This article was originally published at Basler's website. It is reprinted here with the permission of Basler.
Lost in the Jungle of Options?
Faced with the challenge of designing an image processing system, you may find yourself in a veritable jungle of options, amidst a dizzying range of camera models, relevant properties, helpful features and potential applications.
What you need now is a guide. Someone who cuts a path through the brush and helps you reach the clearing at the end of your decision-making journey of selecting the right camera for your vision application.
Join us in our step-by-step exploration of all the relevant criteria. It will help you make the right decisions, one at a time, to choose the best equipment for your requirements.
Start with a clear-eyed self-assessment. Ask yourself two questions:
- What do I need to see with the camera?
- What characteristics are necessary for my camera to deliver precisely that?
The answers usually provide a good initial guidance and will point in one of these two directions:
Network (IP) Cameras
Industrial (MV) Cameras
Decision #1: Network or Industrial Camera?
Cameras for image processing systems are categorized either as industrial/machine vision (MV) or network/IP (Internet Protocol) cameras.
Network cameras record videos. They are frequently used in classical surveillance applications and in combination with industrial cameras. Some of their typical characteristics:
- often placed within robust casings designed to be resistant to jolts and harsh weather, making them suited for use indoors or out.
- a variety of functions such as day/night modes and special infrared filters deliver outstanding image quality even under extremely poor lighting and weather conditions.
- they compress the images they record. This reduces the volume of data to such a degree that it can be stored in the camera. By connecting to a network, a theoretically unlimited number of users can also access the camera.
Industrial cameras, by contrast,
- send the images as uncompressed ('raw') data directly to the PC; The PC is then responsible for processing the relatively large volume of data. The benefit of this method is that no image information is lost.
- Industrial cameras comprise two technologies: area and line scan cameras. They capture images differently, which is relevant to the type of vision application.
Excursion: How area and line scan cameras capture images
Area scan cameras
- are outfitted with a rectangular sensor featuring numerous lines of pixels that are exposed at the same time. The image data is thus recorded in one single step, and is also processed in the same way.
- are typically used in a variety of industrial applications, in the medical and life sciences, in traffic and transportation, or in security and surveillance, often as a supplement to network cameras.
Line scan cameras
- by contrast use one sensor comprised of just 1, 2 or 3 lines of pixels. The image data is captured line for line, with the individual lines then reconstituted into an entire image during the processing stage. The question of whether an area or line scan camera should be used is a question of your applications and its requirements.
- are used universally when products must be inspected as they pass by on conveyor belts — at times at extremely fast speeds. Typical industries include printing, sorting and packaging, food and beverage, and all kinds of surface inspection applications.
Network cameras
- are used for a variety of surveillance tasks, from process controls in shipping lines and packing systems to building and traffic surveillance systems.
- are typically used in places like banks, casinos, company campuses and public buildings, as well as in logistics and transportation centers such as harbors or freight centers.
Decision #2: Monochrome or Color Camera?
A relatively simple decision and one that is usually answered by what your application is all about: the image you require. Do you need it in color to evaluate the results, or is black and white sufficient? If color isn't mandatory, then a monochrome camera is typically the better choice as they are more sensitive and deliver more detailed images. For many applications, for example in intelligent traffic systems, a combination of b/w and color cameras are also frequently used to satisfy the specific national legal requirements for evidence-grade images.
Decision #3: Sensor Types, Shutter Technique, Frame Rates
This step involves picking a suitable sensor, built either around CMOS or CCD sensor technology, and choosing the type of shutter technique: global or rolling shutter. The next consideration is of the frame rate, meaning the number of images that a camera must deliver per second to handle its task seamlessly.
Sensor Types
Excursion: CCD or CMOS?
The fundamental difference between the two sensor technologies is in their technical structure.
In CMOS chips, the electronics to convert the light (and specifically: the photons) into electronic signals (electrons) are integrated directly into the surface of the sensor. This makes them especially quick since they can read the image data more rapidly and allow the user to address the image range flexibly. CMOS sensors are heavily used in the consumer market, such as in SLR cameras.
CCD sensors use the entire sensor surface to capture the light, with no conversion electronics placed on the sensor's surface. This leaves more space for pixels on the surface, which in turn means more light is captured. Sensors of this type are thus extra light-sensitive, a major benefit in low-light applications like astronomy. CCD sensors deliver outstanding image quality in slower applications, although their architecture and the way in which they transport and process image data has increasingly brought them to the limits of their speed.
Over the years, the CMOS technology has progressed so far that it is now suitable for almost any image processing application. CMOS sensors offer
- strong value for the performance
- high frame rates
- high resolution
- low power consumption
- strong quantum efficiency
which helped them gain a foothold in areas previously dominated by CCD sensors. One especially strong selling point of today’s generation of CMOS sensors is their high frame rates without deterioration in image quality.
CMOS area scan sensor and CCD sensor
Shutter Techniques
One simple, but crucial requirement here: the shutter must match the application. The shutter protects the sensor within the camera against incoming light, opening only at the moment of exposure. The selected exposure time provides the right 'dose' of light and determines how long the shutter remains open. The difference between the global and the rolling shutter variants is in the way they handle exposure to light.
Excursion: How global and rolling shutter work
The global shutter opens to allow the light to strike the entire sensor surface all at once. Depending on the frame rate a moving object is thus exposed in a rapid succession. Global shutter is the optimal choice for applications where very fast moving objects must be captured, such as in the traffic and transportation fields, in logistics and in inspections of printed materials.
Rolling shutter exposes the image line-by-line. Depending on the selected exposure time, distortions can occur when objects move during the exposure process – the so-called rolling shutter effect. However, there's no need to abandon the possibility of a rolling shutter just because your application involves moving objects. In many cases, the effect can be circumvented through proper configuration of the exposure times and the use of an external flash.
For the rolling shutter, the exposure time does not begin and end simultaneously, but rather for each individual line respectively: The graphic shows the staggered exposure of the individual lines on the photo.
Learn more about these shutter techniques in our white paper
"Global Shutter, Rolling Shutter — Functionality and Characteristics of Two Exposure Techniques."
Frame Rate
Used synonymously with 'frames per second' or 'fps', or ‘line rate' or 'line frequency', respectively, for line scan cameras. The frame rate describes the number of images that the sensor can capture and transmit per second.
The higher the frame rate, the quicker the sensor. => The quicker the sensor, the more images it captures per second. => The more images, the higher the data volumes.
For fast-moving applications like inspections of printed images, with newspapers moving at high speeds past the camera inspection point, the cameras must be able to 'shoot' in milliseconds.
This is a far cry from some microscopic inspections used in medicine and industry, which typically require only low frame rates.
For area scan cameras these volumes can vary greatly depending on the interface and whether a low rate of 10 fps or a high (fast) speed of 340 fps is being used. Just which frame rates are possible or even necessary depends on what the cameras in the image processing system must record.
Decision #4: Resolution, Sensor and Pixel Sizes
Resolution
Looking up your camera’s specs you read "2048×1088". What exactly does that tell you? It describes the number of pixels per line, in this case 2048 pixels for the horizontal lines and 1088 pixels in the vertical lines. Multiplied together, the numbers indicate a resolution of 2,228,224 pixels, or 2.2 megapixels (million pixels, or 'MP' for short).
To determine which resolution you require for your application, doing some simple maths will help instantly:
Resolution= (Object Size)/(Size of the detail to be inspected)
Excursion: How to Determine the Required Resolution
Let’s say you need to capture a precision image of the eye color of a roughly 2 m tall person standing at a specific point:
Resolution = Height/(Eye detail) = (2,000 mm)/(1 mm) = 2,000 px in x and y = 4 MP
= > To clearly recognize the 1 mm large detail, you need a resolution of 4 megapixels.
Sensor and Pixel Size
Fact #1: The easy part first: large sensor and large pixel surfaces can capture more light. Light is the signal used by the sensor to generate and process the image data. So far, so simple. Now stay with us: The greater the available surface, the better the Signal-to-Noise Ratio (SNR), especially for large pixels measuring 3.5 µm or more. A higher SNR translates into better image quality. A SNR of 42 dB would be considered a solid result.
Fact #2: A large sensor provides larger space onto which more pixels can fit, which produces a higher resolution. The real benefit here is that the individual pixels are still large enough to ensure a good SNR — unlike on smaller sensors, where there is less space available and thus smaller pixels must be used.
Fact #3: And yet large sensors and a large number of large pixels won't achieve much unless the right optics are in place. They can only achieve their full potential when combined with a suitable lens also capable of depicting such high levels of resolution.
Fact #4: Large sensors are also always more cost intensive, since more space means more silicon.
Decision #5: Interfaces and Housing Size
The Interface
The interface serves as the liaison between the camera and PC, forwarding image data from the hardware (the camera sensor) to the software (the components that process the images). Finding the best interface for your application means finding the optimal balance of performance, costs and reliability by weighing a series of different factors against one another.
Depending on what your application demands, you can select between Camera Link, GigE or USB 3.0 to find the best set of properties to transmit image data from the camera to the PC quickly, cleanly and securely.
Excursion: Interface technologies and standards
GigE Vision, USB3 Vision and Camera Link are modern, widely available technology standards that guarantee the compatibility of the camera interface with standard-conformed components and accessories. Each technology is designed to fulfil a specific set of requirements regarding bandwidth, multi-camera setups, of cable lengths, for example.
FireWire and USB 2.0 are older technologies which, due to their limitations, are not recommended without reservation for modern image processing systems anymore.
Comparison of Interfaces
Not sure, which interface technology matches your requirements best?
Check our Interface Advisor for support in making your choice.
Want to explore all the pros and cons of the individual interfaces in detail?
Download our White Paper "Comparison of the Most Common Digital Interface Technologies in Industrial Machine Vision."
Housing
Directly tied to the choice of interface is the size of the camera housing. It is important in terms of the overall integration into the vision system. In applications where cameras are organized next to one another (known as multi-camera setups) to better record the entire width of a material web, each millimeter of space matters.
Size matters, especially in multi-camera setups.
At Basler for example the portfolio of available models ranges from the 29 mm x 29 mm of the Basler ace to the larger dimensions of individual camera with very large (line scan) sensors, such as the Basler sprint series.
Decision #6: Useful Camera Features
All Basler cameras come equipped with a core of helpful features to improve image quality, assess image data more effectively or control processes with greater precision. Check our Features Check List for a comprehensive listing of all features for each camera model.
When designing your image processing system, you will most probably come across these three features:
AOI (Area of Interest)
Allows you to select specific individual areas of interest within the frame, or multiple different AOIs at once. The benefit here is that only those parts of the frame are processed that are of relevance for assessment of the image, thus speeding up the read out of the camera data.
Autofeatures
Basler cameras offer a series of so-called Autofeatures such as, for example, automatic exposure adjustment and automatic gain. By allowing the exposure time and gain parameters to adapt automatically to changing ambient conditions, these two Autofeatures keep the image brightness perpetually constant.
Sequencer
The sequencer is used to read out specific image sequences. This means for example that various AOIs can be programmed and then automatically read out sequentially by the sequencer.
See the Clearing Ahead?
Almost there! After making your way through the jungle of criteria and components, after venturing out into explorations along your path of necessary decisions, you're just steps away now from the final challenge: the right camera!
We offer the right camera for your machine vision system
Fear no more! Let the Basler Camera Selector help you choose precisely what you need: the perfect area scan, line scan or network camera for your individual requirements.
