Figures 10 and 11 show cross-sectional side views of an optical sensor according to this disclosure.

Figure 12 shoes a perspective view of an optical sensor as integrated into an alarm device according to this disclosure. Figure 13 shows a process flow according to this disclosure.

Detailed description

The effectiveness of a safety alarm depends on the availability of an ambient air flow into the safety alarm. Air flowing into the safety alarm is analysed for indicators of smoke, heat, or carbon monoxide such as by optical or chemical means. Therefore, it is important to periodically verify that there is no obstacle or blockage to air entering the safety alarm. Safety alarms may be located in positions to which airflow to the alarm can become blocked. For example, air flow to a safety alarm in a domestic building may become blocked when an occupant moves a large item of furniture adjacent to the alarm. In another example, a user may tape over portions of the safety alarm when painting a ceiling, and subsequently not remove the tape. In these examples, air flow to the safety alarm has been limited and the functionality of the alarm is reduced. It is desirable to remotely monitor the presence and/or distance of obstacles in the proximity of the safety alarm in order to determine air flow to the safety alarm is compromised. It is particularly desirable for the safety alarm to provide a notification or alert when a problematic obstacle is detected in order that a user can take remedial action by removing the obstacle.

In some jurisdictions, legislation mandates that safety alarms include functionality for detecting obstacles for satisfying ‘maintenance/inspection free’ criteria. For example, German regulation DIN 14676 requires the ability to detect obstacles around the perimeter of the device that may block the path of smoke into the device, the ability to detect obstacles directly in front of the device that may block the path of smoke into the device, and the ability to detect when the smoke entries of the device have been covered (as it may be during decoration) with masking tape or a disposable glove i.e. to provide coverage detection.

With reference to Figure 1 , the present disclosure provides a safety alarm device 101 comprising an outer casing 102. As used herein, the term “safety alarm device” may refer to a fire alarm, smoke alarm, heat detector/sensor, carbon monoxide sensor, or any other type of alarm device that detects an emergency condition indicative of a harmful environmental condition based on sensing a property of the ambient air around the device. The outer casing 102 comprises a front surface 104 and a rear surface 103. The rear surface 103 is configured for attachment to an external wall surface (not shown). The front surface 104 opposes the rear surface 103. It would be appreciated that normally, in use, the safety alarm device 101 would be situated with the front surface 104 facing downwards. For example, typically the alarm will be mounted via the rear surface 103 on a ceiling of a room within a building, with the front surface 104 facing towards the floor of the room. The safety alarm device 101 comprises three or fewer optical detectors 105a, 105b (only two being visible in Fig. 1). Each of the optical detectors 105a, 105b comprise a light emitter (not shown) for emitting a light signal along an optical axis (not shown in Fig 1). The optical axis extends outwardly from a perimeter region 106 of the outer casing 102. The term “outwardly” refers to a direction away from the alarm device and typically into the space of a room within which the alarm device is situated. The perimeter region 106 is located between the front surface 104 and the rear surface 103. Each of the optical detectors 105a, 105b further comprises a light receiver (not shown) for receiving a reflection of the light signal from an object (not shown). A controller (not shown) is configured to interrogate each of the three or fewer detectors 105a, 105b to determine the presence and/or distance of the object from the alarm device 101. Typically the controller is located within the outer casing 102 and electrically connected to each of the optical detectors 105a, 105b.

It has been found that utilising three or fewer optical detectors (i.e. one, two, or three optical detectors) provides sufficient obstacle detection coverage around the safety alarm device. It is not necessary to provide obstacle detection coverage around the entirety of the safety alarm device since small objects in proximity to the device are not necessarily problematic for operation of the safety alarm device. It is desirable to utilise a low number of detectors in order to reduce costs, energy consumption, and complexity of the alarm. Furthermore, a lower number of detectors provides improved reliability since there are a reduced number of points of failure. For ease of explanation an example comprising three optical detectors is discussed below, however the discussed principles also apply to examples comprising one or two detectors where possible.

In the example of Fig. 1 , the perimeter region 106 may comprise a distinct intermediate surface in between the front surface 104 and the rear surface 103. The distinct intermediate surface may be at an angle with respect to the rear surface and/or the external wall surface. The perimeter region of the device of Fig. 1 extends around the device which is circular. However, the device may have a different geometric shape with the perimeter region extending around the different geometric shape. The perimeter region may be integral with the front surface. For example, the front surface may comprise a dome-like shape, with the perimeter region extending across a surface of the dome and around a central axis of the dome that extends through and is normal to the crown of the dome.

With reference to Fig. 2, there is a side view of the safety alarm 101 of Fig. 1 as attached to a wall 201 . It would be appreciated that the wall itself does not necessarily form part of the invention as defined in the claims. The wall is typically a ceiling. Typically in use, the rear surface 103 is parallel with, and lies flush with, the wall. The optical axis 202a, 202b of each of the optical detectors 105a and 105b are represented as dashed lines. The angle x of each optical axis 202a, 202b with respect to the external wall is preferably an acute angle greater than 0 degrees. Angle x may be between 30 and 60 degrees, or, 45 degrees. An acute angle x greater than 0 degrees provides for the light signal to be emitted in a downwards direction and thus to intersect and be reflected by objects that are located below the alarm device 101. Advantageously, it is not necessary to place additional detectors on the front surface facing directly downwards for detecting obstacles directly below the alarm device 101. Furthermore, the resulting beams do not interact with the ceiling which could cause false positives. Where the perimeter region comprises an intermediate surface, the angle x of each optical axis may be defined by an angle of the intermediate surface with respect to the external wall 201 and/or rear surface 103.

Fig. 3 is a top-down diagrammatic view of the alarm device 101 of Fig. 1. The light signal is emitted via the light beams 301a, 301 b, 301c. The light beams 301a, 301 b, 301c may have a beam angle y of between 30 to 90 degrees. Preferably, the beam angle is 45 to 75 degrees. More preferably, the beam angle is 60 degrees. The beam angle y is defined as the angle between outer edges of the beam as measured across a plane following the respective optical axis 202a, 202b, 202c.

With continued reference to Fig. 3, each optical axis 202a, 202b, and 202c is angularly spaced from each other by an angle z. The optical axis 202a, 202b, 202c may be equally angularly spaced from one another when viewed from the external wall surface, in which case angle z is 120 degrees when there are three optical detectors 105a, 105b, 105c. Alternatively, each optical axis may be angularly spaced from one another by a different amount. Where there are two optical detectors, angle z may be 180 degrees. In an example with only one optical detector (not shown) there is no angle z. As shown in Fig. 3, the optical axis 202a, 202b, 202c intersect a centroid of the alarm device 101 if extended into the alarm device 101. The light emitter of each optical detector 105a, 105b, 105c may not emit light along an optical axis that intersects a centroid of the alarm device 101. Rather, each light emitter may emit light along an optical axis that is offset to an axis that intersects the centroid of the alarm device 101.

It would be appreciated that the areas between and outside of the light beams 301a, 301 b, 301c are blind-spots within which obstacles would not be detected. The size of the blind spots may only permit objects to be placed close to the alarm device 101 that are small enough to not affect operation of the alarm device 101. The blind spots may be larger, for example a region that has a low risk of problematic obstacle placement can be covered by a blind spot.

Each of the angles x and y discussed above may be different for different ones of the optical detectors 105a, 105b, 105c. For example, the optical detector 105a may have an angle x of 30 degrees, whilst the optical detector 105b may have an angle x of 60 degrees. Furthermore, each angle z may be different for different adjacent pairs of optical detectors. For example, the angle z between optical detectors 105a, 105b may be 30 degrees, whilst the angle z between optical detectors 105a and 105c is 60 degrees. The spacing and orientation of the optical detectors 105a-c may be irregular in order that there is a higher or lower amount of sensitivity for obstacles at a particular location proximal the safety alarm device.

Fig. 4 is an alternative viewpoint to that of Fig. 3 where the profile of the light beams 301a, 301b, 301c is viewed from directly below the alarm device 101 and indicative of the field of view provided by the light beams 301a, 301b, 301c as observable from below the alarm device 101. Fig. 5 is yet a further alternative viewpoint to that of Fig. 3 showing a perspective view of the light beams 301a 301b, 301c and indicating the field of view of the light beams caused by angles x, and y discussed above.

With reference to Fig. 6, each optical detector may be configured to provide a grid array output 701 comprising a plurality of grid values each corresponding to a portion of a field of view of the optical detector. In other words each grid value is related to a particular solid angle of the field of view for one optical detector. Each grid value may indicate the presence and/or distance of the object detected within the corresponding portion of the field of view. The grid array output 701 is typically a computational model that is generated by a processor of the controller processing instructions of computer software that is stored on local memory of the controller. This disclosure also relates to software for undertaking functions of the controller as described herein.

The controller may be configured to determine the size of the object based on the number of grid values of the grid array output indicating the presence and/or distance of the object. With reference to Fig. 7 (including reference numerals according to Fig. 4), an object 801 may be detected by the detector providing the light beam 301 b. In this case, all of the grid values along a particular row of the grid array output 701 may indicate the presence and distance of an object, and therefore a size of the object can be determined. The expression “determine the size of the object” does not necessarily mean that the dimensions of the object are measured. For the purposes of detecting obstacles that could interfere with the operation of the safety alarm device 101 , it is often sufficient to estimate whether the size of the object is above or below a particular threshold. For example, if the size of the object is estimated to be below a threshold (e.g., 1 metre x 1 metre), the object can be deemed to be too small to interfere with the operation of the safety alarm device 101. If the size of the object is below the threshold, the object can be disregarded or otherwise treated as if it were not present. The controller can determine that the size of the object is below the threshold if only a few grid values indicate the presence and/or distance of the object. In this case, the controller can avoid performing more complicated processing operations to determine the dimensions, distance, shape and/or position of the object accurately.

Conversely, if the number of grid values indicating the presence and/or distance of the object is above the threshold, this may signify that the object is sufficiently large to interfere with the operation of the safety alarm device 101. The controller may then perform additional processing operations. For example, and without limitation, the controller may determine the dimensions, distance, shape and/or position of the object accurately. Alternatively or additionally, the controller may transmit an alert signal to a remote server. The controller may be configured to determine the size of the object based on the number of contiguous grid values indicating the presence and/or distance of the object. Two grid values are deemed to be contiguous if they are output by adjacent elements of the grid array, where two elements are deemed to be adjacent if they are next to each other in the horizontal, vertical or diagonal direction. If some or all of the grid values indicating the presence and/or distance of the object are non-contiguous with other grid values indicating the presence and/or distance of the object, this may indicate the presence of an error in the grid values (such as a measurement artefact). Thus, non-contiguous grid values may be disregarded when determining the size of the object.

Generation and processing of the grid-array output may be undertaken by the controller and/or a processor local to each optical detector. The processor local to each optical detector may generate the optical array output 701 , which is subsequently provided to the controller for further processing. In examples comprising a plurality of optical detectors, the controller may receive a grid array output 701 from two or more optical detectors. When grid values corresponding to an edge of each field of view of each of the plurality of optical detectors indicate the presence/distance of an object, the controller may be configured to determine the size of an object spanning the field of view of the optical detectors. For example, with reference to Fig. 7, an object 802 spans the field of view provided by beams 301a and 301c. The controller can deduce the existence of the object 802 based on processing of the grid array outputs provided by the detectors corresponding to beams 301a and 301c. In this case, the grid array output 701 corresponding to the beam 301a would have a right-most grid value indicating the presence of an object, and the grid array output 701 corresponding to the beam 301c would have a left-most grid value indicating the presence of the same object.

By identifying an object spanning across the field of view of two optical detectors in this manner, the controller can identify large objects positioned partly within the blind spot between two beams. More specifically, if an object is detected at adjacent edges of two adjacent detectors’ fields of view, the object might be sufficiently large to interfere with the operation of the safety alarm device 101. However, the size of such an object might not be determined properly if only the number of grid values indicating the presence and/or distance of the object were to be taken into account. By identifying whether the object spans across the field of view of two or more optical detectors, a large object can be detected even when the detectors have blind spots. This, in turn, allows a safety alarm device to be implemented with three or fewer optical detectors, even if those detectors have blind spots. An object which is at the edge of only one detector’s field of view (or an object which is entirely within the blind spot between two beams) can be safely disregarded unless its size is above the threshold, since it unlikely to be large enough to interfere with the operation of the safety alarm device 101.

The technique for identifying objects spanning across the field of view of two optical detectors can be combined with the technique for determining the size of the object based on the number of contiguous grid values indicating the presence and/or distance of the object. For example, non-contiguous grid values at the edge of either detector’s field of view may be disregarded when identifying objects spanning across the field of view of two optical detectors, since those values are unlikely to indicate the presence of a large object. To put this another way, only contiguous grid values at the edge of a detector’s field of view may be taken into account when determining whether an object spans across the field of view of two detectors. In this case, two grid values are deemed to be contiguous if either (i) they are output by adjacent elements of the same grid array, or (ii) they are output by adjacent elements of two different, but adjacent, grid arrays. As before, two elements are deemed to be adjacent if they are next to each other in the horizontal, vertical or diagonal direction.

With reference to Figs. 8 and 9, the safety alarm device 101 typically comprises means for detecting the presence of smoke (the direction of smoke is indicated by arrows 901) within a cavity 905 defined by or within the outer casing. Such means may include additional optical sensors (not shown) configured for detecting a scattering of light emitted from a light source (not shown) within the cavity 905. Increased scattering generally indicates in increased smoke level. In use, smoke flows into the alarm device 101 via one or more apertures 906 in the outer casing. The optical detector 105a may be configured to emit light (via the light beam 301a) in the path of the flow of smoke 901 . Therefore, if the beam 301a of the optical detector 105a is completely intersected e.g. by the object 902, then smoke entry to the safety alarm device 101 is determined to be blocked. In the example of Fig. 8, the smoke entry to the safety alarm device 101 is blocked by object 902, which may be tape that is accidentally left attached to the alarm device 101. In this manner the optical detector 105a is utilised to provide coverage detection. Using the principles discussed in relation to Figs. 9 and 10, each optical detector not only provides obstacle detection for detecting the presence/distance of distant obstacles (e.g. as discussed with respect to Figs. 1 to 7) but also provides coverage detection for detecting a direct coverage of any smoke entry apertures. The principles described herein advantageously provide for both obstacle and coverage detection to be provided by a single set of optical detectors, which saves cost, complexity, and improves reliability in contrast to the use of multiple different types of sensor for obstacle and coverage detection.

Further observable in Fig. 8 is a processor 903 that may take the form of a printed circuit board PCB. The processor 903 may be configured to determine the presence and/or distance of an object based on a time of flight measurement of the reflection of the light signal. For each measurement, a plurality of sample measurements of time of flight may be obtained, and an average result taken to provide a final measurement result. The number of samples may be from 5 to 30. Each measurement may be undertaken periodically, such as every 7 days. A presence of an object may only be determined after a certain number of measurements (e.g. four measurements) indicating the presence of an object have been obtained. During each measurement or sample measurement, the light emitter may emit a light signal and the light emitter may await detection of any corresponding reflection of the light signal. When a reflection is detected, the processor 903 determines a time of flight of the light signal, and based on a known speed of light, the distance of the object causing the reflection can be determined. The determined distance can be transmitted to the controller. The processor 903 may be configured to output a grid array output such as the type discussed in relation to Fig. 7. Where there is a plurality of optical detectors, the controller may individually interrogate the processor 903 of each optical detector in turn and use combined distance information from all of the processors 903 to determine properties of an object such as the size of a face of an object that is proximal to the alarm device. The controller may individually interrogate a particular optical detector by holding the other optical detectors in a reset mode. The controller may be configured to emit a proximity warning when the properties of a detected object are such that the performance of the alarm device is impaired. For example, the controller may be programmed to emit the warning when a distance value of less than a predetermined value such as 60 cm or less than 60 cm is detected by one or more of the optical detectors. The controller may only emit a warning when a certain number of plurality of grid values of grid array outputs provided by a certain number of optical detectors return a distance value that is less than the predetermined value. In this manner, the controller can be programmed to only emit the proximity warning when a particular size/location of obstacle is detected.

Yet further observable in Fig. 8 is the intermediate surface 801 which is located between front surface 104 and rear surface 103. The intermediate surface 801 extends around a perimeter of the safety alarm device. Alternatively, the intermediate surface 801 may partially extend around the perimeter and/or be discontinuous around the perimeter.

The intermediate surface 801 may be angled with respect to the rear surface 103 and/or the external wall surface. The angle of the intermediate surface 801 with respect to the rear surface and/or the external wall may define the angle x (see Fig. 2) of the light beam 301a. Alternatively, the intermediate surface 801 may be substantially normal to the rear surface 103 and/or the external wall. The intermediate surface 801 may not be contiguous with the outer casing.

The optical detector 105a (and any other optical detectors) are located on the intermediate surface 801. The apertures 906 may be located in the intermediate surface 801. It will be appreciated that in other examples there is no such intermediate surface 801 and the optical detector 105a may be mounted in a different location such as on a side of the safety alarm device, with the direction of the beam 301a being independent of the mounting of the optical detector 105a.

The proximity warning may be a proximity warning signal and the controller may be configured to transmit the proximity warning signal to a remote server (not shown) via a network such as the internet. In this case, the alarm device 101 also comprises a networking device such as a Wi-Fi™ transceiver. A remote authority can therefore be notified that operation of the alarm device is impaired due to a problematic obstacle and remedial action can be taken.

With reference to Figs. 10 to 12, a cover 1002 may be in front of each optical detector 103a such that emitted and received light travels through the cover. The cover 1002 may be substantially clear and/or transparent. The cover 1002 prevents dust or contaminants from settling on components of the optical detector 103a which would affect performance. With particular reference to Fig. 12, the device may further comprise an opaque barrier 1004 located in between the light emitter providing beam 301a and the receiver having a receiving field of view 1001. Preferably the opaque barrier 1004 has a height extending from the surface of the optical detector 103a to the cover 1002. The opaque barrier 1004 prevents emitted photons, as indicated by arrow 1003, from being reflected from an underside of the cover 1002 and directly into the receiver, which could cause false positives. The use of the cover 1002 and opaque barrier 1004 improves accuracy of obstacle/coverage detection and also improves reliability. Examples of this disclosure may comprise the dimensions visible in Figs. 10 and 11 , although these dimensions are optional.

Either or both of the cover 1002 or the barrier 1004 may be a separate component to the outer casing, or may be integral to the outer casing.

Any light emitter described herein may be a laser or a light emitting diode (LED). In a particularly favourable example, the light emitter is a vertical cavity surface-emitting laser (VCSEL). In a yet more favourable example, the light emitter is a 940 nm VCSEL. The laser may be a British, European, or International Standard class 1 or class 1 M laser. The laser may be configured to emit light having a wavelength that is not visible for humans. For example, the wavelength may be outside the range that is visible to humans of 400 to 700 nm, or preferably 940 nm. It is advantageous for the emitted light to not be visible in order to avoid distraction to people located in the vicinity of the safety alarm device. The use of lasers means that the device is agnostic to obstacle materials that typically exist in domestic environments. In other words, the behaviour of light emitted by lasers does not significantly vary in behaviour depending on the material of an obstacle that the light impinges. Furthermore, the light emitted by lasers is not significantly affected by low ambient light conditions and can detect surfaces comprising glass. Other technologies such as ultrasound detectors may provide different readings depending on the specific material of an obstacle.

In a particularly favourable example, a safety alarm device for attachment to a flat ceiling surface has 3 class 1 940nm VCSELs positioned evenly around the circumference of the device (i.e. facing directions at 120 degree angles to one another). The beam angle for each laser is 60 degrees. The optical axis of each laser is angled away from the ceiling surface by 30 to 60 degrees. The alarm device has a circular shape with a diameter of around 130 cm. These parameters are found to ensure that placement of any object causing impairment to smoke detection within 60 cm of the alarm device is detected. With reference to Fig. 14, this disclosure also relates to a method for determining the presence and/or distance of an object from a safety alarm device having an outer casing comprising a front surface configured for attachment to an external wall surface, and an opposing rear surface. The method comprises the steps of: 1401 , for each of three or fewer optical detectors, emitting, from a light emitter, a light signal along an optical axis extending outwardly from a perimeter region of the outer casing, the perimeter region located between the front and rear surfaces; 1402, receiving, from a light receiver, a reflection of the light signal from the object; and 1403, interrogating, using a controller, each of the three or fewer detectors to determine the presence of the object and the distance of the object from the safety alarm device. The method may include any additional features discussed herein.

It will be understood that the invention is not limited to the examples and embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.