William L. Lama

Systematic study of the electrical characteristics of the ``Trichel'' current pulses from negative needle‐to‐plane coronas

When a negative dc voltage is applied to a needle in an electronegative gas such as atmospheric air, highly regular corona current pulses (Trichel pulses) are observed under some conditions. Systematic variation of the macroscopic experimental parameters has yielded the functional dependence of the pulse frequency, charge per pulse, and time‐averaged corona current on the applied voltage, needle‐tip radius, and needle‐to‐plane spacing. In combination with a theoretical analysis of the motion of the charge clouds, these results yield new insight into the physical mechanism of this gas‐discharge phenomenon. For example, above threshold voltage, it appears that several negative‐ion‐charge clouds are simultaneously in transit across the gap.

Some radiometric properties of gradient-index fiber lenses

Some radiometric properties of gradient-index fiber lenses and lens arrays are explored. Consideration is restricted to the paraxial region of fibers that produce erect images at unit magnification. In two instances the radiometric properties of these lenses and lens arrays are considerably different from the properties of conventional (nongradient) lenses. First, the off-axis image plane irradiance for a single gradient-index fiber falls off far more rapidly than the familiar cos(4) law. Second, the exposure (integrated image irradiance) for a fiber array is independent of the object-to-image distance.

Modeling vibration-induced halftone banding in a xerographic laser printer

In a raster scanning printer, a laser beam is scanned across a photoreceptor in a direction perpendicular to the photoreceptor motion. When there is vibratory motion of the photoreceptor or wobble in the polygon mirror, the raster lines on the photoreceptor will not be evenly spaced. We analyze the positioning error and show that fractional raster spacing error is equal to photoreceptor fractional velocity error. These raster position errors can result in various print defects, of which halftone banding is the dominant defect. The dependences of halftone banding are examined using a first-order geometry-based printing model, an exposure model, and a more sophisticated laser imaging model coupled with a xerography model. The system model is used to calculate print reflectance modulation due to vibrations in both charged-area and discharged-area development modes using insulative or conductive development. System parameters examined are halftone frequency, raster frequency, average reflectance, vibration frequency, and multiple-beam interlace spacing.

Optical properties of GRIN fiber lens arrays: dependence on fiber length.

The optical imaging and radiometric properties of arrays of GRIN fiber lenses (Selfoc lens arrays) have been studied as a function of the length of the fibers. Experiments measured the object-to-image distance, image quality (MTF), depth of focus, image irradiance distribution, and exposure for a set of Selfoc lenses with differing fiber length. Simple models were used to explain the observed dependences, and analytical formulas were developed for the prediction of these optical properties.

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.

Imaging and radiometric properties of microlens arrays

The imaging and radiometric properties of erect lens arrays made up of small biconvex microlenses are derived from a ray analysis. The lens arrays provide erect, unit magnification images. The relationship between the radii of curvature, the lens thickness, and the one-to-one conjugate distance is derived for both the single-layer case and a double-layer structure, which contains field lenses. Radiometric properties of the microlens and the array are derived for both structures. The results are compared to experimentally measured values obtained from arrays fabricated by a photothermal process.

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.

Reduction/enlargement gradient-index lens array

A novel concept of a gradient-index lens array for reduction and enlargement is described. The array is composed of radial gradient-index rod lenses arrayed in a fanlike manner. Design and radiometric equations are presented. Some image aberrations are discussed in a qualitative fashion. The imaging performance and radiometric properties of a sample lens array are given.