EP3409077B1

EP3409077B1 - Calibrating the output of a light-emitting diode

Info

Publication number: EP3409077B1
Application number: EP17701166.5A
Authority: EP
Inventors: Steinar SÆLID
Original assignee: Prediktor Medical AS
Current assignee: Prediktor Holdco AS
Priority date: 2016-01-25
Filing date: 2017-01-25
Publication date: 2021-01-06
Anticipated expiration: 2037-01-25
Also published as: ES2848184T3; CN107926093A; CN107926093B; WO2017129633A1; EP3409077A1

Description

TECHNICAL FIELD

The present invention relates to calibrating the output of a light-emitting diode (LED) for use in spectroscopic and sensor applications, in particular, calibrating the output based on the environment in which the LED operates.

BACKGROUND

Light-emitting diodes (LEDs) have many uses in modern electronics. Because of their low weight, compact size, and high resilience, LEDs can be used as light sources in devices which are designed to be portable. Because of this, the environment in which an LED operates is rarely consistent. Changes in environment can affect the output or operation of an LED.
In particular, an LED may operate at any of a wide range of temperatures. However, the spectrum and the intensity of the light emitted by the LED vary based on the temperature and operating conditions, such as the LED current. In general, the peak emission wavelength tends to increase with the junction temperature of the LED, and the total emitted energy tends to decrease with increasing temperature. The junction temperature is particularly affected by the ambient temperature in which the LED is located.
US 2016/066384 A1 , US 9 237 620 B1 , US 2015/377695 A1 and WO 2011/123800 A2 describe different methods for calibrating the intensity of light emitted by an LED on the basis of the measurement of temperature, LED current and forward voltage.
One use of LEDs is in a photo-optical sensor arrangement. Light from one or more LEDs passes through or is reflected by a medium and is received by a detector. A connected computer and associated electronics can calculate the attenuation of the light as it passes through the medium or is reflected from the medium, and can output this as a value. The level of attenuation may be related to the composition of the material. For example, in medical device applications, the attenuation may be used to analyse the level of a substance, such as glucose, in a user's blood. However, this attenuation can only be accurately assessed if the initial output of the LED is known with some accuracy. Since the output of the LED differs based on the temperature of the junction (and thus based on the ambient temperature), the output of the detector therefore also depends on the junction temperature (and accordingly the ambient temperature).
The forward voltage of an LED is at least partly dependent on the LED junction temperature. David S Meyaard et al "Analysis of the temperature dependence of the forward voltage characteristics of GalnN light-emitting diodes" provides a model of this dependence: $\frac{{dV}_{f}}{{dT}_{j}} = \frac{k}{e} \ln (\frac{N_{D} N_{A}}{N_{C} N_{V}}) - \frac{{αT}_{j} (T_{j} + 2 β)}{e {(T + β)}^{2}} - \frac{3 k}{e}$
where:

V_f is the junction forward voltage;
T_j is the junction temperature;
k is Boltzmann's constant;
e is the elementary charge;
N_D is the dopant concentration of n-material;
N_P is the dopant concentration of p-material;
N_C is the conduction band electron density;
N_V is the valence band electron density; and
α and β are Varshni parameters (which may be $α = 0.77 \frac{meV}{K}$
and β = 600 K for a GaN-based LED).

In practice, this can be approximated by a linear equation within a limited temperature range by: $V_{f} = a + {bT}_{j} + {cI}_{f}$
where a, b, and c are constants and I_f is the LED forward current.
Bender, Vitor C., et al. in "Electro thermal feedback of a LED lighting system: Modeling and control" describes the following approximate model for the light output emitted from a LED $S_{t} (T_{j}, I_{f}) = (c_{0} + c_{1} T_{j}) (d) (_{0} + d_{1} I_{f})$
Here is

S_t: Total luminous flux from the LED
T_j: Junction temperature
I_f: LED forward current
c ₀,c ₁,d ₀,d ₁: Adapted constant coefficients

Inserting the temperature solved by the first equation (forward voltage) into the second equation (emitted light) shows that the emitted light intensity can be approximated as a quadratic model in the LED forward current and the LED forward voltage alone.
The practical application of this and how a system which makes use of one or more LEDs can be calibrated in view of temperature and other operating conditions, such as LED current, has not previously been explored for spectroscopic use. There is therefore a need in the art to provide a method for calibrating the output of an LED or controlling the output of an LED based on the environment in which it operates.

SUMMARY OF INVENTION

In a first aspect, there is provided a method for calibrating the output intensity of a light emitting diode (LED) for use in spectroscopic applications, the method comprising the steps of: determining a reference forward voltage, V _f0, a reference forward current, I _f0, and a reference detected intensity of emitted light, S ₀, of the LED at an initial junction temperature, T ₀; varying the junction temperature of the LED to provide a plurality of junction temperatures T_j and at each junction temperature: setting the forward current to a plurality of forward current values I_f , and at each forward current: determining the deviation in the forward voltage ΔV_f from the reference voltage V _f0; determining the deviation in forward current ΔI_f from the reference current I _f0; and determining the deviation in detected light intensity ΔS from the reference intensity S ₀; estimating the parameters s_V , s_I , s_IV , s_II , and s_VV such that:S = S ₀ + $Δ S = S_{0} + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}^{2} + s_{VV} Δ V_{f}_{2}$
approximately holds for all sets of forward voltage deviation, ΔV_f, forward current deviation, ΔI_f, and detected intensity deviation, ΔS.
In this manner, a model of the relationship between the intensity of the detected light (which is proportional to the light emitted from the LED) and the LED forward voltage and current can be established (the former varying with junction temperature). Such a model can then be used to control the emitted output of an LED, or to correct the detected light intensity (transmitted by diffuse reflection or by transmission) for the effects of a change in temperature such that a first output and a second output can be compared without an intervening change in the environment limiting the accuracy of any such comparison.
The use of a constructed mathematical model to calibrate for changes in the operating environment of the LED does not require any calibration data to be stored in the device or to be accessed from a remote memory in order to calibrate the output during use. Since the model is non-linear it avoids any need for linear interpolation between stored calibration values and provides an increased precision. There is no requirement for the LED to undergo a special compensation procedure when calibrating or adjusting the LED output. For example the LED operating values do not have to be altered to measure a voltage or current during a calibration routine. In this method normal operating values are used during calibration and when using the calibrated LED such that normal operation of the LED does not have to be interrupted.
The deviation values, indicated by Δ, are the changes in the measured values from some standard reference values (V₀, I₀, S₀) around which the modelling is carried out.
For each deviation measurement in the forward voltage, V_f and the LED current, I_f , the resulting measured intensity deviation ΔS from the standard value S ₀ is measured as well.
The junction temperature is varied by varying the ambient temperature of the LED in steps. The junction temperature does not enter into the prediction model directly, but will cause changes in the forward voltage and current as described above. For model adaptation purposes, the ambient temperature, as well as the LED current, is varied independently during the calibration procedure. The LED current is controlled in standard ways by LED current control electronics which receives a set point for the LED forward current. Based on this procedure, where corresponding values of LED forward voltage and currents (inputs to the model), as well as registered light detection (output from the model) are detected, the model parameters can be estimated by minimizing the model prediction errors based on some objective function. The emitted light is measured by diffuse reflection from a standard reference material disc or otherwise (i.e. transmission).
In some embodiments the variations in ambient temperature and forward current span the planned operating range of the LEDs in terms of temperatures and LED currents. In particular, in some embodiments, the reference values V _f0, I _f0, S ₀, T ₀ are selected such that: the initial junction temperature, T ₀, falls within a predefined expected operational temperature range; and the reference intensity, S ₀, falls within a predetermined desired range for the intended operation.
In certain embodiments the detected reference light intensity S₀ is measured by diffuse reflection or transmission from a standard reference material and is proportional to the emitted light intensity.
Estimating the parameters can be performed in any suitable way. However, in preferred embodiments, estimating the parameters comprises using a partial least squares regression analysis.
In some embodiments, the method can further comprise the steps of: illuminating a target material for measurement; detecting a diffuse reflected or transmitted light intensity from the target material as a detected measurement intensity, σ; determining the deviation in the forward voltage AV_f from the reference voltage V _f0 during the measurement; determining the deviation in the forward current ΔI_f from the reference current I _f0 during the measurement; calculating a predicted intensity S', where S' is the predicted detected intensity given by $S' = S_{0} + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}_{2} + s_{VV} Δ V_{f}_{2};$
and calculating a calibrated attenuation, ω, such that: ω = σ/S', where the calibrated attenuation, ω, provides a measure of the attenuation of the light intensity due to the target material, compensating for temperature variation.
It is conventional for other components to use the uncalibrated output of the LED. However, in preferred embodiments, the method further comprises the step of using the calibrated attenuation, ω, to determine the amount of a specific substance in the target material.
In some embodiments the calibrated output ω may be used to infer the level of a substance in the target medium.
In some embodiments the method further comprises the steps of obtaining a desired intensity, S ₀ ', where the desired intensity may be expressed in terms of the reference intensity by $S_{0}^{'} = S_{0} + Δ S^{'};$
determining the deviation in the forward voltage ΔV_f from the reference voltage V _f0 under present operating conditions; calculating the current set point I_f , where I_f = I _f0 + ΔI_f , such that ΔS = 0 where $S = S_{0}' + Δ S = S_{0}' + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}_{2} + s_{VV} Δ V_{f}_{2};$
and
applying the calculated current set point to the LED, such that S = S ₀ ', providing the desired intensity.
The current set point may be controlled in real time so that the predicted light emission from an LED is kept constant and independent of LED junction temperature.
As will be explained below, it is not essential for junction temperature of the LED to be measured at all, as the precise temperature is not used in calculations. However, it can be useful to ensure that the temperature varies sufficiently to ensure the calibration is accurate. However, it can be difficult to measure the junction temperature directly. Thus, in some embodiments, the junction temperature is varied by varying the substrate temperature, the method further comprising the steps of: measuring the substrate temperature of the LED; and inferring the junction temperature of the LED as the substrate temperature; wherein the deviation in forward current, forward voltage and intensity is determined if the measured substrate temperature differs from a previous substrate temperature at which values were determined, by more than a predetermined amount. This allows the relatively easy-to-measure substrate temperature to be used as an analogue of the relatively difficult-to-measure junction temperature. The data collected in this way allows the model parameter estimation.
In a second aspect, there is provided a device configured to perform the method of the first aspect. Such a device may be a medical analysis device, such as a device for analysing the level of a substance in a user's blood. In some embodiments the level of the substance in a user's blood is interpolated from the calibrated attenuation ω.

BRIEF DESCRIPTION OF FIGURES

Examples of the present invention will now be described with reference to the accompanying drawings, where:

Figure 1 is a schematic diagram of a system for performing the method according to the present invention;
Figure 2 shows a summary of the exemplary method for calibrating the output of an LED in reference conditions and the use of the calibrated LED during operation;
Figure 3 shows an exemplary method for calibrating the output of an LED under reference conditions;
Figure 4 shows an exemplary method for producing a calibrated output of an LED during use;
Figure 5 shows an exemplary method for controlling the output of a calibrated LED during use;
Figure 6 shows a system for use in performing methods of the present invention; and
Figure 7 shows a medical analysis device incorporating a system which makes use of methods of the present invention.

DETAILED DESCRIPTION

As described above, LEDs may be used in spectroscopic and sensor applications, one example of which is to provide information on the constituent substances of a medium. Figure 1 illustrates a system for performing the methods of the present invention in which the attenuation of light from an LED 11 may be measured as it passes through a medium 20. The system comprises one or more LEDs 11, arranged to emit light towards a medium 20 for measurement and a detector 14 arranged to receive the emitted light once reflected or transmitted through the medium 20. A computer comprising a memory 13 and processor 12 is configured to control the current set point of the LED 11 using current controller electronics 15. The computer 13, 12 is further configured to receive the light intensity S detected at the detector in addition to the forward voltage across and current through the LED 11 - the latter measured by appropriate components such as a voltmeter and ammeter.
As discussed, in order to provide a reliable measurement of the attenuation of the light intensity as it passes through a medium 20, the LED must be calibrated for the changing environmental conditions, such as temperature. The following methods provide means to calibrate an LED for environmental effects, output a calibrated output of the LED and control the LED to provide a required intensity.
Figure 2 provides an overview of the methods 100, 200, 300 according to the present invention. The calibration process 100 may take place once, for example following manufacture in the factory, or periodically during the lifetime of a device. The calibration method involves fitting the temperature induced variation of the operational characteristics of an LED to a model. This may be carried out for each individual LED. During operation, the model may then be used in a method 200 to calculate a calibrated output of the LED and in a method 300 to control the emission of the LED to a desired intensity, as will be described.
As noted above, the output of an LED depends approximately linearly on the LED junction temperature which again is approximately a linear function of the forward voltage of the LED at a given current value. Forward voltage is additionally dependent on the current of the LED. In theory, if there is a perfect control of current, there is no reason to measure current in order to compare two outputs from the LED to identify the effects of temperature, as any contribution by current can therefore be included in the constant a in the equation noted above. However, in practice it is very difficult to achieve such perfect current control. Because of this, the actual forward voltage will tend to depend both on the temperature and the current of the LED.
Thus a more practical model can be provided as: $V_{f} = a + {bT}_{j} + cI$
where:

a, b and c are constants; and
I is the current of the LED.

Accordingly, the intensity, S, of the light emitted by the LED, which is dependent both on the current of the LED and the junction temperature of the LED, can be modelled as: $S = s_{0} + s_{V} Δ V_{f} + s_{I} Δ I + s_{IV} Δ I Δ V + s_{II} Δ I^{2} + s_{VV} Δ V^{2}$
where s ₀, s_V, s_I , s_IV , s_II , and s_VV are parameters.
By determining the values of the parameters s ₀, s_V , s_I , s_IV , s_II , and s_VV , a model of the effect of temperature on the intensity of the light emitted from an LED can be established. Such a model can be used to calibrate the output of the LED.
Figure 1 shows an exemplary method 100 for calibrating the output of an LED.

Establishing a model

As described above, the initial calibration of each LED must first be performed before operation. This may take place during or following manufacture of a device comprising the LED. The calibration may then be programmed into the LED including device to account for variations in temperature during later operation of the device by a user. Alternatively the calibration may take place automatically periodically in the lifetime of the device. For example, calibration may take place when the device comprising the LED is charging. Alternatively it may be initiated by a user.
The steps of an exemplary calibration procedure will now be described.
At step 101, a reference forward voltage, V _f0, a reference forward current, I _f0, and a reference detected intensity of emitted light, S ₀, of the LED 11 are determined at an initial junction temperature, T ₀.
These reference values are standard values within the normal or expected operating ranges of the LED and form the basis for constructing the model. In particular, the initial junction temperature, T ₀, and the corresponding reference forward voltage V _f0 may be values near the centre of the normal operating range of the device. As with all methods according to the present invention, it is not necessary to actually know the junction temperature. The reference temperature can be selected by simply running the LED in normal ambient conditions for durations corresponding to those intended during operation. A reference forward current, I _f0, may then be selected such that the detected intensity of emitted light (the reference intensity), S ₀, is close to a desired intensity for use. This intensity is one which provides a detection signal intensity of appropriate magnitude. For example, for application in a medical device configured to analyse the blood, the intensity may be sufficient to pass through the wrist and give an adequate reading at an opposing sensor 14. The reference detected intensity of emitted light S ₀ may be measured under reference conditions by diffuse reflection or transmission from a standard reference material. The detected reference intensity S ₀ is therefore proportional to the actual emitted light intensity S_E , being decreased by a factor corresponding to the attenuation due to the reference material.
The measured intensity may be the intensity of a particular wavelength or range of wavelengths, such as those falling into the ultraviolet, visual, and near-infrared range (about 300 nm to about 2500 nm). The intensity may be recorded by a photo sensor, and the reference target may be a grey tile, a piece of phantom tissue or the like.
At step 102, the ambient temperature of the LED is varied to provide a different junction temperature.
In practice it is difficult to measure the junction temperature of an LED. However, a precise measurement of the junction temperature is not essential in order to establish a model of the effect of temperature on output. As long as the forward voltage, current and intensity are determined at a plurality of junction temperatures which are sufficiently different (regardless of precisely what those temperatures are), the output can be calibrated. Therefore changes in the junction temperature may be applied by changing the ambient temperature of the LED. This may be achieved in many different ways, for example using a heater in close proximity to the substrate to impart the necessary temperature changes.
Since a precise measurement of the junction temperature is not necessary, a useful approximation of the junction temperature is the substrate or the PCB temperature, which is much easier and more practical to assess. The substrate or PCB temperature may be used to infer the junction temperature as long as the on-time for the LED during calibration is intermittent and represents a small percentage of the total time.
Thus, in some cases, the LED (or, more generally, the device comprising the LED) may be brought to a first temperature (such as about 40.0°C) and then gradually brought to a second temperature (such as about 20.0°C). This may be done by heating the LED (or the device) to the first temperature, then allowing natural or controlled cooling to reduce the LED (or the device) to the second temperature. Alternatively, the temperature change may occur during another process of the device, such as charging, which can cause the device to vary its temperature.
Measurements can then be taken at a number of points between the first and second temperatures. These can be at regular intervals, such as at intervals of about 1°C between the first and second temperatures.
In some cases, the temperature of the substrate or PCB may be measured. This can be done in order to ensure that the substrate temperature (and thus the junction temperature) differs sufficiently in order to allow the next set of forward voltage, current and intensity determinations to be made. In other words, determining the forward voltage, current, and intensity may occur only if the substrate temperature (and thus an inferred junction temperature) differs from a previous substrate or junction temperature at which determinations were previously made by a predefined amount.
Initially, following the reference values being determined, the junction temperature of the LED is changed to a first temperature using one of exemplary processes described above.
At Step 103, the forward current I_f is varied with the current control electronics 15 to a plurality of forward current values I_f and at each selected forward current: the deviation in the forward voltage ΔV_f from the reference voltage V _f0 is determined; the deviation ΔI_f in the forward current from the reference current I _f0 is calculated; and the deviation ΔS from the reference intensity S₀ is calculated, with these values stored in a memory.
The variation in temperature from the reference temperature, T ₀ and the change in forward current If will cause the forward voltage across the LED 11 during operation to change. The forward voltage across the LED 11 is therefore measured (for example using a voltmeter) at each value of the forward current set using the current control electronics and this value is obtained by the computer 12, 13. The deviation ΔV_f from the reference voltage V _f0, is calculated and this value is stored in the memory 13.
Furthermore, at each value of the forward current I_f , firstly the deviation ΔI_f in the forward current from the reference current I _f0 is calculated and stored in the memory 13 and the detected light intensity S is measured with the detector/sensor 14. The light intensity is measured as with the reference intensity S ₀ via diffuse reflection or transmission from a standard reference material. The deviation ΔS from the reference intensity S₀ is also calculated and stored in the memory 13. Therefore for each junction temperature (inferred by the substrate temperature) of the LED, a plurality of sets of deviation in forward voltage V_f, forward current ΔI_f and associated deviation in detected light intensity ΔS are stored.
At step 104 the device determines whether to vary the temperature to a new temperature to obtain further current and intensity values.
The steps of 103 may be performed repeatedly over the plurality of junction temperatures. For example, steps 102 to 104 may be performed over each of the plurality of junction temperatures for a predetermined amount of time, or until a predetermined number of measurements have been obtained. In some cases, steps 102 to 104 may continue indefinitely until stopped by an operator or until an error occurs (such as a malfunction of the LED). Each temperature used does not need to be known precisely, since the temperature does not explicitly enter the calibration procedure. It is sufficient that a range of temperatures are used.
When sufficient data has been collected, as determined for example by the predetermined amount of time elapsing or the predetermined number of measurements being determined, when the device comes again to step 104 the method will proceed to step 105.
At step 105, the parameters s_V , s_I , s_IV , s_II , and s_VV are then determined (or at least estimated) such that $S = S_{0} + Δ S = S_{0} + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}_{2} + s_{VV} Δ V_{f}_{2}$
holds (or at least approximately holds) for all sets of forward voltage deviation, ΔV_f , forward current deviation, ΔI_f, and detected intensity deviation, ΔS.
In this step the processor accesses the data acquired during the repeated steps 102 to 104 which is stored in the memory. The parameters are then estimated using an appropriate technique to find the optimum values which get closest to a solution in which the above equation holds for all sets of values. Each unique set of values comprises a corresponding forward voltage deviation, ΔV_f , forward current deviation, ΔI_f, and detected intensity deviation, ΔS.
The determination of these parameters may make use of any appropriate computer-implemented method, including regression analysis. In some cases, a preferred method for determining values for the parameters involves a partial least squares regression analysis.
In some cases, if the loop of steps 102 to 104 is stopped before sufficient data is gathered, step 105 may instead comprise retrieving previously determined values of the parameters.
After step 105 a model is determined which provides the relationship between changes in forward voltage (produced via a change in junction temperature of the LED 11), current and intensity for a specific LED 11. This model can therefore be used to account for the effects of a changing environment in which the LED operates, as will now be described.

Applying the model to calculate a calibrated output

Once the parameters are determined and thus the model established, the model may be applied in order to calibrate a particular output of the LED in order to correct for effects of the local environment in which the LED operates. An exemplary method 200 for providing a calibrated output measurement is illustrated in Figure 4. The subsequent steps of method 200 may occur repeatedly, without the need for performing the steps of method 100 each subsequent time.
Method 200 may take place during use of the device to take a measurement of the attenuation of light emitted from the LED as it travels through or is reflected by the medium of a target material.
At step 201, a target material 20 intended for measurement is illuminated by an LED 11 of the device 10.
The device may be arranged to measure a diffuse reflected or transmitted signal from the target material. When the method is performed by a medical device configured to take a measurement of a substance in the blood, the light emitted by the LED 11, may be directed at the tissue from which a measurement is made. For example in a wearable device the LED may illuminate the one side of the wrist and the transmitted signal detected at the opposite side.
The intensity may be recorded by a photo sensor, and may be assessed or corrected against the reference intensity (from a white/grey tile or the like). The measured intensity may be the intensity of a particular wavelength or range of wavelengths, such as those falling into the ultraviolet, visual, and near-infrared range (about 300 nm to about 2500 nm).
At step 202, the intensity σ of the emitted light is detected after passing through or being reflected by the target medium.
The detector receives the light after passing through the medium and the detected intensity σ, after attenuation by the medium is obtained by the computer 11, 13.
At step 203 the deviation in the forward voltage, ΔV_f , and forward current, ΔI_f , from the reference values I ₀, V ₀ is calculated.
During illumination of the target material with the LED 11, the voltage across and current through the LED is measured with a voltmeter and ammeter (within the device). These values are obtained by the computer 13, 12 which calculates the deviation from the reference values I ₀, V ₀ determined during calibration 100.
At step 204 a predicted intensity S' is calculated, where S' is the predicted detected intensity when measuring the intensity detected from the reference target, as given by $S' = S_{0} + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}_{2} + s_{VV} Δ V_{f}_{2}$
Using the stored value of the reference intensity S ₀, and the calculated deviations from the stored reference forward voltage and forward current ΔV_f , ΔI_f, a predicted intensity S' is calculated. This calculation uses the parameters s_V , s_I , s_IV , s_II , and s_VV determined during the calibration method 100. The predicted intensity S' corresponds to the intensity of the emitted radiation that would be detected at these values under reference conditions - after diffuse reflection or transmission from the reference material. This value therefore takes into account the temperature induced variations in the LED characteristics.
At step 205 a calibrated attenuation of the light intensity, ω, due to the target medium 20 is calculated, where ω = ^σ/ _S' .
The output, ω, gives the fractional reduction intensity due to attenuation in the target material as compared to the reference material - taking into account the difference in junction temperature. That is, ω, is a quantity proportional to the light intensity attenuation due to scattering and absorption on travelling through the target material (such as tissue), compensated for environmentally-derived emitting side intensity variations.
This output can be related to the level of attenuation of the emitted light due to the target material which itself may be related back to the level of a substance in the material. For example, prior investigations can produce a model which describes a relationship between the level of attenuation of a certain wavelength of light and the percentage composition of a specific substance in the target material. This output therefore provides a measure of the change in absorption due to the target material, irrespective of any temperature variations that may have occurred. A measurement of a first target material may therefore be reliably compared to a measurement of a second target material where the temperature of the LED may have changed in the intervening period.
In this manner, the output of the LED is substantially corrected for the environmental differences, thereby allowing a first calibrated output of an LED to be compared with a second calibrated output of an LED, even if the temperature is not consistent. For example, in the case of a medical device configured to detect the attenuation of emitted light as it passes through the tissue of a user, a first reading can be taken in a cold environment (for example outside) and a second reading can be taken in a warm environment (for example within a heated building) and the output values of the relative attenuation in each case may be directly compared. In this way, the environmental effects of temperature do not influence the output of a device 10 incorporating the calibrated LED 11.
The order of steps 202, 203 and 204 shown in method 200 is merely one example, and it will be appreciated that these steps may be performed in any order.

Controlling the LED

In some embodiments, rather than using the calibrated output of an LED, it is desirable to adjust the operation of an LED to emit light at a particular desired intensity. For example, this may be particularly desirable in order to obtain two or more intensity measurements which can be compared without the accuracy of the measurements being reduced by a change of ambient temperature. A preferred method 300 for doing so is shown in Figure 5. Method 300 again makes use of the parameters s_V , s_I , s_IV , s_II , s_VV and reference values V _f0, I _f0, S ₀ obtained in method 100.
At step 301, a desired intensity, S ₀ ',, is obtained, where the desired intensity may be expressed in terms of the reference intensity by $S_{0}^{'} = S_{0} + Δ S^{'} .$
This expression of the desired merely defines the value in terms of its deviation ΔS' from the reference intensity S ₀. The desired intensity may be predetermined, user input or algorithmically calculated. For example the desired intensity may simply be the reference intensity used in method 100 in which case ΔS' = 0, so $S_{0}^{'} = S_{0} .$
At step 302, the deviation in the forward voltage ΔV_f from reference forward voltage V _f0 of the LED is determined for the current operating conditions.
This may occur in the same manner as the forward voltage is determined in method 100, using a voltmeter integral to the device or any suitable piece of equipment. The deviation in forward voltage is measured at that current point in time in which the intensity of the LED is adjusted.
At step 303, a desired current set point I_f is calculated, where I_f = I _f0 + ΔI_f, such that ΔS = 0 where $S = S_{0}^{'} + Δ S = S_{0}^{'} + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}_{2} + s_{VV} Δ V_{f}_{2}$
Again, the formulation of the current set point I_f = I _f0 + ΔI_f, merely expresses the current set point in terms of the change required from the reference current I _f0. In this step the required current set point is calculated such that ΔS = 0. For example in the case where the desired intensity $S$ $_{0}^{'}$
is the reference intensity S ₀ (ΔS' = 0), the required current set point I_f is that which gives a value of deviation forward current ΔI_f, such that $Δ S = s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}_{2} +$
$s_{VV} Δ V_{f}_{2} = 0,$
given the current value of the deviation in the forward voltage.
At step 304, the desired current set point, I_f , is applied to the LED.
This will result in the calibrated output of the LED being substantially equal to the desired intensity, $S_{0}^{'} .$
In some cases, steps 302, 303 and 304 may be repeated continuously or periodically. This can account for changes in, for example, the ambient temperature which would result in a change to the forward voltage of the LED. In this manner, the current applied to the LED will continuously or periodically be adjusted such that the calibrated output continues to substantially match the desired intensity.

System

Figure 6 shows an exemplary system 10 which is suitable for performing methods of the present invention.
System 10 comprises one or more LEDs 11, one or more processors 12 which are in communication with one or more memories 13. One or more of the memories 13 may be a computer-readable medium which comprises computer-executable instructions which, when executed by the processor, causes the processor to perform a method of the present invention.
System 10 may further comprise one or more sensors 14 which are in communication with the one or more processors 12 and which are configured to measure one or more characteristics of the one or more LEDs 11. The sensors 13 may comprise an ammeter which is configured to measure the current of one or more of the LEDs 11, a voltmeter which is configured to measure the forward voltage of one or more of the LEDs 11, a photodetector which is configured to measure the intensity of the light emitted from one or more of the LEDs 11 and/or a temperature sensor which is configured to measure the substrate temperature of one or more of the LEDs 11. Readings from one or more of the sensors 14 may be stored by one or more of the processors 12 in one or more of the memories 13.

Medical Analysis Device

The method according to the invention is used in a sensor arrangement which makes use of an LED. A particular application of the methods noted above is in the field of medical analysis devices which make use of LEDs. One example is in determining the level of a substance (such as glucose) in a user's blood, particularly in a non-invasive manner.
Figure 7 shows a particular embodiment of system 10 which comprises a medical analysis device. The medical analysis device 10 is provided adjacent a user's wrist 20. Such a device may alternatively be located adjacent other portions of a user's body.
Device 10 comprises a near-infrared LED 11. LED 11 is located such that it can emit light towards the user's wrist or some other place at the body 20. One or more photodetectors 14 is provided adjacent (or parallel) to the LED 11 and is configured to detect the intensity of the light which is emitted from the LED 11, passes through the user's wrist 20, and is diffusely backscattered through the tissue in the user's wrist 20 to the photodetector 14, generally along a substantially curved (light scattering) path.
Based on the measurements taken by the photodetector(s) 14, a processor 12 can calculate an estimate of the level of glucose (or another substance) in the user's blood. Such readings may be taken periodically and may be stored in memory 13.
The processor 12 may adjust the current applied to the LED 11 in order to ensure that the intensity of the light emitted by the LED 11 is consistent over time. This allows for the variation in the user's blood glucose level to be mapped and compared over time, without the ambient temperature negatively affecting the accuracy of such comparisons.

Claims

A method for calibrating an output of a light emitting diode, LED, for use in spectroscopic applications, the method comprising the steps of:
determining a reference forward voltage, V _f0, a reference forward current, I _f0, and a reference detected intensity of emitted light, S ₀, of the LED at an initial junction temperature, T₀;

varying the junction temperature of the LED to provide a plurality of junction temperatures T_j and at each junction temperature:
setting the forward current to a plurality of forward current values I_f , and at each forward current:
determining the deviation in the forward voltage ΔV_f from the reference voltage V _f0;

determining the deviation in forward current ΔI_f from the reference current I _f0; and

determining the deviation in detected light intensity ΔS from the reference intensity S ₀;

estimating the parameters s_V , s_I , s_IV , s_II , and s_VV such that: $S = S_{0} + Δ S = S_{0} + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}^{2} + s_{VV} Δ V_{f}^{2}$
approximately holds for all sets of forward voltage deviation, ΔV_f, forward current deviation, ΔI_f , and detected intensity deviation, ΔS.
The method of claim 1 wherein the reference values V _f0, I _f0, S ₀, T ₀ are selected such that:
the initial junction temperature, T ₀, falls within a predefined expected operational temperature range; and

the reference intensity, S ₀, falls within a predetermined desired range for the intended operation.
The method of claim 1 or claim 2 wherein the detected reference light intensity S ₀ is measured by diffuse reflection or transmission from a standard reference material and is proportional to the emitted light intensity.
The method of any preceding claim wherein varying the junction temperature comprises varying an ambient temperature of the LED.
The method of any preceding claim wherein the junction temperature is varied by varying a substrate or device temperature, the method further comprising the steps of:
measuring the substrate temperature of the LED; and

inferring the junction temperature of the LED as the substrate temperature;

wherein the deviation in forward current, forward voltage and intensity is determined if the measured substrate temperature differs from a previous substrate temperature at which values were determined, by more than a predetermined amount.
The method of any preceding claim, wherein estimating the parameters comprises using a partial least squares regression analysis.
The method of any preceding claim, further comprising the steps of:
illuminating a target material for measurement;

detecting a diffuse reflected or transmitted light intensity from the target material as a detected measurement intensity, σ;

determining the deviation in the forward voltage ΔV_f from the reference voltage V _f0 during the measurement;

determining the deviation in the forward current ΔI_f from the reference current I _f0 during the measurement;

calculating a predicted intensity S', where S' is the predicted detected intensity when measuring under reference conditions given by $S' = S_{0} + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}^{2} + s_{VV} Δ V_{f}^{2};$
and

calculating a calibrated attenuation, ω, such that: $ω = σ / S',$
where the calibrated attenuation, ω, provides a measure of the attenuation of the light intensity due to the target material, compensating for temperature variation.
The method of claim 7, further comprising the steps of:
using the calibrated attenuation, ω, to determine the amount of a specific substance in the target material..
The method of any of claims 1 to 6, further comprising the steps of:
obtaining a desired intensity S ₀ ', where the desired intensity may be expressed in terms of the reference intensity by $S_{0}^{'} = S_{0} + Δ S^{'};$

determining the deviation in the forward voltage ΔV_f from the reference voltage V _f0 under present operating conditions;

calculating a current set point I_f , where I_f = I _f0 + ΔI_f, such that ΔS = 0 in the model: $S = S_{0}' + Δ S = S_{0}' + s_{V} Δ V_{f} + s_{I} Δ I_{f} + s_{IV} Δ V_{f} Δ I_{f} + s_{II} Δ I_{f}^{2} + s_{VV} Δ V_{f}^{2};$
and

applying the calculated current set point to the LED, such that S = S ₀ ', providing the desired intensity.
The method of claim 9 wherein the forward current is controlled in real time to maintain the desired intensity output from the LED.
A device configured to perform the method of any preceding claim.
The device of claim 11, wherein the device is a medical analysis device.
The device of claim 12, wherein the medical analysis device is a device configured to analyse the level of a substance in a user's blood.
The device of claim 13, wherein the device is configured to perform the method of claim 7 or 8 and to interpolate the level of the substance in a user's blood from the calibrated attenuation ω.