convenient because in addition to having good sensitivity and range with a temperature coefficient in the order of microvolts k the output voltage is an absolute function of temperature and therefore a comparison temperature calibration feature is not needed in the past cooling systems have been used to counter the heating caused by the laser firing process and such cooling heat exchangers have typically kept the dye solution at about room temperature it has been determined however that so long as the dye solution has been properly specified for operation at higher temperatures the actual temperature to which the solution is held is not critical thus to minimize cooling required by the system the present system preferably runs at about 38 40 u00b0 c to maintain consistent color output that temperature is maintained within 2 5 u00b0 c and preferably within u00b1 1 u00b0 c this higher temperature also provides the added advantage of requiring less dye in the system since at higher temperatures the filter removes less dye as discussed above the concentration of specific solutes dye or additive is monitored at 52 by means of optical absorption at a specific wavelength as illustrated in fig3 the solution being monitored is allowed to flow through a sensing cell 62 the solution is illuminated from one side by a pulsed source 64 of white light an optical interference filter 66.