Patent Description:
A photoplethysmogram is an optically obtained plethysmogram as typically used to obtain a volumetric measurement of arterial blood, although other applications are possible. A light source transmits light through the skin of a user into the user's vasculature. The light is reflected by the vasculature and detected by a photo detector.

In reflective photoplethysmography the light transmitted from the light source is reflected into the light detector and in transmissive photoplethysmography light transmitted by the light source passes through the target into the light receiver. The light received at the light detector is a function of the components and relevant volumes of those components in the offset region and their optical properties.

One common use of a photoplethysmography sensor is as a pulse reader or as a pulse oximeter.

<CIT> discloses a photon detection system for detecting bedsore, decubitus, ulcer caused by pressure-induced ischemia in human breast tissue, fat, tissue and comprises position measurement device tracking position of pressure sores. The device comprises a light source and a detector, and specific optics to bend the light beam between the light source and the detector.

<CIT> discloses an apparatus and an optical sensor module for body fat measurement, including at least one light source, a tilter to control an irradiation angle of the light source to radiate light from the light source into biological tissues at a predetermined angle, and an optical detector to detect an optical signal scattered from the biological tissue, and transform the scattered optical signal into an electric signal to acquire accurate body fat information about deep body fat layers.

An apparatus according to the invention is defined by appended claim <NUM>. Preferred embodiments are defined by dependent claims <NUM>-<NUM>. A method according to the invention is defined by claim <NUM>.

For a better understanding of various examples that are useful for understanding the brief description, reference will now be made by way of example only to the accompanying drawings in which:.

In order to make a compact and potentially small scale apparatus <NUM> it is desirable for a light source <NUM> at a light detector <NUM> to be in close proximity. In examples of <FIG> the light source and the light detector are laterally offset <NUM> by a distance d. Previously it has been necessary to have this lateral offset distance d greater than <NUM> in certain applications such as reflective photoplethysmography. In this constraint it is necessary to ensure that the optical path length from the light source <NUM> to the light detector <NUM> is of sufficient length. The examples described below provide a new type of apparatus <NUM> comprising a light source <NUM>, a light detector and optics <NUM>. The optics <NUM> enable the lateral offset between the light source <NUM> and the light detector <NUM> to be less than has previously been possible, that is less than <NUM>.

<FIG> illustrates an apparatus <NUM> comprising: a light detector <NUM>; a light source <NUM>, laterally offset <NUM> from the light detector <NUM> by a first lateral offset d; optics <NUM> configured to receive light emitted by the light source <NUM> and output the received light, wherein a majority of the light output is directed towards an offset region <NUM> laterally offset <NUM> from the light detector <NUM> by at least a second lateral offset D different to the first lateral offset d.

In order to make a compact and potentially small scale apparatus <NUM> it is desirable for the second lateral offset D to be greater than the first lateral offset d.

<FIG> illustrate other examples of the apparatus <NUM> in which different optics <NUM> are used to redirect a majority of the light <NUM> received from the light source <NUM> towards the offset region <NUM> laterally offset <NUM> from the light detector <NUM> by at least a second lateral offset D greater than the first lateral offset d. These figures also illustrate that light reflected from the offset region <NUM> is detected by the light detector <NUM>.

In some, but not necessarily all embodiments, the first lateral offset is less than <NUM> and the second lateral offset is greater than <NUM>. In some embodiments, the second lateral offset is greater than <NUM>.

According to the invention, the optics <NUM> are configured to diffract the received light <NUM>. As illustrated in <FIG>, the optics <NUM> have a first side <NUM> facing towards the light source <NUM> and a second side <NUM> facing towards the offset region <NUM>. The first side <NUM> comprises a light in-coupling region <NUM> and the second side <NUM> comprises a light out-coupling region <NUM>. The light in-coupling region is configured to in-couple the light <NUM> received from the light source <NUM> at a first angle from a normal to the in-coupling region and the light out-coupling region is configured to out-couple the light <NUM> at a second angle from a normal <NUM> to the out-coupling region <NUM>.

In the example illustrated in <FIG>, according to the invention, the second angle is greater than the first angle and the in-coupling region <NUM> and the out-coupling region <NUM> are in close lateral proximity.

In the example illustrated in <FIG>, the light out-coupling region <NUM> is laterally offset <NUM> by a lateral offset X from the light in-coupling region <NUM>. The lateral offset X is the difference between the second lateral offset D and the first lateral offset d in this example. The optics <NUM> provides a laterally extending light guiding region <NUM> between the in-coupling region <NUM> and the out-coupling region <NUM>. This lateral light guiding region <NUM> is configured to guide the light <NUM> by total internal reflection between the in-coupling region <NUM> and the out-coupling region <NUM>. The total internal reflection traps light inside the optics <NUM> between the in-coupling region <NUM> and the out-coupling region <NUM> The optics <NUM> therefore extends laterally for at least the offset X and in some examples for a distance as large as the second offset D away from the light detector <NUM>. As an example, as illustrated in <FIG> the optics <NUM> may form a window <NUM> overlying both the light detector <NUM> and the light source <NUM>.

According to the invention, the in-coupling region <NUM> is provided by a diffractive structure or, not being part of the invention, by a refractive element or elements. According to the invention, the out-coupling region <NUM> is provided by a diffractive structure pattern or, not being part of the invention, by a refractive element or elements. A diffractive structure may for example be a diffractive optical element, a diffraction grating, a periodic structure or pattern or a series of diffraction lines/slits/grooves.

As illustrated in <FIG>, the window <NUM> comprises an internal surface <NUM> (first side <NUM> of the optics <NUM>) and an external surface <NUM> (second side <NUM> of the optics <NUM>). The internal surface <NUM> is an externally reflective surface <NUM> such that external light passing through the window <NUM> to the reflective surface <NUM> is reflected back through the window <NUM> externally. The reflective surface <NUM> may in some examples be a specular reflective surface, for example, a mirrored surface.

The reflective surface <NUM> comprises an aperture <NUM> for the light <NUM> from the light source <NUM> and an aperture <NUM> for the light detector <NUM>. The aperture <NUM> for the light detector <NUM> is generally aligned with the light detector <NUM> (no lateral offset). The aperture <NUM> for the light <NUM> from the light source <NUM> is generally aligned with the out-coupling region <NUM> of the optics <NUM>. In the example of <FIG>, the aperture <NUM> would be aligned with the light source <NUM> and in the example of <FIG>, the aperture <NUM> would be aligned with the out-coupling region <NUM> is laterally offset <NUM> by a lateral offset X from the light in-coupling region <NUM>.

The reflective surface <NUM> extends between the out-coupling region <NUM> and the light detector <NUM> and increases the amount of light from the light source <NUM> that ultimately reaches the light detector <NUM>.

The reflective surface <NUM>, in the illustrated example, extends over the internal surface <NUM> for a distance beyond the second lateral offset D from the light detector <NUM>. The reflective surface is continuous and uninterrupted except for the apertures <NUM>, <NUM>. The reflective surface <NUM> may extend, in all directions, over the internal surface <NUM> for a distance beyond the second lateral offset D from the light detector <NUM> and, in this case, the aperture <NUM> may have an annular shape and surround the aperture <NUM>.

In the above described examples the light source <NUM> may be any suitable source of light <NUM>. For example, it may comprise one or more light emitting diodes. The one or more light emitting diodes may transmit light at the same frequency or at different frequencies. The light detector <NUM> may be any suitable detector of light. For example it may be a photodetector such as a semiconductor photodetector configured, for example, as a photodiode or as a phototransistor.

In the examples of <FIG> and <FIG>, the light output by the light source <NUM> is directed towards an offset region <NUM> laterally offset <NUM> from the light detector <NUM> by at least a second lateral offset D different to the first lateral offset d. The offset region <NUM> may for example comprise an annulus of regions that are at the same azimuthal distance from an axis through the light source <NUM> or light detector <NUM>. The annulus may, for example be symmetric about an axis through the light detector <NUM> and may have a position displaced along that axis from the light detector <NUM>. The light <NUM> output by the light source <NUM> is directed to the whole of the annulus.

<FIG> illustrates a photoplethysmography system <NUM> comprising the apparatus <NUM>. The system <NUM> comprises driving circuitry <NUM> for driving the light source <NUM> of the apparatus <NUM> and detection circuitry <NUM> for receiving an output from the light detector <NUM> of the apparatus <NUM>.

In order to obtain a desired signal from the offset region, it may be desirable to use signal processing to separate the desired signal from undesired signal.

For example, in some but not necessarily all embodiments the offset region <NUM> may comprise an arterial blood supply that pulsates. The pulsating blood has a time varying absorption which causes the light detector <NUM> to produce a signal with a relatively small time varying component. Extraction of the time varying component associated with the pulse by detection circuitry <NUM> provides information specific to the pulsating blood. For example, according to the Beer Lambert law the absorption will depend upon absorptivity of the blood components, concentration of the blood components and a light path length through the arterial blood.

The apparatus <NUM> may be configured to compare absorption at different frequencies of light for the same offset region <NUM> at the same time.

The apparatus <NUM> may be configured to determine the relative concentrations of analytes in the offset region <NUM> as the light path length through the arterial blood for both light detectors will be the same. This analysis may be performed by the apparatus <NUM>, operating as a pulse oximeter using red and infrared light, to determine a concentration of oxyhaemoglobin.

In some but not necessarily all examples, control circuitry <NUM> is connected to both the driving circuitry <NUM> and the detection circuitry <NUM>. The control circuitry <NUM> may for example operate the light source <NUM> and the light detector <NUM> in a coordinated or synchronized manner to reduce noise. For example, the driving circuitry <NUM> may modulate, in time, the amplitude and/or frequency of the light source enabling the separation of a detected light <NUM> arising from the light source <NUM> from that arising from ambient light.

In some but not necessarily all examples, the control circuitry <NUM> may for example operate the light source <NUM> and the light detector <NUM> in a time division duplex fashion such that they are not simultaneously operational but that they operate successfully with a time period that is dependent upon the distance of the offset region <NUM> from the light source <NUM> and the light detector <NUM>. It may for example be desirable for the light detector <NUM> to be switched off when the light source <NUM> is switched on and for the light detector <NUM> to be switched on when the light <NUM> emitted from the light source <NUM> has been reflected by the offset region <NUM> and is at the light detector <NUM> It will be appreciated that the timing of the detection can control the location of a target region within the offset region <NUM> from which signals are sampled.

The light source performs the function of providing light and may be replaced by any suitable lighting means. The light detector performs the function of detecting light and may be replaced by any suitable light detection means. The optics performs the function of laterally offsetting the light from the light source <NUM> to the offset region <NUM> and may be replaced by any suitable light offsetting means.

In this brief description, reference has been made to various examples. The use of the term 'example' or 'for example' or 'may' in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus 'example', 'for example' or 'may' refers to a particular instance in a class of examples. It is therefore implicitly disclosed that a features described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example.

In the foregoing examples, the second lateral offset is greater than the first lateral offset. In other examples, that do not form part of the claimed invention, the second lateral offset may be less than the first lateral offset.

Claim 1:
An apparatus (<NUM>) comprising:
a light detector (<NUM>);
a light source (<NUM>), laterally offset from the light detector by a first lateral offset, d, in a first direction, defined from the light detector to the light source;
diffraction optics (<NUM>) configured to receive light (<NUM>) emitted by the light source and output the received light, wherein the diffraction optics are configured to output the received light by diffracting the received light,
wherein a majority of the light output is directed towards an offset region (<NUM>) laterally offset from the light detector by at least a second lateral offset, D, in the first direction, wherein the second lateral offset, D, is greater than the first lateral offset, and overlaps and extends beyond the first lateral offset,
wherein the diffraction optics (<NUM>) has a first side towards the light source and a second side towards the offset region, wherein the first side comprises a light in-coupling region (<NUM>), the light in-coupling region being provided by a diffractive structure, and the second side comprises a light out coupling region (<NUM>), the light out-coupling region being provided by a diffractive structure, and
wherein the light in-coupling region is configured to in-couple the received light at a first angle from a normal (<NUM>) to the light in-coupling region and the light out-coupling region is configured to out-couple the light at a second angle (θ) from a normal (<NUM>) to the out-coupling region, towards the offset region (<NUM>), and wherein the second angle (θ) is greater than the first angle,
and wherein the light detector is configured to detect the light reflected from the offset region.