Digital Camera Technology for Today’s Industrial Imaging Applications

At this point, it is necessary to define the number of pixels that will represent each of these points. In this application, it would be sufficient to allow three pixels to define each of the two edges and four pixels to define the span. Therefore, a minimum of 10 pixels should be used to define the 25-mm bottle cap in the image. From this, we can determine that one pixel will represent 2.5 mm of the object itself.

Now we can determine the overall camera resolution. Choosing 400 mm of the object to represent the horizontal resolution of the camera, the camera then needs a minimum of 400/2.5 = 160 pixels of horizontal resolution. Vertically, the camera then needs 250/2.5 = 100 pixels vertical resolution. Adding a further 10% to each resolution to account for variations in the object location within the field of view will result in the absolute minimum camera resolution of approximately 176 (H) × 110 (V).

If a camera with this resolution were available, the images acquired would appear as seen in Figure 1. Notice that a cluster of relatively lighter pixels compared to the dark background represents each bottle cap. In this image, it is possible to discern that there are 24 bottle caps contained within the case, but little else. Choosing a camera with a higher resolution, such as 640 × 480, which is commonly available, will yield a much-improved image that could be used to acquire more detail within the image (such as print), as shown in Figure 2.

Pros and Cons of Increasing Resolution

While a higher-resolution camera will help increase accuracy by yielding a clearer, more precise image for analysis, the downside is slower speed. Digital cameras transmit image data as a series of digital numbers that represent pixel values. A camera with a resolution of 200 × 100 pixels will have a total of 20,000 pixels, and, therefore, 20,000 digital values must be sent to the acquisition system. If the camera is operating at a data rate of 25 MHz, it takes 40 nanoseconds to send each value. This results in a total time of approximately 0.0008 seconds, which equates to 1,250 frames per second.

Increasing the camera resolution to 640 × 480 results in a total of 307,200 pixels, which is approximately 15 times greater. Using the same data rate of 25 MHz, a total time of 0.012288 seconds, or 81.4 frames per second, is achieved. These values are approximations, but it is apparent that an increase in camera resolution will result in a proportional decrease in camera frame rate.

Speed and Exposure

When selecting a digital camera, the speed of the object being imaged must be considered. In the previous example, it was assumed that the object was not moving during exposure; therefore, a relatively simple and inexpensive camera could be used. This scenario is not always the case. Objects move continuously in many applications and, in others, they may be stationary only for very short periods of time.