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Code reading systems

Stationary code reading systems

Lenses

5/30

Siemens ID 10 · 2014

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Overview

With a lens suitable for the respective image evaluation task, the

size of the image field is determined for the camera image for the

required operating distance.

High light intensity and the geometry of the image are extremely

important for image evaluation (code reading, form recognition

and position detection) High light intensity permits short shutter

speeds and consequently a reduction of the blurring due to mo-

tion as well as maximizing the range.

Fault-free image geometry optimizes the fault tolerance of image

analysis with regard to perspective and unevenness of the sur-

face of the depicted object.

Lenses with fixed focal length and a settable aperture and focus

are ideal for this purpose and are therefore preferred.

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Application

Code and text recognition

The algorithms of code and text recognition tolerate variations in

form and size of the marking. In many applications, code read-

ers must tolerate the influence of perspective distortion. Geo-

metric errors resulting from the imaging often reduce the reading

performance of the overall system.

Important selection criteria for the lenses of code readers are

fast shutter speeds which guard against blurring due to motion,

as well as maximization of the reading distance.

All lenses in this accessories list meet the requirements for code

reading and text recognition. In addition, lens accessories (e.g.

filters) are available which in conjunction with the accessories of

the readers support project-specific configurations.

Form recognition and position location

For form recognition with high reproducibility, is a format-filling

high-resolution image is required. Geometric errors resulting

from the imaging often reduce the reading performance of the

overall system.

Important selection criteria for the lenses of code readers are a

fast shutter speed which guards against blurring due to motion,

as well as maximization of the range. It is particularly important

to maximize the range, since the stability of image analysis rises

as the angle of the image field reduces. This is why a large dis-

tance from the test object is advantageous.

All lenses in this accessories list meet the requirements for

shape recognition. In addition, lens accessories (e.g. filters) are

available that in conjunction with the accessories of the readers

support project-specific configurations.

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Function

Image types

The optical path of the lens is defined by its construction.

For spherical lenses the solid angle depends on the focal

length, focus adjustment and aperture; all rays run through the

focal point of the lens (central projection). Objects that are fur-

ther away from the lens are depicted smaller; objects that are

closer to the lens are depicted larger:

The required image field size (height and width of the image),

the size of the sensor chip and the focal length of the lens deter-

mine the operating distance:

d = (f x IS) / b

d = Operating distance (i.e. distance from lens to test object)

in mm

f = Focal length of the lens in mm

IS = Size of image in the plane of the test object in mm

b = effective dimensions of the sensor in mm

In the case of lenses used in image processing systems, the fo-

cal length is fixed, the aperture and focus settings can be fixed.

The focal length, the maximum focal aperture and the focusing

range are normally specified on the lenses.

Focal distance

The focal length makes a statement about the angle of the image

field or magnification of the lens.

The focal length of the lens is determined by the size of the re-

quired image field and the size of the camera chip when a spe-

cific distance has to be maintained. The most common chip

sizes in cameras today are ½", 1/3" and ¼". If the distance to the

object lies below the adjustable focusing range of the lens, i.e.

at close range, the focus can be adjusted using intermediate

rings.

If the back focal length for this camera is dimensioned for

CS-Mount lenses, as for VS 130-2, a C-Mount lens can also be

focused if a 5 mm extension tube is used.

Aperture

Reduction of the light intensity by interrupting the optical path.

Focus

Setting the focus of the lens to a specific distance.

Depth of field

Depth of field is the area within which (in front of and behind the

object) that is displayed with sufficient sharpness of focus. The

larger the aperture (the smaller the aperture number), the

smaller the depth of field.

Lenses with a larger focal length have a smaller depth of field,

the effect is considerable for images at close range.

Sensor

dimension

Focal

length

Working

distance

Field

of view

G_FS10_XX_00096

ID10_2014_EN_NEW.book Seite 30 Mittwoch, 11. Dezember 2013 11:05 11