Process and Apparatus for Measuring Optical Density Changes and Transit Times in Transparent Materials

1. Field of the Invention

The invention is a recirculating optical/electronic closed loop process and apparatus in which nanosecond light pulses are repetitively passed through a light-transparent medium to test the time delay caused by a changing refractive index in the medium and to indicate any needed change in the constituency of the medium.

2. Description of the Prior Art

In process control technologies, there is a wide-spread need for a sensing data and apparatus which meaures and records optical density of materials or measures small distance variations between two or more reference points. The process control has application in fluid control devices or systems which, as examples, record the transmittance or the fluid level in vessels, determine the refractive index of such liquids, and replace displacement measuring devices that are predominantly used in the tool machine industry. In the past, level meters have ued complicated mechanical apparatus, refractometers have used optical devices with read-out techniques that depend on human obervers, and the displacement of materials was often recorded manually or semi-automatically by applying a measuring stick or tape or by some electrical read-out gadget. There is a need for obtaining minute changes in the detected data on the refractive index, over wide ranges of refractive index changes, represented as n+.DELTA.n, in light-transparent media. These refractive index changes may be used in controlling various process control operations.

The present invention employs the time domain of free traveling signals having a pulsed light source producing free traveling optical light pulses that pass through a transparent medium in an optical path and signal processing of the light pulses in an electronic portion of a closed loop feedback system.

The electronic portion is comprised of an externally triggered high rate pulse generator, possibly initially triggered by a lower pulse rate kick pulser, which produces very fast electronic signal pulses of short duration and at a large repetition rate. These electronic signal pulses pass through a pulse counter and repetitively trigger a light source. After each short duration light pulse from the light source travels through the optical path, a light detector cell receives electromagnetic radiation pulses, herein after referred to as optical light pulses but not necessarily limited to the optical range. The demodulated output from the detector cell is then amplified by a broadband amplifier with its output applied to the pulse generator. The pulse generator retriggers another pulse that proceeds through the pulse counter to the light source to again produce a short duration light pulse. The closed loop cycle is repeated with each nanosecond speed electronic and light pulse retriggering the subsequent pulse.

The refractive index of the transparent medium in the optical path determines the transient time of each pulse through the closed loop. In other words, the system uses the generation of optical transients for speed of light measurements in the transparent medium. The system utilizes the integration of minute signal delays during specific units of time. The transparent medium whose refractive index is being measured is preferably a liquid used in some process, such as checking for water in gasoline to checking more complex fluid mixtures, in which the constituents of the medium are being tested and measured. The signal propagation speeds in the transparent medium transmitting materials presupposes the knowledge of the refractive index n.sub.i and the wavelength .lambda..sup.1 of the modulated probing optical signal.

The preferred embodiment of the present invention is the transmissive mode in the optical path with the pulsed light source and the light detecting means facing toward each other on opposite sides of the transparent medium. The system may operate in the reflective mode however wherein the pulsed light source and the light detecting means are on the same side of the medium and a reflective surface is on the opposite side of the medium.

Each element in the closed loop has a different refractive index, including the connections between elements in the electronic portion. Stated another way, the delay time in the closed loop is comprised of many delays representing each element and each connection with only the varying delay of the loop caused by the changing refractive index of the medium after the system has been previously settled down when the medium is not in the optical path. The delay time acts as the frequency which determines or indicates the refractive index of the transparent medium. The accuracy of the frequency measurement elements, such as the pulse counter and related elements, sets the limit of how well the refractive index and other factors are measured.

The speed measuring system in which the time required for a very short pulse, i.e. nanosecond pulse, to cycle through the closed loop varies according to the change in the refractive index of the transparent medium, which may be constantly flowing liquid, being tested in the continuous process, and the possible control system to maintain a set refractive index mixture of the liquid in the process. Circulation of each nanosecond pulse around the entire loop takes a certain amount of time. When operated repetitively wherein the initial pulse keeps retriggering subsequent pulses at the nanosecond frequencies, the pulse repetition rate that is established becomes a unique quantity inversely proportional to the delay caused by the refractive index of the liquid medium being monitored and controlled. The number of pulses recorded on the pulse counter in a precisely timed period of time converts to the refractive index changes in the continuously flowing light transparent medium being monitored. The system allows continuous monitoring of the liquid medium without disturbing the medium itself. The light pulse beam penetrates the medium after passing through beam forming optics. The light pulse beam may be focused in a small portion of the medium to measure the refractive index only in localized areas or defocused over a much larger area. If the frequency is measured by counting pulses for 1 second and the count is accurate to 1 pulse, the frequency is within an accuracy of 1 part in 10.sup.6. The present frequency measurement apparatus is within that accuracy.

The invention will be described with further detail in reference to the lone FIGURE.

The lone FIGURE is a schematic illustration of the process conrol apparatus and system of the present invention.

Referring now to the FIGURE, transmit times of the electromagnetic, i.e. optical signals, are used for sensing the refractive index and/or transparency measurements. Arrows are used to indicate the direction of signal flow. A separate kick pulser generator 14 may be used to trigger an electric pulse signal from pulse generator 16 through a pulse counter 18 to a pulsed light source 20. Pulsed light source 20 amplifies and converts the electric pulse signal to a free-traveling pulse of optical radiation. The light source 20 may be a light emitting diode (LED). Many LED components are available that emit light of various colors, some of which emit non-visible light or mircowave radiation, such as Gunn-oscillators.

An electric pulse signal generated from pulse generator 16 of about 2 volts driving amplitude was found sufficient to modulate a LED with a 5 nanosecond light pulse, at a wavelength .lambda. of 650 nanometer, at a starting pulse repetition frequency of about 15 MHz, but may be up to 20 MHz. Optical elements 22 and 24 of beam forming optics direct the optical radiation pulse through a vessel 26 which has transparent walls through which the radiation passes and which contains the transparent medium, which may be a liquid, that is being tested. The liquid may enter tube 30 and exit tube 28. The vessel 26 itself may be moved in and out of the optical path of elements 22 and 24, say by a slide carriage system. The light source 20 is preferably positioned into the focus of lens 22. The optical radiation signal is passed through the vessel 26 walls and the transparent liquid and is focused into a radiation detecting means 10, such as a light detector cell. The detector cell may be a photoelectron multiplier (PM) since the PM would readily provide the necessary amplification capable of measuring a difference intensity of the light pulse. The detector cell may also be a phototransistor or a photocell.

Upon arrival of each optical radiation pulse at the detector cell 10, a new electric pulse signal is created which enters a wideband amplifier 12 that amplifies the new electric pulse signal which is applied to the pulse generator 16. Another electric pulse signal is then emitted from generator 16 which will transit the closed loop as previous pulses have. The signal circulation rate, i.e. the signal repetition frequency F.sub.r, is automatically repeated. The repetition frequency F.sub.r depends only on the electrical/optical signal transit time of all components in the signal propagation loop. All electronic elements are preferably connected together with low loss coaxial cable. The transparent medium, which is being tested, is made a part of this loop and effects the Fr by its length and characteristic transit time, while all the other transit times of elements in the loop can be arranged to remain constant. In this manner, when a change in the refractive index of the liquid medium occurs by change in the amount of constituent materials in the liquid a different signal transit time also occurs which causes a varying signal circulation rate in the closed loop.

The refractive index n is defined as the ratio of two group velocities of electromagnetic signals, n.sub.i =c.sub.o /c.sub.i, where c.sub.o =3.times.10.sup.10 cm/sec, i.e. the speed of light in vacuum. The equivalent refractive index for the entire loop is therefore: n.sub.i =c.sub.o /c.sub.i =c.sub.o /(s.multidot.F.sub.r), if s is the totaled electromagnetic length of the propagation loop. Alternatively, statements of signal propagation speeds in transmitting materials presuppose the knowledge of the refractive index n.sub.i and the wavelength .lambda. of the modulated probing signal. It is obvious that each element i of the loop may have a different refractive index, which then may be determined from its individual signal transit time n.sub.i =c.sub.i .multidot.t.sub.i /.DELTA.s.sub.i, i.e. from frequency difference measurements .DELTA.f=F.sub.r .+-.f, at the various length settings .DELTA.s of the loop.

Once the process control system has been started, the automatic recirculation of the signals will be repetitive in the same mode, with all round trips over the closed loop counted by the pulse counter 18 and registered as the signal repetition frequency F.sub.r. The pulse counter 18 may be tuned circuits or electronic counters. The quantity F.sub.r reflects the integrated number of signal transits through the loop per unit of time, F.sub.r =(1/T.sub.loop). Obviously, the time for one loop transit T.sub.loop is identical to the sum total of all transit times through the individual elements. Tests have confirmed very uniform triggering action of the pulse generator 16 even for large changes of transit time, which is introduced by the insertion of various time delay elements into the loop.

The signal speed c.sub.n through the vessel 26 and the transparent medium therein can be determined from the measurement of two signal repetition frequencies, one measured when the vessel 26 is empty and one measured when the transparent medium being meaured or tested is present in the vessel, by using the equation,

where, F is the original frequency of signal repetition when the vessel is empty, f is the new frequency of the vessel and the transparent medium therein combined, and .DELTA.s is optical path length of the empty vessel, the signal speed can be determined. Many transparent mediums have a well defined transit time that is known to be better than one part in 10.sup.6, if the wavelength of the transiting signal is known. If such quantity is known, and herein indicated as c.sub.i, while the signal transit time through vessel 26 when empty is indicated as c.sub.o, the ratio of the two transit times equals the refractive index n of the medium: n=c.sub.o /c.sub.n. There is dispersion caused by constituents in the transparent medium. The derivative, represented as D, of the refractive index in respect to the wavelength of the transiting signal is identical with the transparent medium's dispersion: D=dn/d.lambda.. Measurements as to incremental changes of distance, represented by .DELTA.s, across the optical path between the pulse light source 20 and the pulse detecting means 10 are as follows:

where c is the signal transit time through the closed loop and f and F are as defined with reference to Equation (1). The invention provides rapid access to the above data without disturbing the medium and facilitates other applications of process control means to the handling, treatment, or manufacturing of the medium with a minimum of delays. Even though the short duration pulses are explained herein as in nanoseconds and for example about 2 volts driving amplitude, the pulses may be in the millisecond to nanosecond range and in the 2 to 200 volt amplitude range.