Illumination for Vision applications

INDEX

What different types of lighting are available?

Can you tell me more about the different types of lighting?

 

1.   Fibre Optic lighting
2.   LED Lighting
3.   High frequency fluorescent lighting
4.   Halogen lighting

How do I select the right one for my application?

I have a very reflective object. How do I light this? 

How do I ensure even illumination over a large area?

I am using a line scan camera. What options are available to me? 

Introduction

Lighting is one of the most important considerations for any vision application.
Having chosen the right camera, optics and frame grabber if required for the
application most people will next turn their attention to lighting. Without the
right type of lighting the application will not be successful.
The aim of this document is to outline the main considerations when considering
what type of lighting to use. 

What different types of lighting are available? 

There are many different types offered.
These include:-

• Fibre optic lighting
  LED lighting
  High frequency fluorescent lighting
  Halogen lighting                  

 

Can you tell me more about the different types of lighting available? 

To discover this it is first important to fully understand what each offers. 

Fibre Optic lighting 

This type of lighting is generally expensive since any light guide is made
of many fibre optics which are bonded to give a uniform light path from the
light source to the object being illuminated. The major advantage of Fibre
optic lighting is it is a cold light source. In other words the light will generate
no heat on the object it is illuminating.

Fibre optic lighting has the advantage of being able to focus an intense light
over a very short output profile. For example you can attach to the end of the
fibre optic light guide the following type of outputs:- 

1.   Ring light
2.   Flexible or semi rigid straight lightguides
3.   Line lights
4.   Coaxial off axis lighting units 

Ringlights gives as the name suggests a ring of light around the object.
These type of lightguide outputs are very useful for lighting small samples
for example on a microscope so you obtain even around illumination of the
sample. They can be used at varying distances from the object and
sometimes they can be used with a very shallow angle of incidence to
give a three dimensional profile of a textured object for example. 

Flexible or semi rigid light guides- these are straight light guides,
sometimes the light will be channelled down two straight light guides
The advantage here is the light can be focused on a very small area
from say 10mm down to 1 mm (with the addition of focussing lenses).
Twin guides can give to beams of focused light say from both sides of
an object. 

Line lights- Again as the name suggests the light will be focused to
a narrow line. Again the light guide can be fitted with a focussing lens
to really give an intense line of just a few millimetres. This type of
lighting is well suited to line scan camera use. 

Coaxial off axis lighting units-These lighting units include either
one or two mirrors to deflect the light so that the light does not reflect
back into the lens directly. The advantage here is that they can be
used on objects which are highly reflective in their nature. For example
polished ceramic studs. 

A clear advantage of fibre optic lighting is also the ability to adjust the
light intensity easily from the light source and also to change the colour
of the light. You can also apply various filters and optics to the lighting
systems so they provide the user with a great flexibility.

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LED Lighting 

LED Lighting offers users some advantages over Fibre Optic lighting 

1.   Price is lower
2.   The light unit requires no expensive or heavy power supply
3.   Most LED units can be pulsed to give a higher intensity
4.   The unit can be offered in various colour LEDs to suit the particular application 

Disadvantages 

1.   It is a hot light source and will heat the object being illuminated if close to it.
2.   Not as much flexibility as Fibre Optic light system.
3.   Cannot be focussed as well as Fibre Optic light system. 

LED Lighting units are available in the same output types as fibre optic but with
less flexibility.   

LED Lighting with Fibre Optics  

This is a new innovation. This technology combines a collimated LED light unit coupled to fibre optic fibres. This allows remote LED lighting and the benefits of finely collimated light output beam. This reduces the heat near the sample, allows a finely collimated beam if required and different inherent coloured lighting is possible.  

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 High frequency fluorescent lighting 

The main types of high frequency fluorescent lights offered are ring lights and
linear lights. 

The ring lights are cheaper than the fibre optic alternative, but less flexible and
of course generate heat. They are available with different types of bulbs to give
different colour temperature lighting. 

The linear lights have the advantage of giving much diffused lighting and are
ideal where even illumination is required over a large area.

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Halogen lighting 

This type of lighting is really only well suited to the simplest of applications.
It has poor colour temperature for vision applications and while the cheapest
must be considered a very basic form of lighting. 

How do I select the right one for my application? 

This is a very difficult question to answer as all applications are
different but here a few guide lines. 

1. 1)     Halogen lighting can be used if you just want to illuminate an object
   for viewing purposes and for no other purpose. It can only be used with
   standard 25 frames per sec cameras otherwise you will see a mis-match
   in the image with the 50 cycle mains frequency. It is a cheap way of
   lighting an object and is perfectly adequate for a lot of applications.
2. 2)     If the object being illuminated is going to be affected by or changed in
   some way by heat, use a cold fibre optic light source. The examples here
   can be in microscopy applications in biological or bio medical fields where
   the heat from a light source would change the characteristic state of the
   sample. Another application is in paper web inspection using line scan
   cameras where the light source output from a linear light has to be placed
   very close to the object being illuminated. Many applications demand a
   cold light approach so if you think the state of the sample can be changed
   by the light or you want a safe light source for an operator (health and
   safety issues) then a cold light fibre optic is a good choice. Remember
   however they are focussed lights so do not consider them for illuminating
   large areas otherwise uniform illumination will become a problem. If you
   have cameras clocking at greater than 50 Hz ( 25 frames per second you
   should use a DC Fibre optic light source as they will not be affected by
   the 50Hz cycle). The most typical application for this type of light source
   is with line scan cameras.
3. 3)     LED lighting is a much cheaper alternative to fibre optic. Of course it is
   a hot light source so can only be used on samples not affected by the
   heat. Operators need to be careful they do not touch the hot lights.
4. 4)      If you need to illuminate a sample over a larger area LED lighting
   could be a good choice. You can use many different types of LED colours
   to enhance the image being captured.
5. 5)     Many people build their own LED lighting systems. While there is no
   problem in doing this, the construction of such units needs to be considered
   carefully as the shade of coloured LEDs can vary and for example if you
   order LEDs of the same colour you may find within the same type of LED
   will differ in its colour output. This will obviously affect the profile of the light
   on the sample and hence make any measurement results invalid. While
   more expensive it can save you a lot of time and headaches to buy an
   integrated LED unit form a reputable supplier. All our vendors select the
   LEDs for uniformity and you have the guarantee on the overall lighting unit.
6. 6)     High frequency fluorescence lighting is a very useful way of lighting large
   areas. These lights are hot and again care needs to be taken with them
   but they can give very uniform lighting and can be used with higher speed
   camera systems because of there high frequency pulsing characteristic.  

I have a very reflective object. How do I light this? 

Highly reflective objects where the light reflects back into the lens of the camera is
always a problem. In such a case the light needs to shine off axis from the side.
Now there are two ways of doing this.

If you are interested in a specific section of the sample you could shine a beam
of light from a very acute angle at the sample. An example of this would be a
coin and you are trying to extract the information from a feature on one side
of the coin. By shining a beam of light form a very acute angle at the side (dark
field illumination) you will obtain a 3D profile of the information from the coin.
This image will show the profile of the coin over the area being illuminated.
Of course the non illuminated area would be shown in the image as black.

If, however, you want to illuminate the whole coin you could use a coaxial
lighting system. Here the light is above the object being viewed but the light
comes in from the side and is reflected down onto the sample by either one
(Coaxial lighting) or two (Advanced coaxial lighting) mirrors.

These give a very uniform lighting distribution over a whole area of sample.
This type of lighting unit is available in LED and Fibre optic.

They are available in various sizes up to 100 x 100mm. 

How do I ensure even illumination over a large area? 

One way of doing this is build a light tight box into which samples can be
placed and use high frequency fluorescence tubes to illuminate the surface
of the box. The number and size of the tubes required will depend on the
size of the light box but if the tubes are placed with the front face of the
light angle towards the centre of the box homogeneous lighting can be
achieved. 

I am using a line scan camera. What options are available to me? 

Line scan cameras are used for applications like high speed web inspection
of paper, textiles, glass etc. This type of camera has a linear sensor with
up to around 8000 x1 pixels. This type of camera there demands a linear
light guide with the ability to have no influence by the 50 Hz power cycle. 

The choice of light sources is:- 

DC Regulated Fibre Optic light sources with linear light guides 

The DC regulated light source should be used for this application. This light
source will control the light intensity to an even illumination and not be
affected by the mains frequency cycle. The light source can be adjusted
for intensity and colour temperature     

Or 

LED light sources in linear format. 

If an LED light source is used in this application then you will need to
consider light intensity and colour of illumination before buying the light unit.

The light unit can effectively be made of any length and any intensity.
However the higher the intensity the more expensive this option becomes
because you need to dissipate the heat through a large heat sink or water
or air cool the lighting system.

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Further information available from Geoff Smith.

Alrad Imaging, a trading division of Alrad Instruments Ltd.
Alder House, Turnpike Road Industrial Estate Newbury, Berkshire RG14 2NS

Telephone: +44(0) 1635 30345   Fax No. +44(0) 1635 32630
Email: sales@alrad.co.uk