Thomas J. Hammond

Analytical model for low-pressure gas discharges: application to the Hg + Ar discharge.

A general technique for analyzing complicated gas discharges has been developed and applied to the Hg + Ar (fluorescent lamp) discharge. The theoretical model includes electron excitation and deexcitation, two-state ionization through a saturated metastable level, and proper treatment of the self-absorption of the resonance radiation. The analysis yields simple analytic expressions for the electron temperature, the resonance radiation, and the electric field. When applied to Hg + Ar discharges, these analytic expressions yield good quantitative agreement with the available absolute data on the dependence of the electron temperature, the Hg 2537-A radiation, and the electric field on mercury pressure and current.

Voltage‐current relationship for pulsed arc discharges

A theoretical treatment of the electrical behavior of pulsed discharges in gas‐filled flashlamps is presented. The theory is based on a simple model of the plasma and a balance of electrical power input and radiation output. It is assumed that the plasma radiates as a greybody with an emissivity that depends on the electron density. The principal result is an analytic expression for the arc voltage as a function of current, with initial voltage, arc length and diameter, and atom density as parameters. The expression describes the entire voltage pulse, including the initial fall, the following minimum and the peak when the current peaks. The predicted voltages substantially agree with measurements, including dependences on lamp length and diameter and initial voltage. The theory is an improvement on the empirical V‐I relation commonly used in flashlamp circuit analyses.

Arc–acoustic interaction in rare gas flashlamps

High frequency oscillations have been observed in rare gas flashlamp voltage and light output pulses. Experiments have shown that the frequency of the oscillations increases with the square root of the input electrical energy density. At fixed energy density input, the period of the oscillations increases linearly with the cylindrical lamp radius and with the square root of the atomic mass of the rare gas. These measured dependences suggest an acoustic generation mechanism with the gas temperature proportional to the input energy density. This interpretation allows a determination of the instantaneous gas temperature from the measured oscillation frequency.

Theoretical model of visible radiation from rare gas flashlamps.

A simple theoretical model of visible light emission from xenon flashlamps is presented. The continuum light emission is calculated from the rate of electron-ion recombination in the xenon plasma, which is treated as a greybody in thermal equilibrium. The effect of radiation reabsorption is calculated in terms of the temperature-dependent greybody emissivity. The model predictions of radiated power and energy are compared to measured data. Reasonable agreement is obtained over a wide range of parameters of practical interest. Thus the model provides a useful analytical tool for first-order engineering design of xenon flash-lamp illumination systems.

Effect of Electron Deexcitation and Self-Absorption on the Intensity of the Hg 2537-A Radiation from Hg + Ar Discharges (ac Fluorescent Lamps).

The intensity of the Hg 2573-A radiation from Hg + Ar discharges was measured as an independent function of mercury pressure (0.2-50 mTorr), ac current (50-2100 mA) and tube radius (0.79 cm and 1.27 cm) at a constant Ar pressure of ~4 Torr. For various constant mercury pressures, the Hg 2537-A intensity initially rises linearly with increasing current, but then tends to bend over and approach an asymptotic limit. The nonlinear, asymptotic behavior is due to electron deexcitation of the Hg 6(3)P(1) state at the higher currents in the presence of Hg 2537-A self-absorption. The Hg 2537-A intensity was also measured as a function of mercury pressure at various constant currents. The intensity rises to a peak (which defines an optimum Hg pressure) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. For high ac currents, the optimum Hg pressure is independent of current but varies inversely with the tube diameter. All this behavior is relevant to the problem of obtaining high efficiency from fluorescent lamps at high powers.

Flashlamp voltage and light-pulse shifts caused by gas heating

The gas temperature in a flashlamp is noticeably asymmetric, higher in the second half of the current pulse than in the first. This asymmetry shifts the maxima in lamp voltage and electron temperature from the lamp current peak. Because of the asymmetric gas heating, the voltage maximum preceeds the current peak while the electron temperature lags behind the current. Since the light emission is determined mainly by the electron temperature, a shift of the light peak occurs so that the light peak lags the current peak. A theoretical description of these shifts is presented. Under the assumptions that the current I = K0V1/2, and the gas temperature, power, and current shapes are similar near the maxima, the voltage and light shifts τ have the same magnitude and are given by τ/Δ = 0.136, where Δ is the one-sided current halfwidth. Experiments verified the direction and the approximate magnitudes of the shifts due to gas heating.

Direct current cataphoretic effects on Hg 2537-Å radiation from Hg + Ar discharges (dc fluorescent lamps)

The Hg 2537-A intensity from Hg + Ar discharges was measured as an independent function of mercury vapor pressure (as influenced by a cold spot temperature) at various constant currents and tube radii with ~4-Torr argon. As a function of mercury pressure, the intensity rises to a peak (which defines an optimum mercury cold spot temperature) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. The behavior of the optimum mercury cold spot temperature is dependent upon ac or dc conditions. For ac, the optimum mercury pressure is ~7 mTorr (corresponding to a Hg cold spot temperature of ~40 degrees C) and comparatively insensitive to current. By contrast, the optimum mercury cold spot temperature for the dc case is dependent upon whether the anode end or cathode end is cooled. The dc lamp with anode end cooled yields an optimum mercury cold spot temperature less than 40 degrees C and decreases with increasing current, while the optimum with cathode cooled is greater than 40 degrees C and increases with increasing current. We believe that the peak intensity always occurs at the same real mercury density (because that determines the self-absorption), but the Hg cold spot temperature required to achieve this density is affected by dc cataphoretic pumping phenomena.

LED printbar image plane characterization

Light emitting diode (LED) printbars are finding wider use as imaging devices in digital copiers and printers. The key to their acceptance in high quality printers requires tight control of the image plane irradiance profile. As a first step in understanding the imaging capabilities of an LED printbar a measurement system was developed to characterize the radiometric performance of the printhead from a slit scan of the image plane. From this scan of the image profile, statistics on the pixel to pixel irradiance, pixel to pixel placement, and pixel to pixel image size and shape are developed for an entire LED printhead.