the present invention utilizes a spectral imaging system to generate images of the skin . one suitable spectral imaging system includes a custom - made camera ( forth photonics , athens , greece ) with a 25 mm macro lens and a linear polarizing filter capable of multispectral ( up to about 30 narrow band filters ) and / or hyperspectral ( greater than about 30 narrow band filters ) imaging ( hereafter , both multispectral and hyperspectral are collectively “ ms / hs ”). the camera contains multiple narrow band filters , for example from 4 to 50 narrow band filters in the spectral range of about 700 to about 1000 nm . the settings for the image acquisition including filter selection , exposure time , and camera gain are preferably controlled through computer software . in another embodiment , a commercial camera ( e . g ., nikon g10 ) is used with a filter wheel in front of the camera . the filter wheel can be preferably computer controlled utilizing the appropriate software or commands to select a desired filter and then subsequently change the filter wheel position to the desired filter ( e . g ., moving the filter from 800 nm to 970 nm ). a linearly polarized broadband light source is preferably utilized in the methods of this invention . one suitable linearly polarized light source is the v600 , syris , gray , me . the following terms used herein have the meanings ascribed to them below : “ orthogonal or cross polarization conditions ”: means imaging conditions that include a first polarizer filter in front of the light source and a second polarizer filter in front of the camera lens with the two filters having their polarization axes orthogonal to each other . “ parallel polarization conditions ”: imaging conditions that include a first polarizer filter in front of the light source and a second polarizer filter in front of the camera lens with the two filters having their polarization axes parallel to each other . in one embodiment , the methods of this invention include the step of taking a polarized photograph of the subject . what is meant by “ polarized photograph ” is a photograph of the subject taken ( i ) with a light source that emits light through a polarizing filter and / or ( ii ) through a polarized filter that filters light prior to entering the camera &# 39 ; s lens . as used herein , the identifier “ xp ” shall mean cross - polarized or orthogonal . in one embodiment , the camera and one or more light sources , preferably one , are on about the same plane as the subject &# 39 ; s skin to be photographed , and the light sources are placed so that the angle formed by each light source ( s ), subject &# 39 ; s skin , and camera is about 15 to about 70 degrees , more preferably , about 45 degrees . in one embodiment , a polarizing filter is placed in front of each light source . what is meant by a “ polarizing filter ” is a filter that filters incoming light to emit polarized light . examples of polarizing filters include , but are not limited to , polarizing plates such as those available from edmund scientific ( barrington , n . j . usa ), polarizing prisms such as glan thomson polarizing prisms , or a polarizing reflector that reflects light at about the brewster angle . polarizing filters may be linear or circular polarizing filters . measurements are preferably performed at two wavelengths . the first wavelength , λ 1 , is preferably in the range from about 700 to about 900 nm , more preferably 800 nm , a range in which water does not exhibit strong light absorption . the second wavelength , λ 2 , is preferably in the range from about 900 to about 1000 nm , preferably 970 nm , a range in which light absorbance by water is high . “ specular reflection ” or “ glare ” is the part of the light that changes direction upon incidence on a surface of a material with a different index of refraction than the medium from which the light emanates . in the case of skin imaging the first medium is air and the second is the stratum corneum . in this case , specular reflection is about 4 % of the incident light intensity ( based on fresnel &# 39 ; s law ). the rest of the incident light penetrates the skin tissue and undergoes absorption . the light in the skin tissue will either exit the skin after a few scattering events in the stratum corneum ( thereby maintaining its polarization state ) or penetrate deeper and undergo multiple reflection events before exiting the skin as diffuse reflected light . in one example of the methods of this invention ( and as shown in fig1 ), a halogen light source ls 1 emits light that goes through polarizer p 1 before arriving to the skin surface . the light reflected from the skin then goes through a second polarizer p 3 . p 1 and p 3 are two different polarizers such that one may be oriented orthogonal or parallel to the other . for example , if p 1 is set in a particular plane , then p 3 may be rotated such that it is orthogonal or parallel to p 1 . as shown in fig1 , diffuser d 1 may be used in order to obtain more uniform illumination . however , the diffuser d 1 as shown in the embodiment of fig1 , is not necessary to the methods of this invention . the light from ls 1 goes through p 1 targeting a specific skin area sut . this light is then reflected off the target skin at a specific angle ( θ 1 ), travels through the second polarizer ( shown as p 3 in fig1 ) and filter wheel f to be captured by a digital camera . filter wheel f 1 is capable of switching between band pass filters with bands centered at different wavelengths . preferably the band pass filters are narrow band ( with bandwidth of 5 - 20 nm ) filters . preferably the filters are of high transmission (& gt ; 80 %). preferably the filters are interference ( or dichroic ) filters . in one embodiment the filter wheel may be replaced by a liquid crystal tunable filter ( lctf ) system . for the present invention , the preferred wavelengths are from about 700 to about 900 nm , more preferably 800 nm and from about 900 to about 1000 nm , more preferably 970 nm . to analyze the skin surface , four images are captured or taken by the digital camera : 1 . a digital image of skin under parallel polarization conditions filtered at a first wavelength λ 1 of from about 700 to about 900 nm ; 2 . a digital image of skin under orthogonal ( cross ) polarization conditions filtered at a first wavelength λ 1 of from about 700 to about 900 nm ; 3 . a digital image of skin under parallel polarization conditions filtered at a second wavelength λ 2 of from about 900 to about 1000 nm ; 4 . a digital image of skin under orthogonal ( cross ) polarization conditions filtered at a second wavelength λ 2 of from about 900 to about 1000 nm ; in another example of the present invention shown in fig2 , two halogen light sources ( ls 1 and ls 2 ) are shown in combination with optional diffusers ( d 1 and d 2 ) and polarizers ( p 1 and p 2 , respectively ). in a preferred embodiment , p 1 and p 2 are oriented in the same manner . a third polarizer ( shown as p 3 in fig2 ) is oriented orthogonal or parallel to p 1 and p 2 . in fig2 , the light from ls 1 and ls 2 is aligned through p 1 and p 2 to a specific skin area . this light is then reflected from the target skin , passed through p 3 and a filter wheel and then captured by a digital camera . in the example shown in fig2 , the angles of reflectance θ 1 and θ 2 are preferably the same . the recorded images are put into an equation . the equation is applied on a pixel by pixel basis , which results in a reflectance value ( number ). the numbers are then used to create a map of the skin surface . this map correlates to the hydration of the skin . “ reflectance values ”: the light intensity being recorded by the camera at a particular wavelength and a particular polarization state ( orthogonal or parallel ). the orthogonal polarized image and the parallel polarized image obtained at two wavelengths is preferably used to determine light absorption due to water , as defined by the following equation : as used herein , the term “ α ” is a term that is adjusted to account for the amount of diffused reflected light in the parallel polarized image . in a preferred embodiment of the invention , α is preferably between 0 and 1 , more preferably between about 0 . 2 and about 0 . 4 , preferably about 0 . 3 . as used herein , “ λ ” shall mean wavelength measured in nanometers or nm at which the reflected light is filtered before entering the camera detector . in another embodiment of the method of this invention , a light diffuser is placed between the light source unit and the polarizing filter . in yet another embodiment of the method of this invention , a linear polarizing filter is used at the light source and the linear polarizing filter is arranged such that the electric field of the emitted light is about parallel or perpendicular ( orthogonal ) to the plane formed by the light source , the person &# 39 ; s skin , and the camera . in another embodiment of the method of this invention , the polarized light source ( s ) are positioned on a vertical plane above the camera and the subject &# 39 ; s skin so that the angle formed by the light source , subject &# 39 ; s skin and camera is about 15 to 70 degrees , preferably about 45 degrees . the light source ( s ) are filtered with a linear polarizing filter that is placed with the transmitted electric field in the vertical direction ( e . g ., parallel to the plane ). example of crossed - and parallel - polarized images taken at 800 and 970 nm the imaging system was set up in the configuration shown in fig1 . the optional diffuser d 1 was not used to acquire these images . the angle θ 1 was approximately 25 degrees . using the skin area from the lower left leg of a 28 year old male volunteer exhibiting dry skin , a hydration cream , about 0 . 1 ml of enydrial ® lait hydratant corps ( available from roc , france ) was applied evenly to a specified area of the lower leg with bare fingers . any excess cream was removed by blotting the surface with a paper towel . the application time took approximately one minute and the measurements were taken within about five minutes after application . four images were taken . the first image ( fig3 ) was taken at 800 nm under parallel polarization conditions . the second image ( fig4 ) was taken at 800 nm under cross - polarization conditions . the third and fourth images ( fig5 and 6 ) were both taken at 970 nm under parallel polarization and cross - polarization conditions , respectively . the images were then processed using the following equations to obtain the deep and surface water content : deep ( viable epidermis and dermis ) water map ( fig7 ): - log ⁡ ( reflectance ⁢ ⁢ at ⁢ ⁢ 970 ⁢ λ xp reflectance ⁢ ⁢ at ⁢ ⁢ 800 ⁢ λ xp ) - log ⁡ ( reflectance ⁢ ⁢ at ⁢ ⁢ 970 ⁢ λ pp - α ⁢ ⁢ reflectance ⁢ ⁢ at ⁢ ⁢ 970 ⁢ λ xp reflectance ⁢ ⁢ at ⁢ ⁢ 800 ⁢ λ pp - α ⁢ ⁢ reflectance ⁢ ⁢ at ⁢ ⁢ 800 ⁢ λ xp ) the increased hydration from the moisturizing cream is visible in the surface water map .
the methods of this invention relate to means for measuring skin hydration . the methods of this invention utilize at least two wavelengths filtered by at least two polarizers to create digital images of skin treated with personal care products . the methods are useful for demonstrating the efficacy of skin care products intended to increase skin hydration and / or protect the skin from dehydration , even when such dehydration is not apparent to the naked eye .