Imaging System Resolution

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Definition

Resolution quantifies the ability of an imaging system to be able to differentiate one artifact from the other found within the same field of view. The higher the resolution, the greater the level of detail of the imaged object.

An imaging system, in its simplistic form, consist of optics, an imaging sensor, and a processor.

For example, the human visual system:

1. Optics - the lens system consisting of a lens and iris which focus visible light from external objects such as a tree
2. Imaging sensor - the retina which collects or receives the light delivered by the lens/iris
3. Processor - the retina delivers the information collected from the retina to the brain via the optical nerve for final processing

In the visual inspection world:

1. Optics - the lens system consisting of an objective or zoom lens
2. Imaging sensor - a CCD or CMOS sensor and delivers the sensor information via signals such as component, RGB, S Video, Component, Composite.
3. Processor - electronic circuits which convert and eventually output the signals from the sensor to a monitor, which then serves as input to the human vision system.

Pixels

When working in the remote visual inspection world, resolution is very easily quantified by the use of pixels. An image sensor (camera) is comprised of many individual pixels which detect brightness (and with filters, color). The pixels are arranged in a uniform grid; the earliest were 320 pixels wide by 240 pixels high.

For an image sensor, assuming the optics supplying the image to the sensor is not the limiting factor, resolution is often determined by the sensor's ability to "resolve" line pairs. A test pattern with alternating black and white lines at fixed distances is placed in front of the camera, if the resulting image clearly shows the distinct black and white lines, the camera passes the resolution test. Test patterns with smaller and smaller (and closer together) lines are used until the resulting image from the camera system begins to look gray.

In theory, it should only require 2 pixels to resolve a line pair, the black line projected on one pixel, the white line on the other.

In practice however, most image sensors would need to have two or three pixels per the dark line, then another two or three pixels for the white space. This has to do with the quality of the sensor and the close proximity of the pixels to one another.

Pixel count is now typically used to define an imager's resolution. Multiply the number of horizontal rows by vertical rows to get a total pixel count. For standard VGA resolution (640×480) you have 307,200 pixels or .3 megapixels (1 megapixel = 1 million pixels)

Optics

Pixels are not the only way to obtain resolution from an imaging system, in fact, multi-megapixel systems are a relatively new development. The traditional method for obtaining higher resolution is by using optics. As an example, microscopes have been giving us high resolution images for decades.

Conclusion

Once the required resolution of a visual inspection application has been defined. The system designer can begin to choose optics and image sensors which will provide the required results. Taking into account additional factors such as depth of field, field of view, light sensitivity, etc. The system may incorporate short focal length optics with a multi-megapixel sensor, or longer focal length optics with a lower pixel count imager, both options will deliver the same results. However, there will be differences in packaging, depth of field, field of view, viewing requirements for monitors, cost, etc.

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