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? 

      Generally the choice will be governed by the following factors:-

•         Price – for those applications were you want a low cost camera, then analogue has historically nearly always been the best choice. They have been considerably lower priced than digital cameras. However, in the last few years there has arrived on the market some very attractively priced digital cameras with 8 bit fire wire or USB2.0 interface. So the price advantage that analogue cameras have had over digital is slowly being 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? 

      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 does my image need to be acquired? 

      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:-

•     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.
•     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.
• Gig-E – Gigabit Ethernet interface is relatively new but it will grow in popularity because it allows huge amounts of data to be transferred quickly and over long distances

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,Gigabit Ethernet 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.

We have two very useful lens pdf documents you can download:-

• CCTV LENS CALCULATOR
• CCTV LENS TERMINOLOGY

This guide is not extensive and is no substitute for speaking to one of experienced sales engineers who will be able to assist in the selection of the right camera and lens combination for your application. Also visit the lens section of our web site. We have now included an index selection guide that helps you select the right manufacturers for the type of lens you are looking for.  Or please call our technical sales desk on 01635 30345.

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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