CN107926093B - Method and apparatus for calibrating output intensity of light emitting diode in photoelectric sensor - Google Patents

Abstract

A method for calibrating the output of a Light Emitting Diode (LED) and a method for generating a calibrated output of an LED and controlling a calibrated LED are disclosed. The method comprises the following steps: at the initial junction temperature T₀Determining a reference forward voltage V of the LED_f0Reference forward current I_f0And reference to the detected intensity S of the emitted light₀. Varying the junction temperature of the LED to provide a plurality of junction temperatures T_jAnd at each junction temperature: setting the forward current to a plurality of forward current values I_fAnd at each forward current: determining the forward voltage and the reference voltage V_f0Deviation Δ V of_f(ii) a Determining the forward current and the reference current I_f0Deviation Δ I of_f(ii) a And determining the detected light intensity from a reference intensity S₀The deviation Δ S of (d); estimating a parameter s_V、s_I、s_IV、s_IIAnd s_VVSo that: s ═ S₀+s_VΔV_f+s_IΔI+s_IVΔIΔV+s_IIΔI²+s_VVΔV²Forward voltage deviation Δ V approximately applicable to all groups_fForward current deviation Δ I_fAnd the detected intensity deviation Δ S.

Description

Method and apparatus for calibrating the output intensity of light-emitting diodes in a photoelectric sensor

technical field

The present invention relates to calibrating the output of light emitting diodes (LEDs) for spectroscopy and sensor applications, and in particular to calibrating the output based on the environment in which the LEDs operate.

Background technique

Light emitting diodes (LEDs) have many uses in modern electronics. Due to their light weight, compact size and resilience, LEDs can be used as light sources in devices designed to be portable. Because of this, the environments in which LEDs operate are rarely consistent. Changes in the environment can affect the output or operation of the LED.

In particular, LEDs can operate at any of a wide range of temperatures. However, the spectrum and intensity of light emitted by LEDs varies based on temperature and operating conditions such as 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 exposed.

One use of LEDs is in photo-optical sensor arrangements. Light from the one or more LEDs passes through or is reflected by the medium and is received by the detector. A connected computer and associated electronics can calculate the attenuation of light as it travels through or reflected by the medium, and can output the attenuation as a value. The degree of attenuation may be related to the composition of the material. For example, in medical device applications, attenuation can be used to analyze 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 is different based on the temperature of the junction (and thus the ambient temperature), the output of the detector is also dependent on the junction temperature (and accordingly on the ambient temperature).

The forward voltage of an LED depends at least in part on the LED junction temperature. David S Meyaard et al. "Analysis of the temperature dependence of the forward voltage characteristics of GaInNlight-emitting diodes" provide a model for this dependence:

in:

V _f is the junction forward voltage;

T _j is the junction temperature;

k is the Boltzmann constant;

e is the primary charge;

_ND is the doping concentration of the n-type material;

NP is the doping concentration of the _p -type material;

N _C is the conduction band electron density;

_NV is the valence band electron density; and

且β＝600K)。α and β are Varshni parameters (for GaN-based LEDs, it can be

and β=600K).

In practice, this can be approximated by a linear equation over a limited temperature range as:

V _f =a+bT _j +cI _f

where a, b, and _c are constants, and If is the LED forward current.

In "Electro thermal feedback of a LED lightingsystem: Modeling and control" Bender, Vitor C et al describe the following approximate model for the light output emitted from an LED:

S _t (T _j , If )=(c _{0 +} c ₁ T _j )(d ₀ + _{d 1} If )

here is

S _t total luminous flux from LED

T _j junction temperature

I _f LED forward current

Constant coefficients suitable for c ₀ ,c ₁ ,d ₀ ,d ₁

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 by a quadratic model of LED forward current and LED forward voltage alone.

Its practical application and how a system utilizing one or more LEDs can be calibrated in view of temperature and other operating conditions such as LED current has not been previously explored for spectroscopy use. Accordingly, there is a need in the art to provide a method for calibrating or controlling the output of an LED based on the environment in which it operates.

SUMMARY OF THE INVENTION

近似适用于所有组的正向电压偏差ΔV_f、正向电流偏差ΔI_f和检测出的强度偏差ΔS。In a first aspect, there is provided a method for calibrating the output of a light emitting diode (LED), the method comprising the steps of: determining, at an initial junction temperature T ₀ , a reference forward voltage V _f0 of the LED, a reference Forward current I _f0 and reference detected intensity S ₀ of emitted light. Change the junction temperature of the LED to provide a plurality of junction temperatures _Tj and at each junction temperature: set the forward current to a plurality of forward current values _If , and at each forward current: determine The deviation ΔV _f of the forward voltage and the reference voltage V _f0 ; the deviation ΔI _f of the forward current and the reference current I _f0 is determined; and the difference between the detected light intensity and the reference intensity S ₀ is determined; Deviation ΔS; parameters S _V , S _I , S _IV , s _II and S _VV are estimated such that:

The forward voltage deviation ΔV _f , the forward current deviation ΔI _f and the detected intensity deviation ΔS are approximated for all groups.

In this way, the relationship between the intensity of the detected light (proportional to the light emitted from the LED) and the LED forward voltage and current (the former varies with junction temperature) can be modeled. This model can then be used to control the emitted output of the LED for the effects of changes in temperature, or to correct the detected light intensity (transmitted by diffuse reflection or transmission) so that the first output can be compared with the second output , without intermediate environmental changes that limit the accuracy of any such comparisons.

Using the constructed mathematical model to calibrate changes in the operating environment of the LED does not require any calibration data to be stored in the device or accessed from remote memory 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 greater accuracy. LEDs do not need to undergo special compensation procedures when calibrating or adjusting the LED output. For example, it is not necessary to change the LED operating value to measure voltage or current during a calibration routine. In this method, normal operating values are used during calibration and when using the calibrated LEDs so that the normal operation of the LEDs does not have to be interrupted.

The deviation value, represented by Δ, is the change in the measured value relative to some standard reference value (V ₀ , I ₀ , S ₀ ) around which modeling is performed.

For each deviation measurement of forward voltage V _f and LED current If , the measured intensity deviation ΔS resulting from the standard value S _{0 is} also measured.

The junction temperature is changed by gradually changing the ambient temperature of the LED. The junction temperature does not directly enter the prediction model, but will cause the aforementioned changes in forward voltage and current. For the purpose of model adaptation, the ambient temperature as well as the LED current are varied independently during calibration. The LED current is controlled in a standard manner by LED current control electronics that receive a set point for the LED forward current. Based on this procedure where the corresponding values of LED forward voltage and current (input to the model) and registered light detections (output from the model) are detected, it can be estimated by minimizing the model prediction error based on some objective function model parameters. The emitted light is measured by diffuse reflection or other means (ie transmission) from a standard disc of reference material.

In some embodiments, the changes in ambient temperature and forward current span the planned operating range of the LED in terms of temperature and LED current. Specifically, 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 predetermined expected operating temperature range; and The reference intensity S ₀ falls within a predetermined desired range of intended operation.

In some embodiments, 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.

Estimation of parameters can be performed in any suitable manner. However, in a preferred embodiment, the estimation of the parameters involves the use of partial least squares regression analysis.

并且计算出校准的输出ω，使得：ω＝σ/S′其中，所述校准的输出提供了对由于所述目标材料引起的光强度的衰减的测量，补偿了温度变化。In some embodiments, the method may further comprise the steps of: irradiating a target material for measurement; detecting the intensity of light diffusely reflected or transmitted from the target material as the detected measurement intensity σ; determining the Deviation ΔV _f of the forward voltage from the reference voltage V _f0 ; determine the deviation ΔI _f of the forward current from the reference current I _f0 during the measurement; calculate the predicted intensity S', where S' is the predicted detected intensity, given as

And the calibrated output ω is calculated such that: ω = σ/S' where the calibrated output provides a measure of the attenuation of light intensity due to the target material, compensating for temperature changes.

It is conventional for other components to use uncalibrated LED outputs. However, in a preferred embodiment, the method further comprises outputting the calibrated output ω as the output of the LED.

In some embodiments, the calibrated output ω may be used to infer the level of species in the target medium.

In some embodiments, the method further comprises the steps of: obtaining a desired intensity S ₀ ', wherein the desired intensity can be expressed as S' ₀ =S ₀ +ΔS' according to the reference intensity; in the current run The deviation ΔV _f of the forward voltage and the reference voltage V _f0 is determined under the condition; the current set point If is calculated, wherein _{If =I f0 +ΔI f , so} that ΔS=0, where:

并且将计算出的电流设置点施加至所述LED，使得S＝S₀′，提供期望的强度。

And applying the calculated current set point to the LED such that S=S ₀ ' provides the desired intensity.

The current set point can be controlled in real time so that the predicted light emission from the LED remains constant and independent of the LED junction temperature.

As will be explained below, measuring the junction temperature of the LED is completely unnecessary because the exact temperature is not used in the calculations. However, it can be useful to ensure that the temperature varies enough to ensure that the calibration is accurate. However, directly measuring the junction temperature can be difficult. Accordingly, in some embodiments, the junction temperature is changed by changing 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, if the measured If the substrate temperature differs from the previous substrate temperature of the determined value by more than a predetermined amount, the deviation of the forward current, the deviation of the forward voltage and the deviation of the intensity are determined. This allows the relatively easy-to-measure substrate temperature to be used as an analog to the relatively difficult-to-measure junction temperature. Data collected in this way allows model parameter estimation.

In a second aspect, an apparatus configured to perform the method of the first aspect is provided. Such a device may be a medical analysis device, such as a device for analyzing the level of a substance in a user's blood. In some embodiments, the level of the substance in the user's blood is interpolated from the calibrated output ω.

Description of drawings

Examples of the present invention will be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a system for carrying out the method according to the invention;

FIG. 2 illustrates an exemplary method for calibrating the output of an LED under reference conditions and an overview of the use of the calibrated LED during operation;

FIG. 3 illustrates an exemplary method for calibrating the output of an LED under reference conditions;

FIG. 4 illustrates an exemplary method for producing a calibrated output of an LED during use;

5 illustrates an exemplary method for controlling the output of a calibrated LED during use;

Figure 6 shows a system for performing the method of the present invention; and

Figure 7 shows a medical analysis device incorporating a system using the method of the present invention.

Detailed ways

As mentioned above, LEDs can be used in spectroscopy and sensor applications, one example of which is to provide information about the constituent substances of a medium. FIG. 1 shows a system for carrying out the method of the present invention, wherein the attenuation of light from an LED 11 can be measured as it passes through a medium 20 . The system comprises one or more LEDs 11 arranged to emit light towards the medium 20 for measurement and a detector 14 arranged to receive the emitted light once it is reflected or transmitted through the medium 20 . A computer including memory 13 and processor 12 is configured to use current controller electronics 15 to control the current set point of LED 11 . The computers 13, 12 are further configured to receive the light intensity S detected at the detector as well as the forward voltage across and current through the LED 11 - the latter being measured by suitable components such as voltmeters and ammeters.

As mentioned above, in order to provide a reliable measurement of the attenuation of light intensity as light passes through medium 20, the LEDs must be calibrated for varying environmental conditions, such as temperature. The following method provides a means of calibrating the LEDs for environmental influences, outputting the output of the calibrated LEDs, and controlling the LEDs to provide the desired intensity.

Figure 2 provides an overview of methods 100, 200, 300 according to the present invention. The calibration process 100 may be performed, for example, once after manufacture in a factory, or periodically during the life of the device. The calibration method involves fitting temperature-induced changes in the operating characteristics of the LED to a model. This can be performed for each individual LED. During operation, this model can then be used in method 200 to calculate the output of the calibrated LEDs and in method 300 to control the emission of the LEDs at a desired intensity, as will be described.

As mentioned above, the output of an LED depends approximately linearly on the LED junction temperature, which in turn is approximately a linear function of the forward voltage of the LED at a given current value. The forward voltage in turn depends on the current of the LED. In theory, if there is a perfect control of the current, there is no reason to compare the two outputs from the LED to identify the temperature The current is measured under influence. In practice, however, it is very difficult to achieve such perfect current control. Because of this, the actual forward voltage will tend to depend on both the temperature and the current of the LED.

So a more realistic model can be provided as:

V _f =a+bT _j +cI

in:

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 depends on both the LED's current and the LED's junction temperature, can be modeled as:

S=s ₀ +s _V ΔV _f +s _I ΔI+s _IV ΔIΔV+s _II ΔI ² +s _VV ΔV ²

Among them, 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 , the effect of temperature on the intensity of the light emitted from the LED can be modeled. This model can be used to calibrate the LED output.

FIG. 1 shows an exemplary method 100 for calibrating the output of an LED.

Modeling

As mentioned above, an initial calibration of each LED must be performed prior to operation. This can be done during or after the manufacture of the device including the LED. Calibration can then be programmed into the LEDs included in the device to account for changes in temperature during subsequent operation of the device by the user. Alternatively, calibration may be performed automatically periodically over the life of the device. For example, calibration can be performed while the device including the LED is charging. Alternatively, calibration can be initiated by the user.

The steps of an exemplary calibration procedure will now be described.

At step 101, the reference forward voltage V _f0 , the reference forward current I _f0 and the reference detected intensity S _{0 of the emitted light of the LED 11 are determined at the initial junction temperature T 0 .}

These reference values are standard values within the normal or expected operating range of the LED and form the basis for building the model. In particular, the initial junction temperature T ₀ and the corresponding reference forward voltage V _f0 may be values close to the center 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 operating the LEDs under normal ambient conditions for a duration corresponding to the duration expected during operation. The reference forward current I _f0 can then be selected such that the intensity S ₀ (reference intensity) of the detected emitted light is close to the desired intensity for use. The strength is that which provides the detected signal strength of the appropriate magnitude. For example, for application in a medical device configured to analyze blood, the strength may be sufficient to pass through the wrist and give an appropriate reading at the opposing sensor 14 . The reference detected light intensity S ₀ of the emitted light can be measured by diffuse reflection or transmission from a standard reference material under reference conditions. The detected reference intensity S ₀ is thus proportional to the actual emitted light intensity S _E , which is reduced by a factor corresponding to the attenuation due to the reference material.

The measured intensity can be that of a particular wavelength or range of wavelengths, such as those falling within the ultraviolet, visible, and near-infrared ranges (about 300 nm to about 2500 nm). The intensity can be recorded by a light sensor, and the reference target can be a grey tile, a piece of phantom tissue, or the like.

At step 102, the ambient temperature of the LEDs is varied to provide different junction temperatures.

In practice, measuring the junction temperature of an LED is difficult. However, in order to model the effect of temperature on the output, an accurate measurement of the junction temperature is not necessary. The output can be calibrated as long as the forward voltage, current, and strength are determined at sufficiently different junction temperatures (regardless of how precisely those temperatures are). Therefore, a change in junction temperature can be applied by changing the ambient temperature of the LED. This can be achieved in many different ways, for example using heaters in close proximity to the substrate to impart the necessary temperature changes.

Since an accurate measurement of the junction temperature is not necessary, a useful approximation to the junction temperature is the substrate or PCB temperature that is easier and more realistic to evaluate. As long as the on-time of the LEDs during calibration is intermittent and represents a small fraction of the total time, the substrate or PCB temperature can be used to infer the junction temperature.

Thus, in some cases, the LED (or, more generally, the device including the LED) may be brought to a first temperature (eg, about 40.0°C), and then gradually a second temperature (eg, about 20°C). This can be achieved by heating the LED (or device) to a first temperature and then allowing natural cooling or controlled cooling to lower the LED (or device) to a second temperature. Alternatively, the temperature change may occur during another process of the device (such as charging) that may cause the device to change its temperature.

Measurements can then be taken at multiple points between the first temperature and the second temperature. It may be at regular intervals, such as at intervals of about 1°C between the first temperature and the second temperature.

In some cases, the temperature of the substrate or PCB can be measured. This can be done to ensure that the substrate temperatures (and thus the junction temperatures) are sufficiently different to allow for the next set of forward voltage, current and intensity determinations. In other words, changes in forward voltage, current and intensity can only occur if the substrate temperature (and thus the inferred junction temperature) differs from the previous substrate temperature or junction temperature (at which the previous determination was made) by a predetermined amount. Sure.

Initially, after the reference value is determined, the junction temperature of the LED is changed to the first temperature using one of the above-described exemplary procedures.

At step 103, the forward current If _is changed with the current control electronics 15 to a plurality of forward current values If, and at each selected forward current: the difference between the forward voltage and the reference voltage _Vf0 is _determined deviation ΔV _f ; the deviation ΔI _f of the forward current from the reference current I _f0 is calculated; and the deviation ΔS from the reference intensity S ₀ is calculated, and these values are stored in the memory.

Changes in temperature from the reference temperature To and changes in forward current If will _cause the forward voltage across LED ₁₁ to change during operation. The forward voltage across the LED 11 is thus measured at each value of the forward current set using the current control electronics (eg using a voltmeter) and obtained by the computer 12 , 13 . The deviation ΔV _f from the reference voltage V _f0 is calculated and the value is stored in the memory 13 .

Furthermore, at each value of the forward current I _f , the deviation ΔI _f of the forward current from the reference current I _f0 is first calculated and stored in the memory 13 , and the detected value is measured with the detector/sensor 14 light intensity S. This light intensity is measured as the reference intensity _S0 via diffuse reflection or transmission from a standard reference material. The deviation ΔS from the reference intensity S ₀ is also calculated and stored in the memory 13 . Thus, for each junction temperature of the LED (inferred from the substrate temperature), sets of deviations in forward voltage _Vf , deviations in forward current _ΔIf and associated deviations in detected light intensity ΔS are stored.

At step 104, the device determines whether to change the temperature to the new temperature to obtain further current and intensity values.

Step 103 may be repeated for multiple junction temperatures. For example, steps 102 to 104 may be performed at each of the plurality of junction temperatures for a predetermined amount of time, or until a predetermined number of measurements are obtained. In some cases, steps 102 to 104 may continue indefinitely until stopped by an operator or until an error occurs (eg, failure of an LED). Because temperatures are not explicitly entered into the calibration routine, it is not necessary to know exactly every temperature used. It is sufficient to use the temperature range.

When sufficient data has been collected (as determined, for example, by elapse of a predetermined amount of time or determination of a predetermined number of measurements), the method will move to step 105 when the device comes to step 104 again.

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

The forward voltage deviation ΔV _f , the forward current deviation ΔI _f and the detected intensity deviation ΔS apply (or at least approximately apply) to all groups.

In this step, the processor accesses the data stored in the memory acquired during the repetition of steps 102 to 104 . The parameters are then estimated using appropriate techniques to find the optimal value that is closest to a solution where the above equation applies to all sets of values. Each unique set of values includes the corresponding forward voltage deviation ΔV _f , forward current deviation ΔI _f and detected intensity deviation ΔS.

Determination of these parameters can utilize any suitable computer-implemented method including regression analysis. In some cases, the preferred method for determining the value of the parameter involves partial least squares regression analysis.

In some cases, if the loop of steps 102 to 104 is stopped before sufficient data is collected, step 105 may instead include retrieving previously determined parameter values.

After step 105, a model is determined for a particular LED 11 that provides the relationship between changes in forward voltage, changes in current, and changes in intensity (generated via changes in the junction temperature of the LEDs 11). As will now be described, the model can thus be used to account for the effects of changing environments in which the LEDs operate.

Apply the model to calculate the calibrated output

Once the parameters are determined and thus the model is established, the model can be applied to calibrate the specific output of the LED in order to correct for the effects of the local environment in which the LED is operating. An exemplary method for providing calibrated output measurements is shown in FIG. 4 . Subsequent steps of method 200 may be repeated without performing the steps of method 100 at each subsequent time.

Method 200 may be performed during use of an apparatus to measure the attenuation of light emitted from an LED as it passes through or is reflected by a medium of a target material.

At step 201 , the target material 20 for measurement is illuminated by the LED 11 of the device 10 .

The device may be arranged to measure the diffuse reflection or transmission signal from the target material. When the method is performed by a medical device configured to measure substances in the blood, the light emitted by the LED 11 may be aimed at the tissue being measured. For example, in a wearable device, an LED can illuminate one side of the wrist and detect the transmitted signal on the opposite side.

The intensity can be recorded by the light sensor and can be evaluated or corrected relative to a reference intensity (from white/grey tiles, etc.). The measured intensity can be that of a particular wavelength or range of wavelengths, such as those falling within the ultraviolet, visible, and near-infrared ranges (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 computer 11, 13 obtains the detected intensity σ after being attenuated by the medium.

At step 203, the deviation ΔV _f of the forward voltage from the reference value V ₀ and the deviation ΔI _f of the forward current from the reference value I ₀ are calculated.

During illumination of the target material with the LED 11, the voltage across the LED and the current through the LED are measured with a voltmeter (in the device) and an ammeter. These values are obtained by computers 13 , 12 which calculate deviations 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 below

Using the stored value of the reference intensity S ₀ , and the calculated deviation ΔV _f from the stored reference forward voltage and ΔI _f from the stored reference forward current, the predicted intensity S' is calculated. This calculation uses the parameters s _V , s _I , s _IV , s _II and s _VV determined during calibration method 100 . The predicted intensity S' corresponds to the intensity of the emitted radiation at these values under reference conditions - after diffuse reflection or transmission from the reference material to be detected. This value therefore takes into account the temperature that causes a change in LED characteristics.

At step 205, the calibrated attenuation ω of light intensity due to the target medium 20 is calculated, where ω=σ/S'.

The output ω gives the fractional reduction in intensity due to attenuation in the target material when compared to the reference material - taking into account the difference in junction temperature. That is, ω is an amount proportional to the attenuation of light intensity due to scattering and absorption through the target material (such as tissue), compensating for ambient-induced intensity changes on the emission side.

This output may be related to the degree of attenuation of the emitted light due to the target material, which itself may be related to the level of species in the material. For example, previous research can produce models that describe the relationship between the degree of attenuation of a certain wavelength of light and the compositional percentage of a particular species in the target material. This output thus provides a measure of the change in absorption due to the target material, regardless of any temperature changes that may have occurred. It is thus possible to reliably compare the measurement of the first target material with the measurement of the second target material in which the temperature of the LED may have changed during this time.

In this way, the output of the LED is substantially corrected for differences in the environment, allowing the first calibrated output of the LED to be compared to the second calibrated output of the LED, even if the temperatures are not consistent. For example, where the medical device is configured to detect the attenuation of emitted light as it passes through the user's tissue, a first reading may be obtained in a cold environment (eg, outdoors) and may be obtained in a warm environment (eg, under heating) within the building) to obtain a second reading and can directly compare the relative attenuation output values in each case. In this way, the effects of the temperature of the environment do not affect the output of the device 10 incorporating the calibrated LEDs 11 .

The order of steps 202, 203, and 204 shown in method 200 is merely an example, and it should be understood that the steps may be performed in any order.

Control LEDs

In some embodiments, instead of using the calibrated output of the LED, it is desirable to adjust the operation of the LED to emit a particular desired intensity of light. For example, this may be particularly desirable in order to obtain two or more intensity measurements that can be compared without the accuracy of the measurements being reduced by changes in ambient temperature. A preferred method 300 for doing so is shown in FIG. 5 . The method 300 in turn utilizes the parameters s _V , s _I , s _IV , s _II and s _VV and the reference values V _f0 , I _f0 , S ₀ obtained in the method 100 .

At step 301, the desired intensity S ₀ ' is obtained, where the desired intensity can be expressed as S ' ₀ =S ₀ +ΔS' according to the reference intensity.

This expression of expectation defines a value only in terms of its deviation ΔS' from the reference intensity S ₀ . The desired intensity can 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 ΔV _f of the forward voltage of the LEDs from the reference forward voltage V _f0 is determined for the current operating conditions.

This can occur in the same manner as determining the forward voltage in method 100, using a voltmeter integrated into the device or any piece of suitable equipment. The deviation of the forward voltage is measured at this current point while the intensity of the LED is being adjusted.

At step 303, the desired current set point If is calculated, where _{If = If0 + ΔIf ,} such that ΔS=0, where

考虑到正向电压的偏差的当前值。Furthermore, the formula I _f = I _f0 +ΔI _f for the current set point expresses the current set point only in terms of the required change with respect to the reference current I _f0 . In this step, the required current set point is calculated such that ΔS=0. For example, where the desired intensity S' ₀ is the reference intensity S ₀ (ΔS'=0), the required current set point If _{is the current set point that gives the value of the deviation ΔV f of} the forward current, such that

The current value taking into account the deviation of the forward voltage.

At step 304, the desired current _setpoint If is applied to the LED.

This will result in a calibrated output of the LED substantially equal to the desired intensity _S0 '.

In some cases, steps 302, 303 and 304 may be repeated continuously or periodically. This can account for, for example, changes in ambient temperature that will cause the forward voltage of the LEDs to change. In this way, the current applied to the LED will be adjusted continuously or periodically so that the calibrated output continues to substantially match the desired intensity.

system

FIG. 6 shows an exemplary system 10 suitable for carrying out the methods of the present invention.

System 10 includes one or more LEDs 11 , one or more processors 12 in communication with one or more memories 13 . One or more of the memories 13 may be computer-readable media comprising computer-executable instructions that, when executed by a processor, cause the processor to perform the methods of the present invention.

System 10 may further include one or more sensors 14 in communication with one or more processors 12 and configured to measure one or more characteristics of one or more LEDs 11 . The sensor 13 may include an ammeter configured to measure the current of one or more of the LEDs 11 , a voltmeter configured to measure the forward voltage of one or more of the LEDs 11 , a voltmeter configured to measure the voltage from the LEDs 11 . One or more photodetectors emitting the intensity of the light and/or temperature sensors configured to measure the substrate temperature of one or more of the LEDs 11 . Readings from one or more of sensors 14 may be stored in one or more of memories 13 by one or more of processors 12 .

Medical Analysis Equipment

A particular application of the method described above is in the field of medical analysis devices utilizing LEDs. One example is determining the level of a substance, such as glucose, in a user's blood, in particular in a non-invasive manner.

FIG. 7 shows a specific embodiment of a system 10 including a medical analysis device. The medical analysis device 10 is positioned adjacent to the user's wrist 20 . Such a device may alternatively be adjacent to other parts of the user's body.

Device 10 includes near infrared LEDs 11 . The LED 11 is positioned so that it can emit light towards the user's wrist or other location 20 at the body. One or more photodetectors 14 are positioned adjacent (or parallel to) the LEDs 11 and are configured to detect the intensity of light that is typically emitted from the LEDs 11 along substantially curved (light scattering) paths, Passes through the user's wrist 20 and is diffusely backscattered by tissue in the user's wrist 20 to the photodetector 14 .

Based on the measurements made by the one or more photodetectors 14, the processor 12 may calculate an estimate of the level of glucose (or another substance) in the user's blood. Such readings may be obtained periodically and may be stored in memory 13 .

The processor 12 can adjust the current applied to the LEDs 11 to ensure that the intensity of the light emitted by the LEDs 11 remains consistent over time. This allows changes in the user's blood glucose levels to be mapped and compared over time without the ambient temperature negatively affecting the accuracy of such comparisons.

Claims (14)

1. A method for calibrating the output intensity of a Light Emitting Diode (LED) in a photosensor for spectroscopy applications, the method comprising the steps of:

at the initial junction temperature T₀Determining a reference forward voltage V of the LED_f0Reference forward current I_f0And a reference intensity S of the detected emitted light₀；

Varying the junction temperature of the LED to provide a plurality of junction temperatures T_jAnd at each junction temperature:

setting the determined forward current to a plurality of forward current values I_fAnd at each forward current value:

determining a forward voltage corresponding to the forward current value and the reference forward voltage V_f0Deviation Δ V of_f；

Determining the forward directionCurrent value and the reference forward current I_f0Deviation Δ I of_f(ii) a And is

Determining the detected light intensity from the reference intensity S₀The deviation Δ S of (d);

estimating a parameter s_V、s_I、s_IV、s_IIAnd s_VVSo that:

forward voltage deviation Δ V applicable to all groups_fForward current deviation Δ I_fAnd the detected intensity deviation Δ S.

2. The method of claim 1, wherein reference is made to a forward voltage V_f0Reference forward current I_f0Initial junction temperature T₀Reference intensity S₀Is selected such that:

the initial junction temperature T₀Within a predetermined expected operating temperature range; and is

The reference intensity S₀Within a predetermined desired range of expected operation.

3. Method according to claim 1 or 2, wherein the reference intensity S₀Measured by diffuse reflection or transmission from a standard reference material and proportional to the intensity of the emitted light.

4. A method according to claim 1 or 2, wherein varying the junction temperature comprises varying an ambient temperature of the LED.

5. A method according to claim 1 or 2, wherein the junction temperature is varied by varying the substrate or device temperature, the method further comprising the steps of:

measuring a substrate temperature of the LED; and is

Inferring a junction temperature of the LED as a base temperature;

wherein the forward current value is the reference forward current I if the measured substrate temperature differs from a previous substrate temperature of a determined value by more than a predetermined amount_f0Deviation Δ I of_fThe forward voltage and the reference forward voltage V_f0Deviation Δ V of_fAnd the detected light intensity and the reference intensity S₀Is determined.

6. The method according to claim 1 or 2, wherein estimating the parameters comprises using partial least squares regression analysis.

7. The method according to the preceding claim 1 or 2, further comprising the steps of:

irradiating a target material for measurement;

detecting the intensity of light diffusely reflected or transmitted from the target material as a detected measured intensity σ;

determining the forward voltage corresponding to the forward current value and the reference forward voltage V during the measurement_f0Deviation Δ V of_f；

Determining the forward current value and said reference forward current I during the measurement_f0Deviation Δ I of_f；

The predicted intensity S 'is calculated, where S' is the predicted detected intensity when the measurements are taken under the reference conditions, given as

And is

The calibrated attenuation ω is calculated such that:

ω＝σ/S′

wherein the calibrated attenuation ω provides a measure of the attenuation of the light intensity due to the target material, compensating for temperature variations.

8. The method of claim 7, further comprising the steps of:

using the calibrated attenuation ω to determine the amount of the particular substance in the target material.

9. The method of claim 1 or 2, further comprising the steps of:

obtaining a desired intensity S₀', wherein the desired intensity can be based on the reference intensity S₀And is expressed as S'₀＝S₀+ΔS′；

Determining the forward voltage and the reference forward voltage V under current operating conditions_f0Deviation Δ V of_f；

Calculating a current set point I_fIn which I_f＝I_f0+ΔI_fSo that Δ S is 0, in the model:

and is

The calculated current set point is applied to the LED such that S-S0' provides the desired intensity.

10. The method of claim 9, wherein the forward current is controlled in real time to maintain a desired intensity output from the LED.

11. An apparatus configured to perform the method of any preceding claim.

12. The device of claim 11, wherein the device is a medical analysis device.

13. The device of claim 12, wherein the medical analysis device is a device for analyzing a level of a substance in blood of a user.

14. The apparatus of claim 13, wherein the level of the substance in the user's blood is interpolated according to the calibrated attenuation ω.

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