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Frequently Used Terminology

Angle Error

Is the inability of a lens to project an angle correctly. It's a lens defect and can be repaired at the factory. Caused by damage to cemented pair of lenses.


Is a precision instrument that is used to verify the alignment of a projection lens axis to the mirror and screen. When the Axiscope is installed in the Comparator in place of the projection lens, its optical axis occupies the same position as that of the projection lens. Cross hair rectile lines on the axis of the Axiscope may be focused at any point within the Comparator and represent the location of the optical axis at that point.

Compound Lens System

Is a group of lenses which products an inter- mediate image within the system, previous to producing the final image on the screen. This final image is erect.

In a compound system:

The objective lens is nearest the object and forms the intermediate image.

The ocular is the lens nearest the screen and forms the final image.

Usually objectives and oculars are separate assemblies and can be used in different combinations to produce different magnifications.

Condensing Lens System

Is a lens which receives diverging light from a lamp filament placed at its principal focus and emits parallel rays.

Depth of Focus

The distance along the optical axis through which the object may be moved and still have acceptable focus on the screen.


Occurs when there is a symmetrical variation in magnification over the entire field. A straight pin may appear barrel shaped or pin cushion shaped, if there is distortion present.

Erect Image

Is an image on the screen that has the same orientation and movement as the object.

Erecting Lens System

Is a simple lens system that is used to form an intermediate image for a simple projection lens system. The purpose is to produce a final erect image on the screen.

Field Aperture

Is the maximum diameter of lens system that will produce an image on the screen. The diameter of the screen, divided by the magnification of the projection lens, will give the field for any Comparator lens system.

Focal Clearance

Is the distance from the focal plane to the front of the lens cell; the maximum clearance for fixtures or objects.

Focal Plane

Is a plane perpendicular to the optical axis, located in such a position in front of the lens system that an object placed there will produce a sharply defined image on the screen. conversely, if an object appears in sharp focus on the screen, it is in the focal plane.

Front Surface Mirror

Is a plane polished surface that has been aluminized to reflect light without passing through the glass.

Inverted Image

Is an image on the screen that has the opposite orientation and movement as the object.


A piece of glass, having opposite regular surfaces, usually spherical, which is used either singly or combined in an optical instrument for forming an image by changing the direction of rays of light by refraction.

Lens Cell - Mirror Cell

Housing for the optical elements.

Matched Projection Lenses

Are lens systems which are preset to a fixed standard projection distance and will produce specified magnification without any adjustment of the Comparator.


Is a surface which changes direction of light by reflection.

Mirror Chart Image Chart

Is a chart where the lines are on the surface away from the operator when the chart is in use; overlay charts.

Optical Axis

Is the imaginary centerline of an element or system about which all optical surfaces are symmetrical, the line on which all centers of curvature of an element or system are located; of a Comparator, an imaginary line extending from the lamp filament to the center of the screen when the Comparator is properly aligned, the geometrical axis of a lens system.

Optical Element

A single part, which has optical characteristics.

Prefocused Projection Lenses

Are matched lenses which are preset to a fixed focal plane and which will allow changing magnification of the object without refocusing the object.


Is a system of lines in the focal plane of an optical system; a system of lines on optical glass.

Principal Focus

Is the position of a point to which parallel rays are re-fracted by a lens. This point is commonly called the crossing point in a Telecentric system and will be on the optical axis only when the condensing lens axis and filament are also on the optical axis.


The return or bending back of light rays by a surface. Also referred to as Surface illumination which illuminates the front facing side of a part.


Is bending a light ray from a straight path in going through a transparent medium.

Right Image Chart

Is a chart where the lines are on the surface closest to operator when the chart is in use. On J&L Comparators 30" diameter, 14” diameter, 6-3/4 x 9-3/4” when not used as overlay.

Simple Lens System

Is a single lens, or group of lenses acting as a unit, which forms a single image. The image is inverted

In a simple system composed of more than one lens.

The eye lens is the one nearest the object.

The field lens nearest the screen.

Any other lenses are intermediate.

Telecentric Optical System

An optical system designed to use only parallel light.

Telecentric Stop

Is a diaphragm placed on the optical axis at the -principal focus of lens system for the purpose of eliminating non-parallel light.

Uneven Magnification

Is an unsymmetrical variation in magnification. A straight pin will measure differently from one side of the screen to the other in other words -big on one side small on the other. Caused by misalignment of the optical elements of the Comparator.


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