DE69117517T2 - FINGERLING SENSOR - Google Patents

Description

The invention relates to a finger receptor for use with a non-invasive monitoring device for determining the concentrations of various blood components.

It is known to provide a finger receptor with an elongated recess to accommodate a finger. However, previous receptors are not sufficiently powerful to achieve accurate and reproducible results in different types of users of the device.

Furthermore, previous devices produce variable results due to extraneous light, pulse rate, movement of the finger during testing, changing path lengths, different finger sizes, insufficient blood concentration within the finger and/or different finger colors. A finger sensor is described in German Patent No. 1 466 900. This sensor has a solid shape with a circular cross section and is used to determine pulse and blood pressure. Unfortunately, this sensor is not suitable for fingers of different sizes. As can be seen from Figure 3, light from the light source can travel around the finger to the light detector. Also, a high degree of accuracy is not required to determine pulse and blood pressure, so the sensor of the German patent would not be sufficiently accurate to be used for non-invasive determination of blood constituents. European Patent Application No. 0204459 describes a finger sensor for use with an oximeter. The Sensor has two housing sections that pivot together in such a way that they move away from each other when a finger is inserted.

According to the invention there is provided a finger receptor for use with a non-invasive monitoring device as defined in claim 1.

The finger receptor for use with a non-invasive monitoring device for receiving a user's finger is used with a light source. The receptor has a base containing an elongated channel sized to receive a finger. This channel has an opening to receive the finger and two sides with a light path entry point on one side and a light path exit point on the other side. The entry point and the exit point are sized and arranged such that at least a portion of the light passing through the entry point is received at the exit point. The channel is shaped such that a finger properly inserted into the channel completely fills a zone located between the entry point and the exit point. The light from the light source forms an optical path from the entrance point to the exit point with detection means for determining when the finger is correctly positioned in the channel.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of an optical system using a receptor with a non-invasive measuring device;

Figure 2 is a perspective view of an embodiment of the receptor;

Figure 3 is a front view of the receptor according to Figure 2;

Figure 4 is a plan view of a receptor with a plunger in a rest position;

Figure 5 is a top view of the receptor with a finger inserted into a channel and the plunger pushed back;

Figure 6 is a sectional view along the channel of the device;

Figure 7 is a front view of the receptor with an empty channel and a neutral density filter over a light path exit point;

Figure 8 is a side view of the receptor, showing the focusing lens and its armature in a rest position;

Figure 9 is a front view of the device, with a finger in the channel and the lens positioned at the light path entrance point to focus the light on the finger; and

Figure 10 is a sectional view showing the neutral density filter in a rest position.

Description of a preferred embodiment

In Figure 1, a finger 2 is arranged in a receptor 4. The light 6 from a light source 8, preferably a polychrome light source, passes through a collimating lens 10 and then through a scanning focusing lens 12 into a light path entrance path 14 in the receptor 4. The light 6 passes through the finger 2 and leaves the receptor through the light path exit point 16. The scanning focusing lens 12 focuses the light 6 onto the entrance point 14. The light 18 from the exit point 16 is scattered and broadened, and a portion of the light 18 falls on a focusing lens 20. This light 22, which falls on the focusing lens 20, is focused onto a slit 24 in a partition 26.

From the slit 24, the light 22 passes through a collimating lens 28 and onto a diffraction grating 30. From the diffraction grating 30, the light 22 passes through a focusing sensor lens 32 which focuses the light onto a linear photodiode array 34. The array 34 is connected to a printed circuit 36 for processing the data obtained from the light intensity differences as the light passes through the finger.

The non-invasive monitoring device is not discussed in detail since the claimed invention is the receptor. The receptor can be used with various non-invasive monitoring devices, one of which is described in detail in U.S. Application No. 07/362,342 (US-A-5,361,758), filed June 7, 1989.

The scanning focusing lens 12 is mounted on a shaft 38 by means of an armature 40. The shaft 38 is rotatable about its longitudinal axis in both directions by operation of a motor (not shown in Figure 1) so that the lens 40 can be moved into and out of the light path for finger and reference measurements. While a finger is in the receptor 4, the lens 12 is moved into the optical path and a scanning motor (not shown in Figure 1) moves the lens 12 back and forth relative to the entrance point 14. A neutral gray filter 42 is mounted on a carrier 44 on the same arm 38. When a reference measurement is to be made with the monitor, no finger is in the receptor and the motor (not shown in Figure 1) rotates the shaft 38 to move the filter 42 so that it aligns with the exit point 16 and to move the lens 12 away from the entry point 14.

Figures 2 and 3 show a perspective view and a partial view of a portion of the receptor 4. The receptor 4 has a base 46 which a longitudinal channel 48 adapted to receive the finger 2. The channel has a mouth 50 and two sides with a light path entrance point 52 on one side and a light path exit point 54 on the other side. The entrance point 52 and the exit point 54 are dimensioned and arranged such that at least a portion of the light passing through the entrance point is captured at the exit point. The entrance point 52 is circular and the exit point 54 has an elongated shape with one long side running parallel towards the finger so that the exit point can capture an increased amount of the light passing through the entrance point. An entrance channel 55 has a circular cross-section which converges towards the channel 48. An exit channel 56 extends from the exit point 54 and has parallel sides.

A torsion spring 57 has one end supported by a rod 58 and the other end hooked over the fitting 59. The torsion spring is wound around a small axle supported by a small block 53. The fitting 59 carries a roller 60 rotatably mounted thereon. The spring 57 provides a constant pressure on the user's finger within a certain magnitude. The roller 60 is oriented and arranged to roll along the finger as the finger is inserted lengthwise into the channel 48. The roller applies a slight pressure to the finger to press the finger against the bottom of the channel. Furthermore, the pressure exerted by the roller is of sufficient magnitude to slow but not stop the flow of blood within the finger and to concentrate blood in the finger near the entry and exit points and thereby To increase the amount of blood in the part of the finger to be monitored.

In Figure 3, it can be seen that the channel 48 has a first side 62 that is relatively steep and a second side that has a moderate slope. The steep side 62 allows light entering through the entrance point 52 to cross the finger at an angle that is close to 90º. From Figure 3, it can be seen that the channel has a generally parabolic cross-section. The zone containing the light entrance and exit points and the inner surface of the channel extending between the entrance and exit points also has a parabolic cross-section. It is desirable to have the entrance point 52 as close to 90º to the light path as possible, as this maximizes the absorption of the light and minimizes the reflection of the light by the surface of the body part. The moderately sloped side 64 allows the channel to better accommodate fingers of different sizes. It can be seen that the inlet channel 55 has a tapered shape, with the inlet channel converging as it approaches the side wall 62. The exit channel 56 extends from the side 64, but neither diverges nor converges.

In Figures 4 and 5, the roller and torsion spring have been removed to expose a plunger 66 slidably mounted in the channel 48. A spring 68 is disposed between the plunger 66 and a plate 70 mounted over an inner end of the channel 48. When the plunger is at rest, the spring 68 urges the plunger outward toward the orifice 50 as shown in Figure 4.

When a finger 2 has been inserted through the opening 50 into the channel 48, the plunger is in Figure 5 inwardly so that the spring 68 is compressed and a projection 72 on the plunger 66 abuts a microswitch 74 mounted on a top of the base 46. The switch 74 is connected to activate the non-invasive monitoring device, measure variations in light intensity by activating the light source and allowing the light to pass through the finger 2. Software monitoring of the microswitch while the measurements are being taken ensures that the switch is closed and the finger is not moved. The spring 68 is located in an opening 76 within a support plate 78 above an inner end of the channel 48.

Surrounding the mouth 50 of the channel 48 is a foam collar 106 which includes an opening 107. The opening 107 is flush with the channel 48 so that when a finger is inserted through the opening, the collar forms a seal around the finger to prevent external light from entering the channel. A lens hood 104 prevents external light from entering the area of the light source. Further, a platform 102 is a rectangular plate arranged to support a user's hand when the finger is inserted into the channel. The platform 102 is arranged at an angle of 90º to the channel, but can be mounted to be arranged at a lesser angle, for example 30º.

The inlet channel 55 (shown in dashed lines) is stepped to converge at the side 62. An outlet channel 56 has parallel sides that are spaced much further apart in the horizontal direction than the sides of the inlet point 52 on the wall 62. The actual intersection of the inlet point 52 and the outlet point 54 is not shown in Figure 5, since these intersections are covered by the finger 2. The plunger 66 has an opening 84 (shown in dashed lines) to connect the entry point 52 to the exit point 54 when the plunger is in a rest position as shown in Figure 4. In this way, light can be directed through the entry channel 55, through the plunger 66 and out the exit point 54 and the exit channel 56 into the neutral density filter (not shown in Figures 4 and 5) when the monitor is calibrated using the neutral density filter shown in Figure 1. The plunger 66 has a recess to accommodate the user's fingernail.

In Figure 6 it can be seen how the finger 2 is conformed to the shape of the channel by the base of the channel 48 so that a zone between the entry point 52 and the exit point 54 is filled and connected by an inner surface 86 of the channel which runs between the entry point 52 and the exit point. It can be seen that the zone is significantly smaller than the finger. In this way the path length from the entry point to the exit point is constant for users who have fingers of different sizes and shapes. An outline of a typical finger bone 88 is also shown, where it can be seen that the bone is positioned above the light path running through the finger from the entry point to the exit point. The light can travel from the entry point 52 to the exit point 54 through a fleshy portion of the finger.

In Figures 7, 8, 9 and 10 the operation of the lens 12 is shown, with the neutral density filter 42 shown in greater detail than was described in Figure 1. The lens 12 is attached to the armature 40, which in turn is attached to a shaft 38. The shaft 38 extends through the base 46 parallel to the channel 48. At one end of the shaft 38, near the exit channel 56, the carrier 44 is attached to the shaft 38. The neutral gray filter 42 is mounted on the carrier 44. An electric motor 108 has a shaft 109 to which a positioning disk 110 is attached. The positioning disk 110 includes a pin 112 which extends from the outer circumference of the disk 110 into a slot 114 of the carrier 44. The carrier 44 is slotted along a line 116 and bent along a fold line 116 so that the neutral filter 42 is mounted at an angle relative to the exit channel 56. In this way, the filter 42 does not reflect light exiting the exit channel 56 back into the channel. As the motor 108 rotates the shaft 109, the disk 110 also rotates and the pin 112 moves within the slot 114 to pivot the carrier 44 about the pivot located on the shaft 38 and move the neutral density filter either from the position shown in Figure 10 to a position where the filter is aligned with the exit channel 56, or vice versa. As the carrier 44 rotates, the shaft 38 also rotates, which in turn causes the armature 40 to rotate about the pivot on the shaft 38. As the armature 40 rotates, the lens 12 moves from the position shown in Figure 8 to a position where the lens is aligned with the entrance channel 55. As this occurs, a lower edge 120 rests on an eccentrically mounted cam 122. The cam 122 is mounted on a shaft 124 of a scanning motor 126. When the lens is generally aligned with the entrance channel 55, the scanning motor 126 is energized to rotate the eccentric cam 122. This rotation causes the lens 12 to constantly move in a small cycle with respect to the entrance channel 55. Thus, when the finger is inserted into the channel 48 and the scanning motor 126 is energized, the lens will move continuously. and in turn continuously move the light entering the entrance channel with respect to the finger.

The non-invasive monitoring device is based on the principle of measuring the absorbance of mid-infrared radiation passing through a part of the body. According to the Lambert-Beer law, the concentration of the constituents is proportional to a proportionality constant (the attenuation coefficient), the path length and the absorbance (log [1/T], where T is the transmission, i.e. the ratio of light of a given wavelength transmitted through the matrix). By measuring the absorbance at a range of given wavelengths, some of which will limit the path length, it is possible to calculate the concentration of a given constituent.

There are several ways in which this absorbance measurement can be carried out, without limiting the present invention, two methods are as follows:

(1) Use the light from a scanning monochromator, pass it through a selected body part and collect the light transmitted through it on a silicon detector. A second measurement involves a measurement of the light transmitted in the absence of the body part. From these two measurements the transmission and hence the absorptivity can be calculated; (2) Use a polychromic light source, pass it through the body part to be measured, collect the light, direct it parallel to a diffraction grating and focus the different wavelengths of light on a linear matrix detector. Each element of the matrix will then measure the intensity of the light for a narrow band of wavelengths. A corresponding measurement in the absence of the body part (comparative scanning) will then calculate the transmittance for each element Since the different elements of the matrix have different dark loss currents, it is necessary to record a dark current and subtract it from both the sample scan and the comparison scan before calculating the transmittance and absorptivity.

With regard to the measurement of absorbance, a number of observations can be made.

First, as already noted, the absorption of light for a specified component at a particular wavelength is a function of the attenuation coefficient for the component at that wavelength and the effective path length.

Second, the effective path length is determined by a combination of the actual path length and the scatter.

Third, the attenuation coefficient tends to increase, sometimes by an order of magnitude, as the wavelength increases. The consequence of increasing attenuation coefficients is that the effective path length must be reduced to compensate in order to keep the absorbance measurements within a useful range for standard measuring instruments (for example, 0.2 - 2.3 OD).

Fourth, at wavelengths above about 1600 nm, the attenuation coefficient for water reduces the optimal path length to about 1 mm in non-scattering media. Since most useful sites for an in vivo measurement have a tissue thickness of more than 1 or 2 mm, the absorbance measurement in these cases and for these wavelengths is limited to a reflectance measurement. It should be remembered that for a reflectance measurement, the tissue penetration depth is about one and a half times the path length. This presents problems when using longer wavelengths, as discussed below.

The method of using polychromic light described above is the preferred method because a single rapid scan of the array allows the user to acquire data for all wavelengths at the same time. The monochromator allows the user to acquire data only sequentially at a rate determined by the scan speed, the detector integration time, and the number of wavelength steps scanned. For example, a typical monochromator requires 500 msec to scan 680 to 1150 nm. For a diode array, scanning all elements (680 to 1150) may take 5 msec. If a pulse (100 msec) from the heart increases the effective path length and thus the absorbance during the time of data acquisition, all wavelengths will be affected equally. In a monochromator, the pulse would increase the path length and thus the absorbance selectively for the wavelengths measured during the occurrence of the pulse.

There are several parts of the body where the measurements discussed here can be made. These include, for transmission, the finger, lip, earlobe, a fold of skin at the waist, and the skin between the thumb and forefinger. Virtually any skin surface can be used for reflectance. The question of appropriate path length is important for both reflectance and transmittance. In the case of reflectance, some of the scattered light may be reflected back to the detector. In some cases, there may be one or more structures that have a highly reflective surface (e.g., the periosteum of the skull) that reflects the light back to the detector after it has covered a short path length. To measure significant changes in absorption, a sufficient path length must be used. through the tissue before the light is measured at the detector.

Similarly, the body part must have a sufficient light transmittance path length to produce useful absorptivity values.

Regardless of the method used to measure absorbance, the detector has inherent limitations. In the wavelength range from 700 nm to 1200 nm, silicon detectors are commonly used.

The performance of these detectors has usually been optimized for around 750 nm. This means that the peak performance for a hypothetical light source with uniform intensity at all wavelengths would be at 750 nm. At longer wavelengths in the 1100 nm range, the response would drop to about three percent of the response at 750 nm. At shorter wavelengths, a similar reduction in response would occur, to about three percent at 300 nm. The reduction in response at the upper and lower limits of the detector means that the measurement of absorbance in these ranges will be more noisy, i.e. have an increasing measurement error. Since in this patent application the absorbance is to be measured at the longer wavelengths, it is necessary to average the results of several scans to obtain a better estimate of the absorbance.

Light entering the body is scattered, and the light leaving it spreads out in virtually any direction. This light scattering means that a limited part of the light can be captured by a lens system placed at a point where the light exits the body part. Furthermore, the light captured for diffraction on a diode matrix must be directed parallel, which means that the the useful light available to the measuring instrument is further reduced. The consequence is that to measure the light incident on a diode grid it is necessary to integrate the light over a period of about 200 milliseconds. This period of time provides a useful amount of light for measuring the transmitted light. To prevent saturation of the grid/detector during the reference scan it is necessary to place a neutral density filter in the reference beam path. The absorptivity can be expressed in the usual way, and by adding the constant OD of the neutral density filter at a given wavelength to the calculated absorptivity it is possible to indicate the total effective density resulting from both absorptivity and scattering.

There are several goals in designing a tissue interface:

(1) Minimizing variability and maximizing

Repeatability of readings;

(2) Passing the light through the desired body tissue;

(3) Optimizing the path length and minimizing its change between objects; and

(4) Maximizing light throughput during measurement.

Absorption begins at the point where the light enters the tissue. In the case of transmission, as the light passes through the tissue, more and more light is absorbed as the path length increases. Of course, if the path length is too long, very little light remains for measurement and the calculation of the absorbance will be subject to considerable error due to interference.

The constitution of the tissue through which the light is directed determines the absorption of the light at the wavelengths measured. If the components to be measured are in the blood, it is preferable to send the light through the capillary bed beneath the skin. For the same reason, layers of fat or connective tissue should be avoided in order to minimize absorption in these tissues. The fingertip is considered the body part of choice for the following reasons.

The shape of the entrance port 52 is circular and a suggested size is 4 mm in diameter. At the exit port the light is scattered almost equally in any direction up to 60º from the optical path. Since the light is to be collimated, only the light emitted in accordance with the optical system is used, so the plane of the exit port is far less critical and can be located at an angle between 45º and 90º to the central optical axis. A suggested size for the exit port 54 is 4 mm by 10 mm with the long side parallel to the finger to increase the area from which the light can be collected.

With ideal positioning of the finger, the path length will be constant for different users. In practice, however, there will still be some deviations in the path length for different users due to finger positioning. To ensure that the measurements of the absorbance are as reproducible as possible, the receptor is designed so that for different individuals the finger can be placed as consistently as possible in the same position on repeated occasions. This reproducibility of placement in the receptor applies not only to the position along the length of the finger, but also to the location of the entry point and Exit points in the canal on the lateral views of the finger. This reproducibility of finger placement is best achieved by creating a surface on which a palm can be rested while at the same time positioning the hand so that the middle finger can be inserted vertically into the receptor. When the finger is fully inserted into the canal, the roller will apply pressure over the first knuckle.

Even under ideal conditions, the path length will increase with each pressure pulse associated with the user's heartbeat.

Preferably, the hand is placed on a board that is at an angle of approximately 30º to the angle of the receptor. When the finger is inserted, a hood, preferably made of an opaque fabric, covers the hand and fingers while the measurements are being taken.

Preferably, a constant torsion spring is used to maintain a constant pressure on the finger for fingers of different sizes. A leaf spring could also be used, but the pressure applied to the finger will change with the size of the finger.

The combination of the roller pressing on the first knuckle of the finger and the shape of the channel results in the finger being molded into the shape of the channel.

Averaging a number of measurements at a particular wavelength will minimize the interference and give a better estimate of the true "value" of the absorbance at the particular wavelength chosen for averaging. The same averaging approach can be used to give a better estimate of the "true" finger absorbance for a particular wavelength by eliminating the effects of variability in finger placement. and finger shape are averaged out. There are two ways that can be used to perform this averaging.

One way is to repeatedly place the finger in the light path and take a measurement each time. This way is obviously time consuming and tiring for the person. The second way involves some form of mechanical movement of the finger or light path, which eliminates the need to move the finger out of the measuring instrument during the measurement.

It is possible to move the subject's finger relative to the light path, which would simulate repeated insertion of the finger and allow an average of the results. In this process, it is important that the positioning of the finger in the light path is carried out in as non-systematic a manner as possible; otherwise a systematic error in the average could occur. Conversion of the movement to random characters can be carried out by randomly varying the length of time the finger is moved. There are two ways in which the finger can move in this manner:

(a) Movement between measurements. This can be accomplished by moving the finger, then scanning the finger to take an average absorption, moving the finger again, scanning again, and repeating this process as many times as desired to achieve an averaged placement; and

(b) It is also possible to move and scan the finger continuously, so that an average is formed simultaneously for both the movement and the scanning, thereby reducing the time required to capture the necessary data is needed. The danger of this approach is that the synchronicity of the sampling and movement speed distorts the averaged results.

Since the movement of the finger with respect to the optical path is the same as the movement of the optical path, the results of the finger movement can also be achieved by moving the optical path either as a whole or as a part. In fact, tests have shown that simply by making small movements using the single lens that focuses the light on the finger, it is possible to average the finger position in a manner that corresponds to the repeated placement of the finger.

The light trajectory can be varied relative to the finger so that regardless of how the finger is positioned, the light trajectory through the finger can be averaged to reduce the variation between repeated measurements. Preferably, the light trajectory is varied by changing the focal point of the light entering the finger. Collated light is directed to a lens which focuses the light onto the finger, the placement of the lens in the collimated beam being varied by an eccentric cam as measurements are taken. The light is scattered after entering the finger. Some of the light is absorbed, some is scattered and reflected, and the remaining part is transmitted through the finger. Of the light transmitted through the finger, a portion is collected by an optical system, collimated, passed through a diffraction grating, and focused onto a linear array detector. The absorptivity is calculated by expressing the light transmitted through the finger as a percentage of the light used to illuminate the finger.

When the finger is removed to allow a measurement of the reference light intensity for calibration purposes, it is preferable to pass collimated light through the finger receptor. To make the reference measurement, the second motor moves the scanning lens 12 just beforehand out of the collimated light beam. The collimated beam passes through the receptor's entrance and exit ports, which are aligned with the optical system. Since the light passing through the receptor is much more intense when the finger is removed, it is necessary to position a neutral density filter in the path of the reference emitter to prevent saturation of the linear array detector. This is accomplished by using the second motor so that the filter is positioned just behind the exit channel at the same time that the focusing lens is removed from the light path. Since the attenuation of the neutral density filter is a constant, the absorptivity measurement can be expressed in relative terms or calculated by adding the constant to all the absorptivity readings.

Alternatively, a two-beam optical system can be used and, for the purpose of taking a reference measurement, a portion of the light can be reflected around the receptor by the same light collection system used with the receptor. Since only a portion of the light is reflected, the light intensity can be reduced by reducing the amount of light reflected and the neutral density filter can be eliminated.

Claims (22)

1. Finger receptor (2) for use with a non-invasive monitoring device for determining the concentrations of various blood or tissue components, the receptor being used with a light source for receiving fingers (4) from various users and having a base (46) containing a longitudinal channel (48) which is dimensioned for individually receiving fingers, the channel having an opening (50) for receiving the finger and an inner surface (86) with two sides and an entry point (52) for a light path on one side and an exit point (54) for the light path on another side, the inner surface extending between the entry point and the exit point, the entry point and the exit point being dimensioned and arranged such that at least a portion of the light passing through the entry point is captured at the exit point, and the receptor having sensing means (74) for determining whether a finger is correctly positioned in the channel, characterized in that the channel has a fixed cross-sectional shape, that the inner surface extends between the entry point and the exit point and forms a zone and that the zone has a cross-sectional shape such that fingers of different sizes, individually correctly inserted into the channel, completely fill the channel zone located between the entry point and the exit point, so that no light from the entry point can reach the exit point without passing through the finger arranged in the receptor from the entry point to the exit point, the light originating from the light source forming an optical path from the entry point to the exit point. 2. A receptor according to claim 1, wherein means are provided for reducing the intensity level of the light source when a reference measurement is made using the monitoring device. 3. Receptor according to claim 1, wherein the channel has a cross-sectional shape with a bottom that curves in a continuous manner to a first side (62) and a second side (64), that the first side is provided with the entry point arranged therein and is relatively rigid and that the second side is provided with the exit point arranged therein and has a small slope compared to the first side. 4. A receptor according to claim 3, wherein means are provided for moving the optical path relative to the finger as light from the light source is directed along the light path. 5. Receptor according to claim 1, wherein pressure means are provided to slightly press the finger into a suitable position when it is inserted into the channel, the pressure exerted being sufficient large enough to slow but not stop blood flow in the finger and to concentrate blood adjacent to the entry and exit points. 6. Receptor according to claim 4, in which the means for moving the optical path relative to the finger are formed by a movable lens (12) which is mounted between the light source and the finger when the finger has been properly inserted into the channel. 7. A receptor according to claim 2, wherein the lens is mounted on a shaft (38) connected to and movable by a scanning motor (126), the motor moving the lens relative to the finger when the monitoring device is in operation and the finger has been properly inserted into the channel. 8. Receptor according to claim 7, in which the means for reducing the level of light intensity are formed by a filter (42) mounted between the light source and the monitoring device. 9. A receptor according to claim 8, wherein the filter is mounted on the same shaft as the lens, but is offset from the lens so that when the shaft is moved by a motor (108) to position the lens between the light source and the finger, the filter is not positioned to reduce the light intensity, and when the shaft is moved by the motor to position the filter to reduce the light intensity from the receptor, the lens is not positioned between the light source and the finger. 10. A receptor according to claim 1, wherein the sensing means are connected to automatically activate the monitoring device when the finger is properly positioned in the channel. 11. A receptor according to claim 1, wherein a platform (102) is arranged outside the channel for supporting a hand of the user so that the movement of the finger of the hand is minimized during operation of the monitoring device. 12. Receptor according to claim 2, in which the means for reducing the level of light intensity are formed by a filter (42) arranged in a path of the light. 13. A receptor according to claim 2, wherein the means for reducing the level of light intensity are reflection means by means of which a portion of the light can be monitored. 14. Receptor according to claim 2, in which the means for reducing the level of light intensity are realized by a reduction of the integration time when performing a measurement. 15. Receptor according to one of claims 1 or 2, in which the channel is arranged at an angle of about 30º to the horizontal. 16. A receptor according to claim 13, wherein the reflecting means are formed by a series of reflectors which reflect a portion of the light from the light source around the receptor so that the The amount of light received by the monitoring device is reduced in intensity. 17. A receptor according to claim 1 or 2, wherein a plunger is slidably mounted in the channel which is spring loaded and biased towards the opening so that the plunger is urged away from the opening by the finger when the finger is inserted into the plunger, the sensing means being triggered when the finger is properly positioned in the opening. 18. Receptor according to one of claims 1 or 2, in which the pressure means are formed by a roller (16) rotatably mounted on an armature (59) loaded by a torsion spring which is biased towards the channel and arranged to come into contact with the finger when the latter is inserted and arranged in the channel. 19. Receptor according to claim 1, wherein the light entry point into the channel has a circular shape and the light exit point from the channel has an elongated shape and is larger than the entry point. 20. A receptor according to claim 19, wherein an entry channel (55) through the base converges toward the channel and an exit channel (56) has a constant size from the channel through the base. 21. A receptor according to claim 20, wherein the entry point converges in three stages, the side walls of the entry point being arranged parallel to each other. 22. Receptor according to one of claims 1 or 2, in which the user's finger has a phalanx (88) and the channel is dimensioned such that the zone is substantially smaller than the finger and is shaped such that the light path extending between the entry point and the exit point is located near the bottom of the channel under the phalanx when the finger is properly inserted into the channel.