of the present invention is 6 7 dimethoxycoumarin which when excited by excitation radiation having a wavelength of 337 nm emits fluorescent radiation at a wavelength of 435 nm the characteristic excitation and emission spectra of 6 7 dimethoxycoumarin are illustrated in the accompanying drawings as described hereinafter it is to be understood that when reference is made herein to a particular wavelength for example with respect to excitation or emission it is intended to mean that wavelength which is most representative of the condition being described most typically the peak of a curve illustrating the spectrum which fully represents said condition thus as shown by the curve for the excitation spectrum 6 7 dimethoxycoumarin is excited by radiation over a spectrum of wavelengths from 310 to 380 with an optimum excitation at the peak wavelength of 337 nm for convenience unless otherwise defined the wavelengths quoted herein are the peak wavelengths for the phenomenon in question the preferred second indicator used in the first embodiment of the invention is hpta in the preferred first embodiment of the invention excitation radiation having a wavelength i e a peak wavelength u03bb o of 337 nm for example from a nitrogen gas laser is transmitted from the proximal end of an optical fiber through the distal end where it excites a first indicator preferably 6 7 dimethoxycoumarin which emits fluorescent radiation having a wavelength u03bb 1 of 435 nm this fluorescence emission in turn excites the second indicator preferably hpta to emit fluorescent radiation having a wavelength u03bb 2 of 510 nm the intensity of the fluorescence.