To the Editor:
The performance of a fluorescence microscope depends largely on the quality of its optical elements, predominantly its objective lenses. However, this quality is not easy for users to assess. We wrote PSFj (http://www.knoplab.de/psfj/ and Supplementary Software) to enable automated analysis of a microscope's performance with respect to resolution, chromatic aberrations and planarity across the entire image field.
Modern fluorescence microscopy methods demand state-of-the art instruments that perform at their diffraction limit (the maximum theoretical resolution a given microscope is able to deliver when equipped with a specific set of optical elements) and that provide a flat image field over a wide range of wavelengths. Although high-performance objectives are optimized to fulfill these demands, variation in the quality and alignment of components in the microscope as well as potential ill-usage (scratches, dirty lenses, etc.) will unavoidably lead to deviations from optimal performance.
The performance of objective lenses is typically assessed by measuring their point spread function (PSF), i.e., by employing published protocols1 to image a point-like object (usually a subresolution-sized fluorescent bead). PSF width and centroid position at different wavelengths and positions in the field of view (FOV) provide measures of resolution, chromatic aberrations and image field planarity and are usually analyzed using either open-source software (such as MetroloJ, http://imagejdocu.tudor.lu/doku.php?id=plugin:analysis:metroloj:start) or custom routines1. However, no protocol exists for the rapid quantification of these measures throughout the FOV, making it generally difficult and tedious to judge whether a microscope is performing optimally. As a consequence, a microscope's performance is assessed only occasionally (if at all) and is often based on a few PSF measurements. These measurements are typically made in the center of the FOV (where microscopes generally perform best), providing only a glimpse—an often misleading one—of the actual performance. To profit from the development of cameras with large complementary metal-oxide semiconductor (CMOS) sensors that can capture most of the FOV, including the edges where objective lens performance often deteriorates, users must conduct a careful assessment of each instrument's performance.
PSFj provides an automated full-FOV analysis of resolution, chromatic aberrations and planarity based on image stacks acquired from typical PSF test slides with multicolor, subresolution fluorescent beads. Using robust routines, PSFj identifies all beads in the FOV and analyzes their fluorescence intensity distributions (Supplementary Note). For each individual intensity distribution, it retrieves a number of parameters (Fig. 1) including the centroid position (x0, y0, z0), the smallest and largest full-width at half-maximum (FWHM) in the xy plane (FWHMmin and FWHMmax, respectively), the angle (q) between the x axis and the direction of FWHMmax, and the axial FWHM (FWHMz). FWHMmin and FWHMmax serve as measures for lateral resolution, whereas their ratio A = FWHMmin/FWHMmax and q describe the lateral resolution anisotropy. The axial positions (z0) of all the beads, which are attached to the surface of a coverslip, provide a measure of the planarity of the image field. In addition, dual-color analysis allows for the determination of wavelength-dependent differences in bead centroid position (Δx0(l1, l2), Δy0(l1, l2), Δz0(l1, l2)), which provides a measure for lateral and axial chromatic aberrations. For dual-camera systems, this also quantifies the degree of misalignment between the cameras. Keeping different applications of such measurements in mind, we designed PSFj to offer three reporting options (Supplementary Methods).
Results are visualized directly in the form of pseudocolored heat maps (Fig. 1), providing a quick and comprehensive performance check and inviting routine use. Second, the software can generate a summary report of the current system performance or a full report containing all individual PSF measurements and associated fitting parameters. Third, a table with the extracted resolution, planarity and colocalization data can be exported. This can be used for subsequent analysis, such as in an image processing or restoration pipeline. In addition, an average PSF from a user-selectable region of interest can be exported, for example, for image deconvolution.
We used PSFj to quantify the performance of various high–numerical aperture (NA) objectives and to track day-to-day and system-to-system variation. The results showed substantial performance differences and allowed us to identify strengths and weaknesses of individual objectives as well as general shortcomings (Supplementary Figs. 1 and 2). In particular, we found that whereas lateral resolution performance generally fell short (∼20–30%), axial resolution often met or exceeded expectations from the scalar approximation of the PSF commonly used in textbooks2 (Supplementary Note). Planarity was usually well corrected with variations over the FOV below the axial resolution and allowed for the detection of tilted slides caused, for example, by dust particles or misaligned slide holders or stages. Axial chromatic shifts were usually small, with little variation across the FOV (Supplementary Table 1). In contrast, chromatic shifts often showed circular symmetry and increased toward the edge of the FOV, which is a sign of lateral chromatic aberrations. Day-to-day performance variation of most objectives was relatively small (∼2–6%) and comparable to single-measurement FOV variations (Supplementary Table 2). Furthermore, testing a limited number of identical objectives identified objective-to-objective and microscope-to-microscope variations of about 10% (Supplementary Tables 3 and 4).
The PSFj software is open source and based on libraries from various sources, including ImageJ3 and μManager4, and it runs as a stand-alone application on the three major operating systems (using Java).
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We thank H. Lorenz and C. Hoerth from the Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH) imaging facility, and F. Bestvater and D. Krunic from the Deutsches Krebsforschungszentrum (DKFZ) light microscopy facility, for assistance in acquiring some of the image stacks, and a number of friends for testing PSFj and providing expert feedback. This work was supported in parts by a grant from the German Research Foundation (DFG), Sonderforschungsbereich (SFB) 1036 (TP10).
The authors declare no competing financial interests.
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Theer, P., Mongis, C. & Knop, M. PSFj: know your fluorescence microscope. Nat Methods 11, 981–982 (2014). https://doi.org/10.1038/nmeth.3102
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