Method and apparatus for measuring moisture content of skin

There are disclosed a method and an apparatus for measuring moisture content of skin. The skin surface is illuminated with polarized light incident at the Brewster angle. The degree of polarization of the light refracted within the skin and reflected out is measured. The measurement is a function of the refractive index of the skin which, in turn, is a function of the moisture content. To avoid measurement variations due to such things as skin color, output readings are in the form of a ratio of the reflected polarized light intensity to the reflected total light intensity.

This invention relates to a method and an apparatus for measuring the 
moisture content of skin, and more particularly to the taking of in vivo 
measurements in a rapid, simple and accurate manner. 
The medical profession and the cosmetic industry have long sought a 
practical instrument which is capable of measuring skin moisture content 
in a rapid and simple manner, and with a degree of precision. Prior art 
attempts to provide such an instrument have not been successful. One prior 
art technique for measuring skin moisture content, based upon a 
capacitance effect, is disclosed in Copeland et al U.S. Pat. No. 
4,013,065, entitled "Moisture Dermatometer," which issued on Mar. 22, 
1977. 
Light has also been used in the prior art to measure skin characteristics. 
In DeWitt et al U.S. Pat. No. 4,029,085, entitled "Method for Determining 
Bilirubin Concentration from Skin Reflectance," which issued on June 14, 
1977, there is disclosed a technique for measuring the spectral 
reflectance of skin to determine bilirubin concentration. This technique 
entails the illumination of the skin at different wavelengths, and 
measuring the intensity of the reflected light at each wavelength. But the 
technique is not applicable for determining the moisture content of skin. 
It is also known in the prior art to use polarized light for taking 
measurements of different kinds. For example, in Gee U.S. Pat. No. 
3,904,293, entitled "Optical Method for Surface Texture Measurement," 
which issued on Sept. 9, 1975, there is disclosed a method for measuring 
the textural characteristics of a surface by illuminating the surface with 
polarized light and measuring the degree of depolarization of the 
reflected light. The use of polarized light transmitted through an object 
of interest for comparing one pattern with another is disclosed in Green 
et al U.S. Pat. No. 3,982,836, entitled "Method and Means for Enhancing 
Prints for Direct Comparison," which issued on Dec. 28, 1976. Of perhaps 
greater interest to the subject invention are Aspnes U.S. Pat. No. 
3,985,447, entitled "Measurement of Thin Films by Polarized Light," which 
issued on Oct. 12, 1976, and Dill et al U.S. Pat. No. 4,053,232, entitled 
"Rotating-Compensator Ellipsometer," which issued on Oct. 11, 1977. In 
both of these patents, there are disclosed techniques for using polarized 
light to measure the thickness and/or refractive index of a thin film. But 
these patents do not suggest the use of polarized light to measure the 
moisture content of skin. 
Even should it be thought that light (e.g., polarized) might be used to 
measure moisture content of skin, there is a logical reason for rejecting 
such an attempt. It is apparent that the intensity of the light which is 
reflected from the human skin necessarily depends on its texture and 
color. Since there is a wide range of skin color, it would appear that it 
would not be feasible to relate the intensity of the reflected light to 
moisture content (even assuming that, quite apart from skin color effects, 
the reflected light intensity is somehow a function of moisture content). 
The scientific basis for my invention is predicated upon the following 
principles of optical physics. A ray of unpolarized light which is 
incident on an object is separated into a reflected ray and a refracted 
ray. With respect to the reflected ray, the angle of reflection equals the 
angle of incidence. The angle of the refracted ray is determined by the 
well-known Snell's Law. A special case, covered by Brewster's Law, is that 
in which the angle of incidence (to a line normal to the surface) is the 
Brewster angle (about 57.degree. for ordinary glass). In such a case, it 
is found that the reflected light, while being only a small fraction of 
the total incident light, is plane-polarized. Furthermore, the reflected 
and refracted rays are 90.degree. apart. It is also well known that the 
intensity of the reflected plane-polarized light can be increased by 
stacking a pile of glass plates one on top of the other. The refracted ray 
in the top plate is partially reflected (plane-polarized) from the bottom 
surface of the plate. As the unreflected part of the ray moves on to the 
upper surface of the second plate, part of it is reflected 
(plane-polarized). This process continues as the reflected ray, which gets 
lower and lower in intensity, is partially reflected at both surfaces of 
each plate. All of the reflected rays are plane-polarized and parallel 
with the plane-polarized ray reflected from the uppermost plate surface. 
The net result is that the composite ray reflected from the pile of plates 
(at a reflection angle which equals the angle of incidence) is not only 
plane-polarized, but can also have an intensity which is an appreciable 
fraction of the intensity of the incident unpolarized light. These effects 
are described, for example, in Jenkins and White, "Fundamentals of 
Optics," McGraw-Hill Book Company, Inc., 1957, pages 488-494. 
The skin consists of epidermis and dermis as the two major divisions. The 
epidermis is a transparent structure that may be structurally divided into 
four layers. The outer layer is the stratum corneum, the second layer is 
the granular layer, the third layer is called the prickle layer, and the 
lowest layer is known as the basement membrane. Each of these layers has a 
distinct cellular structure. Since the epidermis consists of layers of 
transparent cells, light passing through the epidermis may be treated in 
the same manner as light passing through a pile of parallel glass plates. 
Unpolarized light striking the epidermal surface at the Brewster angle 
will be mostly transmitted through the epidermis and thus refracted. Some 
of the vibrations perpendicular to the plane of incidence are reflected 
(plane-polarized) at each surface, and all of the vibrations parallel to 
the plane of incidence are refracted. The net result is that the reflected 
rays are all plane-polarized in the same plane, and the refracted beam 
having lost most of the perpendicular components of vibration is partially 
polarized in the other plane. 
In the case of a stack of glass plates, the degree of polarization of the 
reflected light is a function of both the number of plates in the stack 
and their refractive index (see, e.g., page 493 of the Jenkins and White 
text). Consequently, since the skin affects incident light as does a pile 
of parallel glass plates, the degree of polarization of the reflected 
light is a function of the refractive index of the skin. 
The refractive indices of many biological fluids are functions of their 
concentrations of proteins and salts. The concentration of any biological 
fluid is inversely related to the water content in that fluid matrix. 
Therefore, if unpolarized light is incident upon a skin surface, the 
degree of polarization of the reflected light is necessarily a function of 
the refractive index of the skin which, in turn, is a function of the 
moisture content of the skin. The underlying technique of my invention is 
to direct a ray of light toward the skin surface, and to measure the 
degree of polarization of the reflected light to determine the moisture 
content of the skin. 
There are two refinements of this basic technique, however, which make it 
both practical and accurate, and are incorporated in the preferred 
embodiment of the invention. The first refinement concerns the use of an 
incident beam of light which is polarized rather than unpolarized, and 
which impinges on the skin at approximately the Brewster angle (within the 
range 45.degree.-60.degree. incident to the normal, with an angle of 
54.degree. being preferred). When unpolarized light is used, it is found 
that the degree of polarization of the reflected light is a function of 
the moisture content of the skin, but even large variations in moisture 
content result in relatively small variations in the degree of 
polarization. In other words, any instrument which uses unpolarized light 
has a small dynamic range which can lead to inaccurate results. But when 
polarized light is used, and especially when it is incident at 
approximately the Brewster angle (45.degree.-60.degree. to the normal, 
with 54.degree. being preferred), there are large variations in the degree 
of polarization of the reflected light depending on the skin moisture 
content. The light which is reflected from the upper surface of the skin 
is scattered due to the non-smooth texture of the skin. While this portion 
of the reflected light is partially polarized, since only a small part of 
the scattered light is detected it does not affect the overall measurement 
to a significant degree (as is desired inasmuch as the light reflected 
from the upper skin surface is not a function of moisture content). It is 
the light which is refracted and then reflected within the skin whose 
intensity varies greatly with the moisture content. Thus the use of 
polarized light incident at the Brewster angle is clearly preferred 
because it provides a much wider dynamic range. 
The second refinement takes into account measurement variations which would 
otherwise arise from such things as skin textures and especially skin 
colors of different patients. Considering skin color, for example, it is 
apparent that the total light which is internally refracted and reflected 
necessarily depends upon the degree to which the incident light is 
absorbed in the skin and this, in turn, depends on the skin color. An 
instrument which is calibrated to provide moisture content readings as a 
function of the intensity of the reflected light could not service the 
general population, because the calibration would be accurate only for 
patients having more or less the same skin color. 
For this reason, the output readings of the instrument of the preferred 
embodiment of my invention are not simply a direct function of the degree 
of polarization of the reflected light. Two measurements are actually 
taken for each patient. The first is the intensity of the polarized light 
in the reflected beam. The second is the total intensity of the reflected 
beam. (The two measurements can be taken simply by inserting and then 
removing an analyzer in the path of the reflected light, as is known in 
the art.) The output measurement is proportional to the ratio of the first 
reading to the second. Skin texture and color, as well as other variables, 
affect both readings to approximately the same degree. Thus, both 
measurements for one patient may be just half of both respective 
measurements for another, but the ratio of measurements for both patients 
will be the same if they have equal skin moisture contents. It must be 
understood that the "normalization" introduced by taking ratio 
measurements is not a normalization which is the same for all patients; 
the denominator is not a constant for all patients. On the contrary, the 
normalization is different for each patient in that any measurement of 
reflected polarized light intensity is normalized relative to the 
reflected total light intensity for the same patient. The instrument may 
be calibrated in terms of ratio measurements versus average moisture 
content determinations (using standard accurate, albeit expensive and 
time-consuming, techniques) for a large population; the ratio measurement 
for each patient subsequently tested can thus be compared with a standard 
"ratio versus moisture content" curve to determine the skin moisture 
content of the patient, without any regard being given to other skin 
variables which might otherwise render the measurement inaccurate. 
Further objects, features and advantages of my invention will become 
apparent upon consideration of the following detailed description in 
conjunction with the drawing which depicts a preferred embodiment of the 
invention.

Referring to the drawing, light source 12, which may be a source of 
unpolarized light, directs a beam of light 14 through filter 16 and 
polarizer 18 to the skin surface 10. The filter is designed to provide a 
relatively narrow bandwidth (preferably, 5-10 nanometers) of wavelengths, 
although it is not essential to the practice of the invention. The 
preferred wavelengths are those which are not absorbed by the skin so that 
a maximum amount of light may be reflected from the skin. Also, since the 
refractive index of any material is, to at least a small degree, dependent 
upon the wavelength of radiation, it is preferable to use a narrow band of 
wavelengths. In general, the entire range of wavelengths from 450 
nanometers to 700 nanometers has been found to be practical, although 
wavelengths centered as closely as possible to 700 nanometers are 
preferred. (The greater the intensity of the light source, the narrower 
may be the wavelength bandwidth; what is necessary, of course, is to have 
a total quantity of incident light which allows measurements to be taken 
on the reflected light.) A conventional polarizer 18 is utilized in order 
to polarize the light which is incident on the skin surface. 
The light is incident at an angle .phi..sub.i to the normal N. The 
reflected light 26 is at the same angle .phi..sub.i to the normal, and the 
refracted light 24 within the skin is at an angle .phi..sub.r to the 
normal. As described above, the refracted light undergoes numerous 
reflections within the skin, all of the reflected rays exiting the skin 
surface in a direction parallel with ray 26. 
A disc 28 is provided with two apertures. Aperture 28b is clear, while a 
conventional analyzer 28a is placed in the other aperture. The analyzer is 
oriented to pass light which is plane-polarized in the orientation of beam 
14. (The final reading displayed thus increases with the degree to which 
the polarization of reflected beam 26 increases as a function of skin 
moisture content.) The disc is turned by motor 34 so that the reflected 
light 26 passes through analyzer 28a on its way to photomultiplier tube 32 
when the disc is in the position shown in the drawing, or the light simply 
passes through aperture 28b when the motor has rotated the disc 
180.degree.. In either case, the output of the photomultiplier tube on 
conductor 42 is extended to control circuit 40. 
The signal on conductor 42 is extended by the control circuit over either 
conductor 46 or conductor 48 to one of respective operational amplifiers 
50 and 52. The amplified signals are extended over conductors 54 and 56 to 
a conventional ratiometer 60 which forms the ratio of the two signals on 
the respective input conductors, and applies a signal proportional to the 
ratio over conductor 62 to the input of any conventional display unit 64 
which then displays the result. 
Control circuit 40 first extends a signal over conductor 44 to motor 34 
which controls the rotation of disc 28 so that aperture 28b is in front of 
the photomultiplier tube 32. The resulting "normalizing" (denominator) 
signal on conductor 42 is extended by the control circuit over conductor 
46 to operational amplifier 50. Thereafter, the control circuit extends a 
signal over conductor 44 to cause the motor to rotate disc 28 180.degree. 
(to the position shown in the drawing). The resulting "degree of 
polarization" signal on conductor 42 is now extended by the control 
circuit over conductor 48 to operational amplifier 52. The only function 
of the control circuit is to establish a connection between conductor 42 
and one of conductors 46 or 48 depending upon the position of disc 28 as 
determined by the signal applied to conductor 44. The ratiometer then 
forms the ratio of the signal level on conductor 56 to the signal level on 
conductor 54, with the resulting ratio being displayed on display 64. 
The control circuit can cause disc 28 to step continuously between its two 
positions so that continuous signals appear on conductors 54 and 56 to 
facilitate the formation of the ratio signal on conductor 62. In order to 
maintain the signal on each of conductors 54 and 56 while the other signal 
is being derived, if that is necessary for the operation of the 
ratiometer, the output of each operational amplifier may be provided with 
a conventional sample-and-hold circuit. 
As described above, the angle of incidence .phi..sub.i is the angle 
(54.degree. in the preferred embodiment) which gives the largest 
variations in the signal on conductor 48 as a function of skin moisture 
content. Variations in the output readings which would otherwise be caused 
by variables such as skin texture and color are substantially eliminated 
by taking a "normalized" measurement for each patient--the numerator and 
denominator of the ratio are both affected by approximately the same 
factor for each skin variable except moisture content. Thus the resulting 
ratio is primarily a function of moisture content only. 
The instrument may be calibrated initially based upon measurements taken on 
excised skin samples of a large population. For each patient in this 
population, the wet weight of his skin sample is recorded. The instrument 
is then used to take a moisture content ratio reading with some arbitrary 
settings of the gains of the two operational amplifiers. The skin specimen 
is then partially dried in an oven, re-weighed, and again placed under the 
instrument for the taking of a second reading. This process is continued 
until the dry weight of the tissue is obtained. From this data, a curve 
may be plotted for each patient of instrument reading versus moisture 
content. The individual plots are then averaged so that there results an 
"average" plot of instrument reading versus moisture content. In order to 
directly relate the instrument readings to the moisture content values in 
absolute terms, the gains of one or both operational amplifiers may be 
adjusted so that each instrument reading (a ratio) actually represents the 
respective moisture content value. Once the gains of the operational 
amplifiers are determined in this manner, these gains are used for all 
subsequent instruments which are produced. 
This assumes, however, that the calibration curve of instrument reading 
versus moisture content is a linear function, but the relationship is not 
this exact. For this reason, for greater accuracy the calibration curve 
itself can be used when testing any patient. The ratio which is displayed 
is not necessarily the exact moisture content. The ratio is simply used to 
consult the calibration curve from which the actual moisture content is 
read. 
Although the invention has been described with reference to a particular 
embodiment, it is to be understood that this embodiment is merely 
illustrative of the application of the principles of the invention. For 
example, each ratio reading may be the ratio of the "normalizing" signal 
to the "degree of polarization signal," rather than the reverse. Thus it 
is to be understood that numerous modifications may be made in the 
illustrative embodiment of the invention and other arrangements may be 
devised without departing from the spirit and scope of the invention.