The following is a guest post by Barry Wheeler, Digital Projects Coordinator, Office of Strategic Initiatives
In part 1 of this blog series, we saw that manufacturers claimed “resolution” is based on the number of steps per inch a small motor moves the scanner assembly (the rows) and the number of tiny sensors per inch the manufacturer puts on the assembly (the columns.) But the International Standards Organization does not consider this the scanner’s resolution. The ISO defines the steps and sensors as the “sampling” rate because the sensors can only attempt to measure (sample) the brightness at each point. The ISO defines “resolution” as the actual result on the screen, not the number of sensors and steps that attempt to read each point.
We can think of the primary difference between measuring each point and actually resolving each point as “efficiency.” Some of these differences come about because the light may scatter and miss the sensor, the motor step may not be sufficiently precise, or the collected value may be inaccurate. Inside every scanner or camera, between the sensor and the screen is a small, highly specialized computer called a digital signal processor. This processor must work very hard to link a dot on the page to a dot on the screen. Unfortunately, sometimes a manufacturer attempts to compensate for scanner inefficiencies by over-processing the image data, introducing new problems.
The Library follows the ISO standards and scans a “target” such as this one in Figure 1.
Then computer software applies sophisticated ISO specified formulas to measure the actual resolution. Targets and technology are necessary in a production environment, but you can do the same using your own images and your own software – your brain – to evaluate the resolution of an image. The targets used by the Library include figures to visually evaluate a scan. I’ve enlarged the central region of our target in the images that follow. By showing you how we examine this region, I hope to help you learn how to evaluate the resolution of your own scans.
Any numbers are only approximate but the analysis will show you if the resolution is sufficient for your own needs. Figure 2 shows an almost perfect capture of the target area from a very high priced scanner operated at a very, very slow speed.
Figure 3 shows a capture of the same area from a high quality camera with an excellent lens.
Figure 4 shows the capture from an inexpensive scanner (before it was dissected for our earlier blog!).
While Figure 4 shows us an unacceptable scan, Figure 5 shows a much improved scan from the same scanner! By experimenting with the scanner software controls – and carefully examining the results – a much improved scan was produced.
Now we can answer the question, “What resolution should I use?”
Assuming you will look at, or print, the image at the same size as the original (enlargements and film scanning will require another blog post), begin with a scan resolution of 300 dpi or 400 dpi. Choose a document to scan with some areas of fine lines and detail. Examples might be eyelashes, eyebrows, hair, small tree branches, perhaps detail in clothes, or many small windows in a building. Examine the resulting scan carefully. Look for clean solid lines, distortion, poor contrast between lines, false color, edge and halo artifacts around text. Then change the resolution, or the contrast, or the “sharpening” – change any one control and run another scan. Repeat several times. With practice, pick the scan you find best.