Improvements to UV sensing

Abstract

The present disclosure relates to improvements to UV sensing. In one example there is provided a UV sensing system comprising: a UV photodiode which provides a UV signal responsive to excitation by incident radiation wavelengths within a predetermined in- band range of wavelengths, the provided signal being modified by a) changes in temperature of the UV photodiode; and b) the presence of one or more out-of-band wavelengths in the incident radiation, wherein the excitations due to out-of-band wavelengths are not additive, thereby evading identification by traditional monochromatic characterisation techniques as contributing to the spectral response of the UV photodiode; wherein the UV sensing system is configured to mitigate and / or compensate for the modification of the UV signal.

Description

[0001] IMPROVEMENTS TO UV SENSING

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to improvements to UV sensing. More particularly, but not exclusively, it relates to mitigating and / or compensating for errors in UV measurements obtained from a UV photodiode, and a UV sensing system with components to facilitate such mitigation / compensation.

[0004] BACKGROUND OF THE INVENTION

[0005] Sunburn and skin cancers are associated with exposure to UV radiation, but not all UV radiation harms skin to the same extent. UV radiation generally refers to wavelengths shorter than 400 nm, but the most damaging rays are of wavelengths below 320 nm. These rays are generally called UVB radiation.

[0006] These rays' relative ability to damage skin can be expressed as a function of wavelength, and this function is called the erythemal action spectrum, or EAS (erythema means reddening of the skin). The EAS is a normalized function, with a value of 1 at wavelengths below 298 nm. These rays cause the most damage. At longer wavelengths, the ability to damage skin drops off very quickly (approximately -90% per 10 nm). A ray at 320 nm is only 1% as damaging as an equally bright ray at 298 nm.

[0007] Solar radiation spans the UV, visible, and infrared parts of the spectrum. Solar wavelengths that cause the most damage are attenuated by atmospheric absorption, particularly by ozone, which blocks practically all sunlight with wavelengths below 298 nm.

[0008] The spectral distribution of wavelengths that have the potential to cause skin damage is found by multiplying the spectrum of incident solar irradiance times the erythemal action spectrum (EAS). There is almost no erythemal UV radiation at wavelengths below 298 nm, because there is very little incident radiation due to atmospheric absorption. Erythemal UV radiation can become significant between 298 and 320 nm. It tapers off at wavelengths above 320 nm, because of the steep drop-off of the EAS. The total potential for skin damage is equal to the integral of erythemal UV radiation across all wavelengths. This integrated irradiance is commonly referred to as UVEry, UVBEry, or UVREry.

[0009] Measuring UVEry is essential in UV dosimetry, which is the study of a person's exposure to UV radiation over time. It is also a common radiation measurement in weather stations used for meteorology and public safety. UVEry is the measurement that determines the UV index, which is defined as erythemal UV radiation (in W / m2) times 40. For example, when incident UVEry is 0.25 W / m2, the UV index is 10. Variations in UVEry, and hence the UV index, are caused by many factors including the solar path length through the atmosphere, atmospheric ozone concentration, aerosol optical depth, albedo of the surrounding landscape, and solar distance from Earth. Total mid-day solar radiation can be 1030 W / m2, 4120 times more than the peak UVEry measurement. High quality measurement of UVEry is thus very difficult, as it requires excluding 99.97% of solar radiation from the measurement, and the remaining 0.03% must be measured with enough resolution to quantify small variations of UVEry over time.

[0010] Measurement of UVEry has traditionally been done with a temperature-stabilised pyranometer or photomultiplier, shielded by one or more optical filters that are designed to attenuate longer wavelengths of light in a manner that simulates the EAS. This method is relatively expensive to build, and requires a significant amount of power to keep the temperature stable.

[0011] Gallium Nitride, or GaN, is a semiconductor that responds to UV wavelengths shorter than 360 nm. This is a spectral cutoff, above which the photodiode loses more than 90% of its sensitivity. A cutoff wavelength of 360 nm is unsuitable to use for UVEry measurements, but it is used for general UV measurements (e.g. sensors marketed as UVA sensors, or UV sensors in consumer electronics).

[0012] Aluminium Gallium Nitride, or AIGaN is a semiconductor made from Gallium Nitride doped with Aluminium. In this material, the ratio of Aluminium to Gallium determines the cutoff wavelength. A ratio of 1:3 shifts the cutoff wavelength to approximately 310 nm, which is more suitable for measurements of UVEry. AIGaN photodiodes are much cheaper than optical filters and temperature-controlled pyranometers, and UV dosimeters using AIGaN photodiodes have been available on the market for at least 15 years.

[0013] Although AIGaN photodiodes are low-cost and low-power, they suffer from low accuracy. This has generally rendered them insufficient for scientific applications, for which the more complex and expensive instruments are still dominantly used. It is thus desirable to improve the accuracy of AIGaN photodiodes (or photodiodes using other semiconductors with similar properties), to facilitate the provision of UV sensing systems which are cheap, compact, and accurate enough to be suitable for scientific applications.

[0014] It is an object of the present invention to provide a UV sensing system and method which overcomes or at least partially ameliorates some of the abovementioned disadvantages or which at least provides the public with a useful choice.

[0015] BRIEF DESCRIPTION OF THE INVENTION

[0016] According to a first aspect the invention broadly comprises a UV sensing system comprising: a UV photodiode which provides a UV signal in proportion to in-band irradiance, attenuated by the presence of out-of-band irradiance; wherein the UV sensing system is configured to mitigate and / or compensate for out-of-band irradiance of the UV photodiode.

[0017] The in-band range of UV wavelengths corresponds to a range of wavelengths that cause a response in the UV photodiode, generating a positive electrical signal in proportion to the in-band UV irradiance. The term out-of-band in relation to the UV photodiode means wavelengths not in the range corresponding to in-band. In some types of photodiodes, out- of-band irradiance causes attenuation of the signal generated in response to in-band irradiance. In some aspects, the UV sensing system is configured to reduce or compensate for this attenuation. In some examples this may be implemented in the optical domain, using an optical filter that reduces or eliminates those out-of-band wavelengths which cause attenuation of the positive signal generated by in-band wavelengths. In some examples, the compensation may be implemented in the electrical domain, by measuring the out-of-band irradiance with a secondary light sensor, and modifying the signal generated by the UV photodiode by applying a correction function based on the measurement of out-of-band wavelengths, to the effect of reversing some or all of the attenuation of the UV photodiode signal generated by in-band wavelengths.

[0018] In some examples, the UV sensing system may be a UV dosimeter. According to another aspect the invention broadly comprises a UV sensing system comprising: a UV photodiode which provides a UV signal responsive to excitation by incident radiation wavelengths within a predetermined in-band range of wavelengths, the provided signal being modified by the presence of one or more out-of-band wavelengths in the incident radiation; wherein the excitations due to out-of-band wavelengths are not additive, thereby evading identification by traditional monochromatic characterisation techniques as contributing to the in-band spectral response of the UV photodiode; wherein the UV sensing system is configured to mitigate and / or compensate for the out-of-bound wavelengths and their modification of the UV signal.

[0019] According to another aspect, the UV sensing system further comprises an optical filter positioned in front of the UV photodiode and configured to pass UV light corresponding to the in-band range of wavelengths and to block light corresponding to one or more out-of-band wavelengths, so as to reduce or prevent out-of-band excitation of the UV photodiode.

[0020] According to another aspect, the optical filter functions as a shortpass filter with a cutoff wavelength above 320 nm, excluding light of longer wavelengths; a bandpass filter having an upper cutoff wavelength above 320 nm; or a bandstop filter having a lower cutoff wavelength above 320 nm.

[0021] According to another aspect, the UV sensing system further comprises one or more secondary light sensors which provide a signal corresponding to one or more out-of-band wavelengths of radiation incident on the UV photodiode, wherein the UV sensing system compensates for out-of-band excitation by applying an out-of-band correction function to the UV signal based on the secondary light signal.

[0022] According to another aspect, a secondary light sensor is a broadband light sensor, responsive to a range of visible and / or infrared wavelengths.

[0023] According to another aspect, a secondary light sensor is configured to measure light in a wavelength range of approximately 400-1000 nm.

[0024] According to another aspect, the out-of-band correction function is nonlinear.

[0025] According to another aspect, the out-of-band correction function comprises a plurality of coefficients which are functions of the secondary light signal.

[0026] According to another aspect, the UV sensing system further comprises a temperature sensor which provides a temperature signal, wherein the UV dosimeter compensates for cutoff shift error by applying a cutoff correction function to the UV signal, the cutoff correction function comprising: a spectral response function of the UV photodiode, measured at a known reference temperature by a scanning monochromator, indicating the cutoff wavelength above which the photodiode becomes effectively blind; a cutoff shift function which shifts the cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function.

[0027] According to another aspect, the UV sensing system compensates for dark current error by applying a dark current correction function to the UV signal based on the temperature signal.

[0028] According to another aspect, the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal. According to another aspect, the UV photodiode is a GaN or AIGaN or SiC photodiode.

[0029] According to another aspect, the UV sensing system further comprises one or more processors, and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: obtaining measurements from the UV photodiode; and correcting the UV measurements by applying one or more correction functions.

[0030] According to another aspect, the UV sensing system further comprises a strap or clip to facilitate use as a wearable device.

[0031] According to another aspect, the UV sensing system further comprises a wireless communication subsystem for wireless transmission of sensor data to another device.

[0032] According to another aspect, the UV sensing system further comprises at least one output device for indicating information to a user about measured UV.

[0033] According to another aspect, the invention broadly comprises a method of UV dosimetry comprising: obtaining measurements from a UV dosimeter, the measurements comprising a UV signal from a UV photodiode; mitigating or compensating for out-of-band excitation of the UV photodiode; and indicating and / or recording a resulting UV reading.

[0034] According to another aspect, the invention broadly comprises a method of improving the accuracy of a UV photodiode. The method comprises obtaining measurements from the UV photodiode which provides a UV signal responsive to excitation by wavelengths of incident radiation within a predetermined in-band range of wavelengths, the provided UV signal being modified by the presence of one or more out-of-band wavelengths in the incident radiation; mitigating or compensating for the modification of the UV signal; and indicating and / or recording a resulting UV reading. According to another aspect, out-of-band excitation is mitigated by attenuating out-of-band light before it reaches the UV photodiode.

[0035] According to another aspect, mitigating or compensating is performed by filtering the incident radiation to pass UV light corresponding to the in-band range of wavelengths and to block light corresponding to one or more out-of-band wavelengths.

[0036] According to another aspect, the measurements further comprise a secondary light signal from a secondary light sensor responsive to out-of-band wavelengths, and wherein out-of-band excitation is compensated for by an out-of-band correction function based on the secondary light signal.

[0037] According to another aspect, the out-of-band correction function is nonlinear.

[0038] According to another aspect, the out-of-band correction function comprises a plurality of coefficients which are functions of the secondary light signal.

[0039] According to another aspect, the measurements further comprise a temperature signal from a temperature sensor; and wherein the method further comprises correcting the UV signal by a cutoff correction function comprising: a spectral response function for the UV photodiode including a known cutoff wavelength measured at a reference temperature; a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function or approximation.

[0040] According to another aspect, the solar irradiance spectrum function is based on a radiative transfer model.

[0041] According to another aspect, the solar irradiance spectrum function utilises time, date, and geographical location. According to another aspect, the solar irradiance spectrum function utilises atmospheric path length and / or solar zenith angle.

[0042] According to another aspect, the method further comprises correcting the UV signal by a dark current correction function which compensates for dark current based on the temperature signal.

[0043] According to another aspect, the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal.

[0044] According to another aspect, the UV photodiode is a GaN, AIGaN, or SiC photodiode.

[0045] According to another aspect, the method further comprises transmitting the measurements and / or the resulting UV reading from the UV dosimeter to an external device.

[0046] According to another aspect, the invention broadly comprises a non-transitory computer-readable storage medium comprising instructions which, when executed by one or more processors, cause the one or more processors to perform operations comprising: receiving measurements obtained from a UV dosimeter, the measurements comprising a UV signal from a UV photodiode and a secondary light signal from a secondary light sensor responsive to out-of-band radiation; and correcting the UV signal by an out-of-band correction function which compensates for out-of-band excitation of the UV photodiode based on the secondary light signal.

[0047] According to another aspect, the out-of-band correction function is nonlinear.

[0048] According to another aspect, the out-of-band correction function comprises a plurality of coefficients which are functions of the secondary light signal.

[0049] According to another aspect, the measurements received further comprise a temperature signal from a temperature sensor, and the operations further comprise correcting the UV signal by a cutoff correction function comprising: a spectral response function for the UV photodiode including a known cutoff wavelength at a reference temperature; a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function or approximation.

[0050] According to another aspect, the operations further comprise correcting the UV signal by a dark current correction function which compensates for dark current based on the temperature signal.

[0051] According to another aspect, the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal.

[0052] According to another aspect, the invention broadly comprises a UV sensing system comprising: a UV photodiode which provides a UV signal; and a temperature sensor which provides a temperature signal; wherein the UV sensing system is configured to compensate for cutoff shift error by applying a cutoff correction function to the UV signal, the cutoff correction function comprising: a spectral response function for the UV photodiode including a known cutoff wavelength measured at a reference temperature; a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function or approximation.

[0053] According to another aspect, the system further comprises one or more processors, and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: obtaining measurements from the UV sensing system; and correcting the UV signal by applying the cutoff correction function.

[0054] According to another aspect, the invention broadly comprises a UV sensing system comprising: a UV photodiode which provides a UV signal; and a temperature sensor which provides a temperature signal; wherein the UV sensing system is configured to compensate for dark current error by applying a dark current correction function to the UV signal based on the temperature signal.

[0055] According to another aspect, the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal.

[0056] According to another aspect, the system further comprises one or more processors, and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: obtaining measurements from the UV sensing system; and correcting the UV signal by applying the dark current correction function.

[0057] According to some aspects, there is provided a UV sensing system comprising: a UV photodiode which provides a UV signal responsive to excitation by incident radiation wavelengths within a predetermined in-band range of wavelengths; a compensation mechanism configured to reduce or eliminate attenuation of the UV signal caused by exposure of the UV photodiode to predetermined out-of- band wavelengths.

[0058] In some examples, the compensation mechanism is an optical filter configured to attenuate the predetermined out-of-band wavelengths. In some examples, the compensation mechanism comprises applying a correction function to the UV signal.

[0059] Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings.

[0060] As used herein the term "and / or" means "and" or "or", or both.

[0061] As used herein "(s)" following a noun means the plural and / or singular forms of the noun.

[0062] The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in the same manner.

[0063] In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.

[0064] For the purpose of this specification, where method steps are described in sequence, the sequence does not necessarily mean that the steps are to be chronologically ordered in that sequence, unless there is no other logical manner of interpreting the sequence.

[0065] BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The invention will now be described by way of example only and with reference to the drawings in which:

[0067] Figure 1 shows a block diagram of the electronics of a UV dosimeter;

[0068] Figure 2 shows a perspective view of the UV dosimeter configured as a wearable device;

[0069] Figure 3A shows a flow chart of a method of UV sensing, including applying successive corrections to a UV signal;

[0070] Figure 3B shows a flow chart of a modified method of UV sensing involving filtering out visible light;

[0071] Figure 3C shows a simplified diagram of a UV photodiode with a visible light filter;

[0072] Figure 4 shows a graph of dark current in UV photodiodes as a function of temperature;

[0073] Figure 5A shows a graph of out-of-band error for a UV photodiode at constant level of UV and varying intensities of several wavelengths above 400 nm; Figure 5B shows a graph of out-of-band error for a UV photodiode at varying intensities of out-of-band light for several different levels of constant UV;

[0074] Figure 5C shows a graph comparing uncorrected UV readings and readings corrected for out-of-band error at six different levels of UV intensity;

[0075] Figure 5D shows a graph comparing UV readings with and without filtering of visible light at constant UV intensity;

[0076] Figure 6A shows a graph showing a cutoff shift correction function based on temperature; and

[0077] Figure 6B shows a graph showing a spectral solar irradiance function, a typical spectral response function of a UV photodiode, and the change in photodiode signal when the cutoff wavelength is shifted by a temperature change.

[0078] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0079] According to various aspects of the various embodiments of the present invention as illustrated in figures 1-6B, there is provided a UV dosimeter 100 which will now be described. Whilst in these embodiments the invention is described with respect to a UV dosimeter, the invention is not so limited and could be applied to any suitable UV sensing system or method. Examples may include a smartwatch, a home weather station or a public UV index or exposure service such as a display billboard installed at a beach or park. Such systems or methods may be employed to correct or compensate for errors in some photodiode measurements or output signals caused by previously unknown and unexpected effects which the inventor has discovered.

[0080] For the purposes of this specification, the term in-band in relation to a UV photodiode means a range of wavelengths to which the UV photodiode generates a positive electrical signal in proportion to incident UV irradiance. The term out-of-band in relation to the UV photodiode means wavelengths not in the range corresponding to in-band.

[0081] As shown in figure 1, the UV dosimeter 100 comprises a UV photodiode 102, being the primary sensor involved in UV dosimetry. The UV photodiode 102 is effectively a UV photodiode with a spectral response suitable for measurement of UVEry, and hence it preferably has a cutoff wavelength of close to 315 nm such that it is substantially insensitive to radiation that is out-of-band (i.e. not of interest as it is above 315 nm). The UV photodiode 102 may be any such photodiode which exhibits one or more of the errors described herein, so as to benefit from the corresponding improvements. Preferable examples include suitably configured GaN, AIGaN, and SiC photodiodes, due to their low cost.

[0082] The UV dosimeter 100 may further comprise various ancillary or auxiliary sensors, some of which may be utilised in correcting measurements from the UV photodiode 102 as will subsequently be described. The ancillary or auxiliary sensors may include a temperature sensor 104, a secondary light sensor 106, a UVA sensor 108, an accelerometer 110, and a proximity sensor 112. The secondary light sensor 106 may comprise a visible light sensor or a light sensor with a sensitivity range extending over some or all of the visible light spectrum and into the infrared (IP) spectrum. Some sensors may not contribute directly to the dosimetry functions of the UV dosimeter 100, and may instead serve other functions.

[0083] The UV dosimeter 100 preferably further comprises at least one storage medium 114 (i.e. non-transitory memory) operably connected to one or more processors 116. The storage medium 114 stores instructions that can be executed by the one or more processors 116 to carry out UV dosimetry functions. This involves obtaining measurements from the sensors, and potentially also performing some level of processing of the data. It may also store various settings, sensor calibrations and the like. The at least one storage medium 114 may also provide for storage of collected sensor data for future processing or transmission.

[0084] Although for each above sensor a single sensor may be sufficient to perform the functions described herein, additional sensors of any particular type may also be provided. Additional sensors may facilitate averaging of multiple readings of the same quantity, providing redundancy in case of component failure, measuring multiple spectral components of a desired spectral response, or measuring values at different locations on the UV dosimeter 100.

[0085] The UV dosimeter 100 preferably further comprises at least one wireless communications subsystem 106, for example a WiFi, Bluetooth / Bluetooth Low Energy, or radio module. This facilitates transmission of sensor data to an external device 120, for example a smartphone or a PC, which may occur in real time as the measurements are collected or at a later time by transmitting data stored in persistent memory. The wireless communications subsystem 106 may also facilitate communication from the external device 120 to the UV dosimeter 100, for example to adjust settings, update firmware, manage data or the like. A dedicated software application or other user-interface may be provided on the external device 120 for purposes of interacting with the UV dosimeter 100.

[0086] The UV dosimeter 100 may also or alternatively be provided with a suitable interface for a wired connection to the external device 120, for example a standard USB port, facilitating wired transmission of sensor data or other interactions as described above. This interface may also act as a charging port for rechargeable batteries of the UV dosimeter 100, although charging could additionally or alternatively be enabled by a separate port and / or a wireless power transfer receiver. The UV dosimeter 100 may also be provided with removable batteries instead of or in addition to providing a charging interface.

[0087] The external device 120 may forward data received from the UV dosimeter 100 to a backend server 122 for processing and / or storage. However, if provided with a suitable wireless communications subsystem 106, the UV dosimeter 100 may communicate with the backend server 122 directly. Corrections to the UV measurements may be processed on the UV dosimeter 100, on the external device 120, on the backend server 122, or be split between some combination thereof. The corrections may be performed in real time as the measurements are collected, or during post-processing of stored data.

[0088] The UV dosimeter 100 may further comprise one or more output devices 124, for example LED indicators, a display, a speaker, or a vibration motor. The output devices 124 may be suitable for providing some indication of UV exposure to a user. For example, current or cumulative UV exposure may be indicated by LED brightness level or flashing frequency, by displaying values on a display, by playing a sound effect, or by vibration. However, such indications / notifications may additionally or alternatively be provided through the external device 120 via the wireless communications subsystem 106 or via wired UART connection. Output devices 124 may also be used to provide status information about the UV dosimeter 100, or to provide indications of other sensor readings. The accelerometer 110, if provided, may be used as simple input device through the use of a tap detection algorithm. Taps on the UV dosimeter 100 may thereby trigger various functions, for example providing indications via the output devices 124, toggling between different modes of the UV dosimeter 100, or managing connection with the external device 120. However, other input devices such as buttons, touch screens or the like could alternatively be used to trigger such functions. The accelerometer 110 may also be used to provide data about physical activity of the user. It may be provided as part of an inertial measurement unit which collects data related to acceleration, orientation and the like in multiple axes.

[0089] The proximity sensor 112, if provided, may provide an indication of whether the UV dosimeter 100 is being worn. This may provide quality assurance for the UV dosimetry data. The UV dosimeter 100 may be configured to only record increases in cumulative dose while being worn, and / or to reset the cumulative dose if the UV dosimeter 100 is removed for a sufficient period.

[0090] As shown in figure 2, the UV dosimeter 100 is preferably configured as a wearable device by provision of a strap 200 connected to a housing 202 to allow the UV dosimeter 100 to be worn on the wrist or arm. The housing 202 is thus preferably no larger than 50 mm in diameter. However, the UV dosimeter 100 could alternatively be configured to be worn in some other way without the use of a strap 200, or configured as a laboratory instrument for which larger dimensions may be suitable.

[0091] Where one of the output devices 124 is a display, the UV dosimeter 100 may be a smartwatch that performs various other functions besides UV dosimetry. Such functions may include any of the typical functions of smartwatches (facilitated by ancillary or auxiliary sensors or other peripherals where needed), for example measuring heart rate, counting steps, providing GPS navigation, managing media playback, displaying notifications, installing apps, and the like.

[0092] As shown in figures 3A-3C, the method of UV dosimetry associated with the UV dosimeter 100 involves accounting for various sources of error associated with UV photodiodes 102 which are herein described. Accounting for these sources of error may involve correction of the UV signal based on ancillary or auxiliary sensor data, and / or mitigation / prevention e.g. by the use of certain filters.

[0093] In a measurement step 300 of the method as shown in figure 3A, a UV signal is obtained from the UV photodiode 102, and auxiliary sensor signals (e.g. temperature, visible light, and UVA) are also obtained. Such signals are obtained from operation of the UV dosimeter 100, but may be transmitted to the external device 120 or to the backend server 122 before some or all of the subsequent steps are performed.

[0094] In a dark current correction step 301 of the method, dark current error in the UV signal is corrected using a temperature signal from the temperature sensor 104. 'Dark current' is a background signal from the UV photodiode 102 which is present at all times. Increased temperature of the UV photodiode 102 reduces the energy required for electrons and holes to jump across the semiconducting barrier, thereby increasing the baseline rate at which these jumps occur, causing the dark current to increase. Experiments performed by the applicant have indicated that temperature-induced variations in dark current produce significant errors in commonly used UV photodiodes.

[0095] In an out-of-band correction step 302 of the method, out-of-band error is corrected through the use of a secondary light signal from the secondary light sensor 106. Experiments performed by the applicant have indicated a previously unrecognised error which arises with commonly used UV photodiodes, in which additional light (out-of-band) outside of the usual spectral response band (in-band) of the UV photodiode 102 produces an unexpected decrease in a UV signal from the photodiode.

[0096] Without wishing to be bound by theory, it is thought that this unintended out-of- band photoresponse is caused by parasitic Schottky in the 'ohmic' region of the semiconductor. The effect is not observed when characterising the spectral response of a UV photodiode through the standard laboratory method of repeatedly applying a calibrated monochromatic beam of light to the sensor until all wavelengths have been scanned. It is only when out-of-band wavelengths are applied simultaneously to in-band wavelengths that the parasitic Schottky effect occurs, and the spectral response ceases to be additive as would be expected. The out-of-band wavelengths have thus evaded identification as contributors to the overall photoresponse of the sensor.

[0097] The error caused by out-of-band wavelengths is significant with the addition of visible light (wavelengths between about 400 to 700 nm), and infrared light (wavelengths above 700 nm). Hence, to facilitate this correction the secondary light sensor 106 is preferably a broadband light sensor with sensitivity that extends across the visible band and at least somewhat into the infrared, e.g. 400-1000 nm. In some instances, the broadband range of wavelengths may be sampled by multiple secondary sensors each measuring various wavelengths, and a combination of the multiple signals may be used in lieu of a single broadband light sensor. The terms 'secondary light sensor', 'visible light sensor', 'visible light signal' and the like, as used throughout the specification and appended claims, are therefore to be interpreted broadly.

[0098] In a cutoff shift correction step 303 of the method, cutoff shift error is corrected through the use of a temperature signal from the temperature sensor 104. Experiments performed by the applicant have indicated a previously unrecognised error which arises with commonly used UV photodiodes, in which the normal cutoff wavelength (e.g. around 310 nm for AIGaN photodiodes typically used to measure UVEry) shifts with temperature. In practice, an increase in temperature causes increased sensitivity to unwanted wavelengths adjacent to the cutoff wavelength. The cutoff shift correction step 303 reduces this error by modelling the wavelengths in the vicinity of the cutoff wavelength, and using the temperature signal to determine how much of the UV signal is due to the unwanted wavelengths.

[0099] In an indication step 304 of the method, corrected readings (cumulative and / or instantaneous) are somehow indicated to a user by the UV dosimeter 100 (via output devices 124) or by the external device 120. This may involve producing a notification when measured UV exceeds a certain threshold, continuously indicating current and / or cumulative readings, or displaying a summary of UV data collected over some time period. Where some or all of the UV correction steps are performed on the backend server 122, this may involve the corrected result being transmitted back to the external device 120 or to the UV dosimeter 100. The indication may be produced in real time via continuous processing of measurements, or the measurements may be recorded / stored on the UV dosimeter 100, the external device 120, and / or the backend server 122 for later processing / review.

[0100] It will be appreciated that the correction steps could be applied in any order, and some may be more significant than others - however, applying any of the correction steps alone will provide some improvement to the quality of the UV signal. In some examples, only one or two of the correction steps may be performed.

[0101] As shown in figure 3B, in an alternative embodiment of the method, the out-of- band correction step 302 may be replaced by an optical filtering step 305 of optically filtering out out-of-band light before the UV signal is obtained i.e. before light reaches the UV photodiode 102. This passive alternative for mitigating out-of-band error requires no processing of additional sensor data, which may be preferable over the use of a visible light sensor 106. However, the two methods may also be combined such that any out-of-band radiation not filtered out is subsequently corrected for by applying the out-of-band correction step 302 (using the visible light sensor 106). The filter may be configured to allow transmission of in-band wavelengths, and reduce some or all out-of-band wavelengths. This helps to ensure that the positive signal produced by the incidence of in-band radiation on the photodiode is preserved, whilst the subtractive or negative signal produced by incidence of any out-of-band radiation is reduced, in order to enhance the accuracy of the signal output from the photodiode.

[0102] As shown in figure 3C, the UV dosimeter may achieve such filtering with an optical filter 306 positioned in front of the UV photodiode 102 and configured to pass UVB light and block visible light. The optical filter 306 is preferably a shortpass filter with a cutoff somewhere between 320 to 400 nm, which will also block infrared to some extent. However, it will be appreciated that another type of filter, such as a bandstop, may also be suitable provided it sufficiently blocks at least part of the visible spectrum. If the optical filter 306 is also used to shield a UVA sensor 108, the cutoff wavelength is preferably high enough to avoid blocking UVA. The optical filter 306 may be provided instead of the visible light sensor 106, or in combination with it as previously described. As shown in figure 4, a dark current correction function 400 (being a function of temperature) can be used in the dark current correction step 301. This function compensates for dark current based on the temperature signal from the temperature sensor 104, and is obtainable for a particular UV photodiode 102 via a measurement of the baseline photodiode output across a range of temperatures. Experiments performed by the applicant indicate that a function comprising an exponential term and a constant term suitably approximates the temperature response for a typical UV photodiode 102.

[0103] As shown in figure 5A, experiments performed by the applicant indicate that increasing intensity of monochromatic wavelengths throughout the visible spectrum (400- 700 nm) causes steep drop-offs in the output of the UV photodiode 102 when it is also excited by a constant level of UVB. The drop-offs follow characteristic curves of slightly different shapes at different visible wavelengths. Given the similarity of the curves, a broadband visible light sensor 106 is suitable for developing and applying an out-of-band correction function, as its spectral response can account for all visible wavelengths in aggregate. The range of the broadband visible light sensor 106 may extend at least slightly into the infrared range, for example it may be sensitive from 400-1000 nm.

[0104] As shown in figure 5B, further experiments performed by the applicant indicate that increasing intensity of solar irradiance (stripped of its UV component), as measured by a visible light sensor 106, likewise causes drop-offs in the output of the UV photodiode 102 at a constant level of UVB. Repetition at different fixed levels of UVB produces a family of characteristic nonlinear curves. A suitable out-of-band correction function (being a function of visible light intensity) may be obtained by measuring the output of the photodiode 102 while applying a fixed visible light input and a varying UVB input. This may be repeated at different visible light levels to produce a family of curves, and then the variation in the coefficients of curvature between the individual curves can be characterised by secondary functions. This effectively combines the family of curves into a single out-of-band correction function. As shown in figure 5C, further experiments performed by the applicant indicate that applying such an out-of-band correction function significantly improves the quality of the UV signal at varying levels of UVB and broad-spectrum visible light (i.e. white light). Filtering out visible light entirely may provide similar or improved results, as shown in Figure 5D, but filters can be imperfect and some out-of-band error may remain. The largest improvement may therefore be achieved by the combination of filtering and active correction.

[0105] As shown in figures 6A and 6B, for any solar spectrum (calculated or approximated by geolocation, atmospheric conditions, etc.), a cutoff correction function 600 (being a function of temperature) can be used in the cutoff shift correction step 303. The cutoff shift correction function 600 may comprise the standard spectral response function of the UV photodiode 102 at a known reference temperature, a solar spectral irradiance function 602 (or an approximation thereof) as shown in figure 6B, and a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature. Figure 6B illustrates the impact of cutoff shift on the photodiode output when subject to solar irradiance, which is compensated for by the cutoff shift correction function 600.

[0106] The cutoff shift function may utilise a constant rate of spectral shift with temperature i.e. it may be a linear function. However, an alternative model could also be used. In any case, the combined cutoff correction function 600 will generally be nonlinear due to the nonlinear shapes of the solar spectrum and the photodiode spectral response. The cutoff correction function can be stored as instructions on the storage medium 114, in the external device 120, or on the backend server 122.

[0107] It will be appreciated that various alternative models other than those examples described above may provide comparable improvements to the quality of the UV signal. Different UV photodiodes 102 may also exhibit different responses, such that it is preferable to obtain appropriate data in order to fit each function for that photodiode. However, one set of functions may generalise sufficiently well to all photodiodes of a particular type. It will be appreciated that any UV photodiode can be characterised using the principles described above, and will benefit from the improvements of the present invention should it exhibit errors as described - thus, the present invention may be applicable to photodiodes other than GaN, AIGaN and SiC photodiodes which were the primary basis of the experimental results discussed.

[0108] The functions described above may be stored as instructions on the storage medium 114, on the external device 120, or on the backend server 122, such that the corrections to the UV signal are implemented in software. However, at least some of the functions could alternatively be implemented in hardware by providing dedicated electronic circuits on the UV dosimeter 100 (e.g. appropriately configured arithmetic circuits). The correction steps may also be implemented by some combination of software instructions and dedicated electronics.

[0109] To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims.

[0110] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Claims

CLAIMS1. A UV sensing system comprising: a UV photodiode which provides a UV signal responsive to excitation by incident radiation wavelengths within a predetermined in-band range of wavelengths, the provided signal being modified by the presence of one or more out-of-band wavelengths in the incident radiation; wherein the UV sensing system is configured to mitigate and / or compensate for the modification of the UV signal.

2. The UV sensing system of claim 1, further comprising an optical filter positioned in front of the UV photodiode and configured to pass UV light corresponding to the in-band range of wavelengths and to block light corresponding to one or more out-of-band wavelengths, so as to reduce or prevent out-of-band excitation of the UV photodiode.

3. The UV sensing system of claim 2, wherein the optical filter comprises one or more of: a shortpass filter with a cutoff wavelength above 320 nm; a bandpass filter having an upper cutoff wavelength above 320 nm; a bandstop filter having a lower cutoff wavelength above 320 nm.

4. The UV sensing system of any one of the preceding claims, further comprising a secondary light sensor which provides a signal corresponding to one or more out-of-band wavelengths of radiation incident on the UV photodiode, wherein the UV sensing system compensates for out-of-band excitation by applying an out-of-band correction function to the UV signal based on the signal from the secondary light sensor.

5. The UV dosimeter of claim 4, wherein the secondary light sensor is a broadband light sensor responsive to a range of visible and / or infrared wavelengths.

6. The UV sensing system of claim 5, wherein the secondary light sensor is configured to measure light in a wavelength range of approximately 400-1000 nm.

7. The UV sensing system of any one of claims 4 to 6, wherein the out-of-band correction function is nonlinear.

8. The UV sensing system of claim 7, wherein the out-of-band correction function comprises a plurality of coefficients which are functions of the secondary light signal.

9. The UV sensing system of any one of the preceding claims, further comprising a temperature sensor which provides a temperature signal, wherein the UV sensing system modifies the UV signal responsive to the temperature signal.

10. The UV sensing system of claim 9, configured to compensate for cutoff shift error by applying a cutoff correction function to the UV signal, the cutoff correction function comprising: a spectral response function for the UV photodiode including a known cutoff wavelength measured at a reference temperature; a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function.

11. The UV sensing system of claim 9 or 10, wherein the UV sensing system compensates for dark current error by applying a dark current correction function to the UV signal based on the temperature signal.

12. The UV sensing system of claim 11, wherein the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal.

13. The UV sensing system of any one of the preceding claims, wherein the UV photodiode is a GaN photodiode, an AIGaN photodiode, or a SiC photodiode.

14. The UV sensing system of any one of the preceding claims, further comprising one or more processors, and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: obtaining measurements from the UV photodiode; and correcting the measurements by applying one or more correction functions.

15. The UV sensing system of any one of the preceding claims, comprising a UV dosimeter or smartwatch and further comprising a strap or clip to facilitate use as a wearable device.

16. The UV sensing system of any one of the preceding claims, further comprising a wireless communication subsystem for wireless transmission of sensor data to another device.

17. The UV sensing system of any one of the preceding claims, further comprising at least one output device for indicating information to a user about measured UV.

18. A method of improving the accuracy of a UV photodiode, the method comprising: obtaining measurements from the UV photodiode which provides a UV signal responsive to excitation by wavelengths of incident radiation within a predetermined in-band range of wavelengths, the provided signal being modified by the presence of one or more out-of-band wavelengths in the incident radiation; mitigating or compensating for the modification of the UV signal; and indicating and / or recording a resulting UV reading.

19. The method of claim 18, wherein mitigating or compensating is performed by filtering the incident radiation to pass UV light corresponding to the in-band range of wavelengths and to block light corresponding to one or more out-of-band wavelengths.

20. The method of claim 18 or 19, wherein the measurements further comprise a signal from a secondary light sensor which provides a signal corresponding to one or more out-of- band wavelengths of radiation incident on the UV photodiode, and wherein the modification of the UV signal is compensated for by an out-of-band correction function based on the signal from the secondary light sensor.

21. The method of claim 20, wherein the out-of-band correction function is nonlinear.

22. The method of claim 21, wherein the out-of-band correction function comprises a plurality of coefficients which are functions of the signal from the secondary light sensor.

23. The method of any one of claims 18 to 22, wherein the measurements further comprise a temperature signal from a temperature sensor which is used modify the UV signal responsive to the temperature signal.

24. The method of claim 23, wherein the method further comprises correcting the UV signal by a cutoff correction function comprising: a spectral response function for the UV photodiode including a known cutoff wavelength measured at a reference temperature; a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function or approximation.

25. The method of claim 24, wherein the solar irradiance spectrum function is based on a radiative transfer model.

26. The method of any one of claims 24 or 25, wherein the solar irradiance spectrum function utilises time, date, and geographical location.

27. The method of any one of claims 24 to 26, wherein the solar irradiance spectrum function utilises atmospheric path length and / or solar zenith angle.

28. The method of any one of claims 23 to 27, further comprising correcting the UV signal by a dark current correction function which compensates for dark current based on the temperature signal.

29. The method of claim 28, wherein the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal.

30. The method of any one of claims 18 to 29, wherein the UV photodiode is an AIGaN, GaN, or SiC photodiode.

31. The method of any one of claims 18 to 30, further comprising transmitting the measurements and / or the resulting UV reading to an external device.

32. A non-transitory computer-readable storage medium comprising instructions which, when executed by one or more processors, cause the one or more processors to perform operations comprising: receiving measurements obtained from a UV sensing system, the measurements comprising a UV signal from a UV photodiode and a secondary light signal from a secondary light sensor; and correcting the UV signal by an out-of-band correction function which compensates for out-of-band excitation of the UV photodiode based on the secondary light signal.

33. The storage medium of claim 32, wherein the out-of-band correction function is nonlinear.

34. The storage medium of claim 33, wherein the out-of-band correction function comprises a plurality of coefficients which are functions of the secondary light signal.

35. The storage medium of any one of claims 32 to 34, wherein the measurements received further comprise a temperature signal from a temperature sensor, and the operations further comprise correcting the UV signal by a cutoff correction function comprising: a spectral response function for the UV photodiode including a known cutoff wavelength measured at a reference temperature; a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function or approximation.

36. The storage medium of any one of claims 32 to 35, wherein the operations further comprise correcting the UV signal by a dark current correction function which compensates for dark current based on the temperature signal.

37. The storage medium of claim 36, wherein the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal.

38. A UV sensing system comprising: a UV photodiode which provides a UV signal; anda temperature sensor which provides a temperature signal; wherein the UV sensing system is configured to compensate for cutoff shift error by applying a cutoff correction function to the UV signal, the cutoff correction function comprising: a spectral response function for the UV photodiode including a known cutoff wavelength measured at a reference temperature; a cutoff shift function which shifts the known cutoff wavelength based on a difference between the temperature signal and the reference temperature; and a solar irradiance spectrum function or approximation.

39. The system of claim 38, further comprising one or more processors, and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: obtaining measurements from the UV photodiode; and correcting the measurements by applying the cutoff correction function.

40. A UV sensing system comprising: a UV photodiode which provides a UV signal; and a temperature sensor which provides a temperature signal; wherein the UV sensing system is configured to compensate for dark current error by applying a dark current correction function to the UV signal based on the temperature signal; wherein the dark current correction function includes an exponential function of temperature that produces an offset to apply to the UV signal.

41. The system of claim 40, further comprising one or more processors, and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: obtaining measurements from the UV photodiode; and correcting the measurements by applying the dark current correction function.

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