Page:Calligraphy for computers (Hershey, 1967) (IA DTIC AD0662398).djvu/13

Acuity

A limiting factor is the acuity of the eye. Any resolution in excess of the amount which can be perceived would be wasted. The acuity of the eye varies among individuals, and the acuity varies with the type of perception. Insofar as the perception of separation between lines is a gauge of acuity, the angle of resolution¹⁰ is 30" of arc or a quarter of a raster unit.

Diffraction

An interesting factor is the diffraction of electrons or light in the printer system. The diffraction pattern of a circular aperture consists of alternating bright and dark rings around the geometric center. The angle $\theta$ which is subtended by the diameter of the first dark ring is given by the equation

$\theta =2.44-{\frac {\lambda }{d}}$

(1)

where $\lambda$ is the wave length and $d$ is the diameter of the circular aperture. The wave length for electrons is given by the equation^{[n 1]}

$\lambda ={\sqrt {\frac {150}{V}}}\times 10^{-8}{\textrm {cm}}$

(2)

where V is the voltage through which the electrons have been accelerated before diffraction.

The paths of the electrons which enter an aperture of the matrix have some dispersion of direction because of the finite aperture of

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1. This equation is given in the Encyclopaedia Britannica⁸ but not in the Handbook of the American Institute of Physics⁹!

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