Abstract:
A description is given of a sensor for measuring the UV fraction of a radiation, comprising a photochromic transducer consisting of a photochromic material ( 3 ) with characteristics of reversible photochromism, whose absorbance varies as a function of the UV fraction of the radiation (R) incident on the photochromic material. An optical path ( 7 ) is provided to guide a light signal through said photochromic material and collect a return light signal, whose value is proportional to the level of absorbance of the photochromic material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a sensor for determining the UV fraction of a radiation which strikes the sensor. In a further aspect, the invention relates to a sensor for determining the UV irradiance. 
     The invention also relates to a measuring device comprising an optical fiber sensor for determining the UV fraction and if necessary the UV irradiance, together with a corresponding method. 
     BACKGROUND OF THE INVENTION 
     All materials of organic origin are at risk if exposed to incorrect illumination, owing to the chemical reactions initiated by exposure to light. A typical reaction is photo-oxidation, in other words the formation of free radicals following the absorption of a photon by an organic molecule and the subsequent binding of the radical with an oxygen molecule. The peroxide radical formed in this way gives rise to a series of chemical reactions which lead to the progressive deterioration of the organic material. The damage caused to the organic material is cumulative and irreversible. 
     This problem is particularly serious in the field of the fine arts, since excessive illumination of the works displayed in museums, art galleries and other exhibition sites causes irreversible damage. 
     It has therefore been necessary to set limits of illumination in the museum environment, specified in precise standards. By way of example, the limits stipulated in the Italian UNI standards (CTI E02.01.304.0) are shown in the following table. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 UV 
                 Annual 
               
               
                   
                 Illuminance, 
                 fraction, 
                 light dose 
               
               
                   
                 lux 
                 mW/lm 
                 (Mlx*hr/yr) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Sensitive materials: textiles 
                 50 
                 75 
                 0.2 
               
               
                 watercolors, objects made from 
               
               
                 paper and paper derivatives 
               
               
                 Moderately sensitive materials: 
                 150 
                 75 
                 0.5 
               
               
                 oil paintings, paintings on canvas, 
               
               
                 wooden objects 
               
               
                   
               
             
          
         
       
     
     The term illuminance (or illumination) denotes the luminous intensity of the light incident on a unit of surface area. This value is measured in lux (lx), and is equal to the luminous flux per unit of surface area: lx=lm/m 2 , where lm (lumen) is the unit of measurement of the luminous flux. The intensity of the UV radiation incident on the unit of irradiated surface area (measured in W/m 2 ) is called the UV irradiance. The UV fraction is defined as the ratio between the UV irradiance and the illuminance and is measured in W/lm. This provides an indication of the quantity of UV radiation present in the total radiation incident on the unit of surface area. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a sensor, a measuring device and a method of measurement for the determination of the UV fraction. 
     The object of an improved embodiment of the present invention is to provide a sensor which can also be used to measure the UV irradiance. 
     Essentially, the sensor according to the invention comprises a photochromic transducer consisting of a material with characteristics of reversible photochromism, for example a fulgide, whose absorbance varies as a function of the UV fraction of the radiation incident on the photochromic material, and at least a first optical path which guides a light signal through said photochromic material and collects a return light signal whose value is proportional to the level of absorbance of the photochromic material. The optical path preferably consists of an optical fiber or a bundle of optical fibers. The measurement may be made by reflection or by transparency. In the first case, the optical fibers for transmitting the radiation and for capturing the return signal are located on the same side of the photochromic material and associated with a diffusing or reflecting surface located on the opposite side. The possibility of positioning the outgoing and return fibers facing each other, with the photochromic material placed between them, is not excluded. 
     As will be explained below, the transducer assumes a coloration and consequently a level of absorbance which varies with the UV fraction of the radiation incident on it. It is therefore possible to interrogate the sensor by sending a light signal which passes through the fulgide and by capturing the attenuated signal, whose degree of attenuation provides a measurement of the UV fraction. 
     In an improved embodiment of the invention, the sensor may also comprise a further optical path for guiding toward the fulgide transducer a light radiation having characteristics such that a decoloration of the photochromic material is caused. The behavior of the fulgide material in the time interval following complete decoloration, under the effect of the external radiation incident on it, depends on the UV irradiance, in other words on the intensity of the ultraviolet radiation incident on it. It is therefore possible, by means of the same light signal used for the determination of the UV fraction, to determine the shape of the curve of attenuation with time and to derive the UV irradiance from this by the method described below. 
     With a sensor of this type it is therefore possible to implement a measurement method comprising the steps of: a) making the radiation to be measured strike a photochromic material, in other words a fulgide; b) making a light signal pass through the fulgide; c) determining the UV fraction of the radiation as a function of the attenuation of the light signal as it passes through the fulgide. 
     In its improved embodiment, the sensor according to the invention can be used to implement a method of measuring the UV irradiance, comprising the following steps: a) striking the fulgide transducer with a light beam having a wavelength such that it causes a decoloration of the fulgide, until its decoloraton is achieved; b) making a light signal pass through the fulgide while the latter is being struck by the radiation to be measured; c) detecting the variation with time of the attenuation of the light signal passing through the fulgide transducer, thus determining an attenuation curve; d) determining the UV irradiance of said radiation from the slope of the attenuation curve. 
     The various steps described above can be repeated at more or less regular intervals, to obtain a plurality of measurements over time and consequently the variation with time of the UV irradiance. When the fulgide transducer has reached the stable state, the UV fraction is determined by means of said transducer. 
     Further advantageous characteristics of the sensor and method according to the invention, and of the device which makes use of these, are indicated in the attached claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the description and the attached drawing, which shows a practical example, without restriction, of the invention. In the drawing, 
     FIG. 1 shows the absorption spectra of a fulgide as a function of the wavelength and of the values of the UV fraction of the radiation incident on it; 
     FIG. 2 shows an attenuation curve of the interrogation signal of the sensor according to the invention; 
     FIG. 3 shows a diagram of the sensor; 
     FIG. 4 shows a diagram of the measuring device using the sensor of FIG. 3; 
     FIG. 5 shows the calibration curve of the sensor for measuring the UV fraction; 
     FIG. 6 shows a diagram comparing the response of the sensor according to the invention with that of a reference probe for measuring the UV fraction; 
     FIG. 7 shows the calibration curve of the sensor for measuring the UV irradiance; and 
     FIG. 8 shows a diagram comparing the response of the sensor according to the invention with that of a reference probe for measuring the UV irradiance. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The distinctive characteristic of fulgides is their completely reversible photochromism: when the fulgide is irradiated with ultraviolet radiation, it becomes colored, whereas when it is irradiated with light having a wavelength in the visible band, the material can be returned to the initial conditions (decoloraton). 
     At the molecular level, the absorption of a photon having the energy hν (where ν is in the UV band) leads to a transformation of the fulgide molecule. The absorption of a photon having the energy hν′ (where ν′ is in the visible band) can return this molecule to its initial form (reversibility of the phenomenon). If E is the molecule in the “noncolored” state, and C is the molecule in the “colored state”, the reaction can be shown schematically as follows:                           
     In the case of a system of E and C fulgide molecules irradiated simultaneously with visible and ultraviolet radiation, the system will tend to move toward a state of photostationary equilibrium in which the two reaction velocities, namely that of coloring and that of decoloring, are equal. In particular, if it is irradiated with constant quantities of UV and visible light, the system of fulgides will reach the asymptote or photostationary state according to the exponential equation of the type: 
     
       
           c ( t ) a  sin  t′+A′·e   −B·t   (1) 
       
     
     where c(t) is the number of colored (type C) molecules. The photostationary state a sin t′ depends on the antagonism of two physical parameters of ultraviolet radiation and visible light:                a                 sin                   t   ′       =     D   ·         Φ   UV     ·     ɛ   EC             Φ   UV     ·     ɛ   EC       +       Φ   VIS     ·     ɛ   CE                     (   2   )                                
     the time constant B depends on the simultaneous action of the two quantities: 
     
       
           B=Φ   UV ·ε EC +Φ VIS ·ε CE   (3) 
       
     
     and the constant A′ depends on the initial conditions: 
     
       
           A′=c ( 0 )− a  sin  t′   (4) 
       
     
     In (2) and (3), Φ UV  and Φ VIS  denote, respectively, the flux of photons of UV radiation and the flux of photons of visible light, and ε EC  and ε CE  denote, respectively, the probabilities of the coloring and decoloring reactions. Equation (2) is used for constructing the UV-fraction sensor. The constant D in Equation (2) is determined by assuming Φ VIS =0, in other words by irradiation with UV only, and is equal to the asymptote reached by the completely colored specimen; 
     
       
           D=a  sin  t′UV   (5) 
       
     
     The quantity Φ UV ε EC  is proportional to the UV irradiance, and Φ VIS ε CE  is proportional to the illuminance. Therefore, according to (2) and the definition of the UV fraction, we obtain;                UV                 fraction     =         irradiance   illuminance     ∝       a                 sin                   t   ′           a                 sin                   t   ′        uv     -     a                 sin                   t   ′             =         F     UV                       ɛ   EC           F     VIS                       ɛ   EC                   (   6   )                                
     Therefore, the photostationary state a sin t′, to which a set of fulgides irradiated with UV and visible light tends, is directly related to the UV fraction. It is not necessary to measure the UV irradiance and illuminance separately and find the ration of the two values. 
     Essentially, therefore, the irradiated fulgide has a variable coloration, and therefore a variable absorbance, according to the UV fraction of the radiation incidents of it. By way of example, FIG. 1 shows the absorption spectra found when a specimen of fulgides incorporated in a polymer matrix is irradiated with known UV fractions. As shown in FIG. 1, the photochromic material has an absorbance varying as a function of the UV fraction of the radiation incident on it. The variation of absorbance is significant for wavelengths from 480 to 640 nm, and particularly for wavelengths in the region of 540-570 nm. 
     Another equation which is useful for constructing a sensor capable of measuring UV irradiance is obtained from the following argument: if a specimen of fulgides which is completely decolored, in other words one in which only type E molecules are present, is irradiated with UV and visible light, the only possible reaction in the first instants of irradiation is the coloring reaction, and therefore the only physical parameter which generates modifications in the specimen is the UV irradiance, regardless of the quantity of visible light (filters for blocking the visible light are not required). This can be formulated in the following equation:                       c          t            (     t   =   0     )       ∝   irradiance           (   7   )                                
     As will be explained subsequently, it is possible to apply this equation to make use of the transient state of the fulgide, and particularly the coloring velocity, to determine the UV irradiance of the radiation incident on it. 
     Fulgides in a polymer matrix can be used as the transducer of an optical fiber sensor by inserting the matrix containing the fulgide material in the optical path of a light signal used to interrogate the sensor, this signal being guided by a system of optical fibers and suitably modulated in a known way. The attenuation of the signal, which is proportional to the absorbance of the fulgide material, is used to determine the UV fraction with which the fulgide is irradiated. 
     The quantity c(t) (Equation 1) is related to the attenuation of the light guided by the optical fibers according to the following equation (Lambert&#39;s and Beer&#39;s laws):                ln        I     I   0         ∝         -   d     ·   c          〈   t               (   8   )                                
     where I 0  is the intensity passing through the fulgide material when c=0 (specimen decolored), I is the intensity at a generic instant, and d is the thickness of fulgide through which the beam has passed. If the fulgide material is irradiated with constant quantities of UV and visible light, then by combining Equations (1) and (8) the variation of the attenuation with time can be expressed by the following exponential equation;                ln        (     I     I   0       )       =       -   asympt     +     A   ·            -   B     +   t                   (   9   )                                
     FIG. 2 shows an attenuation curve obtained by irradiating a specimen of fulgides with constant quantities of UV and visible light (UV=117.6 mW/m 2 ; VIS=1119 lux), with the theoretical curve according to Equation 9 superimposed on it. 
     The following equation can therefore be used to measure the UV fraction (a sin t is a measurable quantity);                UV                 fraction     =       irradiance   illuminance     ∝     asympt       asympt   UV     -   asympt                 (   10   )                                
     For measuring the UV irradiance, it is simply necessary to relate the slope of the attenuation curve (9), in the first instants of measurement following the complete decoloring of the specimen, to the UV irradiance              irradiance   ∝              [     ln        I     I   0         ]            t            (     t   =   0     )               (   11   )                                
     Construction of the Sensor 
     One embodiment of the sensor is shown in FIG.  3 . The sensor is indicated in a general way by the number  1  and has a transducer  3  consisting of a block of parallelepipedal shape consisting of a polymer matrix containing the fulgide material, for example benzylidene succinic anhydride. The dimensions of the parallelepiped may be, for example, 2×2×5 mm. One of the lateral faces, indicated by  3 A, of the block  3  is exposed to the ambient radiation RF whose characteristics are to be determined by means of the sensor. 
     A diffusing surface is applied to one of the bases  3 B of the block  3 , while a first end of a bundle of optical fibers, indicated by  7 , is applied to the opposite base  3 C. The end of the bundle of optical fibers  7  is applied adjacent to the lateral face  3 A struck by the ambient radiation R, to increase the sensitivity of measurement, the exposed side being the area of the transducer  3  which is most exposed to the radiation R and which is therefore colored to the greatest extent. The bundle of optical fibers  7  consists of five fibers  9  connected to a light source, indicated schematically by  11 , and consisting, for example, of a laser diode, whose emission wavelength is such that it causes the decoloration of the fulgide material incorporated in the transducer  3 . The bundle  7  of fibers also comprises two optical fibers  13  and  15  of a measuring line. The optical fiber  13  carries a light radiation from a source  17 , for example an LED which emits at 570 nm, or in any case at a wavelength at which the fulgide material used has an absorbance varying markedly as a function of the UV fraction of the ambient radiation R incident on it. The light beam guided by the input fiber  13  passes through the whole length of the transducer  3  and is diffused by the diffusing surface  3 B. The optical fiber  15  receives at its input a fraction of the light diffused by the surface  3 B and guides it to a photodetector, for example a phototransistor or a photodiode  19 . 
     The sensor  1  is inserted in a measuring device of the optical fiber type, which, in an experimental embodiment, has the layout shown in FIG.  4 . The LED  17  emitting at 570 nm is housed in a casing to which the input fiber of a directional coupler  20  is connected. One of the two output fibers from the coupler  21  is used as the reference arm  21  for compensating any fluctuations of the source and is associated with a photodiode  23  interfaced with an amplification circuit  25 . The other output optical fiber  13  from the coupler  20  forms part of the measuring arm which guides the interrogation light signal to the photochromic transducer  3  inserted in the sensor  1 . The photodiode  17  which receives the light signal, which is attenuated after passing through the transducer of the sensor  1 , is associated with an amplification circuit  27 . The two amplification circuits  25 ,  27  are interfaced, through analog inputs and a DAP acquisition card, with an electronic computer  29 . The computer  29  also provides an analog output for the drive current of the LED  17  and a digital output for modulating the LED, connected to an amplification and modulation circuit  31 , which controls the LED  17  which emits the interrogation signal for the sensor  1 . In the diagram in FIG. 4, the laser diode  11  is omitted. 
     To measure the UV fraction with the device of FIG.  4  and the sensor of FIG. 3, it is simply necessary to “measure” the color of the transducer  3  at a given instant: for this purpose, the transducer  3  is exposed to the radiation whose UV fraction is to be measured, and the attenuation of the signal guided by the optical fibers  13 ,  15  is measured with the apparatus shown in FIG.  4 . 
     Calibration and Practical Tests 
     The sensor was calibrated in the laboratory by illuminating the transducer  3  with different values of the UV fraction, using a mercury lamp as the UV source and a tungsten lamp as the visible-light source. The response of the optical fiber sensor was compared with that of a commercially available reference sensor. The calibration graph is shown in FIG.  5 . 
     The operation of the sensor for measuring the UV fraction was tested in real operating conditions in the Pollaiolo Room at the Uffizi Gallery in Florence. The results are reproduced in FIG. 6, where the time is shown on the horizontal axis and the measured UV fraction (expressed in μW/lm) is shown on the vertical axis. The diagram shows the response of the sensor according to the invention in black, and the response of a reference probe of a commercial type in gray. 
     The sensor was placed in the proximity of a picture exposed to the light from the windows. The color of the fulgide transducer varies with the variations in illumination, and since the variations of the natural levels are sufficiently slow, the fulgide transducer can be considered to be in the photostationary state in each successive instant. 
     The agreement between the responses of the sensor according to the invention and of the reference probe is high at times when the venetian blinds which partially obscure the windows are open. Additionally, the commercial reference probe shows a greater response to the effects of disturbance caused by the passage of visitors in front of the measurement point, while the sensor according to the present invention provides a more accurate result, since it is not affected by these disturbance factors. 
     The following procedure was devised for measuring the UV irradiance by using Equation (11): starting with the decolored fulgide specimen, this is exposed to the ambient radiation for a sufficient time to determine the slope of the absorbance curve of the fulgide specimen. In this stage of coloring, the attenuation of the signal is measured by means of the two outward and return optical fibers  13 ,  15 . 
     The transducer  3  is then illuminated with visible light, emitted by the laser diode  11  and guided by means of the optical fibers  9  to the transducer, for a time sufficient for the decoloring of the fulgide material forming the transducer. The light emitted by the laser diode  11  is sufficient to return the transducer  3  to the completely decolored state, so that the system is ready for a new measurement of the slope of the attenuation curve. The optimal times for the two stages were found to be as follows: 2 minutes for the decoloring; 3 minutes for the measurement of the slope. Thus the apparatus can capture one data element every 5 minutes. 
     The results of the calibration of the probe carried out in the laboratory by illumination with the mercury lamp are shown in FIG.  7 . 
     The test in real conditions, the results of which are shown in FIG. 8, was carried out in the Pollaiolo Room. The black curve shows the response of the fulgide sensor, while the gray curve represents the response of the reference probe. In this case also, the response of the optical fiber sensor matches the response of the commercial reference probe, and is less affected by the passage of visitors than that of the reference probe. 
     It is to be understood that the drawing shows only one embodiment of the invention, which can be varied in its forms and arrangements without departure from the scope of the guiding principle of the invention.