The invention concerns a method and an apparatus for re¬ producible measurement of reflected energy from an object, wherein the object is irradiated with at least one source of energy, the reflected energy is detected, and an inten¬ sity related signal is formed for at least one spectral band in said detection.

Devices capable of determining the colour of an object are known in a plurality of embodiments, said devices usually measuring the reflectance of the object in a plurality of spectral bands. These bands may e.g. be formed by causing several alternate lighting sources to illuminate the ob¬ ject, where the light energy reflected from the object is detected with a wide-band detector. On the basis of the intensity related signal collected from the detector it is possible to generate a plurality of signals with suitable signal processing which represent the spectral bands formed with the individual light sources and optionally the background light. Another example is described in DE 3 244 286, where a wide-band light source illuminates an ob¬ ject, the light energy reflected from the object being passed to three detectors which each gives an initial re¬ presentation of the intensity of the light impinging on the detectors after passage through suitably selected fil¬ ters. These devices are useful in connection with colour or reflection measurement, where the device can be engaged with the object, or where the position of the object can be determined accurately with respect to the device. This is a significant drawback when the object whose colour is to be determined has a deformable surface, where liquid, drainable by pressure impact, contributes significantly to the colour or reflectance of the object.

The object of the invention is therefore to provide a method with which it is possible to perform reproducible measurements on objects whose state is changed by pressure impact.

This object is achieved in that the detector and the ob¬ ject are moved mutually, during which movement the object is present within visual field of the detector. During this movement a plurality of intensity related signals is currently collected with the detector, one of which being selected, and successive signal levels thereof being com¬ pared. When these successive signal levels in the selected intensity related signal describe a mutual predetermined relation, timewise associated signal levels in the inten- sity related signals are scanned for representation of the energy reflected from the object. It may be expedient in some cases that several intensity related signals are selected and evaluated simultaneously.

When the signals are pulsed as stated in claim 2, it is possible to eliminate the intensity originating from back¬ ground radiation. When light is used as stated in claim 3, it is possible to determine the colour of the object, and when using the source detector configurations stated in claims 4 and 5 the energy reflected from the object may be classified within well-defined spectral bands or as back¬ ground noise or radiation. Claim 6 defines a particularly advantageous object on which the method may be used.

It is stated in claim 7 how a signal representing a spec¬ tral band defined by the source detector configuration is used as a trigger signal in that the signal, after engage¬ ment with e.g. a tissue surface, exhibits a well-defined horizontal tangent, which may be used for providing a re- producible set of signal levels in the spectral band in question.

A trigger signal essentially independent of the reflection properties of the object is obtained by selecting the in¬ tensity related signal as a representation of the back¬ ground energy or radiation. Thus, e.g. the inflection of the curve may be used for selecting the set of signal values which represent the colour or reflection properties of the object.

Claim 9 defines a measuring apparatus with a known source detector configuration, characterized in that the appara¬ tus comprises means for calculating intensity related data signals in a plurality of frequency bands as a function of time while the apparatus is moved with respect to the ob¬ ject, means for storing the intensity related signals, means for comparing successive values in a selected inten¬ sity related data signal and for selection of associated values among the intensity related signals as a represen¬ tation of the energy dissipation properties of the object when succesive values of the selected intensity related data signal exhibit a predetermined mutual relation. The means for storing the intensity related data signals can both comprise analog storage means, e.g. in the form of magnetic tapes, or electronic storage means for storing selected (sampled) data signals. The values may also be entered in respective buffer registers where signal pro¬ cessing takes place currently, the buffer values being rejected if the selected intensity related data signal does not comply with the predetermined relation. Then new values may be entered in the buffer register so that the procedure may be continued until the predetermined rela¬ tion is achieved and the measure of the energy dissipation properties of the object thus determined may be read out and used for the purpose where a measure of the energy dissipation properties of the object is needed.

The method and the apparatus of the invention are particu¬ larly useful for measurements on e.g. humans since reflec¬ tance, absorption and transmission of electromagnetic ra¬ diation, including light, are used for measurement on living organisms (e.g. measurement of erythema, pigment concentration, concentration of various substances and tissue fluid, blood sugar in veins or tissue fluid). Since the colour of living organisms depends upon the pressure applied by the measuring probe to the surface, the measur- ing probe is approached manually or automatically to the object while light emitter and light detector are acti¬ vated. Thus, e.g. the intensity of false light/background radiation and reflectance (and thereby absorption) from the measurement object are currently measured. The desired set of measurement values is selected (calculated) automa¬ tically or manually on the basis of comparison of the curve inclination of background light and/or the inclina¬ tion (e.g. inflections) of reflected light curves. This calculation can be performed during collection of measure- ment data, but may also be performed after the termination of the measurement procedure. Further, the apparatus can automatically initiate storage of data from a point of time where the level of false light is below a certain limit such that storage space in the memory will be saved. It may also e.g. be expedient merely to examine the be¬ haviour of a signal and to initiate collection of data values for the other signals when this signal exhibits a predetermined level.

It is also contemplated to use e.g. ultrasound transducers instead of light sources, where a detector then collects the reflected signal. Instead of refractive index, it is then the acoustic impedance and changes of it which give a characteristic image of the reflected energy.

The invention will be explained more fully below with re¬ ference to the drawing, in which

fig. 1 shows a preferred embodiment of a measurement appa- ratus according to the invention,

fig. 2 shows the component mounting plate in the measure¬ ment apparatus shown in fig. 1,

fig. 3 schematically shows the signal processing part which is connected to the components shown in figs. 1 and 2,

fig. 4 shows the signal collected by the detector of fig. 2 as a function of time,

fig. 5 shows an example of a data set collected with the method of the invention, where a measurement apparatus is placed against an object with varying engagement pressure, and

fig. 6 shows the same with other parameters.

The apparatus shown in fig. 1 comprises a window disc 10 which is of a transparent plastics material in the pre¬ ferred embodiment, but may alternatively be of a glass material or the like. The window disc 10 forms an end face in a cylindrical measurement chamber which is also defined by a substantially solid measurement head 15 and a compo- nent mounting plate 20, which has a plurality of light sources 25a-d on the side facing the measurement chamber which are formed by four light diodes (LEDs) in the shown example which are arranged around the wide-band detector 22. The light diodes may e.g. emit light with a band width of 20 nm and have respective center wavelengths of 500 nm, 540 nm, 580 nm and 650 nm. The assembly of the measurement

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chamber takes place in a known manner by means of gluing, hot welding or the like. Supplies to the electronic compo¬ nents 22, 25 on the component mounting plate 20 may be passed through said plate for further signal processing, which may take place internally in the apparatus, but the signals may also be transferred to an external unit where data processing can be performed there, optionally after termination of the measurement procedure.

The window disc 10 is shown as a plane plastics plate, but may be provided with lens effect if this is expedient in the case in question. This may be done with ordinally con¬ vex or concave lenses, but in addition it is also possible to use holographically produced lenses. The window disc 10 may be provided with filter effect in addition to the one already present, which is conditional upon the material. Antireflection layers and polarization layers may be applied as needed. In many cases it is not necessary to use a window disc, which can therefore be omitted.

Time control and signal processing may e.g. be integrated in a measurement apparatus such that a time control unit 40 control the light diodes 25a-d so that they are alter¬ nately supplied with voltage. It has been found expedient e.g. to have four light diodes lighting intermittently so that a cycle will be performed with eight time intervals of substantially the same length, where the first, the second, the third and the fourth light diode 25a-d, re¬ spectively, lights in the first, the third, the fifth, and the seventh time interval, while none lights in the last half of the time intervals. As will be seen in fig. 4, the sensor 22 can collect a continuous signal which, in the first, the third, the fifth and the seventh time interval, is the sum of the background radiation or noise and the light reflected from the objects which originates from one of the four light diodes 25a-d. In the second, the fourth,

the sixth and the eighth time intervals the signal gene¬ rated with the sensor 22 exclusively originates from the background radiation. The output signal from the detector 22 may be processed analogously for obtaining the informa- tion concerning colour or reflection properties of the ob¬ ject which is contained in the signal. However, it is pre¬ ferred to apply the output signal to an analog-to-digital converter 42 where the signal is thus sampled, which can expediently take place with a sampling frequency so that the voltage value in the output signal is scanned in the middle of the time intervals mentioned previously. Alter¬ natively, the voltage level may be integrated over the individual intervals to form a representation of the inci¬ dent light intensity.

Suitable subtraction of the signal values thus digitized or sampled will thus provide five intensity related sig¬ nals as a function of time, one for the background light or noise and four for the spectral bands formed with the light diodes. This takes place in a signal divider circuit 44 and corresponds approximately to demultiplexing. These sampled data signals define the light intensity caused by various sources as a function of time. The four data sig¬ nals caused by the light diodes 25a-d represent the colour of a possible object which is placed in front of the measurement apparatus, while the last signal represents false light or noise.

In case of measurements on living organisms, e.g. measure- ment of erythema, pigment concentration, concentration of various substances in tissue fluid, blood sugar in veins or tissue fluid or e.g. in case of measurement of water content in cheese, it is not readily possible, on the basis of pressure dependent liquid drainage, to measure the colour or reflectance with the apparatus engaged with a deformable surface of the object. The best example of

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such an object is the surface of skin, where it is attempted to determine the skin content of pigments by colour measurement and thereby also tan as well as skin redness. This may be expedient if it is attempted to evaluate the period of time a person can be exposed to sunlight with a given intensity, without this giving the person irritation problems because of sun-scorching and the like.

This is done in that the previously mentioned colour measurement apparatus is approached to the object manually or automatically while the light sources are activated, as described before, and the detector currently emits an out¬ put signal. One optionally more intensity related signals are selected among those mentioned before, successive signals being compared or individual values being compared with a predetermined threshold value. When a predetermined relation is achieved, the point of time of the collection of the value is determined, or the sample number of the value and associated values of the wavelength bands gene¬ rated by the light diodes 25a-d are read out as a repre¬ sentation of the colour or reflectance of the object.

Fig. 3 shows how a voltage supply circuit 35 supplies the components of a signal processing circuit with voltage, including the four light diodes 25a-d shown in figs. 1 and 2, through a time control unit 40, the voltage being sup¬ plied in the manner described before. A photoactive ele¬ ment 22, such as a photodiode, generates the signal in response to the intensity of the light reflected from the object, said signal being passed to an analog-to-digital converter 42. The analog digital converter 42 samples the signal shown in fig. 4, the sample frequency being con¬ trolled by the time control unit 40. Sampling can advan- tageously be performed such that the scanned values re¬ present the voltage level with the time intervals pre-

viously mentioned. The output signal from the analog-to- digital converter 42 is applied to a signal divider cir¬ cuit 44, whose function corresponds to demultiplexing of an arrived digital data signal, such that four signals will be formed on the output of said circuit 44 in this example, representing the spectral bands formed by the source detector configuration, and a signal which repre¬ sents background noise. The signal representing background noise is applied to a determination circuit 46 in which successive signal levels are compared or a single one is compared with a fixed threshold value. If the comparison of data values describes a predetermined relation, the determination circuit 46 applies a trigger signal which is passed to a circuit 48 for storage and reading out of data. This storage circuit may be built in many known ways and may e.g. consist of a buffer store in which informa¬ tion concerning the measured colour is entered currently and is read out in case of a trigger signal for represen¬ tation of the colour measurement result under reproducible conditions. Until the trigger signal arrives, the oldest values in the buffer store are replaced by new measurement values. Alternatively, all five signals may be stored and read out at the termination of the measurement procedure to an external, central processing unit. After the desired measurement data have been found, these are transferred for use in a unit irrelevant to the invention.

Fig. 5 shows an example of reflectance measurement where the measurement head is approached to the surface of the skin. The X-axis represents time, in the form of the sample number, which corresponds to the number of the digitized value with respect to the start time. Each sample contains measurements which represent three spec¬ tral bands (in contrast to the example with four bands described before), as well as a background measurement. The sample frequency is selected to 10 ms in this case.

The Y-axis may be any representation of intensity and is in this case selected as reflectance indicating the inci¬ dent intensity with respect to the intensity spread from an optical white face. It will be seen from the curves that the signal level of the three spectral bands defined by the source detector configuration increases until sample number 30, and then a substantially constant level is maintained since the engagement pressure of the probe against the measurement area is constant. It will likewise be seen that the background radiation B decreases until the same sample number, following which this signal too is constant. It should be noted that the representation of the background measurement is scaled down with a factor 100. It will be readily apparent that the engagement with the skin surface takes place precisely in the inflection of each of the signals in the colour bands formed. They can likewise define unambiguous limit values of the signal of the background radiation.

Fig. 6 shows what happens when the measurement head is pressed against the skin after engagement. It will be seen that the levels change both absolutely and relatively so that the reproducibility disappears. The reason is that the blood volume of the skin changes, and the measurement is therefore not performed correctly if it is not per¬ formed precisely at the engagement moment before pressure is exerted on the skin.