James S. Cybulski; James Clements; Manu Prakash

“ Focusing is achieved using the same positioning of one's thumbs, except the thumbs are pulled apart (or pushed together) . This causes tension (or compression) along the optics stage, resulting in −Z (or +Z) deflection of the micro-lens due to flexure of the supporting structure of the sample-mounting stage (figure 1C) . Unlike traditional microscopes, the Foldscope anchors the sample at a fixed location while the optics and illumination stages are moved in sync. ”

“ Many children around the world have never used a microscope, even in developed countries like the United States. A universal program providing “a microscope for every child” could foster deep interest in science at an early age. ”

“ Structural loops in folded paper provide kinematic constraints as a means for passive self-alignment. This light, rugged instrument can survive harsh field conditions while providing a diversity of imaging capabilities, thus serving wide-ranging applications for cost-effective, portable microscopes in science and education. ”

“ Using both upper and lower pieces of transparent tape creates a cavity for mounting wet specimens such as live algae suspended in water. Since the paper microscope slide is less than half the thickness of a standard glass slide, a spacer is required to elevate the sample closer to the lens. ”

“ Zemax software was used to model the Foldscope optics to assess optimal aperture radius and resolution. The basic model of the system consists of a ball lens, an aperture, an object at infinity, and an image plane (see figure S5A) . This model requires two parameters to be independently optimized ― lens-image distance and aperture radius. The analysis is carried out in four steps: 1) optimize lens-image distance in model by minimizing focusing metric (figure S5C) ; 2) determine search space for aperture radius as defined by empirically chosen limits on Strehl Ratio, 0.75–0.98 ”

“ For example, in situ examination of specimens in the field provides important opportunities for ecological studies, biological research, and medical screening. Further, ultra-low cost DIY microscopes provide means for hands-on science education in schools and universities. ”

“ The diffuser helps to evenly distribute light from the small LED over the aperture area, while the condenser focuses a hollow cone of light onto the specimen. Since the specimen must be placed at the focal point of the condenser, the slide thickness has to match the back focal length of the condenser plus the spacing from the condenser to the slide. A 140X Darkfield Foldscope was used to image 6 μm polystyrene microspheres as shown in figure 2D.
Capillary Encapsulation Lens Mounting
The process of precisely mounting micro-optics to an aperture crucially governs lens performance. ”

“ For a 300 μm sapphire ball lens: EFL = 172 μm, BFL = 22 μm, and MAG = 1,450X. Thus substantial magnification can be obtained, but the sample must be separated from the lens by only a fraction of the thickness of a human hair. Three additional optical performance metrics include field of view radius (FOV) , numerical aperture (NA) , and depth of field (DOF) . These depend on aperture radius (a) , the optimization of which is discussed below. For the previous example, the normalized optimal aperture radius is nOAR = a/r = 0.51, giving: FOV = 88 μm, NA = 0.44, DOF = 2.8 μm (see table 2) . ”

“ Modern techniques such as micro-scale plastic molding and centerless ball-grinding have grown to serve numerous applications, including telecommunication fiber couplers, cell phone cameras, and medical endoscopes. ”

“ Darkfield condenser aperture shown in inset has 1.5 mm inner diameter and 4.0 mm outer diameter. (E–H) Schematic cross-sections of Brightfield, Fluorescence, Lens-Array, and Darkfield Foldscope configurations, showing the respective arrangements of ball lenses, filters, and LEDs. See table 2 for ball lenses used for specific magnifications. Fluorescence
Conventional fluorescence microscopy typically requires an expensive illumination source for high-intensity broad-spectrum illumination and multiple optical elements with precisely defined spectral profiles. ”

“ C) . Thus, the best achievable resolution is attained at the expense of a reduced field of view. When a digital sensor is used in place of the naked eye, the lens fixture effectively reduces the field of view to roughly the optimal field of view.
Foldscope imaging modalities. (A) Brightfield Foldscope image of a monolayer of 1 μm polystyrene microspheres (Polysciences 07310-15) using a 1,450X lens. (B) Fluorescent Foldscope image of 2 μm polyfluorescent microspheres (Polysciences 19508-2) using a 1,140X lens with Roscolux gel filters #19 and #80. ”

“ File:Pone.0098781.s003.tifClick here for additional data file. Figure S4 Numerical Modeling Characterization of Optimal Field of View. (A) Plot of Wavefront Error over the full field of view defined by the aperture for a 300 μm Sapphire ball lens with a 147 μm aperture. With increasing field coordinate, the Wavefront Error becomes very large and the image will be out of focus. An “optimal field of view” is defined at a field coordinate of 21 μm, where the Wavefront Error is approximately 1/5 wave number. ”

“ Foldscope components and tools used in the assembly, including Foldscope paper components, ball lens, button-cell battery, surface-mounted LED, switch, copper tape and polymeric filters. (E) Different modalities assembled from colored paper stock. (F) Novice users demonstrating the technique for using the Foldscope. (G) Demonstration of the field-rugged design, such as stomping under foot. Table 1 "Bill of Materials. (10.1371/journal.pone.0098781.t001) "File:Pone.0098781.t001Summary of unit costs for Foldscope components in volumes of 10,000 units, not including assembly costs. ”