Quiz 3

Q01Selecting Effective Lighting

The key to solving a vision application is choosing the correct lighting. Depending on the illumination type or position, the way in which the inspection target can be seen varies greatly.

Match up the following vision applications (1 though 3) to the illumination type (a, b or c) that is the most suitable lighting for each of the targets.
Choose from A to C, the correct combination of target type to illumination type.

Target Type

Text inspection on a shiny plastic package Normal lighting Using illumination
Engraving inspection of a metal processing component Normal lighting Using illumination
Inspection for chipped edges on a rubber disk Normal lighting Using illumination

Illumination Type

Low angle lighting Principle Provides light at a low angle to produce high contrast on edges, recesses or protrusions on the target surface.
Dome lighting Principle Provides indirect light at multiple angles, creating an image with no shadows or glare.
Coaxial vertical illumination Principle Provides great contrast between flat areas (specular reflection) and non-flat areas (diffuse reflection) of the target.
  1. 1+(a), 2+(c), 3+(b)
  2. 1+(c), 2+(b), 3+(a)
  3. 1+(b), 2+(c), 3+(a)

Correct answer:C.1+(b), 2+(c), 3+(a)

1.Text inspection on a shiny plastic package Normal lighting Using illumination + (b) Dome lighting is the most suitable! Dome lighting is the most effective method for the inspection of the text on the plastic package as the lighting irradiates from all directions, creating uniform lighting throughout the wavy target surface.
2.Engraving inspection on a metal processing component Normal lighting Using illumination + (c) Coaxial vertical illumination is the most suitable! Coaxial vertical illumination is the most effective for inspecting the metal processing component as the diffuse reflection from the engraving shows up dark while the specular reflection from the flat shiny surface shows up very bright.
3.Inspection for chipped edges on a rubber disk Normal lighting Using illumination + (a) Low angle lighting is the most suitable! Low angle lighting is the most effective method for inspecting the chips on the rubber disk as the shallow angle of the lighting allows you to obtain the reflection from the chips along the edges of the disk.

Q02Finding the Maximum Line Speed

When checking for abnormalities on a continuously moving sheet using a vision camera, it is important to determine what the maximum speed of the target should be.

Testing on the target has confirmed that in order for the abnormalities to be properly detected, a field of view of 100mm (3.94") is needed. In addition, the image processing time taken to conduct the inspection using this field of view was 25 ms.

Of the following options, which one is the maximum line speed that detection can occur under these conditions without missing any abnormalities?

  1. 60 m/minute
  2. 120 m/minute
  3. 240 m/minute

Correct answer:C.240 m/minute

As shown in the equation below, the maximum line speed for a given application can be obtained by dividing the field of view by the processing time.

Maximum line speed = Field of view ÷ Image processing time

In this example,

  • Field of view = 100 mm (3.94") square
  • Image processing time = 25 ms = 0.025 seconds

Therefore,Maximum line speed=100 mm (3.94")÷0.025 seconds
=400 mm (15.75") /second
=240 m (787.4’) /minute.

Also, since image processing times can vary greatly depending on the image sensor being used and the settings, the above equation can be reversed to obtain the actual image processing time required under a given maximum line speed/field of view. This is an effective method in which to find these necessary parameters in advance.

Q03Lens Depth of Field

The depth of field indicates the Z-axis range in which the target image captured stays in focus.

For example, if you are using a particular lens under a certain condition and the working distance in which the target stays in focus is 100 mm to 105 mm (3.94" to 4.13"), the depth of field is considered to be 5 mm (0.2"). The deeper the depth of field, the less likely the image will be out of focus.

In order to increase the depth of field as much as possible, which of the following options A to D indicate the correct combination?

  1. “Select a lens with a long focal point distance (telephoto)”+“Narrow the aperture as much as possible”
  2. “Select a lens with a long focal point distance (telephoto)”+“Open the aperture as much as possible”
  3. “Select a lens with a short focal point distance (wide angle)”+“Narrow the aperture as much as possible”
  4. “Select a lens with a short focal point distance (wide angle)”+“Open the aperture as much as possible”

Correct answer:C.“Select a lens with a short focal point distance (wide angle)”+“Narrow the aperture as much as possible”

When using a wide angle lens with a shorter focal distance, it is easier to focus the image on the CCD. Focusing can also be enhanced by using a more narrow lens aperture. A wide angle lens has a longer range of image formation due to the shallow angle of light streams being focused on the CCD. A smaller (narrow) aperture allows less light to enter the lens environment, resulting in a more focused image formation on the CCD.

When the aperture is closed When the aperture is open

Q04Line Speed and Shutter Speed

In general, to prevent a high speed moving target from being out of focus when capturing images, a fast shutter speed is required. On the other hand, if you are capturing the image of stars in a dark night sky, a longer exposure time is required and a slow shutter speed is needed.

For this example, the image sensor has a field of view of 100mm (3.94") and the CCD size is 500 × 500 pixels*. The target will pass the image sensor at a speed of 1000 mm/second.
Which of the following shutter speeds would be best in order to keep the image (in the above example) in focus under the optimum lighting conditions?

*For ease of calculation, the CCD size has been made 500 pixels × 500 pixels, however in reality this CCD size does not exist.

  1. 1/500 second
  2. 1/1000 second
  3. 1/10000 second
  4. 1/50000 second

Correct answer:C.1/10000 second

The information outlined here can also be utilized in the manual operation of a digital single-lens reflex camera, so please ensure you know and understand the information thoroughly.

In order to keep an image in focus, it is best to use a fast shutter speed. However, if the shutter speed is too fast, the image may end up too dark as there is not enough exposure. In order to obtain an optimum image capture without losing focus, set the shutter speed using the time it takes for the target to move 1/2 of a pixel as a reference. This is in order to complete exposure in half the time it takes to move the distance of 1 pixel.

  1. First, find the size of 1/2 a pixel.Field of view is 100 mm(CCD size: 500 pixels ×500 pixels)
    100 mm ÷ 500 pixels ÷ 2 = 0.1 mm
  2. Next, find the time it takes to move 0.1 mm.
    0.1 mm ÷ 1000 mm/second = 0.0001 seconds = 1/10000 second

Therefore, the answer is 1/10000 second.

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