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OCULAR—see optics
OPTICS—the collective term for all optical parts of a Famous
Trails system which includes the objective lens and the ocular.
• The ocular increases projection size of the image.
• The objective lens gathers ambient light, focusing it onto the
image intensifier. Can also provide an increase in size of
image view. Short focal lengths (image size is 5x or less) and
high-speeds (f2 or faster) are found on the best objective
lenses. They are coated for the best efficiency in the near-
infrared bandwidth.
PHOSPHOR SCREEN—renders a visible green image by exhibiting
fluorescence when impinged by the electron flow in the intensifier
tube. (The human eye picks up on contrasts in green more easily
than other colors) The phosphor screen is positioned at the back
of the intensifier tube.
PHOTONIC SPECTRUM—the span of electromagnetic energy
ranging from 0.1mm (x-ray) to 1,000mm (far-infrared). The
visible range within this span is from 400 to 750nm.
PHOTOCATHODE— a component that changes light (photon
energy) into electrons (electrical energy) which in turn move with
increasing speed in the intensifier tube. The objective lenses point
light on the photoelectric surface of the photocathode, which is
excited and passes electrons through the intensifier tube.
PHOTOSENSITIVITY—the measure of the photoelectric
current from the photocathode per the input light flux from a
tungsten lamp. It is expressed in microamperes per lumen and
indicates the photocathode’s sensitivity to light.
RESOLUTION—how well an image is rendered and displayed
by a Famous Trails device. A constant, it can be expressed as the
maximum number of lines per millimeter (lp/mm) that can be
distinguished when a white-and-black striped pattern is focused
on the photocathode. The best resolution in most Famous Trails
systems is at the center of the view, with blurring towards the edges.