Showing papers in "Review of Scientific Instruments in 1965"
TL;DR: In this article, a high speed fountain pen ink stream is divided into a uniform procession of drops, each of which is independently charged in proportion to an input signal voltage, and the charged drops are collected on a moving chart to form an instantly visible, permanent record of the input signal.
Abstract: A high speed oscillograph, using ordinary ink and paper, has been developed that provides a new approach to the old problem of producing instantly visible, high frequency records with inexpensive writing materials. A high speed jet of ordinary fountain‐pen ink is divided into a uniform procession of drops, each of which is independently charged in proportion to an input‐signal voltage. After projection through a constant transverse‐deflecting field, the charged drops are collected on a moving chart to form an instantly visible, permanent record of the input signal. Drops are typically formed at a rate of 100 000/sec; each has an independent trajectory and makes an individual mark representing an independent sample of the input waveform. The ink stream may be switched on or off at high speed by providing, between the drop‐launching point and the record surface, a collector that intercepts drops having a specific trajectory. Besides oscillography, which is discussed in detail, the technique has applications in other fields requiring marking at high speed or marking without pressure or physical contact.
245 citations
TL;DR: In this paper, a new technique is described for measuring the position history of the free surface of an impacted specimen, and measurements can be made to 0.025 μ accuracy at free surface velocities up to 1 mm/μsec.
Abstract: A new technique is described for measuring the position history of the free surface of an impacted specimen. Measurements can be made to 0.025 μ accuracy at free surface velocities up to 0.1 mm/μsec. Problems and possible solutions for monitoring higher velocities are indicated.
175 citations
TL;DR: In this article, a method to determine stress in thin films deposited on single crystalline silicon wafers is described, where the film is treated as an elastic membrane attached to the edges of a thin circular disk.
Abstract: A convenient method to determine stress in thin films deposited on single crystalline silicon wafers is described. The film is treated as an elastic membrane attached to the edges of a thin circular disk. Due to the stress forces of the film, the substrate deforms elastically and assumes a parabolic curvature. This parabolic deflection is determined by measuring the substrate profile on the surface with a light section microscope. Knowing the curvature and the elastic constants of the substrate wafer, one can calculate the stress forces exerted by the film. The experimental accuracy of the method is about ±10%.
126 citations
TL;DR: In this paper, the continuous channel electron multiplier is operated in a gain saturated mode such that single, charged particles entering the input mouth of the channel will initiate output pulses whose amplitude and shape are both uniform and independent of the character of the excitation radiation.
Abstract: It has been found to be possible to operate the continuous‐channel electron multiplier, a recently developed type of windowless electron multiplier, in a gain saturated mode such that single, charged particles entering the input mouth of the channel will initiate output pulses whose amplitude and shape are both uniform and independent of the character of the excitation radiation. By suitable design of the channel this saturated pulse can be made stable against changes in such operating conditions as ambient pressure and applied voltage. When operated in this mode, the efficiency of the channel for the detection of electrons over the energy range 250 eV to 10 keV is estimated to be greater than 50%.
120 citations
TL;DR: In this paper, the ability of a photomultiplier system to resolve time differences is presented in graphic form and the results are presented in parametric form and hence may be applied to a wide variety of scintillator•photomULTiplier combinations, provided the system is reasonably approximated by the model used in this work.
Abstract: Results on the ability of a photomultiplier system to resolve time differences are presented in graphic form. The results are presented in parametric form and hence may be applied to a wide variety of scintillator‐photomultiplier combinations, provided the system is reasonably approximated by the model used in this work. In addition to the usual effects included (scintillator lifetime, pulse shape, transit time spread), the model includes the effects of finite risetime in the scintillation process.
116 citations
TL;DR: In this paper, a theoretical analysis of the time averaging method with special emphasis on the correlation effects caused by non-white noise is presented, and it is shown that only in special cases is the signalto-noise ratio improved exactly according to the square root of the number of successive scans through the investigated spectrum.
Abstract: A theoretical analysis of the method of time averaging is made with special emphasis on the correlation effects caused by non‐white noise. It is shown that only in special cases is the signal‐to‐noise ratio improved exactly according to the square root of the number of successive scans through the investigated spectrum. This is shown in detail for random noise with an exponential‐cosine autocorrelation function. In the case of random noise with a pure exponential autocorrelation function, the deviation from the proportionality to the square root of the number of scans does not exceed a factor √2. If the frequency dependence of the power spectrum of the noise is given by |f|−λ, 0≤λ≤1, then the proportionality to the square root of the number of scans holds exactly. A peak in the noise power spectrum at a multiple of the repetition frequency of the scanning does affect the time averaging only if its width is smaller than the repetition frequency.
113 citations
TL;DR: In this paper, a new vapor deposition technique was developed for the production of insulating thin-film CdS and ZnS piezoelectric transducers, which were used to generate either shear or compressional waves in dielectric materials at frequencies in the gigacycle range.
Abstract: A new vapor deposition technique has been developed for the production of insulating thin‐film CdS and ZnS piezoelectric transducers. These high efficiency transducers have been used to generate either shear or compressional waves in dielectric materials at frequencies in the gigacycle range. The thickness, which is measured by means of a quartz crystal microbalance, is controlled to produce films which operate at their fundamental resonance. The mode of the generated waves is determined by the orientation of the driving electric field with respect to the crystal axes of the film transducer.
107 citations
TL;DR: In this paper, the transverse electromagnetic interaction of an intense azimuthally uniform beam of particles with itself, including the effect of a resistive vacuum tank, is investigated theoretically.
Abstract: The transverse electromagnetic interaction of an intense azimuthally uniform beam of particles with itself, including the effect of a resistive vacuum tank, is investigated theoretically. It is shown that a beam of particles all having velocity ν is unstable against the development of transverse waves having a phase velocity close to (1 − v/n)ν, where v is the number of transverse free betatron oscillations occurring in one revolution and n is a positive integer greater than v. The growth rate for the instability is proportional to N/σ½, where N is the number of particles in the beam and σ is the conductivity of the surface material of the vacuum tank. Stabilizing mechanisms are examined by means of the Vlasov equation and it is shown that a spread in the quantity (n−v)ν, evaluated for particles in the unperturbed beam, will prevent the instability. A criterion for the spread required is shown, in the limit of walls of high conductivity, to depend upon the beam intensity and energy as well as upon certain...
102 citations
TL;DR: In this paper, the effect of finite resistance in the vacuum-tank walls on the longitudinal stability of an intense beam of particles in an accelerator is investigated theoretically, and it is shown that even if the particle frequency is an increasing function of particle energy, the wall resistance can render the beam unstable against longitudinal bunching.
Abstract: The effect of finite resistance in the vacuum-tank walls on the longitudinal stability of an intense beam of particles in an accelerator is investigated theoretically. We show that even if the particle frequency is an increasing function of particle energy, the wall resistance can render the beam unstable against longitudinal bunching. In the absence of frequency spread in the unperturbed beam, the instability occurs with a growth rate that is proportional to (N/{sigma}){sup 1/2}, where N is the number of particles in the beam and {sigma} is the conductivity of the surface material. By means of the Vlasov equation a criterion for beam stability is obtained. In the limit of highly conducting walls the criterion involves the frequency spread in the unperturbed beam, the number of particles N, the beam energy, geometrical properties of the accelerator, but not the conductivity {sigma}. A numerical example presented indicates that certain observations of beam behavior in the MURA 40-Mev-electron accelerator may be related to the phenomenon we investigated.
98 citations
TL;DR: In this article, a circuit diagram of the current supply is given, including a transistor switch for pulsing the current, and construction parameters are given for coils to produce field gradients for use in NMR spin-echo diffusion studies.
Abstract: Construction parameters are given for coils to produce field gradients for use in NMR spin‐echo diffusion studies. The coils are shaped to give the greatest field gradient homogeneity in the region of the sample, taking into account image currents in the magnet pole faces. A circuit diagram of the current supply is given, including a transistor switch for pulsing the current.
96 citations
TL;DR: In this article, a new solution calorimeter is described which combines the thermometric titration techniques with the precision and accuracy of conventional solution calcorimetry, which has a low heat leakage, k = 1.1×10−3 min−1, and a short equilibration period, 1-3 sec.
Abstract: A new solution calorimeter is described which combines the thermometric titration techniques with the precision and accuracy of conventional solution calorimetry. The calorimeter through use of a unique design has a low heat leakage, k = 1.1×10−3 min−1, and a short equilibration period, 1–3 sec. Measurements of quantities of heat as small as 4 calories with an accuracy of 0.1% are possible. The heat of ionization of water at zero ionic strength has been determined to be 13.34±0.02 kcal/mole which is in excellent agreement with the value of 13.34±0.01 kcal/mole determined by conventional solution calorimetry. The large amount of information obtained from a single run makes this type of calorimeter a valuable addition to the field of solution calorimetry.
TL;DR: In this paper, the delay time between arrival of the laser pulse and current flow across the gap was studied as a function of total laser beam power (0 −80 MW), dielectric gas (SF6, N2, air), gas pressure (100 −1400 Torr), electrode spacing (0.4 −1.5 cm), gap electric field (10 −100 kV/cm), and focus point location between two 5 cm diam stainless steel spheres.
Abstract: The influence of parameters affecting the laser triggering of a high voltage electrical sphere‐sphere gap has been experimentally investigated. Of primary interest was the delay time between arrival of the laser pulse and current flow across the gap. This delay was studied as a function of total laser beam power (0–80 MW); dielectric gas (SF6, N2, air); gas pressure (100–1400 Torr); electrode spacing (0.4–1.5 cm); gap electric field (10–100 kV/cm); and focus point location between two 5 cm diam stainless steel spheres. Delay times less than 10 nsec were observed in SF6 at atmospheric pressure with corresponding low jitter. For the cases studied delay times varied inversely with the electric field, gas pressure, and focus point distance from the anode surface. Above a certain laser beam power the delay time was not a significant function of laser power for the range studied. Applications of laser triggering are discussed with a description of current and future research areas.
TL;DR: In this article, a phase fluorometer with high intensity light with a high degree of modulation was constructed using new commercial equipment including a Pockels effect light modulating device, and the output of the phase detector was recorded and precise phasenull found graphically.
Abstract: Using new commercial equipment including a Pockels‐effect light modulating device, a phase fluorometer with the following characteristics has been constructed: high intensity light with a high degree of modulation (typically m=43% at 1 kV modulating voltage and 462 mμ); pure sine modulation of variable frequency (14.0110, 21.0170, 27.0040 Mc); narrow wavelength regions for excitation and emission; very small contamination of detected light with exciting light (<1%); spectral range of the modulated light from 200 to 1000 mμ. The phase difference between fluorescent light and scattered light is measured with a high precision wide band phasemeter employing fixed and variable coaxial delay lines. The output of the phase detector is recorded and precise phasenull found graphically. Maximum time resolution is 0.02 nsec. The average over‐all precision in general use is Δτ/τ=2% in the range from 2 to 63 nsec; a wider range can be measured with slightly reduced precision. Systematic errors caused particularly by e...
TL;DR: In this article, a method for measuring the pyroelectric coefficient, dc dielectric constant, and volume resistivity of a polar material over a broad temperature range is described.
Abstract: A method is described for measuring the pyroelectric coefficient, dc dielectric constant, and volume resistivity of a polar material over a broad temperature range. The technique is based on the observation of the capacitive charging of a pyroelectric sample by the pyroelectric current generated during a continuous temperature change. This method has the advantages of requiring neither a heat flux measurement nor the attaining of isothermal conditions and does not alter the domain structure of a ferroelectric material. Values of the above properties were measured for Clevite PZT‐5 (lead‐zirconate‐titanate ceramic) over the temperature range from −200 to 200°C. The pyroelectric coefficient of Clevite PZT‐4 (lead‐zirconate‐titanate ceramic) was measured by a simplified method over the temperature range of 25 to 75°C.
TL;DR: In this article, the decay time at the amplitude-limiting section of the system and the pulse shape at the final output were specified as independent parameters, which can be reduced by orders of magnitude without degrading the signal-to-noise ratio.
Abstract: The techniques of modern network theory have been applied to some of the problems of pulse shaping in nuclear pulse amplifiers. A technique referred to as pole‐zero cancellation has been evolved which permits a system designer to specify the pulse decay time at the amplitude‐limiting section of the system and the pulse shape at the final output as completely independent parameters. The deadtime of the amplifier system can be reduced by orders of magnitude without degrading the signal‐to‐noise ratio. In addition, the usual charge‐injection system has been examined, and a simple, linear lumped‐element network has been evolved which makes possible an improved evaluation of the amplifier response to detector charge pulses.
TL;DR: In this paper, it was shown theoretically that a magnet stable to 35 MA/m (440 kOe) can be built from hard drawn copper if a reinforcing steel cylinder is used at a certain radius inside the coil.
Abstract: Magnet coils are investigated which can produce magnetic fields of 15–40 MA/m (200–500 kOe) during working times of the order of 0.1 sec. These fields are suited for measurements in highly conducting materials. The coils are precooled to 20°K or 77°K and the Joule heat is not removed during the pulse. In Sec. I the thermal limitations of this method are reviewed. Plots are given for the optimum dimensions and current densities for coils producing a specified field during a specified time, minimizing the necessary power supply. Details of a magnet constructed along these lines are given. The maximum field produced by this coil, 16 MA/m (200 kOe) during 0.3 sec, however, proved to be determined by mechanical instabilities. These problems are reviewed in Sec. II. Calculations of the Lorentz force and the internal stresses resulting from it were carried out taking the elastic and plastic properties of copper and insulation into account. It is shown theoretically that a magnet stable to 35 MA/m (440 kOe) can be built from hard drawn copper if a reinforcing steel cylinder is used at a certain radius inside the coil.
TL;DR: In this paper, electrical and sputtering characteristics of a 10 cm-diam inverted magnetron configuration were investigated in the transition region from positive to negative space charge modes, and the ion energy distributions were obtained for both modes.
Abstract: Electrical and sputtering characteristics of a 10‐cm‐diam inverted magnetron configuration have been investigated in the transition region from positive to negative space‐charge modes. Radial voltage and ion energy distributions were obtained for both modes. The positive space‐charge mode resulted in the most efficient transport of sputtered material from cathode to anode. Dependence of space‐charge sign reversal on apparatus dimensions and magnetic field strength are discussed showing that operation in the 10−5 Torr range can be expected for a 40‐cm‐diam configuration. The deposition profile at the anode was in good agreement with the calculated profile showing that ion current and hence sputtering is uniform over the effective cathode area.
TL;DR: In this article, a simple system capable of continuously recording complete magnetization curves for hysteretic superconductors is described, which allows the applied field to be cycled slowly in order to avoid transient effects in the specimen material.
Abstract: A simple system capable of continuously recording complete magnetization curves for hysteretic superconductors is described. It allows the applied field to be cycled slowly in order to avoid transient effects in the specimen material. A voltage proportional to the differential susceptibility of the material is integrated by a chopper stabilized solid state amplifier, and plotted on an X‐Y recorder as a function of applied field. The system is capable of recording magnetization curves over periods of 15 min with an over‐all uncertainty of 5% or less.
TL;DR: The application of electron spin echo methods to spectrometry is discussed theoretically and illustrated with experimental examples in this article, where a pulsing and detection apparatus with a bandwidth of 8 Mc, constituting a relatively simple addition to a conventional X band cw spectrometer, is described.
Abstract: The application of electron spin echo methods to spectrometry is discussed theoretically and illustrated with experimental examples. A pulsing and detection apparatus with a bandwidth of 8 Mc, constituting a relatively simple addition to a conventional X band cw spectrometer, is described. Computations have been made for the echo waveforms generated in the case where the microwave field intensity is of the same order as or less than the linewidth. The optimization of experimental conditions is considered, and an expression has been derived for the minimum number of spins detectable by the echo method. This is essentially the same as the number detectable by cw spectrometry at the same average power level and in the same time of observation. The general applicability of the method is limited by requirements regarding phase memory in the material studied. It is possible, however, that for some measurements, especially those involving broad lines, the echo method may possess advantages over cw spectrometry because of the fundamental difference in the way in which the resonance signal is obtained.
TL;DR: In this paper, a hysteresis loop tracer employing an alternating current electromagnet capable of providing fields up to 3600 Oe was described, and the performance of the present system over other types of loop tracers was pointed out.
Abstract: A hysteresis loop tracer employing an alternating current electromagnet capable of providing fields up to 3600 Oe is described. Merits of the present system over other types of loop tracers are pointed out. Some typical hysteresis loops obtained using the apparatus are also presented.
TL;DR: In this paper, a mutual inductance bridge for measuring ac magnetic susceptibility at low frequencies, using standard components, is described, and the equivalent of a decade mutual indicator is achieved by using a commercial decade autotransformer in combination with a small fixed mutual inductor.
Abstract: : A mutual inductance bridge for measuring ac magnetic susceptibility at low frequencies, using standard components, is described. The equivalent of a decade mutual indicator is achieved by using a commercial decade autotransformer in combination with a small fixed mutual inductor. The Gertsch RT-61 transformer is used at 30-400 cps and lower, while the RT-60 is used at approx 1 kc. Both transformers have a nominal accuracy of 0.001%, which is more than sufficient in this use. A lownoise preamplifier followed by a lock-in amplifier is used for null detection. (Author)
TL;DR: In this article, a method for finding the local resistivity of a semiconductor crystal by measuring the radio frequency spreading resistance of a small probe placed on a flat surface of the sample is described.
Abstract: A method is described for finding the local resistivity of a semiconductor crystal by measuring the radio frequency spreading resistance of a small probe placed on a flat surface of the sample. The method is nondestructive and does not require the attachment of electrical contacts. The resistivity measured is that of the material lying within a hemisphere of about 1 mm radius; measurements have been made over the range 0.1–100 Ω cm with good accuracy above 0.5 Ω cm.
TL;DR: The conversion efficiency of explosive into electromagnetic energy has been as high as 10% in the most favorable cases as discussed by the authors, and different compression experiments, including Bz and Bφ configurations in various geometrical arrangements, are described in detail and their results analyzed and discussed.
Abstract: Chemical explosives are used to compress magnetic flux trapped within a metallic liner system. In this way, final magnetic fields from 0.5 MG in a volume of 40 cm3 to 3.7 MG in 1.5 cm3 have been produced and carefully measured. The conversion efficiency of explosive into electromagnetic energy has been as high as 10% in the most favorable cases. The different compression experiments, including Bz and Bφ configurations in various geometrical arrangements, are described in detail and their results analyzed and discussed.
TL;DR: In this paper, the critical role played by first stage gain in shaping the output pulse height distribution and detection efficiency of an electron multiplier used as an ion detector is studied theoretically and experimentally.
Abstract: The critical role played by first stage gain in shaping the output pulse height distribution and detection efficiency of an electron multiplier used as an ion detector is studied theoretically and experimentally. In a well‐focused electron multiplier it is found that anode pulse height distributions observed for different ion species agree well with distributions calculated by using compound Poisson statistics. Models are developed to explain measurement bias in an electron multiplier which is used in either the dc or pulse counting modes and it is shown how these results can be applied to isotope ratio measurements. Design and operating characteristics are described for a 20 stage electron multiplier which was developed for high speed pulse counting and high detection efficiency. The surface of the first dynode is coated with a thin film of Al2O3 to provide a stable first stage yield of 7–9 secondary electrons/incident ion for monatomic alkali ions with 20 kV impact energy. The polyatomic ion Na2BO2+ yields about 16 secondary electrons/incident ion at 20 kV impact energy. Resolving time of the entire detector system is reduced to 10 nsec by using a specially designed solid state discriminator and pulse shaper. Also, a technique is described for the simultaneous detection and pulse counting of two ion beams.
TL;DR: In this paper, a sensitive ion cyclotron resonance spectrometer designed to study low energy ion-molecule interactions in a weakly ionized gas is described, which is capable of determining simultaneously the collision frequency and average concentration of ions having a particular charge-to-mass ratio.
Abstract: A sensitive ion cyclotron resonance spectrometer designed to study low energy ion‐molecule interactions in a weakly ionized gas is described It is capable of determining simultaneously the collision frequency and average concentration of ions having a particular charge‐to‐mass ratio General design considerations discussed include sensitivity, linewidth, resolution, and line broadening at high E/P Effects of electron buildup are also discussed The measurement chamber is intended to operate within a 1500 G, 102 cm long solenoid Ion sources and electrode arrangements are described Power absorption is detected by a narrow band, derivative type system for which circuit diagrams are given The spectrometer has a resolution of about 3% and can detect an average concentration of 1–10 ions/cm3
TL;DR: Low frequency field modulation (FMT) as mentioned in this paper is a commonly used modulation technique for measuring the de Haas-van-Alphen effect at high magnetic fields and is applicable to a wide variety of magnetic measurements and is particularly valuable for magnetic transition studies.
Abstract: Low frequency field modulation techniques (FMT) for measurements of differential magnetic susceptibility are discussed and specific applications to the de Haas‐van Alphen effect are analyzed. The method utilizes a small ac modulating field which is superimposed on a large applied dc magnetic field. Differential changes in magnetization of the sample are synchronously detected with the aid of a suitable arrangement of pickup coils. The FMT is applicable to a wide variety of magnetic measurements and is particularly valuable for de Haas‐van Alphen or magnetic transition studies at high magnetic fields. Advantages of this technique include: simplicity, versatility, direct applicability to axial high field water‐cooled and superconducting solenoids or transverse magnets, high differential sensitivity (dσ/dB=3×10−6 emu/gG), convenient access to the sample environment, and use of standard commercially available electronic components. Details of the detection coil assembly, sample rotation devices, and low temperature accessories are presented along with a general discussion of the data processing techniques. Modifications for transverse field modulation and accurate orientation of the dc or modulation field with respect to the sample are also discussed.
TL;DR: In this article, a photocell slit system was used to eliminate all position drifts by starting each new cycle from the same position, which can be used to analyze the velocity of the spectrometer.
Abstract: In order to eliminate drifts encountered in electromechanical Mossbauer spectrometers, a drive has been developed which is drift free with regard to velocity as well as position. The spectrometer can be driven in two modes: constant acceleration and constant velocity. The constant acceleration input signal is the analog output of the address scalers of a multichannel analyzer running as a multiscaler (time mode). The constant velocity is obtained from a photocell slit system which drives the input of the spectrometer with a rectangular wave. In both modes, the photocell system is used to eliminate all position drifts by starting each new cycle from the same position. To analyze the velocity of the spectrometer a voltage‐to‐frequency converter and multichannel analyzer are used.
TL;DR: In this paper, the theory and design of a magnetically shielded solenoid with correction coils and no magnetic shielding are discussed. And the calculations of the magnetic field due to a soleneoid inside a closed cylinder of infinite permeability are summarized.
Abstract: The theory, design, and construction of a magnetically shielded solenoid are described. Three correction coils are employed in addition to the main solenoid winding. The solenoid is enclosed in three concentric magnetic shields which serve to screen the inner region of the solenoid from external magnetic fields. First the theory and design of a solenoid with correction coils and no magnetic shielding are discussed. Then calculations of the magnetic field due to a solenoid inside a closed cylinder of infinite permeability are summarized. These calculations show that a properly constructed shield can improve the homogeneity of the field due to a single solenoid by an order of magnitude. Optical pumping measurements of the field homogeneity in the central region and of the field distribution along the axis of the solenoid are reported. The measurements of the field distribution agree with the calculations to within a few hundredths of a percent. The shields reduce the disturbance due to changing external fields by a factor of 100. The main solenoid is 91.44‐cm long and has an inner diameter of 32.41 cm. The solenoid field is 18.6 G/A and the solenoid dissipates approximately 320 W when producing a field of 60 G. The solenoid‐shield system has at 60 G a homogeneity of 1 part in 105 over an 8‐cm‐diam sphere at the center of the solenoid. Construction details of the shields and coils are given and the solenoid power supply is described.
TL;DR: In this article, a simple and reliable calibration technique, using conventional laboratory electronic equipment, was developed to measure the absolute sensitivity of magnetic probes in the frequency range of 1 − 50 Mc. The use of these probes is briefly mentioned.
Abstract: The details of the construction and calibration of high frequency magnetic probes, used to measure the transient magnetic fields in a linear pinch device, are described. A simple and reliable calibration technique, using conventional laboratory electronic equipment, was developed to measure the absolute sensitivity of magnetic probes in the frequency range of 1–50 Mc. Factors affecting the frequency response of magnetic probes are considered, and high frequency design and construction are discussed. A double probe, which measures two magnetic field components simultaneously is described. The use of these probes is briefly mentioned.
TL;DR: In this paper, an instrument was developed which measured the rate of phototaxis in a suspension of microorganisms by recording the increase in optical density which results when the organisms accumulate in a beam of stimulating light.
Abstract: An instrument has been developed which measures the rate of phototaxis in a suspension of microorganisms by recording the increase in optical density which results when the organisms accumulate in a beam of stimulating light. Intensity and spectral quality of the actinic light, as well as of randomly scattered background illumination, can be controlled. Density changes are measured with a dual chopped beam arrangement using infrared light. Thermostatic control of the temperature of the sample is provided.