the subject of the present invention is a novel instrumental method using infrared thermography imaging coupled with a mechanical deformation system of the skin . this method is useful for characterizing rheological properties of the skin . according to one of its aspects , the invention deals with a method of determining a skin elasticity value for the skin of an individual , said method using infrared thermography imaging coupled with a mechanical deformation system comprising at least two metallic plates intended to pinch a skin sample of a person . in a preferred embodiment , the method deals with determining the elasticity of skin face . in this embodiment , “ elasticity ” as used in the context of the present invention may correspond to one or several clinical criteria such as firmness , tonicity , suppleness , sagging skin , oval of the face , bags beneath the eyes , nasolabial folds or ptosis of the lower part of the face . such clinical criteria are usually estimated by the person him / herself , or by a dermatologist . they can be defined for example by using a graduated , validated , visual scale such as one described in bazin , r ., doublet , e ., in : med &# 39 ; com ( eds ), skin aging atlas , volume 1 caucasian type , paris , france , 2007 , pp : 46 - 57 . the volunteers &# 39 ; self - assessment was specifically quantified by the method of the invention , particularly as regards firmness , elasticity and skin sagging . the method of the invention is an interesting alternative to current subjective methods and enables to quantify the rheological properties of the skin . the method of the invention resolves the difficulty of measuring the subjective perception of the global phenomenon of skin firmness , elasticity and sagging . the inventors showed that the measurement performed with the method of the invention were consistent with the volunteers &# 39 ; subjective assessment of their skin &# 39 ; s elasticity . a — contacting the metallic plates of the mechanical deformation system with the skin sample and recording a first thermal image of the skin on which thermal prints of the metallic plates on skin can be displayed , b — pinching the skin sample with the mechanical deformation system intended to trap said skin sample , and subsequently releasing the system , c — recording a second thermal image of the skin on which thermal prints of the metallic plates on skin can be displayed , d — calculating a value associated with an elastic recovery of the skin sample . in step b , the pinching of the skin is preferably made under a controlled pressure being between 20 and 40 n , more preferably between 25 and 35 n , typically being 31 n . the metallic plates are preferably parallel so that contact between the metallic plates with the skin generates parallel markings displayed with infrared thermography imaging . the value associated with an elastic recovery of the skin sample corresponds to the ratio between a first distance corresponding to the distance between the thermal prints of the metallic plates measured after pinching in step c , and a second distance corresponding to the distance between the thermal prints of the metallic plates measured before pinching in step a . a visual reproduction of the deformation can be obtained using infrared thermography imaging thermacam ™ sc500pal camera , flir system ™, sweden . once the system is removed from the skin after application , a thermal image of the face is usually recorded . the contact between the system &# 39 ; s metallic bars ( cold temperature ) with the skin ( warm temperature ) generates clearly discernible markings . the distance ( in cm ) from the centre of each of the marks left on the skin is calculated . the same distance is also measured before pinching . to evidence rheological variations , the measurements of the distance separating the two imprints left by the system in the image are taken before ( av ) and immediately after the restriction ( ap ), and the ratio of both distances is calculated . this calculation can be made either manually by experts or using software . the closer this ratio is to the value of 1 , the greater the rheological properties of the skin ( increased elasticity and suppleness ). if this ratio is low , the skin has reduced elasticity and suppleness . in one embodiment , the pinching system is formed by an internal spring and two metallic plates fixed to two sliding sections ( fig1 ). the two sections slide under a constant , controlled pressure of between 20 and 40 , for example 31 n . to carry out measurements , the pinching system is placed at certain points of the skin surface in static position to set up contact with the metallic plates of the system . then , using the fingers , the two outer parts of the system are pulled inwards as far as an abutment thus trapping part of the skin within the created space . the system is then relaxed to release the skin allowing it to return to its initial state . the mechanical deformation system can be placed at certain points of the skin surface in static position to set up contact with the metallic plates of the system . the method of the invention is preferably conducted on the face , more preferably on the lower part of the cheek . according to another aspect , the invention deals with a method of measuring the impact of various cosmetic products on improving the subjective perception of firmness . one object of the invention is a method for measuring the effect of a cosmetic product on improving the firmness or elasticity of the skin of a person , said method comprising : calculating a first value associated with an elastic recovery of the skin sample according to the method of determining a skin elasticity value previously described , applying the cosmetic product on the skin sample , calculating a second value associated with an elastic recovery of the skin sample according to the method of determining a skin elasticity value previously , comparing the first value and the second value . the invention is illustrated more in detail in reference to the following examples . instrumental evaluation according to the invention and correlation with clinical assessment 116 caucasian women , allocated to 5 different age groups ( group “ 20 - 29 ”, n = 20 , mean = 24 . 3 +/− 3 . 1 years ; group “ 30 - 39 ”, n = 23 , mean = 34 . 4 +/− 2 . 8 years ; group “ 40 - 49 ”, n = 23 , mean = 44 . 4 +/− 3 . 0 years ; group “ 50 - 59 ”, n = 24 , mean = 54 +/− 3 . 1 years and the group “ 60 - 70 ”, n = 26 , mean = 64 . 9 + 7 − 3 . 4 years ), were recruited for this study . the volunteers were invited to sit down in a comfortable armchair in order to prevent any head movements during measurement . an infrared thermography camera set up a fixed distance away from the volunteer , was used to capture the image of the skin before and after mechanical deformation . a system comprising an internal spring and 2 metallic plates fixed to 2 sliding sections was used for this study ( see fig1 ). to record the measurement , the pinching system was placed on the skin in a static position in order to create contact between the system &# 39 ; s metallic plates and the skin . then , using the fingers , the two outer parts of the system were pulled inwards as far as the wedge ( the two sections slide at a controlled constant pressure of 31 n ), thus trapping part of the skin within the space created ( see fig2 ). finally , the two sections were relaxed thus releasing the skin and allowing it to return to its initial state ( see fig4 ). skin deformation was analysed using an infrared thermography camera ( thermacam ™ sc500pal , flir system ™, sweden ). a first thermal image was recorded before pinching ( see fig3 ). the contact between the system &# 39 ; s metallic bars ( cold temperature ) with the skin ( warm temperature ) generates clearly discernible vertical and parallel markings . the distance ( in cm ) from the centre of each of the marks left on the skin is calculated . once the system was removed from the skin after application after pinching , a second thermal image was recorded . the distance separating the two vertical prints on the image is measured before ( av ) and immediately after ( ap ) the restriction and the ratio of both distances ( r = distance ap / distance av ) is calculated . two way analysis of variance ( anova ) and linear regression were used to assess the effect of age . the correlations were studied on the basis of linear regression ( two by two ) and principal component analysis ( pca ). all of the statistical tests were analysed with a risk α = 5 %. a study was conducted to verify the repeatability and reproducibility of the measurements . three experts quantified the distances before and after pinching of the skin . four photos were taken by panellists : 2 photos before pinching and 2 photos after pinching . the results clearly show that the measurements are fully repeatable and reproducible ( see the table i below ). the older the person , the more the rheological ap / av index is reduced . the reduction of ap / av index goes along with the loss of the skin &# 39 ; s rheological characteristics ( suppleness , elasticity ) ( see table ii and fig5 ). statistical analysis of the ap / av distance ratio revealed a significant age - related correlation ( r =− 0 . 23 , p = 0 . 02 ). c - study on the correlation of the ap / av index with subjective evaluation of rheological criteria the coherency between the measurements taken using the method of the invention ( pinching combined with infrared thermography ) and a subjective evaluation previously carried out has been verified . each volunteer was asked , in front of a mirror , to answer the self - assessment questionnaire ( based on the information collected during the interviews ), scoring each item on a scale of 1 to 10 ( 1 being the lowest score and 10 being the highest ). all of the evaluations were carried out on skin without cream and make - up . some of the evaluations were carried out in front of the mirror in order to record the extent of sagging skin in three areas using a graduated , validated , visual scale as described in bazin , r ., doublet , e ., in : med &# 39 ; com ( eds ), skin aging atlas , volume 1 caucasian type , paris , france , 2007 , pp : 46 - 57 , such as bags under the eyes , nasolabial folds and ptosis of the lower part of the face . a zero score denoted no visible presence of sagging whilst 10 indicated the highest level of sagging . when there are significant differences between age groups for each item , letters indicate the age groups to which it belongs . if the groups have the same letter , they are comparable , if not they are they significantly different . the results have shown a marked reduction in the firmness , elasticity and oval of the face with age . increased sagging of the skin is also clearly apparent with advancing age . as regards the analysis of the various visual grades , the results have shown a significant increase in value for the three facial areas studied ( bags beneath the eyes , nasolabial folds and oval structure of the face ). at the same time , fine lines and wrinkles are logically accentuated with age . a clinical evaluation was carried out by a dermatologist at the same time as the subjective self - evaluation . the same items than the ones listed in the self - assessment questionnaire , and the same visual scales for skin sagging were used for the clinical evaluation . the results of the clinical evaluation were very similar to those of the self - assessment except for perceived moisture levels , which fell significantly with age . overall , the results clearly showed that the ap / av ratio gives a coherent description of the subjective assessment made by women on the elasticity and firmness of their skin . data analysis emphasized significant links with the volunteers &# 39 ; self - assessment for certain items and according to age (“ firmness f ”: r = 0 . 32 , p & lt ; 0 . 01 ; “ elasticity e ”: r = 0 . 28 , p & lt ; 0 . 01 ; “ sagging skin ss ”: r = 0 . 37 , p & lt ; 0 . 01 ). a study on the efficacy of a cosmetic product was conducted over a period of 3 weeks in france in 40 individuals . the measurements were made on nude skin before and after 3 weeks &# 39 ; treatment . the treatment consisted in a twice - a - week application for 3 weeks of a product marketed under the trade name masque revitalisant — dior prestige de christian dior ( mask in fabric ). the ap / av ratio was measured in the same conditions as described in example 1 . the results are reproduced in table iv . under the study conditions , the treatment allowed a significant improvement in the suppleness / elasticity of the skin as expressed by a significant increase in the ap / av ratio . p * time was calculated with a risk of α = 5 %
the invention relates to a method of measuring the elasticity and firmness of skin . the invention also relates to methods of measuring improvements in a person &# 39 ; s skin health by measuring firmness and elasticity before , during and after a cosmetic treatment . the invention further relates to methods of measuring the improvement in a person &# 39 ; s skin firmness and elasticity that a cosmetic agent may cause when applied on the skin .