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machine vision optics

Machine Vision optics generally refers to the lenses that are used. Again here with machine vision, there are quite a variety of factors to consider.

Magnification

Resolution

Field of View

Distance to object

Depth of Field

Amount of light available...

Camera sensor size

So as you can see it isn't as easy as it looks. It's also quite a specialised area. Many people are familiar with concepts such as ethernet interfaces or processor speed, but this is different. This needs some research before you take the plunge.

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Types of Machine Vision Optics

Lenses fall into a number of categories but can largely be summarised as follows:

General Purpose Lenses - For non-critical CCTV applications when using cameras with a small sensor.
Precision Lenses - For machine vision applications where Mega Pixel cameras are being used.
Telecentric Lenses - For machine vision applications where object measurement is the objective and distortion caused by the lens is not allowed.
Macro Lenses - For Hi Magnification applications.
Motorised lenses - For applications where it is necessary to have remote control of Focus and Iris functions via small motors on the lens.

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Optics - The basics

One of the first questions everyone involved with machine vision optics asks is ' what lens do I need'.

To begin to understand the concepts think of what a lens does. It focuses the light entering it so that the beams of light intersect behind the lens at a specific point. This is known as the 'focal point'.

Remember this is OK for a single point of light. However for most everyday applications we want to image more than a single point of light. We want to look at widgets and labels etc. For these the light coming into the lens doesn't impact on a single point. It hits in a number of places on the same plane. To cope with this we need to use a 'compound lens'...This is the type of lens most commonly used in machine vision.

They know the size of what they want to inspect, they know how far away they need to place the camera, so they've already answered the first few questions. Another key one is what camera are they using and more importantly what's the size of the array? Is it 1/4", 1/2" etc?

The most basic formula to calculate the focal length of a lens is as follows:

Focal Length =

working Distance * Size of Image

size of object + size of image

However in reality we need to consider the size of the CCD array we are imaging to.

So the formula now becomes

Focal Length =

working Distance * length of CCD array

size of object + length of CCD array

If you camera has a 1/4" or a 1" CCD, then this dimension is fixed.

There are some other complications but these are the three fundamentals....The key things to remember are that your camera, the size of the object and the working distance are enough to get you started.

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Choosing a lens

In the previous paragraph we learned the basics of how to choose a lens in machine vision optics. There are a few complications however which need to be taken into account.

1 In the manufacturing process for lenses in machine vision, there are optical flaws around the edges of the lens, so always over spec your lens.

2 Always choose a lens that is bigger than the CCD. For example, a lens that is spec'd for a 1/4" CCD will not work on a camera with a 1/2" CD

3 Your camera will be configured to accept different format attachments: C Mount, F Mount etc...Take care to choose the right one.

4 Extension rings do not affect depth of Field, they are designed to alter the Working Distance.

5 If you want increased depth of field, close the iris as much as possible and increase the level of your illumination.

6 Increasing the F number will increase your depth of field but your image may become quite dark as it leaves in less light.

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