Machine vision requirements for better performance and higher resolution continue driving developers to incorporate digital cameras into their solutions. This trend will likely accelerate as the price and performance of digital cameras improves. This article will provide you with information on digital camera technology and key factors to consider when choosing a digital camera and associated frame grabber — assuming that all upfront analysis has been performed and that a digital imaging solution is required.
Why Use a Digital Camera?
Analog cameras continue to be the dominant choice in most machine vision applications for several reasons:
- Huge installed base,
- Mature technology with well-known standards,
- Performance is often adequate for the application, and
- Inexpensive, readily available cabling.
Until recently, there were several significant disadvantages to using a digital camera. Digital cameras typically were priced higher than similarly performing analog models. And, in addition to being expensive and bulky, digital camera cabling often was not easily interchangeable between different types of cameras and frame grabbers. However, with the introduction of the Camera Link™ and GigE Vision standards, digital camera cabling issues have largely been eliminated, while their performance continues to steadily improve, in many cases far exceeding that of any analog camera.
So why use a digital camera? Digital cameras can deliver higher data rates, higher resolution, and higher bit depths than analog cameras. Digital transmission is also inherently less susceptible to noise than is analog — a key consideration for plant environments.
One of the ways that digital cameras achieve these high data rates is by providing data on multiple taps, or channels. While this has been done with analog cameras, synchronizing the taps is more difficult with analog, as the frame grabber must be able to lock with subpixel accuracy from multiple sources. Since a digital camera performs the data alignment, synchronizing and interfacing to multiple taps using these sources is assured.
Digital Camera Standards
When they were first introduced, commercial digital cameras used parallel TTL (Transistor-Transistor Logic) level signal outputs. These single-ended, 5V signals were prone to noise interference and could not be transmitted over long distances. As frame rates and resolutions increased, it became necessary to transition to a signaling standard that could accommodate the higher data rates.
RS-422 was the next step in the evolution of commercial digital camera standards. RS-422 parallel output cameras provide potentially higher data rates than TTL by using a lower voltage and higher clock frequencies. The differential signal strategy provides better noise immunity, and because it is a differential signal, the number of data lines doubles over that of a single-ended TTL signal and, therefore, cabling becomes an issue. Cables for RS-422 digital cameras typically are unique to the camera/frame grabber pair, so cable costs are high.