Control of light source of pulse oximeter

Abstract

To provide a method for realizing reduction in the power consumption of a battery-powered pulse oximeter.SOLUTION: A method for controlling a light source 3, 3a, 3b of a pulse oximeter 1 comprises: receiving a first input value 17 indicating an electrical variable in relation to the light source, the electrical variable including an electrical current flowing through the light source and / or an electrical voltage present at the light source; receiving a second input value 19 indicating an amplitude of a sensor signal 11 generated by a light sensor 5 of the pulse oximeter; determining at least one limit value for the electrical variable using the second input value, the at least one limit value being assigned to the minimum or maximum target value of a target value range in which the amplitude should lie; determining at least one deviation value indicating a deviation of the first input value from the at least one limit value; generating a control signal 23 for controlling the light source using the at least one deviation value, such that the amplitude approximates the target value range.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for controlling a light source of a pulse oximeter.The invention further relates to a control unit, a computer program and a computer readable medium for carrying out this method, as well as to a pulse oximeter. [Background technology]

[0002] A pulse oximeter is a device that non-invasively detects arterial blood oxygen saturation by measuring the absorption or emission of light when viewed through (transilluminating) body tissue through which blood flows. Such pulse oximeters can also be used to monitor pulse rate.

[0003] Pulse oximeters are increasingly being used in portable, battery-powered applications. For example, pulse oximeters can be attached to patients during emergency transport and kept close to them as they move between different departments in a hospital. Furthermore, pulse oximeters can be used as plug-in modules in multi-parameter patient monitors with limited energy budgets. Such applications are driving the demand for pulse oximeters with low power consumption. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention can be seen as providing a method by which the power consumption of a pulse oximeter, in particular a battery-operated pulse oximeter, can be reduced without significant loss of performance. Another object of the present invention can be seen as providing a control unit, a computer program and a computer-readable medium for carrying out such a method, as well as a corresponding pulse oximeter.

[0005] These problems are solved by the subject matter of the independent claims. Advantageous embodiments of the invention are set out in the dependent claims, the following description and the accompanying drawings. [Means for solving the problem]

[0006] A first aspect of the invention relates to a method for controlling an (electrical) light source of a pulse oximeter, the pulse oximeter further comprising a light sensor configured to convert light components transmitted and / or reflected by a body part when illuminated with light from the light source into an (electrical) sensor signal. The method comprises receiving a first input value indicative of an electrical quantity related to the light source, the electrical quantity including a current through the light source and / or a voltage present at the light source, receiving a second input value indicative of an amplitude of the sensor signal, identifying (determining, setting) at least one limit value of the electrical quantity using the second input value, the at least one limit value being assigned to a minimum or maximum target value of a target value range in which the amplitude should be, identifying at least one deviation value indicative of a deviation of the first input value from the at least one limit value, and generating a control signal using the at least one deviation value to control the light source, thereby causing the amplitude to approximate (approach, meet) the target value range.

[0007] This method makes it possible to conserve power during operation of the pulse oximeter, for example by reducing the current through the light source, hereinafter referred to as the light source current, while the quality of the photoplethysmogram (PPG) from the sensor signal is still sufficient and of the required accuracy, and by increasing it when stronger noise appears or the quality of the sensor signal gradually deteriorates.

[0008] To ensure sufficient quality when measuring oxygen saturation, the pulsatile component, also called AC component, of the sensor signal in the PPG must not fall below a certain value.

[0009] Typically, the amplitude of the sensor signal is approximately linear with respect to the light source current for any given patient because physiological factors such as finger thickness and skin color that affect the light absorption of blood-carrying body tissue do not change significantly during the measurement.

[0010] In order to reach the target value, the light source current can be adjusted accordingly during operation. However, to avoid excessive and / or frequent adjustments, a target value range for the amplitude must be defined. As long as the AC component is within the target value range, there is no need to adjust the light source current. If the AC component is below the minimum target value of the target value range, the light source current is increased accordingly. Conversely, if the AC component is above the maximum target value of the target value range, the light source current is reduced accordingly.

[0011] The method can be computer-implemented.

[0012] By "sensor signal" it is possible to understand an analog or digital signal. By "amplitude" it is possible to understand in particular the amplitude of the AC component of the sensor signal.

[0013] An "electrical quantity" can also be understood as an electrical quantity based on a current and / or a voltage and / or an electrical quantity that influences the current and / or the voltage. For example, the electrical quantity can be at least one of the following quantities: an adjustable series resistor connected upstream of the light source, the frequency with which the light source is switched on and off, the time relationship (time ratio) between the switch-on phases in which the light source is switched on and the switch-off phases in which the light source is switched off. For example, the frequency can be a clock frequency and / or the time relationship can be a duty cycle associated with the pulse width modulation of the light source.

[0014] A "target value" such as a "minimum target value" or a "maximum target value" can be understood as a predetermined constant. However, it is also possible that the minimum or maximum target value or both target values ​​can be changed during operation, for example in response to (measured or estimated) changes in specific physiological factors of the respective patient.

[0015] A second aspect of the invention relates to a control unit, which comprises means (elements) configured to carry out the methods described above and below.

[0016] The means may include hardware and / or software modules. In particular, the means may include a processor configured to execute the (computer-implemented) method. Furthermore, the means may include a data communication interface and / or a storage device for wireless and / or wired data communication with peripheral devices, e.g. a smartphone, a smartwatch, a tablet, a laptop, a PC or a ventilator. Alternatively, the control unit may be implemented solely as hardware, e.g. in the form of an ASIC element (application-specific integrated circuit) or an FPGA element (field-programmable gate array).

[0017] It is noted that features of the methods described above and below may also be features of the control unit (and vice versa).

[0018] A third aspect of the present invention relates to a pulse oximeter. The pulse oximeter comprises a light source, a light sensor and a control unit as described above and below. The light sensor is configured to convert light components transmitted and / or reflected by a body part when illuminated with light from the light source into an (electrical) sensor signal. The pulse oximeter, e.g. its control unit, may be configured to determine oxygen saturation (sO2) and / or pulse rate from the sensor signal by absorption spectroscopy.

[0019] A "light source" can be understood as, for example, a light emitting diode, a laser diode, an incandescent lamp, or a combination of at least two of these examples. In particular, the light source can be designed to emit light in at least two different predetermined wavelength ranges, for example red light, (near) infrared light, or green light. Examples of suitable wavelengths and wavelength ranges are 660 nm, 750 nm to 850 nm, and 905 nm to 940 nm.

[0020] By "light sensor" it is possible to understand for example a photodiode, a photocell, a CMOS sensor (CMOS: complementary metal-oxide-semiconductor), a CCD sensor (CCD: charge-coupled device) or a combination of at least two of these examples.

[0021] "Pulse oximeter" can also be understood as CO-oximeter.

[0022] The pulse oximeter may be designed as a clip for attachment to a body part such as, for example, a finger, an earlobe, or a wrist.

[0023] The pulse oximeter can be designed so that the light source and light sensor can be placed on the same side of the body part or on opposite sides of the body part.

[0024] Another aspect of the invention relates to a computer program and a computer readable medium on which the computer program is stored.

[0025] The computer program comprises instructions which, when executed on a processor, cause a processor, for example a processor of a control unit as described above and below, to carry out the methods as described above and below.

[0026] The computer-readable medium may be a volatile or non-volatile data storage device. For example, the computer-readable medium may be a hard disk, a universal serial bus (USB), a random-access memory (RAM), a read-only memory (ROM), an erasable programmable memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a flash memory, or a combination of at least two of these examples. The computer-readable medium may be a data communication network that allows the program code to be downloaded (for example, via the Internet), or a cloud.

[0027] It is noted that features of the methods described above and below may also be features of a computer program and / or a computer readable medium (and vice versa).

[0028] Various embodiments of the present invention are described below, which should not be understood as limiting the scope of the present invention.

[0029] According to one embodiment, the at least one limit value may be determined by multiplying the first input value by a quotient of a minimum or maximum target value and a second input value, in other words, the at least one limit value may change during operation depending on changes in the first input value, i.e. the electrical quantity related to the light source, and / or the second input value, i.e. the amplitude of the sensor signal.

[0030] According to one embodiment, the at least one deviation value may be determined by subtracting the first input value from the at least one limit value.

[0031] According to one embodiment, the at least one limit value may include a lower limit value assigned to a minimum target value. In this case, the first deviation value may be determined by subtracting the first input value from the lower limit value. The control signal may then be generated using the first deviation value.

[0032] According to one embodiment, the lower limit

number

number

[0033] According to one embodiment, the at least one limit value may include an upper limit value that is assigned to a maximum target value. In this case, a second deviation value may be determined by subtracting the first input value from the upper limit value. The second deviation value may then be used to generate a control signal. The upper limit value may be, for example, a value of an electrical quantity at which the light source, for example its brightness and / or color, is set so that the amplitude of the sensor signal (approximately) matches the maximum target value.

[0034] According to one embodiment, the upper limit

number

number

[0035] According to an embodiment, the first deviation value and the second deviation value can be compared with each other, in particular their signs. In this case, a control signal can be generated or an average value can be determined only if the sign of the first deviation value coincides with the sign of the second deviation value. Thus, it is possible to check with only a small amount of calculations whether the first input value, for example the current light source current, is within or outside the limit range set by the lower and upper limits, which also makes it possible to infer whether the current amplitude is within or outside the target value range due to the (almost) linear relationship between the current light source current and the current amplitude of the sensor signal. If the current amplitude is within the target value range, there is no need to adjust the light source current.

[0036] According to one embodiment, an average value can be determined from the first deviation value and the second deviation value. The control signal can then be generated using the average value as at least one deviation value. The average value can be, for example, an arithmetic mean value, a geometric mean value, or a square mean value. This can potentially simplify subsequent calculation steps.

[0037] According to an embodiment, the adjustment value can be determined using at least one deviation value and an allocation rule, which assigns an adjustment value to each possible deviation value (every deviation value). In this case, the first input value and the adjustment value can be used to determine an output value, in particular by adding the first input value and the (positive or negative) adjustment value. The output value can then be used to generate a control signal. An "allocation rule" can be understood as, for example, a mathematical function or a look-up table. The allocation rule can be, for example, stored in a storage device of the control unit as described above and below.

[0038] According to one embodiment, the allocation rule can be or be based on a sigmoid function. A "sigmoid function" can be generally understood as a function of an S-shaped graph. A sigmoid function can include linear or nearly linear parts. Such parts can be important, among other things, for the stability of the method.

[0039] According to one embodiment, the allocation rules may be defined as follows:

number

[0040] According to one embodiment, term 2 -Δ An approximation P of can be specified based on a series expansion, preferably a Taylor series, and particularly preferably a Maclaurin series. In this case, the allocation rule can be defined as follows:

number

[0041] This means that instead of P, we use term 2 -Δ This allows for improved computational efficiency compared to the embodiment in which t is calculated, thereby further reducing the power consumption of the pulse oximeter, and in addition, this facilitates the implementation of the method as hardware and / or software.

[0042] According to one embodiment, the approximation P can be defined as follows: If -Δ ≥ 0,

number

number

[0043] Here, N represents a predetermined order of the series expansion, and k represents a predetermined coefficient, for example, N may be a natural number between 1 and 10, preferably 5, and / or k may be a percentage value between 0 and 1, preferably between 0.5 and 1.0, particularly preferably 0.75.

[0044] This allows for a particularly computationally efficient and / or particularly easy to implement approximation, with little impact on the accuracy of the method.

[0045] According to one embodiment, the difference between the minimum target value and the maximum target value may be less than or equal to 10 nanoamps, preferably less than or equal to 5 nanoamps, particularly preferably less than or equal to 1 nanoamp.

[0046] According to one embodiment, the minimum target value may be 1 to 5 nanoamps, preferably 1 to 3 nanoamps, particularly preferably 2 nanoamps.

[0047] In practice, a good compromise between accuracy and energy efficiency could be achieved with such current values.

[0048] In that case, the light source current can theoretically be between 3 mA and 50 mA, although in practice the light source current can be outside this range depending on the individual patient.

[0049] According to one embodiment, the pulse oximeter may further comprise a battery for powering at least one electrical or electronic component of the pulse oximeter, in particular the light source, or the entire pulse oximeter, which allows the pulse oximeter to be carried (worn) when outdoors.

[0050] According to one embodiment, the pulse oximeter may further comprise a display unit for displaying at least one value determined by means of the sensor signal, for example oxygen saturation or pulse rate.

[0051] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. Neither the description nor the drawings should be understood as limiting the scope of the present invention. [Brief description of the drawings]

[0052] [Figure 1]FIG. 1 illustrates a pulse oximeter according to an embodiment of the present invention. [Diagram 2] FIG. 1 illustrates a range of target values ​​for use in a method according to an embodiment of the present invention. [Diagram 3] FIG. 1 illustrates allocation rules for use in a method according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] The figures are only schematic and are not to scale. When the same reference signs are used in different drawings, these signify the same or similar functional features.

[0054] 1 shows a pulse oximeter 1 comprising an (electrical) light source 3, a light sensor 5 and a control unit 7. The light sensor 5 is configured to convert light components transmitted and / or reflected by a body part 9, e.g. a finger, an earlobe or a similarly thin body part, when illuminated by light from the light source 3, into an (electrical) sensor signal 11.

[0055] In this example, the control unit 7 is configured to determine oxygen saturation (sO2) and / or pulse rate from the sensor signal 11.

[0056] Furthermore, the pulse oximeter 1 may comprise a display unit 13 for displaying at least one value and / or at least one image / graph relating to the sensor signal 11, in particular relating to the oxygen saturation and / or the pulse rate. The display unit 13 may for example be arranged in and / or on a housing of the pulse oximeter 1.

[0057] The pulse oximeter 1 may also include a battery 15 for supplying power to the pulse oximeter 1. This allows the pulse oximeter 1 to be carried around when going out.

[0058] In this example, the light source 3 comprises a first light-emitting diode 3a emitting light in a first wavelength range, for example 660 nm, and a second light-emitting diode 3b emitting light in a second wavelength range different from the first wavelength range, for example 905 nm to 940 nm. The control unit 7 can be designed to alternately switch the light-emitting diodes 3a, 3b on and off during operation of the pulse oximeter 1. The light source 3 can also comprise only one light-emitting diode with a suitably variable wavelength range. Other types of light sources, such as laser diodes or incandescent lamps, are also possible.

[0059] The light sensor 5 may include, for example, a photodiode, a photocell, a CMOS sensor, a CCD sensor, or a combination of at least two of these examples.

[0060] The pulse oximeter 1 can be designed, for example, as a clip for attachment to a body part 9 and / or as a CO-oximeter.

[0061] In this example, the pulse oximeter 1 is configured such that the light source 3 and the light sensor 5 are disposed on opposite sides of the body part 9 (on either side of the body part 9) during operation of the pulse oximeter 1. Thus, the light sensor 5 primarily receives the light component transmitted by the body part 9 when the light source 3 illuminates the body part 9.

[0062] Alternatively, the pulse oximeter 1 can be designed such that the light source 3 and the light sensor 5 are located on the same side of the body part 9 during operation of the pulse oximeter 1 .

[0063] The control unit 7 comprises means configured to execute a particular method for controlling the power supply to the light source 3, as will be explained in detail below. The means may comprise hardware and / or software modules. In particular, the means may comprise a storage device (memory) and a processor. A computer program may be stored in the storage device, and the processor may be configured to execute the method by executing the computer program. Furthermore, the means may comprise a data communication interface for wireless and / or wired data communication with peripheral devices, e.g. a smartphone, a smartwatch, a tablet, a laptop, a PC or a ventilator.

[0064] The control unit 7 can also be implemented solely as hardware, for example in the form of an ASIC or FPGA element.

[0065] The method includes the following steps:

[0066] In a first step, in the control unit 7, for example in a corresponding hardware and / or software module of the control unit 7, a first input value 17 indicative of an electrical quantity related to the light source 3 and a second input value 19 indicative of the current amplitude of the sensor signal 11 are received. The two input values ​​17, 19 may be measured and / or estimated (i.e. calculated) values. The electrical quantity comprises the current currently flowing through the light source 3 and / or the voltage currently present at the light source 3. The electrical quantity may be an electrical quantity based on the current and / or the voltage and / or an electrical quantity influencing the current and / or the voltage. For example, the electrical quantity may be at least one of the following quantities: an adjustable series resistor connected upstream of the light source 3, the frequency with which the light source 3 is switched on and off, the time relationship between the switch-on phases in which the light source 3 is switched on and the switch-off phases in which the light source 3 is switched off. For example, the frequency may be a clock frequency and / or the time relationship may be a duty cycle associated with a pulse width modulation of the light source 3. The amplitude may in particular be the amplitude of the AC component of the sensor signal 11.

[0067] In a second step, at least one limit value of the electrical quantity is determined using a second input value 19, i.e. as a function of the current amplitude of the sensor signal 11. The at least one limit value is assigned to a minimum target value t or a maximum target value t+ε of a target value range 21 in which the amplitude should be (see FIG. 2).

[0068] In a third step, at least one deviation value indicative of the deviation of the first input value 17 from at least one limit value is identified.

[0069] In a fourth step, a control signal 23 is generated for controlling the light source 3 with at least one deviation value such that the amplitude approaches the target value range 21 .

[0070] This method makes it possible to save power during operation of the pulse oximeter 1, for example by reducing the current through the light source 3 while the quality of the photoplethysmogram (PPG) from the sensor signal 11 is still sufficient and of the required accuracy, but by increasing it if more noise appears or the quality of the sensor signal 11 gradually deteriorates.

[0071] The difference between the minimum target value t and the maximum target value t+ε can be, for example, 10 nanoamperes or less, preferably 5 nanoamperes or less, particularly preferably 1 nanoampere or less. Additionally or alternatively, the minimum target value t can be 1 to 5 nanoamperes, preferably 1 to 3 nanoamperes, particularly preferably 2 nanoamperes. In practice, such current values ​​have been found to be particularly suitable with regard to the accuracy achieved and the energy efficiency achieved.

[0072] Electrical quantity lower limit

number

number

number

number

[0073] For example, the lower limit

number

number

[0074] Additionally or alternatively, a ceiling

number

number

[0075] Here, L1 represents a first input value 17, here the current through the light-emitting diodes 3a and / or 3b, and A1 represents a second input value 19.

[0076] Therefore, the two deviation values ​​Δ1 and Δ2 are,

number

number

[0077] Furthermore, from the two deviation values ​​Δ1 and Δ2,

number

[0078] The mean value Δ can be the geometric mean value or the square mean value.

[0079] The average value Δ may then be used to generate the control signal 23 .

[0080] For example, the signs of the two deviation values ​​Δ1, Δ2 can be compared with one another. In this case, the average value Δ is determined only if the signs match. This allows the first input value 17, e.g. the current light source current, to be determined in such a way that it is possible to determine whether the first input value 17, e.g. the current light source current, falls between the two limit values ​​Δ1, Δ2.

number

[0081] An adjustment value can be determined using the average value Δ and an allocation rule 25 (see FIG. 3), which assigns an adjustment value to every deviation value respectively. An output value can then be determined from the adjustment value and the first input value 17, for example by adding the first input value 17 and the (positive or negative) adjustment value. The output value can finally be used to generate the control signal 23.

[0082] The allocation rules 25 may be stored in a memory of the control unit 7, for example in the form of a mathematical function or a look-up table.

[0083] In particular, the assignment rule 25 can be a special sigmoid function. The sigmoid function can be defined as follows:

number

[0084] To reduce the amount of calculations, especially when using a microcontroller, the sigmoid function term 2 -Δ Alternatively, we can calculate an approximation of P, in which case

number

[0085] The approximation is, for example,

number

[0086] Here, N represents a predetermined order of the series expansion and k represents a predetermined coefficient. For example, N can be a natural number between 1 and 10, preferably 5, and / or k can be a percentage value between 0 and 1, preferably between 0.5 and 1.0, particularly preferably 0.75. The number N influences the accuracy of the approximation and the amount of calculations. With N=5, a good compromise between accuracy and amount of calculations can be achieved.

[0087] Such series expansions may also be called Maclaurin series, which allow for particularly computationally efficient and / or particularly easy to implement approximations with little impact on the accuracy of the method.

[0088] As can be seen in FIG. 3, the allocation rule 25 contains a relatively long (almost) linear portion centered around the zero point, which is important for the stability of the calculations.

[0089] Finally, it is noted that the words "having", "comprising", "including", "with" and the like do not exclude other elements or steps, and that the indefinite articles "a", "an", and "an" do not exclude a plurality.

[0090] It is further noted that features or steps described with reference to one of the above embodiments may also be used in combination with features or steps described with reference to other of the above embodiments.

[0091] Any reference signs in the claims should not be construed as limiting the subject matter defined by the claims. [Explanation of symbols]

[0092] 1. Pulse oximeter 3 light source 3a First light emitting diode 3b Second Light Emitting Diode 5. Light Sensor 7. Control Unit 9 body parts 11 Sensor Signal 13 Display unit 15 Battery 17 First input value 19 Second input value 21 Target range 23 Control Signals 25 Allocation Rules S Maximum allowable adjustment or step amount t Minimum target value t+ε Maximum target value Δ Standard deviation, average value

Claims

1. A method for controlling a light source (3, 3a, 3b) of a pulse oximeter (1), the pulse oximeter (1) further comprising an optical sensor (5) configured to convert light components transmitted and / or reflected by a body part (9) when irradiated with light from the light source (3, 3a, 3b) into a sensor signal (11), The method comprises: receiving a first input value (17) indicative of an electrical quantity related to said light source (3, 3a, 3b), said electrical quantity comprising a current flowing through said light source (3, 3a, 3b) and / or a voltage present at said light source (3, 3a, 3b); receiving a second input value (19) indicative of an amplitude of the sensor signal (11); - determining, using said second input value (19), at least one limit value of said electrical quantity, said limit value being assigned to a minimum target value (t) or a maximum target value (t+ε) of a target value range (21) in which said amplitude should lie; identifying at least one deviation value (Δ) indicative of a deviation of said first input value (17) from said at least one limit value; generating a control signal (23) for controlling the light source (3, 3a, 3b) using the at least one deviation value (Δ) so that the amplitude approaches the target value range (21); A method comprising:

2. the at least one limit value is determined by multiplying the first input value (17) by a quotient of the minimum target value (t) or the maximum target value (t+ε) and the second input value (19); and / or said at least one deviation value being determined by subtracting said first input value (17) from said at least one limit value; The method of claim 1.

3. the at least one limit value includes a lower limit value assigned to the minimum target value (t), a first deviation value is determined by subtracting the first input value (17) from the lower limit value, and the control signal (23) is generated using the first deviation value. and / or the at least one limit value comprises an upper limit value assigned to the maximum target value (t+ε), a second deviation value is determined by subtracting the first input value (17) from the upper limit value, and the control signal (23) is generated using the second deviation value. The method according to claim 1 or claim 2.

4. the first deviation value and the second deviation value are compared with each other, and the control signal (23) is generated only if the sign of the first deviation value matches the sign of the second deviation value. The method according to claim 3.

5. an average value (Δ) is determined from the first deviation value and the second deviation value, and the control signal (23) is generated using the average value (Δ) as the at least one deviation value (Δ); The method according to claim 3 or claim 4.

6. an adjustment value is determined using the at least one deviation value (Δ) and an allocation rule (25), and an adjustment value is assigned to each deviation value by the allocation rule; an output value is determined using said first input value (17) and said adjustment value, in particular by adding said first input value (17) and said adjustment value; The control signal (23) is generated using the output value. The method according to any one of claims 1 to 5.

7. The allocation rule (25) is a sigmoid function or is based on a sigmoid function. The method according to claim 6.

8. The allocation rule (25) is defined as follows: [0010] where f(Δ) represents the adjustment value, Δ represents the at least one deviation value (Δ), and S represents the maximum allowable value (S) of the adjustment value. The method according to claim 6 or claim 7.

9. Section 2 －Δ is determined based on a series expansion, preferably a Taylor series, particularly preferably a Maclaurin series, and the allocation rule (25) is defined as follows: [0025] The method according to claim 8.

10. The approximation P is specified as follows: When −Δ≧0, [0030] and and / or When −Δ<0, [0045] and where N represents a predetermined order of the series expansion, and k represents a predetermined coefficient.

10. The method of claim 9.

11. The difference between the minimum target value (t) and the maximum target value (t+ε) is 10 nanoamperes or less, preferably 5 nanoamperes or less, particularly preferably 1 nanoampere or less; and / or The minimum target value (t) is 1 to 5 nanoamperes, preferably 1 to 3 nanoamperes, and particularly preferably 2 nanoamperes; The method according to any one of claims 1 to 10.

12. A control unit (7), comprising means configured to carry out the method according to any one of claims 1 to 11.

13. A pulse oximeter (1), comprising: A light source (3, 3a, 3b); an optical sensor (5) configured to convert light components transmitted and / or reflected by a body part (9) when irradiated with light from the light source (3, 3a, 3b) into a sensor signal (11); A control unit (7) according to claim 12, A pulse oximeter (1).

14. A computer program comprising instructions which, when executed on a processor, cause the processor to carry out a method according to any one of claims 1 to 11.

15. A computer readable medium having stored thereon a computer program according to claim 14.

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