Camera Selection advice guide


Introduction
Camera selection for a specific application can be a daunting task for any user.
We have compiled this guide for people who are new to the use of cameras
and are bewildered by the vast range of product on offer. It is based on frequently asked questions.


Should I choose an Analogue or digital camera ?

1). Generally the choice will be governed by the following factors :-

Price - for those applications were you want a low cost camera, then
analogue will nearly always be your choice. They are generally considerably
lower priced than digital cameras. However within the last year there are
arriving on the market some very attractively priced digital cameras with
8 bit fire wire or USB2.0 interfacing. So the price advantage that analogue
cameras have had over digital will slowly be eroded.

Is your application purely to view a scene or to do measurements? If you
just want to view a scene for example in a security application or in an industrial
application where you are looking for the presence or absence of a part then
an analogue camera will be fine. If however you want to carry out any form of
measurements, processing or analysis on an image then a digital camera will
always be a better choice. Digitising the image in the camera close to image
sensor will always present a more accurate reproduction of the image data and
hence a better result to work with. This does not mean measurements can not
be done with an analogue camera it just means the result will not be so accurate.




What resolution do I need the camera to have?

2). Secondly you must decide what resolution you need. This is governed by the
amount of detail you want to see, the optical lens and the working distance
(i.e.:- lens distance to object). If the object being observed is large then around
standard VGA format of 640 x 480 will be fine. However, if fine detail is required
(for example grain analysis in a microscopy application or small defects in glass
plates) then a higher resolution will be required. A very simple rule of thumb is
to decide the field of view you need and the size of the smallest detail you want
to view and divide one by the other and multiply by 3 (e.g.: you are viewing a
ceramic tile of 150mm wide and you need to see a defect of 1mm - to be sure
of identifying the defect you need three pixels to be covering 1mm defect so
therefore you will need at least 450 pixels to see the defect. If you were looking
for the same 1mm defect in a 500mm tile then you would need at least 1500 pixels. You will need this resolution in both directions and preferably in a 1:1 ratio so square pixel cameras will be desirable.


How fast is my image acquisition?

3). Next you must decide on the speed the acquisition needs to take. If the object is static this is not an issue and any camera will be suffice. If your object is moving along a conveyor it will need to be either a progressive area scan camera or a line scan camera. Basically for fast moving events (faster than the eye can see) there are two types of camera.

Progressive Area Scan- this type of camera has the ability to read the image
as a whole (rather than an interlaced camera that reads two distinct fields (odd
and even lines) separated by 40ms time interval and then the resultant image is
read out as a complete frame). Where on fast moving objects the interlaced camera gives image blur (because of the time difference the two fields are read out by the image sensor). The progressive cameras read all lines within the same scan and therefore no image blur is visible.

Line Scan cameras. Sometimes area scan cameras do not have the speed to
capture data from a moving object. (example paper or textiles which may travel
at many tens of meters a second) These applications demand cameras which can
read a line of data very fast. Also normally in this application the web is very wide
sometimes many meters so therefore a high resolution camera is required. To deal
with these issues a linescan camera is needed. Line scan cameras are a linear
image sensor (generally one row of pixels in the sensor - up to about 8000 pixels).
Linescan cameras read data at many thousands of lines per second so can deal
with defect detection in very fast moving objects.



Should I choose a Monochrome or Colour camera?

Generally our advice here is if you don't need colour detail always choose
monochrome.

There are two reasons
1). Colour image sensors are monochrome sensors with a matrix colour filter
across them. There are a number of different filters used but all filters will
degrade the image sensor sensitivity by around 30 per cent. That means you
will have to compensate with more light or lower iris setting to let more light
into the optic.

2). The other problem is that in single chip colour cameras the resolution of
the colour is degraded. This is because the filter colour for one pixel will be
different from its neighbours. By software correction the resolution is corrected
but it's a assumed correction so it will not be as accurate colour representation
as that derived from a three sensor colour camera where there is a sensor for
each of the prime colours (blue, green and red) and these are converged to
overlap one another by clever optical mirrors in the camera head. For really
true colour representation a three chip colour camera has no equal.


What's the difference between CMOS and CCD?

This is a big subject in itself but we will try and be clear on the main differences

CCD is more sensitive than CMOS mainly because the CCD chips generally
have 100% fill factor where the CMOS is much less ( this means the CCD is 100%
active sensor while the active part of the CMOS will be no more than 70% and
some a lot less.

CCD is much better for low contrast images. This is because of the lower
inherent noise in the sensor.

CMOS has the advantage of being much more flexible than CCD. You can
window CMOS sensors to read out less data at a higher frame rate (i.e.: although
a sensor may have a resolution of say 1280 x 1024 and readout rate of 15 frames
per second by windowing the sensor and only reading out a 640 x 480 portion
of the image you can achieve a frame rate of nearly 70 frames per second could
be achieved.

CMOS sensors have much lower power consumption and therefore are ideal
for portable devices or space applications.


What type of output should my camera have ?

The type of output will be determined by how you want to read the data out
from the camera.
If you want to read it out to a video monitor then an analogue output (either
CCIR for monochrome) or PAL for colour will be the easiest.
If you want to take the data to a PC then the choice is huge but basically as
follows:

Analogue can still be used - you will have to interface to a frame grabber,
there are number of inexpensive ones we can offer you so the cost need not
be too high.

For undemanding applications USB2.0 will be easy and very straight forward.
Fire Wire (IEEE1394) offers a solution that is also "plug and play" and some
cameras are now available with high resolution. Please see our separate FAQ
on the differences between USB2.0 and Fire Wire (IEEE1394) more.

Camera Link- the digital interface standard for those demanding applications.
Is used with many high performance digital area scan and also line scan cameras.
You will need a frame grabber designed for that type of interface. The price of this interface is decreasing so is not as expensive as you may think.

There are also older types of LVDS RS644 interfacing still used for digital cameras
but these are being phased out and replaced by Camera Link or Fire wire so is not
recommended for new developments.


How do I choose a lens for my camera?

A camera is of no value without a lens or a focusing optic. In microscopy applications this is provided by the microscope manufacturer and with a suitable C mount adapter the camera can be directly coupled to the microscope.

For other applications the lens choice depends on:-
Field of view required
Working distance ( distance between front face of lens and object being
viewed)
Size of detail required
Depth of field required
How you are going to use the data captured. (Accurate measurement
applications for example may require the use of a telecentric or machine
vision quality lens)
Features required from lens (most lenses are manual iris and focus but
other options are available such as zoom and motorised lens control.

This guide is not extensive and is no substitute for speaking to an experienced
engineers who will be able to assist in the selection of the right camera and
lens combination for your application.



For more information...
Call ALRAD Imaging on +44(0) 1635 30345


For details of Sensor Suppliers, click here...
 
  


Home - Website - Search - Suppliers - Links - New Products - Catalogues - Magazines Problem Page - Applications - How they work - Tech Tips - Training - Events - Jobs