Figure 1: Relation of Line-Pairs to Square Waves [View Larger Image]
Figure 2: Pair of Red Squares Unresolved (a) vs. Resolved (b) [View Larger Image].
Test targets are useful when evaluating or calibrating an imaging system's performance or image quality. This could include troubleshooting the system, certifying or evaluating measurements, as well as establishing a foundation to ensure the system works well with another. Image quality can be defined by different components, particularly resolution, contrast, modulation transfer function (MTF), depth of field (DOF), and distortion; therefore, one or more types of test targets may be necessary or helpful depending upon the type of system being constructed or what needs to be measured. Fortunately, an array of targets exists that cater towards specific systems including cameras, visual displays, or even a single, thin lens. To be able to choose the correct test target, it is important to first understand the components of image quality.
COMPONENTS OF IMAGE QUALITY
Resolution is an imaging system's ability to distinguish object detail. It is often expressed in terms of line-pairs per millimeter (lp/mm) as seen in Figure 1. A low resolution image usually lacks fine detail and is often blurry, whereas a high resolution image is highly detailed and clear.
To illustrate this concept, imagine two squares being imaged onto pixels of a CCD camera. Assuming that the primary magnification of the lens is such that one square fills one camera pixel (Figure 2a), if there is no space between the filled pixels, it will appear as one large, red rectangle. However, if "white space", or space distinctively different from the original pixel color, is found between the pixels, the camera will be able to distinguish among the two squares (Figure 2b). Hence, the pairing of the red square and "white space" becomes one lp/mm, which corresponds to two separate pixels.
Contrast is a measurement of the separation between the light and dark regions of an image. More specifically, contrast is a change in the intensity or brightness from one point to another. It affects how effectively the differences between the object and the shades of gray in the background are reproduced. An image with the highest contrast is one in which black is truly black and white is truly white, without any shades of gray in between. As contrast is reduced, the distinction between black and white begins to blur, in a very literal sense, and shades of gray appear (Figure 3).
Contrast is often expressed in terms of percentage (%) and is calculated by using Maximum Intensity (Imax) and Minimum Intensity (Imin), as expressed in Equation 1. It can also be represented by a periodic function (i.e. square wave or sine wave), or a function that alternates regularly and instantaneously between two levels.
Modulation Transfer Function (MTF)
Modulation Transfer Function, or MTF, is a measurement of an imaging lens' ability to transfer contrast from the object plane to the image plane at a specific resolution. The object and image planes are the spatial areas where the object and the image preside. The object plane is in front of the imaging system, and the image plane is either in front or behind the imaging system depending on whether the image is real or virtual. MTF is expressed with respect to image resolution (lp/mm) and contrast (%), as seen in Figure 4. Typically, as resolution increases, contrast decreases until a cut-off point, at which the image becomes irresolvable and grey.
Figure 3: Contrast in Relation to Pixels [View Larger Image]
Figure 4: Example MTF Curve of 0.13X PMAG Imaging Lens [View Larger Image]/
Another component of MTF, in addition to the aforementioned resolution and contrast, is diffraction limit. Diffraction limit is a physical limit restricting a lens from being able to image points or edges perfectly. Since it is constrained by the wave nature of light, even a "perfectly" designed and manufactured lens cannot achieve diffraction limited performance. However, designers utilize a variety of methods to reduce aberrations and increase overall system accuracy in order to come as close as possible to reaching a system's ideal diffraction limit.
Correspondingly, a len's geometry contributes to its ability to reproduce good quality image. Lens Diameter (D), Focal Length (f) and F/# (Equation 2) all affect MTF.
F/# is the light gathering ability of a lens. As Lens Diameter increases, F/# decreases. Low F/# lenses collect the most light, thereby making them ideal for light restrictive applications. Although high F/# can improve an imaging lens' performance, increasing it too much can be detrimental because it can cause the diffraction limit to become progressively worse.