With the knowledge of how aberrations and diffraction influence the performance of an optical system, we shall now discuss ways to measure and evaluate image quality. For the infrared, the blur spot size and the radial energy distribution in the image plane are two quantities of great interest. They indicate the minimum detector element size required to collect a certain amount of energy. From this information, one can derive the MTF, a measure of contrast versus resolution.
As with previously discussed subjects, this chapter encompasses a very special and large field. However, the presented approximations will help to quickly obtain some valuable benchmarks for a given situation.
9.2 Blur Spot Measurements
It was pointed out in Chapter 3 that due to diffraction and uncorrected aberrations, the image of an object point is never a point but a diffused disk, usually referred to as the blur circle or blur spot. To confirm the predicted size of a blur spot, several measurement methods are used.
9.2.1 Circular mask
One way of measuring the size of an image blur is to place the lens under test in the exiting beam of a collimator and insert at the image location of the lens a circular mask just large enough to encompass the percentage of the energy the system design asks for. The 100% level is established by recording the signal without any mask. Of course, the detector has to be large enough to accept all the energy transmitted through the lens under test. Next, a mask is inserted, properly sized to the specifications of the desired blur spot size. The signal will indicate if the energy passing through the mask meets the expected percentage level. The principle of such a setup is shown in Fig. 9.1. The use of a field lens provides the necessary clearance for the pinhole aperture insertion.
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