Sharp DV-SL80X User Manual download pdf (Page 38)

I-38

Quick Guide to Precision

Measuring Instruments

■

Erect Image and Inverted Image

An image of an object projected onto a screen is erect if it is

orientated the same way as the object on the stage. If the image is

reversed top to bottom, left to right and by movement with respect

to the object on the stage (as shown in the gure below) it is referred

to as an inverted image (also known as a reversed image, which is

probably more accurate).

■ Magniﬁcation Accuracy

The magnication accuracy of a projector when using a certain

lens is established by projecting an image of a reference object and

comparing the size of the image of this object, as measured on the

screen, with the expected size (calculated from the lens magnication,

as marked) to produce a percentage magnication accuracy gure, as

illustrated below. The reference object is often in the form of a small,

graduated glass scale called a `stage micrometer’ or `standard scale’,

and the projected image of this is measured with a larger glass scale

known as a `reading scale’.

(Note that magnication accuracy is not the same as measuring accuracy.)

■ Type of Illumination

l Contour illumination: An illumination method to observe a

workpiece by transmitted light and is used mainly for measuring

the magnied contour image of a workpiece.

l Coaxial surface illumination: An illumination method whereby a

workpiece is illuminated by light transmitted coaxially to the lens

for the observation/measurement of the surface. (A half-mirror or a

projection lens with a built-in half-mirror is needed.)

l Oblique surface illumination: A method of illumination by obliquely

illuminating the workpiece surface. This method provides an

image of enhanced contrast, allowing it to be observed three-

dimensionally and clearly. However, note that an error is apt

to occur in dimensional measurement with this method of

illumination.

(An oblique mirror is needed. Models in the PJ-H30 series are

supplied with an oblique mirror.)

■ Telecentric Optical System

An optical system based on the principle that the principal ray is

aligned parallel to the optical axis by placing a lens stop on the focal

point on the image side. Its functional feature is that the image will

not vary in size though the image blurs as the object is shifted along

the optical axis.

For measuring projectors and measuring microscopes, an identical

effect is obtained by placing a lamp lament at the focal point of a

condenser lens instead of a lens stop so that the object is illuminated

with parallel beams. (See the gure below.)

■ Working distance

Refers to the distance from the face of the projection lens to the

surface of a workpiece in focus. It is represented by L in the diagram

below.

■ Parallax error

This is the displacement of an object against a xed background

caused by a change in the observer's position and a nite separation

of the object and background planes.

■ Field of view diameter

The maximum diameter of workpiece that can be projected using a

particular lens.

ΔM(%): Magnification accuracy expressed as a percentage

of the nominal lens magnification

L : Length of the projected image of the reference object

measured on the screen

: Length of the reference object

M : Magnication of the projection lens

Optical axis

Projection lens

Focal point on the image side

Telecentric contour illumination

Condenser lens

Workpiece

Projection

screen surface

Principal ray

Object surface

Light source

(lamp)

Parallax error

Projector screen

Projection lens

Workpiece stage

Workpiece

An erect image

An inverted image

Projection screen

Top of the stage

Workpiece

X-axis movement

Y-axis movement

Example: If a 5X magnication lens is used for a projector with a

screen of ø500mm:

Field of view diameter is given by

500mm

= 100mm

Field of view diameter (mm) =

Screen diameter of prole projector

Magnication of projection lens used

L−

ΔM(%)

———

X 100

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Sharp DV-SL80X User Manual Page 38

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