the following description of the preferred embodiment refers to the monitoring of the viscosity of a resin or the degree of cure in a composite material during cure for illustration purposes only . the method and system of this invention can be employed for the continuous in situ monitoring of viscosity of other substances . fig1 is a schematic view of a system for the continuous in situ monitoring of resin viscosity , indicated generally as 20 , incorporating the principles of this invention . system 20 comprises a source of ultraviolet light 22 . ultraviolet light from source 22 passes through filter 24 which only transmits a beam 26 of the desired wavelengths ( typically about 400 - 450 nm ) of ultraviolet - visible light . filtered light beam 26 is then directed to optic fiber waveguide 28 . optic fiber waveguide 28 extends from system 20 into a remote variable temperature oven 30 to a resin sample 32 curing in oven 30 . resin sample 32 could be , for example , tetraglycidyldiaminodiphenylmethane , which exhibits a viscosity - dependent fluorescence , cured with diaminodiphenyl sulfone . resin sample 32 could also be , for example , diglycidyl ether of bisphenol a cured with diethylenetriamine , neither of which have viscosity - dependent fluorescence . in this case a viscosity - dependent fluorescent material must be added , such as about 0 . 1 - 0 . 5 % polyester yellow dye of a series of p -( n , n - diaklyaminobenzylidene malonitriles possessing viscosity - dependent fluorescence . filtered visible - ultraviolet light beam 26 travels through optic fiber waveguide 28 and irradiates resin sample 32 in oven 30 . the viscosity - dependent fluorescent material in resin sample 32 fluoresces under the excitation of beam 26 . the resultant visible light 34 travels back through optic fiber waveguide 28 to system 20 . light 34 is passed through a filter 36 which only transmits a beam 38 of desired wavelengths of light . ( typically about 510 nm ). beam 38 is focused by a lens 40 onto a photodiode detector 42 . the analog signal from detector 40 is boosted by preamplifier 44 . the boosted signal is amplified by amplifier 46 . the amplified signal is then transmitted to a recorder , such as chart recorder 48 . fig2 is a typical plot of fluorescence versus time for a resin under cure as would be determined with the system just described , compared to a graph of viscosity versus time for the resin under cure as determined by rheometrics spectrometer . the resin is tetraglycidyldiaminodiphenylmethane cured with diaminodiphenyl sulfone . fluorescence was monitored in the range of 505 - 525 nm . the viscosity was separately determined by dynamic rheometry . the results show that viscosity can be accurately determined by measuring fluorescence as the inventor has done . in fig3 a system for continuous , in situ monitoring of resin viscosity , indicated generally as 100 , is shown as it would be used to monitor a composite part 102 curing in an autoclave enclosure 104 . part 102 can be made at least in part from a viscosity - dependent fluorescent material such as tetraglycidyldiaminodiphienylmethane cured with diaminodiphenyl sulfone . alternatively , part 102 can be made from non - viscosity - dependent fluorescent materials , mixed with a small amount , e . g . 0 . 5 %, viscosity - dependent fluorescent material , such as a polyester yellow dye of a series of p -( n , n - diakylamino ) benzylidene malonitriles possessing viscosity - dependent fluorescence . ideally , part 102 would be provided with several viscosity - dependent fluorescent substances , each having a pronounced fluorescence change in a different range of viscosities . this is achieved by selecting viscosity dependent fluorescent substances whose molecules have rotating groups of different dimensions . the substances with the larger rotating groups , such as dibutyl amino and dipropyl amino , are sensitive to chances at low and intermediate viscosities during early stages of cure , and substances with smaller rotating groups , such as dimethyl amino and diethyl amino , are sensitive to changes at high viscosities during later stages of cure . system 100 comprises a light source 106 , for example a high - intensity tungsten - halogen lamp . a monochromator 108 is optically connected to light source 106 and filters out undesirable wavelengths , allowing only selected wavelengths of light to pass . generally wavelengths of about 400 - 450 nm are preferred . the filtered ultraviolet light passes through a beam splitter and filter unit 110 to optic fiber waveguides 112 . optic fiber waveguides 112 extend from unit 110 through remotely located autoclave enclosure 104 to part 102 . optic fiber waveguides 112 can be built into the mold or die for shaping part 102 , which would make use of waveguides 112 easier , eliminating the need to separately apply the waveguides to each part made . the filtered ultraviolet light travels through waveguides 112 to part 102 where it irradiates part 102 . the viscosity - dependent fluorescent material fluoresceces under the stimulation of the ultraviolet light . the resultant visible light travels back through optic fiber waveguides 112 to beam splitter and filter unit 110 . the returning light beam is split in unit 110 and each split beam is filtered to isolate a particular desired wave length or range of wave length . each resulting filtered beam is focused as with a lens onto a photodiode detector 114 . the signals from photodiode detector 114 are transmitted to a microprocessor 116 , for processing . such processing may include comparision of the signal representing fluorescence with a predetermined calibration table of corresponding viscosities to determine viscosity , or further translation into an indication of cure state . microprocessor 116 can also be programmed to control the cure process , directing that cure variables , such as the temperature in autoclave 104 , be varied in response to the input signal corresponding to viscosity , all according to a predetermined optimal cure cycle . fig4 illustrates the mounting of a fiber optic cure sensor 120 in a tool or mold 124 for the fabrication of composite parts , as would be done , for example , in system 100 just described . tool 122 is typically made from aluminum , and has a surface 124 for shaping the part . dams or sidewalls 126 surround tool 122 to contain the curing composite material 126 . sensor 120 extends through tool 122 and its distal or sensing end is mounted flush with surface 124 . permanently mounting probe 120 in tool 122 eliminates the need to install or set up the sensors for each manufacturing run . this reduces the time , labor , and cost required to use the system . at high viscosities , particularly in the later stages of cure of epoxy composite , shifts in the wave length of the maximum intensity of emitted fluorescence l max em and shifts in the wave length of the excitation radiation causing maximum intensity of emission l max em have been observed . adding wave length of the fluorescence maxima as a second parameter monitored allows the system to detect changes of viscosity at high viscosities where intensity changes are not as great . monitoring l max em during the cure of a composite allows the system to detect minor changes during the later stages of cure . fig5 is a schematic view of a system for the continuous in situ monitoring of resin viscosity , indicated generally as 200 , capable of monitoring both the intensity and wave length of fluorescence . the system is shown as it would be connected to a tool - mounted fiber optic probe 202 , mounted flush in the surface of a tool 204 as described above with regard to fig4 . a composite part 206 is curing in tool 204 which is positioned in an autoclave 208 . system 200 comprises light source 210 optically connected to a scanning monochromator 212 . scanning monochromator outputs excitation light to excitation optic fiber 214 in the ultra violet range , scanning a range of discrete wavelengths . this range is typically between about 395 and about 430 nm during early stages of cure ( low viscosities ) and between about 430 and 470 nm during later stages of cure ( high viscosities ). the excitation source scans at a rate of about 1 nm / sec . the excitation light is conducted by excitation fiber 214 through an optical y tap 216 to waveguide 202 . waveguide 202 conducts the excitation light to composite material 206 . the excitation light causes composite material 206 to fluoresce . the emitted fluorescent light is conducted by waveguide 202 , through y tap 216 , to detector fiber 218 . the fluorescent light from detector fiber 218 is focused at 220 onto a diffraction grating 222 which splits up the light and focuses the light onto a photodiode array 224 . photodiode array 224 is comprised of a plurality of individual photodiodes 226 each of which measure the intensity of the particular wavelength or range of wavelengths focused on it . the output from photodiode array 224 is conducted to a diode array controller and digitizer 228 , whose output is in turn conducted to microprocessor 230 . microprocessor 230 is programmed to identify the maximum intensity of the fluorescence and the l max em . the scanning of excitation wave lengths by scanning monochrometer 212 and the measurement of emitted fluorescence can be synchronized so that when the maximum intensity and l max em for each excitation scan are identified , the l max em for these values is known . microprocessor 230 can be programmed to compare the fluorescence intensity and l max em with a predetermined calibration table of corresponding viscosities to determine viscosity , or further translate viscosity into some indication of cure state . microprocessor 230 can also be programmed to control the cure process , directing that cure variables , such as the temperature in autoclave 208 , be varied in response to the input signal corresponding to viscosity or degree of cure , all according to a predetermined optimal cure cycle . in fig6 a plane wing 300 made from a carbon epoxy composite is shown as containing a plurality of fiber optic waveguides 302 extending therethrough . waveguides 302 may be laid up with the wing and remain in place with the wing structure . the remote ends 304 of waveguides 302 are located at particularly sensitive or critical areas of wing 300 . the inward ends 306 of waveguides 302 are connected to monitoring system 308 . monitoring system 308 , shown schematically in fig7 comprises a source of ultraviolet light 310 optically connected to the inward ends 306 of waveguides 302 . ultraviolet light from source 310 is conducted by waveguides 302 to the remote ends 304 where it irradiates the composite material at the sensitive or critical areas of the wing . under the excitation or stimulation of the ultraviolet light the composite material or the viscosity - dependant fluorescent material therein , fluoresces . this light is conducted back along waveguide 302 to inward end 306 . the fluorescent light is passed through a filter 312 which only transmits the wave lengths of interest and is focused as with lens 314 onto a photodiode detector 316 . the resulting signal from detector 316 is analyzed and compared to a norm . this can conveniently be done with a microprocessor 318 . microprocessor 318 can be preprogrammed to evaluate changes in fluorescence of the composite . decreases in fluorescence are caused , for example , by water absorption , thermo - oxidation of the composite , or increased temperature . increases in fluorescence are caused , for example , by over curing and embrittlement of the composite . microprocessor 318 can be preprogrammed to evaluate these changes during the use of wing 302 and give appropriate warning when the condition of wing 302 , as determined by fluorescence , deteriorates to an unacceptable condition for anticipated in - service conditions . there are various changes and modifications which may be made to applicant &# 39 ; s invention as would be apparent to those skilled in the art . however , any of these changes or modifications are included in the teaching of applicant &# 39 ; s disclosure and he intends that his invention be limited only by the scope of the claims appended hereto .