Optics

Basics

Basics

An addition to lighting, lenses are right at the front of signal path image processing and shape light information. They need to therefore be sufficiently taken into account with regard to error propagation.

Efficient image processing lenses are more than just "a piece of glass". They are complex and specialised components whose parameters provide significant contributions to image creation, content and their interpretability.

It is lenses in particular that tailor the test object data to the camera. In the following you will find an overview of the essential properties, sizes and contexts.

Wavelenghts used in image processing

Optical glass does not have to be transparent for all wavelengths used in image processing. This is particularly true of UV and infrared light. The test object must therefore be adjusted to the intended wavelength for lighting.

Visual field (object field)

A fundamental gauge for measuring lenses is determining the visual field that the lens can handle. The size of the visual field is either dependent on the focal length and the working distance of the test object (entocentric lenses) or is stipulated by the design (telecentric and hypercentric lenses).

Image size (image field)

Lenses can only create a limited size of the image, depending on the lens design constraints. The image size is measured and given for the lens where the image is depicted in the sharpest definition (→Flange focal length). The surface, upon which the image of a circle appears, is dependent on the rotationally symmetrical design of nearly all lenses. The maximum image circle diameter is thus often given for lenses. Alternatively, the maximum → image sensor size is given.

Scale for enlargement

The scale for enlargements/reductions (image scale) is taken from the ratio of the size of the image sensor to the size of the visual field. It has a considerable influence on pixel resolution, i.e. the smallest details the image sensor can recognise.

In the case of entocentric lenses, the image scale can be changed by altering the working distance. Shortening the distance makes the image scale greater.

In the case of most telecentric lenses, the image scale and therefore the visual field is fixed and cannot be altered.

Image: At a constant image sensor size, the working distance is greater for reduced images (image scale ß`<1) above="" than="" for="" enlarged="" images="" image="" scale=""> 1) (below).

Working distance

The working distance is specified to provide an easily reproducible setting of the distance of the lens from the test object, including for maintenance purposes. It measures from the leading edge of the lens to the nearest part of the test object.

In the case of entocentric lenses, the working distance (really the image scale!) is set using the focal distance ring. Further reductions in the working distance can be achieved using intermediate rings → Accessories.

Image: Lens working distance and free distance do not always have to correspond, particularly when the lighting used requires a short working distance to function.

Lens mount

C mount and CS mounts have established themselves for use in matrix cameras in industrial applications. Lens mounts with a diameter larger than the C mount are predominantly used for line cameras.

CS mount lenses can be used with a 5 mm – intermediate ring on C mount cameras. The reverse is not possible.

The lens mount is closely connected to the flange focal length (see also Cameras), the distance from the lens mounting surface in which the image appears in sharp definition. It is very precisely set and fixed by the lens manufacturer and cannot be altered by users.

Lens mount

Thread

Flange focal length/mm

S mount

M12 x 0.5

not defined

C mount

1" x 1/32"

17,523

CS mount

1" x 1/32"

12.5

F mount

NIKON bayonet

46.5

Focal lenght

In the case of entocentric lenses, the focal length determines the viewing angle of the lens. A short focal length means a large viewing angle; a long focal length creates a smaller viewing angle. Focal length and viewing angle are inversely proportional: halving the focal length doubles the viewing angle.
The focal length must be viewed in combination with the size of the image sensor. Using different sizes of image sensor, the same focal length creates different viewing angles and thus different visual fields.

Due to design constraints, some types of lenses cannot handle all
large-format image sensors.

Lenses whose focal length can be altered (zoom lenses) are suitable for
laboratory use in order to realise experimental set-ups quickly and flexibly.
They are typically only set to a few thousand focal length changes,
which is clearly too few for industrial use. Lenses with fixed focal lengths are therefore
almost always used in industrial applications.

Aperture / luminosity

Apertures are used to control the brightness on the lens. The aperture is set on the aperture ring, which should always be mechanically fixable. Die f-stops in the series 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 are standardised. The light flux is halved from f-stop to f-stop by closing the aperture and is doubled by opening it.

As a matter of principle, no f-stops can be specified for telecentric lenses; the numerical aperture is given instead. "+" means a large aperture; "-" means a small one.
Simpler lenses do not use f-stop markings, only the identification is "O" (for "open") and "C" (for "closed"). For reasons of image quality, the setting "C", should also always be avoided for lenses with aperture. A set-up with mid-range f-stops is generally recommended.

The lowest f-stop denotes the luminosity of the lens. The lower the number, the greater the luminosity of the lens, i.e. images can be captured with less light.

Example: a lens with a luminosity of 1.4 can cope with half the light required by a lens with a luminosity of 2.
In addition to brightness, aperture settings have an effect on → image quality and a considerable influence on the depth of field.

depht of filed / -range

Dependent on the pixel size on the image sensor and the aperture set, an area in front of the lens is created in which the test object can be moved towards and away from the camera without the image sharpness noticeably deteriorating and without the reliable image processing software functions being endangered. This is the depth of field range.

Whether the lens is entocentric, telecentric or hypercentric, the
depth of field range is exclusively dependent on 3 factors: aperture, image scale
and permissible level of blur.

The depth of field range means that during inspection, the
test object does not always have to be at a constant distance from the camera,
but in a distance corridor that can be reliably
recognised.

Image1: Shallow depth of field with completely open shutter (1.4): either front (left) or back (middle) can be depicted in sharp definition. Middle setting (right) will result in both objects appearing out of focus.

Imaging quality/image quality

Imaging quality / image quality

As a matter of principle, all lenses have image errors. The possibilities for a lens to achieve precise imaging is very dependent upon their construction, the materials used and the complexity of their optical construction, which is reflected in the price of the various lenses. Image sensors with smaller pixels also require higher resolution lenses.

Poor lens image quality can lead to high-resolution image sensors not being able to take full advantage of their large number of pixels. This is because the light is robbed of its local details as early as when it passes through the lens (low-pass effect of a poor lens).

The qualitative characterisation of lenses is a complex topic and cannot be represented in just a few parameters.
It is physically impossible and would also be financial madness to attempt to build a "universal high-performance objective lens". It can therefore be very useful to just look at individual criteria to qualify the lens depending on its use:

  • Geometric fidelity
    Distortion parameter: specifies the degree to which test object and image are mathematically similar with regard to their geometry.
  • Image sharpness and contrast fidelity
    Modulation transmission function (MTF) or resolution parameter:
  • characterises which fine details are resolved by the lens glass and with what contrast they can be depicted by the lens.
  • Brightness fidelity
    Vignetting parameter: describes the level of brightness loss that can be expected to the image border.
  • Colour fidelity
    Chromatic imaging error parameters: describe how effective colour transmission is through the lens and/or what colour error effects appear on the edge of the body (colour fringes).

General information on image errors:

  • In general, image errors are least frequent in the centre of images and most frequent on the edge
  • Image errors can be minimised using an average aperture
  • Lenses with short focal lengths always have more image errors than lenses with long focal lengths due to extreme lens curvature.

Lens image quality can only be shown extensively using a wide range of properties and parameters.

Coating / antireflection

Every transition of light from air to glass and glass to air, even within lenses, is associated with loss of brightness and contrast. As modern objective lens designs always contain many lenses and as loss increases from lens to lens, measures must be developed for minimise these losses. This is achieved using thin, optically transparent layers (one or several), which are vapour-coated in a vacuum on the lenses of the objective lens and appear to the human eye as a coloured fog on the lens surface. Even single layers (bluish fog) lead to a considerable increase in transparency and contrast-rich images. As filters are also subject to the phenomenon of brightness and loss of contrast, they should also be coated.

Type of imaging

Types of images

Besides the visual impression of the entocentric perspective that humans are familiar with, telecentric and hypercentric lenses are also used in image processing.

Type of perspective

Entocentric

Telecentric

Hypercentric

Principle

View

Position of the perspective centre

between camera and test object

infinite

behind test object

Objects appear

large when near, small when at a distance

the same size whether near or at a distance

small when near, large when at a distance

Lens length

small to very small

approx. working distance

length > working distance

Lens diameter

small to very small

> test object

>> test object

Working distance

> 0 … ∞

typically < 300 mm

very small

Justification

mainly uncritical

perpendicularity

perpendicularity and concentricity

 

 

 

Entocentric lenses

Entocentric lenses

-> Selection table

Application:
Observation, completeness and presence control, attribute testing (large/small, high/low components), colour control, codes and character reading, object and rotary orientation recognition. Opportunities for metric measurement are extremely limited.

Fixed focal-length lenses

have an entocentric perspective that is fixed by the focal length and are largely characterised by focal length and luminosity data (example Tevidon 2/10: luminosity 2, focal length 10 mm).

Zoom lenses
Entocentric zoom lenses allow the visual field to be altered at a constant working distance by altering the ->focal length.

Macro lenses
have a shorter minimum working distance than standard lenses, which is achieved by increased expenditure in optical construction. They create particularly good quality images at close distances.

Perpendicular lenses
are a particular design of entocentric lenses that can be used where space is restricted. Optical deflection is achieved by the factory use of 90 degree prisms. As a matter of principle, these lenses are only available with a focal length > 25 mm. Perpendicular lenses always produce reversed images.

Telecentric lenses

Telecentric lenses

→Selection table

Because of the implemented optical principle of telecentricity, which works with parallel main beams, changes in distance to the lens have no influence on the image size, as image scale is fixed with telecentric lenses. Only the image sharpness changes with these lenses given changes in distance.

Most telecentric lenses work with object-side telecentricity, i.e. where the test object is. This means that use of the lens stands up to changes in the position of the test object. Changes to shape, position and size, which are otherwise created by perspective, are avoided.

Use:

Telecentric lenses are particularly suitable for precise metric measuring, but also for inspection tasks for presence and completeness controls on large test objects with different height and measurement levels as well as geometrically complex formed parts (spheres, cylinders, free formed surfaces), which cannot be inspected using entocentric lenses. Parts with varied surfaces, shiny and optically active materials such as glass and plastic are also particularly suitable for this type of inspection.

Although telecentric lenses are precision lenses, they cannot be designed and produced infinitely precisely. Residual faults remain in optical correction and production tolerance, which can be estimated by the quality parameters of telecentricity. In practice the main beams of telecentric lenses therefore deviate slightly from being exactly parallel. This results in the fact that the size of the image changes slightly when the test object is moved towards or away from the camera. There is currently no valid definition of the extent to which the image size changes when traversing the field of telecentricity. Values are determined by the individual producers.

Telecentricity has nothing to do with → depth of field. It describes changes in image size. Depth of field on the other hand describes changes in image sharpness. Furthermore, the principle of telecentricity has no influence on the size of the depth of field range. The same conditions apply to telecentric lenses as to entocentric lenses.

Telecentric lenses work particularly effectively and precisely with telecentric back lighting. A particularly low sensitivity to extraneous and interfering light is connected with this.

As a matter of principle, the optical construction of telecentric lenses is relatively long and requires sufficient space. Spatial problems and changes in direction can be solved using → beam deflection.
In each case precise orientation between lens, test object and lighting is necessary on several axes. If this does not take place, there can be considerable problems due to parallel projection, particularly in the case of low test objects. → Justification aids can be used as a means of optical testing for justification.

Image1: With telecentric lenses changes in distance do not affect the size of the image, only the image sharpness (10 mm change in distance between the upper and lower halves of the image.)

Lens selection

Lens selection

  1. Specify lens mount
  2. What is the wavelength dealt with?
  3. What type of perspective is necessary?
    a) Entocentric: determination of focal length, working distance (intermediate ring), image sensor size
    → Focal length calculator
    b) Telecentric
    determination of visual field, image sensor size, working distance
    → Selection table
  4. What image quality is required?
    → Datasheets
  5. Use of light filters necessary
    → Selection table

Accessories

Light filters

Light filters allow for the targeted influencing of light, which can be used for imaging on the image sensor. It can therefore be sensible to suppress certain light components and, in so doing, emphasise others. The use of light filters thus always reduces brightness. The filter (extension) factor is often provided in order to describe the light loss. This describes the factor by which the shutter speed must be increased in order to achieve an image that is as bright as one without a light filter.

Edge filters

→Selection table

Prevent light components within a defined wavelength:

  • UV cut filter: prevents UV light
    For mechanical and dirt protection for the lens
  • Coloured glass filter: prevents UV light and some visible light
    For filtering particular colour components of visible light
  • Daylight cut filter: prevents UV and visible light
    For suppressing extraneous light

Neutral filters

→Selection table

Suppress a wide range of light wavelengths almost simultaneously and thus reduce brightness overall.

For removing excessive light without changing the aperture or shutter speed.

Band-pass filters

  • Infrared cut filter: broadband band pass filter that blocks UV and infrared light and lets in daylight
    The camera's standard built-in filter for contrast-rich images must be removed from the camera before working with infrared light
  • Narrow band band pass filter
    For near-perfect suppression of extraneous light. Precise adjustment with monochrome lighting is essential

Polarisation filters

Only allows light of certain states of polarisation to pass. Used to suppress reflections and to make very low contrast visible on various materials and during special optical processes.

Light filters that are added or removed change optical path lengths and therefore mechanical distances. This effect is particularly obvious in the case of light filters in front of the camera image sensors or on the image side of the lens. Filters that are removed in these areas should always be replaced by an optical flat of the same thickness in order to prevent changes in the mechanical distance.

Light filters are screwed onto the lens using fine threads and should be fixed with screw-locking fluid in the event of vibrations.

Extension ring

Intermediate rings are used to shorten the working distance set on the spacer ring. The use of intermediate rings generally leads to deterioration → Image quality (optical resolution) of the lens used. The maximum length of rings used should therefore not exceed 10% of the focal length of the lens used. The length of the intermediate ring (ZR) is calculated using:

ZR = y’ * f’/y with y’ – image sensor width
f’ – Focal length of the lens used
y – visual field width


Please take care when changing entocentric lenses: even given the same focal length, the exteriors of different models of lenses and lenses by different manufacturers are not mechanically identical, which is why the working distance can change after changing lenses.

The image scale of most telecentric lenses is set during construction (also by using intermediate rings) and cannot be changed.

Beam deflection

In order to be able to implement telecentric lenses even where space is limited, resolution prisms for beam deflection can be mounted on telecentric lenses. The beam path is typically deflected by 90 degrees. Using resolution prisms changes the working distance. The data is specific to the lens and can be read in the datasheets. In addition, attaching prisms to deflect beams always leads to a reversed image.

Adjustment aid

Optical adjustment aid for perpendicular orientation of telecentric lenses towards test objects. This avoids measurement errors that can be caused by the erroneous orientation of test objects to telecentric lenses (parallel projection).

Two optical flats with cross hairs, that are aligned to each other in the factory, are adjusted by justifying (tilting) the telecentric lens around 2 tilting axes. Afterwards it can be assumed that the optical axis of the lens is perpendicular to the supporting surface of the justification aid.

The use of the justification aid requires at least average dimming of the lens so that the two separated cross hairs can be brought into sharp focus → Depth of field range.