GB1590598A

GB1590598A - Apparatus intended to determine the quantity of sebum secreted by a skin

Info

Publication number: GB1590598A
Application number: GB4358177A
Authority: GB
Original assignee: LOreal SA
Current assignee: LOreal SA
Priority date: 1976-10-20
Filing date: 1977-10-19
Publication date: 1981-06-03
Also published as: DE2746994A1; IT1093016B; DE2746994C2

Description

(54) APPARATUS INTENDED TO DETERMINE THE QUANTITY OF SEBUM SECRETED BY A SKIN (71) We, L'OREAL, a French Body Corporate, of 14 Rue Royale, 75 Paris, 8eme, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: It is known that it is frequently necessary to evaluate the optical characteristics of a sample, by way of transparency or reflection.

This is, in particular, the case when comparative measurements are to be made with the aim of establishing the quantity of sebum deposited, by the skin of a subject to be studied, on a ground glass carrier. To obtain perfectly reproducible measurements, methods proposed so far have used a luminous flux which was as nearly as possible monochromatic and avoided the influences of interference radiation for example ambient light or sources of heat-transmitting infra-red radiation. This technique makes it necessary to operate in a dark enclosed environment.

Apparatuses have already been proposed which allow comparative measurements of the sebum secretions of skin by applying a ground glass strip to a zone of the skin and by examining the glass strip to determine its transparency in a substantially monochromatic beam emitted by a spectrophotometer. A "sebumetric apparatus" designed on this principle, generally comprises an emitter bulb, for instance, a mercury vapour lamp, a monochromatic prism or a grating for separarting out an emission extending over a narrow frequency range, and a photomultiplier device. Such apparatus is considerable in size and is expensive. It allows a determination of the quantities of sebum secreted by a zone of the skin since there is a relationship between, on the one hand, the transparency of the ground strip after application to the skin zone to be studied for a predetermined time and with a predetermined pressure and, on the other hand, the quantity of sebum deposited on the said strip.

A simplified apparatus for determining the quantity of sebum emitted by a skin zone has already been proposed and relies on shining a luminous non-monochromatic flux produced by an electric light bulb on a translucent film which has been applied to the skin bone to be studied, and reviewing the flux transmitted by way of transparence through the film on a photo diode which is in circuit with a milliamperemeter and a battery. This apparatus is much simpler in design than the spectrophotometers previously used, but has the drawback that it does not supply completely reproducible measurements, on the one hand because of the ageing of the lamp emitting the luminous flux, and on the other hand because of variations in output of the battery in circuit with the milliamperemeter and the photo diode.

It is an aim of the present invention to provide an apparatus which is simple in design, of small dimensions and cheap, to effect a comparative measurement of the sebum secretion of a skin zone by relying on the relationship existing between the quantity of sebum secreted and the transparence of a trans]ucent strip applied for a determined time and pressure to the skin zone to be studied.

Accordingly the present invention provides apparatus for determining the quantity of sebum secreted by skin, comprising a transparence of a translucent strip applied against a zone of the skin to be studied; an emitter capable of supplying a luminous flux and positioned to direct the flux onto one face of said translucent plate; a receiver for measuring the luminous flux transmitted from the emitter by way of transmission through the said translucent plate to the opposite face thereof; and means for indicating a measurement; the flux emitter being effective to supply a modulated beam at a fixed frequency, and the flux receiver being effective to detect only the received flux which has the modulated frequency of the emitter.

One of the essential advantages of the apparatus according to the invention lies in the fact that the comparative measurement may be effected without isolating the emitting device, the receiving device and the translucent plate from the parasitic radiations of the surroundings such as the surrounding light or neighbouring heat sources, for instance. The apparatus according to the invention may use non-monochromatic luminous flux, for instance a flux obtained by several electro-luminescent diodes.

In a first embodiment, at least one luminous flux emitter and at least one luminous flux receiver are arranged on either side of the translucent plate.

Alternatively, in a second embodiment, the translucent plate is arranged with one of said faces opposite a reflecting device, the emitter and receiver being both arranged on the other side of the translucent plate (i.e. away from the said reflecting device).

The or each emitting element of the luminous flux may be constituted by an electro-luminescent diode; each luminous flux receiving element may be a phototransistor.

In the said second embodiment, in which the translucent plate is associated with a reflecting device, an assembly of two coaxial optical fibre bundles is used with one end of the assembly connected to an emitterreceiver unit and the other end arranged opposite the translucent plate subjected to the luminous flux. The peripheral optical fibre bundle is supplied by the flux emitter and the central optical fibre bundle is connected to the flux receiver; that end of the optical fibre bundle assembly which is not connected to the emitter-receiver unit is arranged opposite that one of said faces of the translucent plate which has been applied to the skin to be studied, the other side of the said translucent plate being applied against a mirror.

For the two alternative embodiments mentinned above, the translucent plate receiving the lum;?noiis flo from the emitter may be made of ground glass.

The flux emitter may comprise a single emitting element whose flux lies within a narrow wave range spread round a mean wave-length, the distribution of the wave lenatll around the mean wave-length being substantally effected according to a Gaussean curve.

Aliclnatively, the flux emitter may comprise a m!l!-ipJicity of emitting elements, and 1or the flux receiver may comprise a mtllti;,'icity of receiving elements. In that case, the apparatus may conveniently comprise n emitter elements with parallel axes emitting radiation in the same wave range (n being an integer higher than 1), and iz receiver elements each associated with one of said n emitter elements, the measurements obtained from each receiver element being summed to give a mean value relating to the whole of the zone of the examined translucent plate.

It may conveniently be arranged for the photo receiver to be connected as a current follower without any amplifying function.

The receiver element may feed a variable gain amplifier set to the modulated frequency of the luminous flux emitted by the corresponding flux emitter element; the selective gain amplifier may conveniently act like a low pass filter and be placed in series with a high pass filter. The selective gain amplifier is preferably followed by an amplifier stage and a rectifier stage for the sinusoidal current. The output of the signal supplied by the flux receiver is effected by a filter stage comprising means for adjusting the level of the output voltage.

One of the essential advantages of the apparatus according to the invention is due to the fact that the modulation of the luminous flux emitted, and the setting of the receiver to the modulated frequency, makes it possible to eliminate any effects of spurious emissions from the surrounding light or heat sources. The whole circuit of the apparatus according to the invention functions entirely in synchronisation with the flux emitter. Thus a perfectly reproducible measurement is obtained without any isolating precaution in relation to the surrounding radiation emissions. The emitter flux amplitude may vary very slowly in the course of time, due to the ageing of the emitter, but this variation has now great importance in view of the fact that the measurements effected are comparative and presuppose an initial calibration adjustment of the apparatus on the basis of a measurement effected on a translucent reference or calibration element.

In the said second embodiment of the invention the accuracy of the measurement is improved without reducing the simplicity, by using the apparatus so as to effect a double passage of the modulated beam through the translucent plate.

The present invention also provides a process for determining the quantity of sebum secreted by the skin; comprising placing a translucent plate in contact with an area of skin for a given time at a given pressure, passing pulsed luminous flux from an emitter through said p]ate and measuring the flux transmitted by the plate using a phototransducer tuned to detect only flux modulated with the pulse frequency of the said luminous flux from said emitter, and comparing the value of flux detected by said transducer with the value obtained w'len the plate has not been applied to the skin.

In order that the present invention may more readily be understood the following description is given merely by way of example, of two embodiments of the apparatus according to the invention. Reference will be made to the accompanying drawings in which: Figure 1 schematically represents an apparatus according to the first embodiment of the invention; and Figure 2 schematically represents an apparatus according to the second embodiment of the invention.

The apparatus shown in Figure 1 is intended to measure the quantity of sebum secreted by a skin zone on a comparative basis. To do so in the known way, a translucent carrier, for instance a glass plate ground on one side, is applied to the skin zone to be studied for a given time and with a given pressure. This application of the carrier against the skin produces a sebum deposit which modifies the transparence of the carrier. In the case of a ground glass plate, the sebum is to be deposited on the ground surface of the plate.

It is known that there then exists a correlation between the transparence of the plate and the quantity of sebum secreted per unit time by the skin zone investigated.

A first calibration measurement is therefore effected using as sample a ground glass plate which has not been applied to the skin, and the apparatus is adjusted so that the measurement of the output current should correspond to a zero value.

The ground glass plate is then replaced by an identical ground glass plate which has been previously applied to the skin to be examined. This latter plate will now be more transparent than the calibration sample and the luminous flux received by the receiver corresponds to a higher output current which then gives rise to a positive nurnerical value relative to the zero setting of the calibration sample.

Referring now to Figure 1, it will be seen that the apparatus comprises a light emitter device 1 which cooperates with a receiver device 2, the output of the receiver device being applied to a display device 3.

The light emitter device 1 is energised with an electric current from a power supply of 5 or 15 volts stabilised to within 5 X 10-5 approximately. This feed is supplied at terminals A, B and C of the circuit diagram.

The emitter device 1 comprises an oscillator 4, supplying a sinusoidal signal, the frequency of which is chosen, for instance, as 3.104 Khz. Oscillator 4 feeds a shaping circuit 5 which supplies rectangular pulses of the same frequency as those of the oscillator. Circuit 5 feeds a power stage 6 which makes it possible to supply current whose intensity may be as high as 80 milliamps.

The electroluminescent diode 7 emits a luminous flux, schematically indicated by arrows 8, adjustable by means of rheostat 9. This luminus flux is modulated to the frequency of oscillator 4 and the flux intensity varies according to the rectangular pulses.

The flux 8 is transmitted to the sample, that is to say, in the particular case described, to the glass plate 10 which has a ground face which faces the incident flux 8. On the ground face, there has previously been deposited or not, according as to whether one is concerned with the effective or the calibration measurement, a certain quantity of sebum (outlined in the drawing by the dotted line 11). The luminous flux which traverses strip 10 falls on to phototransistor 12 supplied by the stabilised power supply of terminal A.

The emitter connection of phototransistor 12 is connected to ground by means of a resistor 13 and feeds a high pass filter 14 constituted by a 110 nano-Farad capacitor 15 and an adjustable resistor 16 having an approximate value of 470 Ohms. Filter 14 feeds the negative input of a variable gain amplifier stage 17.

The amplifier stage 17 comprises a subassembly constituted by a resistor 25 and a capacitor 26 as will be described in detail below. The sub-assembly 25-26 functions as a low pass filler which, associated with the high pass filter 14, only allows a current corresponding to a clearly determined frequency to pass to output lead 17a. Filter 14 and sub-assembly 25-26 are adjusted so that the frequency passing through is the frequency of oscillator 4, so that on output 17a there will be a sinusoidal voltage corresponding to the first sinusoidal component of the voltage of the rectangular wave pulses supplied by phototransistor 12 (which of course will have pulse repetition frequency the frequency of oscillator 4). Amplifier stage 17 is further constituted by an operational amplifier 18 whose negative input is connected to filter 14 and whose positive input is connected to ground. Between the negative input and the output of the operational amplifier 18, a "double T" filter set to the frequency of oscillator 4 has been interposed for feed-back and consisting, for the first T, of two capacitors 19 and 20 whose centre point is connected to ground by a resistor 21 and for the second T by two resistors 22 and 23 whose centre point is connected to ground by a capacitor 24.

Between the negative input of operational amplifier 18 and its output, there has also been arranged, in parallel, with respect to paths 19, 20 and 22, 23, a third path constituted by an adjustable resistor 25 of approximately 120 Khz and a capacitor 26 of 390 picoFarad.

Output lead 17a of the amplifier is connected to the negative input of an amplifier unit 27 as a whole and constituted by an operational amplifier 28 whose positive input is connected to ground by means of resistor 29 and whose output is connected back to the negative input of the operational amplifier by means of a resistor 30. The amplifier 27 allows a gain of 10 to be obtained and its output is applied to the input of a full wave rectifying stage 31. The rectifier stage 31 is constituted by an operational amplifier 32 which receives the output of amplifier 27 on the negative terminal, via a resistor 32, whilst its positive terminal is connected to ground via a resistor 34. Between the output and the negative input of operational amplifier 32, there has been interposed a circuit comprising two parallel paths in each of which there is placed a diode 34, 36 and a resistor 37, 38, and the two diodes 35 and 36 being polarised in opposite directions.

The output of rectifier stage 31 is effected, on the one hand, by a tapping between diode 36 and resistor 38 and, on the other hand, by tapping off in front of resistor 33, these two paths being combined in a filter stage 39 by means of resistors 40 and 41 respectively. The common output of resistors 40 and 41 is connected to the negative input of an operational amplifier 42 which forms part of the filter stage 39. Shunted across the negative input and the output of this operational amplifier 42 is a parallel pin consisting of a capacitor 43 and a resistor 44. The positive terminal of the operational amplifier 42 is connected to the stabilised supply voltage by terminal C with the interposition of an adjustable resistor 45, the said adjustable resistor making it possible to adjust the output voltage of operational amplifier 42 to the required level.

The output of filter stage 39, that is to say, the output of operational amplifier 42 is applied to a digital voltmeter which constitutes the display device 3 of the apparatus according to the invention. The adjustable resistor 45 allows an adjustment to ensure a "zero" display on the display device 3 when the plate 10 is a calibration plate which does not comprise an sebum deposit. In other words, the adjustable resistor 45 permits the initial calibration of the apparatus.

It is clear that the apparatus according to the invention allows a comparative measurement of a sebum deposit to be made because there is a correlation between the transparence of plate 10 and the quantity of sebum deposited on the said plate. It is also clear that the modulation applied to the emission of electroluminescent diode 7 makes it possible to eliminate all spurious emissions including those corresponding to the ambient light (i.e. daylight or room lighting) and to infra-red emissions due to nearby heat sources. Measurements may therefore be effected in the open air which considerably simplifies "sebumetric" measurements. The apparatus is, moreover, small in size and reasonable in price. Finally, it has been found that an apparatus constructed according to the invention was very reliable and ensured a good reproducibility of the comparative measurements effected.

In the embodiment described above, the electroluminescent diode 7 emits a luminous flux of red light but the choice of the mean wave length of the luminous flux 8 emitted has no critical importance for the performance of the apparatus according to the invention.

Figure 2 shows a second embodiment of the apparatus according to the invention, in which the translucent carrier plate 10 is prepared in the same way as for the first embodiment.

Reference to Figure 2 shows that the apparatus there illustrated comprises three supply terminals A1, B1, C1 which receive a current stabilised at 5 or 15 volts to within 5 x 10-5 approximately.

Terminal B1 feeds an oscillator 104, supplying a sinusoidal signal at a frequency of, for instance, 3.104 Khz. Oscillator 104 feeds a shaping circuit 105 which supplies rectangular pulses of the same frequency as those of the oscillator. The shaping circuit 105 in turn feeds a power stage 106 which delivers a current of up to 80 milliamps. The power stage 106 feeds the photoemitter 107 of an emitter-receiver unit 101.

The emitter-receiver unit 101 also comprises a photo transducer 112 and it is associated with a double optical fibre bundle assembly designated 102 in its entirety. The double optical fibre assembly 102 is constituted by a central fibre bundle 102b and a peripheral fibre bundle 102a arranged coaxially therearound, each bundle being capable of transmitting a luminous flux.

Peripheral fibre bundle 102a is connected, by its end secured to unit 101, to the photoemitter 107, and the central fibre 102b is connected by its end linked to unit 101 to the photo transducer 112. The optical fibre bundle assembly 102 has its free end arranged opposite a glass plate 110 which has a ground face opposite the end of fibre bundle assembly 102. On this ground face, on which there may or may not previously have been deposited a certain quantity of sebum outlined in the drawing by the dotted line 111. The other face of the glass plate 110 which does not carry the sebum 111, is made to rest against a flat mirror 110a.

The power stage 106 supplies the photo emitter 107, via rheostat 109, and thus supplies an adjustable flux to peripheral fibre bundle 102a. This luminous flux is chopped at the frequency of oscillator 104, the flux intensity varying according to the rectangular pulses. The flux emitted is transmitted by fibre bundle 102a towards the ground plate 110, which it traverses by way of transmission to be reflected by mirror 1 10a and retraverses the translucent plate 110 to enter central fibre bundle 102b which channels it in the direction of the photo transducer 112.

The photo transducer 112 has an output current which is proportional to the luminous flux which it receives, this current being passed over a grounded resistor 113 to feed a high pass filter 114. The high pass filter 114 is constituted by a capacitor 115 of 110 nano-Farad, in series was an adjustable resistor 116 which has an approximate value of 470 Ohms. Filter 114 feeds the negative input of a variable gain amplifier stage 117.

Amplifier stage 117 contains the elements of a low pass filter which, in association with the high pass filter 114, will only allow a current corresponding to a clearly determined frequency to pass to its output 117a.

The filter 114 and the filter of amplifier 117 are adjusted so that the passing frequency should be that of the oscillator 104, so that on output 1 17a there will be a sinusoidal voltage corresponding to the first sinusoidal component of the voltage of the rectangular pulses supplied by photo transducer 112 at the frequency of oscillator 104. Output 1 17a is connected to the negative input of an amplifier stage 127.

The output of amplifier stage 127 is applied to the input of a full wave rectifier stage 131.

The output of rectifier stage 131 is connected to the ,input of a filter stage 139 whose output is applied to a digital voltmeter which constitutes the display device 103.

The sub-assemblies 117, 127, 131 and 139 outlined above are identical from the point of view of their make up, to the respective elements 17, 27, 31 and 39 described in detail for-the first embodiment. The components making them up have therefore not again been described in detail, but the correspondence with the detailed description of the first embodiment has been indicated by adding the number 100 to the reference of a component defined for the first embodiment, to obtain the reference of the corresponding component of the second embodiment.

As has been indicated for the first embodiment, the adjustable resistor 145 makes it possible to obtain a "zero" (calibration) setting on the display device 103 when the plate 110 is one which does not carry any sebum deposit. In other words, the adjust able resistor 145 allows the initial calibration of the apparatus.

It is clear that this second embodiment allows, like the first embodiment, compara tive measurements of a sebum deposit.

The advantage of this latter apparatus over the first embodiment is due to the fact that the transparent plate 110 is traversed twice by the luminous beam, which increases the sensitivity of the measurement.

The other advantages mentioned for the first embodiment of apparatus are found again in the second alternative. In the second embodiment of the photo emitter 107 sup plies a non-monochromatic luminous flux in the infra-red range. The choice of the mean wave length of the luminous flux emitted has no critical importance of the operation of the apparatus according to the invention.

One may conveniently use the double optical fibre linked to an emitter-receiver unit such as the unit sold under reference S-322-3 L.E.D. by the Society' SKAN-A-MATIC.

For the second embodiment it is possible to provide that the reflecting device linked to the translucent plate 10 should be obtained by metal deposition on one face of a glass plate whose other face is ground, making it possible to obtain, in one and the the same component, the unit formed by the translucent plate 10 and the associating reflecting device shown in Figure 2.

WHAT WE CLAIM IS:- 1. Apparatus for determining the quan tity of sebum secreted by skin, comprising a translucent plate capable of being applied against a zone of the skin to be studied; an emitter capable of supplying a luminous flux and positioned to direct the flux onto one face of said translucent plate; a receiver for measuring the luminous flux transmitted from the emitter by way of transmission through the said translucent plate to the opposite face thereof; and means for indicat ing a measurement; the flux emitter being effective to supply a modulated beam at a fixed frequency, and the flux receiver being effective to detect only the received flux which has the modulated frequency of the emitter.

2. Apparatus according to claim 1, wherein the receiver for measuring the luminous flux includes at least one photo transistor.

3. Apparatus according to claim 1 or 2, wherein the luminous flux emitter and the luminous flux receiver are arranged on either side of the translucent plate.

4. Apparatus according to claim 1, 2, or 3 wherein the translucent plate is arranged in association with reflecting means, the flux emitter and receiver both being arranged on the side of the translucent plate away from the aforementioned reflecting means.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. emitter 107, via rheostat 109, and thus supplies an adjustable flux to peripheral fibre bundle 102a. This luminous flux is chopped at the frequency of oscillator 104, the flux intensity varying according to the rectangular pulses. The flux emitted is transmitted by fibre bundle 102a towards the ground plate 110, which it traverses by way of transmission to be reflected by mirror 1 10a and retraverses the translucent plate 110 to enter central fibre bundle 102b which channels it in the direction of the photo transducer 112. The photo transducer 112 has an output current which is proportional to the luminous flux which it receives, this current being passed over a grounded resistor 113 to feed a high pass filter 114. The high pass filter 114 is constituted by a capacitor 115 of 110 nano-Farad, in series was an adjustable resistor 116 which has an approximate value of 470 Ohms. Filter 114 feeds the negative input of a variable gain amplifier stage 117. Amplifier stage 117 contains the elements of a low pass filter which, in association with the high pass filter 114, will only allow a current corresponding to a clearly determined frequency to pass to its output 117a. The filter 114 and the filter of amplifier 117 are adjusted so that the passing frequency should be that of the oscillator 104, so that on output 1 17a there will be a sinusoidal voltage corresponding to the first sinusoidal component of the voltage of the rectangular pulses supplied by photo transducer 112 at the frequency of oscillator 104. Output 1 17a is connected to the negative input of an amplifier stage 127. The output of amplifier stage 127 is applied to the input of a full wave rectifier stage 131. The output of rectifier stage 131 is connected to the ,input of a filter stage 139 whose output is applied to a digital voltmeter which constitutes the display device 103. The sub-assemblies 117, 127, 131 and 139 outlined above are identical from the point of view of their make up, to the respective elements 17, 27, 31 and 39 described in detail for-the first embodiment. The components making them up have therefore not again been described in detail, but the correspondence with the detailed description of the first embodiment has been indicated by adding the number 100 to the reference of a component defined for the first embodiment, to obtain the reference of the corresponding component of the second embodiment. As has been indicated for the first embodiment, the adjustable resistor 145 makes it possible to obtain a "zero" (calibration) setting on the display device 103 when the plate 110 is one which does not carry any sebum deposit. In other words, the adjust able resistor 145 allows the initial calibration of the apparatus. It is clear that this second embodiment allows, like the first embodiment, compara tive measurements of a sebum deposit. The advantage of this latter apparatus over the first embodiment is due to the fact that the transparent plate 110 is traversed twice by the luminous beam, which increases the sensitivity of the measurement. The other advantages mentioned for the first embodiment of apparatus are found again in the second alternative. In the second embodiment of the photo emitter 107 sup plies a non-monochromatic luminous flux in the infra-red range. The choice of the mean wave length of the luminous flux emitted has no critical importance of the operation of the apparatus according to the invention. One may conveniently use the double optical fibre linked to an emitter-receiver unit such as the unit sold under reference S-322-3 L.E.D. by the Society' SKAN-A-MATIC. For the second embodiment it is possible to provide that the reflecting device linked to the translucent plate 10 should be obtained by metal deposition on one face of a glass plate whose other face is ground, making it possible to obtain, in one and the the same component, the unit formed by the translucent plate 10 and the associating reflecting device shown in Figure 2. WHAT WE CLAIM IS:-

1. Apparatus for determining the quan tity of sebum secreted by skin, comprising a translucent plate capable of being applied against a zone of the skin to be studied; an emitter capable of supplying a luminous flux and positioned to direct the flux onto one face of said translucent plate; a receiver for measuring the luminous flux transmitted from the emitter by way of transmission through the said translucent plate to the opposite face thereof; and means for indicat ing a measurement; the flux emitter being effective to supply a modulated beam at a fixed frequency, and the flux receiver being effective to detect only the received flux which has the modulated frequency of the emitter.

5. Apparatus according to claim 4, and

including an assembly of two coaxial optical fibre bundles, one end of which assembly is connected to an emitter-receiver unit incorporating said flux emitter and receiver and the other end is arranged to be disposed opposite the translucent plate to be subjected to the luminous flux.

6. Apparatus according to claim 5, wherein the assembly comprises a peripheral optical fibre bundle fed by the flux emitter and a central optical fibre bundle connected to the flux receiver.

7. Apparatus according to claim 5 or 6, wherein that end of the optical fibre bundle assembly which is not connected to the emitter-receiver unit is arranged opposite that of the said faces of the translucent plate which is intended to be applied against the zone of the skin to be studied, the other said face of the said translucent plate being applied against a said reflecting means.

8. Apparatus according to any one of claims 1 to 7, wherein the translucent plate is formed of ground glass.

9. Apparatus according to claim 8 when appendant to any one of claims 4 to 7, wherein the reflecting means associated with the translucent element is a flat mirror.

10. Apparatus according to any one of claims 1 to 9, wherein the flux emitter is constituted by at least one electroluminescent diode.

11. Apparatus according to any one of claims 1 to 9, wherein the luminous flux to be emitted by the emitter is a non-monochromatic flux.

12. Apparatus according to any one of claims 1 to 9, wherein the flux emitter comprises a single emitting element whose emitted flux extends over a narrow waveband spread around a mean wave length, the spread of the wave lengths around the mean wave length being substantially effected according to a Gaussian curve.

13. Apparatus according to any one of claims 1 to 10, wherein the flux emitter comprises a multiplicity of emitting elements.

14. Apparatus according to claim 13, wherein the flux receiver comprises a multiplicity of receiving elements.

15. Apparatus according to claims 13 and 14 taken together, and comprising n said emitting elements with parallel axes supplying emissions in the same wave-band, n being an integer higher than 1, and n receiving elements each associated with a respective one of said emitting elements, the measurements obtained from each receiving element being summed to give a mean value relating to the whole of the zone of the translucent plate to be examined.

16. Apparatus according to any one of claims 1 to 15, wherein the photo receiver is used as a current follower without any amplifying function.

17. Apparatus according to claim 16, wherein the photo receiver feeds a variable gain amplifier adjusted to the modulated frequency of the luminous flux emitted by the corresponding flux emitter.

18. Apparatus according to claim 17, wherein the variable gain amplifier acts as a low pass filter and is placed in series with a high pass filter.

19. Apparatus according to claim 18, wherein the variable gain amplifier is followed by an amplifier stage and a rectifier stage for sinusoidal current.

20. Apparatus according to any one of claims 1 to 19, wherein the output of the signal supplied by the flux receiver is effected through a filter stage comprising an adjustment of the level of the output voltage.

21. Apparatus for determining the quantity of sebum secreted by skin; such apparatus being substantially as hereinbefore described with reference to, and illustrated in, Figure 1 or Figure 2 of the accompanying drawing.

22. A process for determining the quantity of sebum secreted by skin; comprising placing a translucent plate in contact with an area of skin for a given time at a given pressure, passing pulsed luminous flux from an emitter through said plate and measuring the flux transmitted by the plate using a photo-transducer tuned to detect only flux modulated with the pulse frequency of the said luminous flux from said emitter, and comparing the value of flux detected by said transducer with the value obtained when the plate has not been applied to the skin.

23. A process according to claim 22, wherein said luminous flux transmitted by the translucent plate is reflected back through the plate for a second pass before being detected by said photo-transducer.

24. A process for determining the quantity of sebum secreted by skin; such process being substantially as hereinbefore described with reference to the accompanying drawing.