Digital Cameras Expand Resolution and Accuracy

Inspection tasks requiring high-accuracy, high-resolution images can benefit from digital cameras.

Jon Titus, Editorial Director -- Test & Measurement World, 6/1/1998

More and more manufacturers are offering cameras with digital outputs. If you’ve used the more common “analog” cameras, you may wonder what advantages digital cameras hold. Basically, the new digital cameras offer several benefits that you can take advantage of in an inspection system, particularly improved spatial resolution and improved accuracy of image information.

The basic difference between analog cameras and digital cameras comes down to how they handle image data. In an analog camera, the internal circuitry preprocesses the signal to make the picture look like a TV image. Then, the circuits add synchronizing and timing information to the video signal and transmit it to a monitor or to a frame-grabber board. The frame grabber, which resides in a computer, digitizes the video signal and reconstructs an image frame.

A digital camera skips the preprocessing steps, and it adds no timing information to the video signal. Instead, it uses an internal analog-to-digital converter (ADC) to digitize the raw analog signal from each pixel on the image-sensing array. The camera then puts out the digitized value for each pixel in a parallel digital form.

How Many Bits?
Typically, an analog camera produces a video signal that a frame grabber can resolve to about 6 to 7 bits of gray-scale information for an image. The nature of the analog video signal limits the practical accuracy of the signal to about 32 to 64 gray levels. In a digital camera, accuracy starts at about 8 bits (265 gray levels) and can go up to 12 bits (4096 gray levels) without much difficulty. You can purchase cameras that digitize with a 14-bit resolution. In general, an 8-bit camera provides sufficient accuracy for most inspection tasks, but if you need to inspect fine features on a semiconductor wafer, you might choose a 12-bit camera.

Because a digital camera provides an internal ADC, the analog-to-digital conversions take place in an environment that the manufacturer can control. The converter doesn’t feel the effects of the electrically noisy environment inside a PC, so accuracy improves.

An analog camera operates like a TV camera in that it interlaces image-scan lines. In effect, it produces two images, one of the even scan lines and another of the odd scan lines. The frame grabber must merge the even and odd fields to reproduce a complete image. Unfortunately, a frame grabber may pick up the odd field from the previous image and the even field from the current one. Merging them can produce a blurry image if the object moves during the scan. A digital camera scans a complete image—a progressive scan—all at once and produces unambiguous digital information.

Most new digital cameras provide an image resolution of 1kx1k pixels. Most analog cameras used in inspection applications provide 640x480 pixels. So, digital cameras provide more image resolution than analog cameras for a given area. Conversely, you can use a digital camera to acquire an image at the same spatial resolution as an analog camera, but the digital image covers more area.

If you need even more resolution, or if you need a rectangular image, you can find digital cameras with resolutions of 1300x1k pixels. As the features on components such as BGAs continue to get smaller, a digital camera may offer a way to keep up with their inspection requirements.

You can also buy digital line cameras. Instead of providing an array of sensors, the line camera supplies a single line of photosensors. The camera relies on a product’s motion past the line of sensors to scan an image. If the thing you want to image doesn’t move, you can’t use a line camera.

You can expect that camera manufacturers will continue to increase the spatial resolution of the imaging arrays in their digital cameras. But as the number of pixels increases, the amount of light that reaches each pixel decreases. In some applications, such as those in which you need to observe emissions from components on a powered IC, you may need to place an image intensifier in front of the sensor to increase the number of photons.

In addition to continually increasing the resolution and accuracy of digital cameras, manufacturers will move more intelligence into the camera body. After all, if the manufacturers can digitize the image in the body, they can add a bit more circuitry and process image information, too. Manufacturers plan to have the camera’s microprocessor or digital signal processor start by performing simple tasks such as detecting edges, reading bar codes, recognizing characters, and extracting data from two-dimensional codes.

Cost Remains High
Digital cameras have so many advantages over analog cameras that they may seem to be the obvious choice for an inspection system. But there’s another important factor: cost. Because analog cameras have many consumer uses, they are produced in high volumes, and their prices start at about $600 to $700. Because digital cameras employ higher-resolution and include higher-accuracy image sensors that are produced in limited runs, they usually cost at least five times as much—enough to make first-time buyers go into price shock. Before digital cameras find widespread use in inspection applications, their price needs to drop to about $2000.

So, before you decide to replace the analog cameras in an inspection system, carefully weigh the benefits of digital cameras. Be sure your application will benefit from their capabilities.

In addition to cost, digital cameras have another drawback: a lack of standards for the interface that connects them to a computer. Although the signal levels conform to a standard, the signal connections to the camera and to the computer do not.

Most manufacturers use standard RS-422 signal levels to transfer image data from a camera to a computer. (A few cameras may use TTL signal levels.) The differential RS-422 signals provide high noise immunity and let you run cables for many meters.

Camera connections, however, have no such de facto standard. The Automated Imaging Association (Ann Arbor, MI) made an attempt to standardize camera connections so manufacturers could simplify the task of connecting a camera and a computer. Unfortunately, the effort never went beyond standardizing on a 68-pin connector, the equivalent of a SCSI-type connector for computer peripherals.

Also, the format, or protocol, of the data being transmitted to and from the camera varies from manufacturer to manufacturer. The computer transmits TTL, RS-232C, and RS-422 signals to the camera to control exposure, gain, and other camera variables. Manufacturers show no action toward proposing a standard for these signals.

Find the Right Frame Grabber
Because confusion exists at the camera end of the interface circuit, you can also expect some confusion at the computer end. Most camera systems use a frame-grabber board in the computer to acquire image data from a camera and to produce image information that the computer can use. The camera designers and the frame-grabber designers don’t always communicate about new designs. So, frame grabbers usually cannot accept signals from just any digital camera, and you need to do some research when assembling an inspection system.

Camera manufacturers can recommend specific brands and models of frame grabbers that will work with their cameras. Some cameras require dual-channel frame grabbers that can accept alternating lines of data from a camera. Some cameras put out their data in unusual formats that the frame grabber must capture and rebuild into an image.

You’ll also need a frame grabber that can provide signals to control a digital camera. Because RS-422 signals travel well over fairly long distances, be sure that the control signals going to the camera can also travel similar distances without degradation.

You may wonder why, if a camera produces digital information, it can’t transfer data directly into the computer. Some can. Sony currently offers a 640x480-pixel camera that communicates with a computer over the IEEE 1394 bus, commonly called the “FireWire.” Today the speed of the IEEE 1394 bus isn’t high enough for anything but standard 640x480 cameras operating at 30 frames/s. The bus provides a standard high-speed serial communication path for peripherals such as cameras, printers, scanners, and other peripherals. PC manufacturers will be offering IEEE 1394 interfaces on their computers routinely by 1999.

As you examine digital camera hardware, don’t forget about the software that you’ll use to process and analyze your images. In most cases, commercial imaging software should work with 8-, 10-, and 12-bit data coming from a digital camera with a 1kx1k-pixel array. To ensure success, check with the software supplier about compatibility with your camera’s data. And make sure that the software supports the frame grabber board you plan to use.

If you have written your own image-processing software, be sure it can handle the larger amounts of data as well as any “wider” 10- and 12-bit data from a digital camera. The increasing bits of resolution and the much larger sets of data from a 1kx1k-pixel array can easily bog down software. So, when purchasing a digital camera, you may find that you need to purchase a faster computer, too.  T&MW