Subscriber access provided by MT ROYAL COLLEGE
Letter
Absorption-induced image resolution enhancement in scattering media Mehbuba Tanzid, Nathaniel J. Hogan, Ali Sobhani, Hossein Robatjazi, Adithya K. Pediredla, Adam Samaniego, Ashok Veeraraghavan, and Naomi J. Halas ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00558 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
Absorption-induced image resolution enhancement in scattering media Mehbuba Tanzid,1, 2 Nathaniel J. Hogan,1, 2 Ali Sobhani,1, 2 Hossein Robatjazi,1, 2 Adithya K. Pediredla,1 Adam Samaniego,1 Ashok Veeraraghavan1 and Naomi J. Halas1, 2 1
Department of Electrical and Computer Engineering and 2Laboratory for Nanophotonics, Rice University, MS-366, 6100 Main Street, Houston, Texas 77005, United States
ABSTRACT:
Highly scattering media pose significant challenges for many optical imaging applications
due to the loss of information inherent to the scattering process. Absorption can also result in significant degradation of image quality. However, absorption can actually improve the resolution of images transmitted through scattering media in certain cases. Here we study how the presence of absorption can enhance the quality of an image transmitted through a scattering medium, by investigating the dependence of this enhancement on the medium’s scattering properties. We find that absorption-induced image resolution enhancement is substantially larger for media consisting of isotropic scatterers (e.g., dielectric nanoparticles) than for strongly forward-scattering media (e.g. biological tissue). This work leads to a broader understanding, and ultimately control, of the optical properties of strongly absorbing, scattering media. Keywords: light scattering, absorption, resolution, imaging, diffusive transport
ACS Paragon Plus Environment
ACS Photonics
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 18
Light propagation through predominantly scattering media, known as diffusive transport, is a physical process of broad, general interest. Effects such as Anderson localization of light,1–4 and applications such as bioimaging,5,6 seeing through turbid media,7–12 and diffusive electromagnetic cloaking13,14 all fall under this regime. In these studies, optical absorption is either negligible or intentionally minimized. In reality, however, absorption is always present, even in “lossless” materials, and for many types of real-world scattering media, optical absorption is far from negligible. For example, biological tissue, naturally occurring sand or dust, manufactured powders, and vapor clouds typically exhibit both scattering and absorptive properties. The realistic inclusion of optical absorption is necessary to understand how to detect or to resolve images through such media. In many cases the presence of both scatterers and absorbers between an object and its image at the detector result in a degradation of image quality (the loss of information between object and image). However, the manner in which absorption and scattering result in image quality degradation are radically different. Absorption results in fewer photons reaching the detector, which causes a loss of signal strength and consequent loss in signal to noise ratio (SNR). In regimes where incident light is not severely limited, any loss due to absorption can be compensated for by an increase in incident photon flux or by an increase in detector exposure time. On the other hand, scattering degrades image quality by re-directing photons originally headed to the same pixel to multiple pixels, with minimal loss in the total number of photons reaching the detector. Scattering results primarily in an image blur, rather than a loss in SNR. Since multiply scattered photons have longer path lengths through the scattering medium than ballistic photons, when absorbers are present, multiply scattered photons are preferentially absorbed. This results in an increase in the relative amount of ballistic photons that reach the detector, increasing image resolution. 15– 19
Previous studies have examined this effect in the context of imaging through predominantly forward-
scattering media, such as biological tissue phantoms, using coherent light,15,16 comparing this effect to time-gating from coherent, pulsed illumination sources,17 and examining the effect of absorption inhomogeneity on transmission eigenchannels.18,19 ACS Paragon Plus Environment
Page 3 of 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
In the present work, we investigate the phenomenon of absorption-induced image resolution enhancement in scattering media from several new perspectives. We examine how the scattering properties of the medium itself, determined by the size and refractive index contrast of its scatterers, influence image quality. We find that, although initially studied in the context of forward-scattering biological media, absorption-induced image enhancement is actually far more dramatic when the medium is composed of isotropic scatterers. We quantitatively assess absorption-enhanced improvements in image quality in two distinct ways. We quantify the improvement in the perceived transmitted image by using the Structural SIMiliarity index (SSIM), a well-known image quality matrix compatible with visual perception.20–22 We also quantify the improvement in spatial resolution of a transmitted image by determining the increase in transmitted spatial frequencies induced by absorbers.23 Absorption-induced image enhancement is illustrated schematically in Fig. 1. When an image is transmitted through a scattering medium, the transmitted image is degraded due to the scattering of photons into trajectories not associated with the original image (Fig. 1A). By introducing absorbers into the scattering medium, image quality can be improved, since the absorbers will selectively remove more scattered photons than ballistic photons (Fig. 1B). The 5× zoomed-in images show that higher resolution images are recovered with the addition of absorbers (carbon black) into the scattering medium (polystyrene).
ACS Paragon Plus Environment
ACS Photonics
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 18
Figure 1: Absorption-enhanced imaging through scattering media. A USAF resolution target (left) transmitted through a scattering medium (schematic, center), resulting in a transmitted image (right). (A) scattering medium (polystyrene spheres) without absorbers; (B) same scattering medium (polystyrene spheres) with the addition of absorbers (carbon black). The right-most images are 5× zoomed-in images, showing the improvement in image resolution obtained by the addition of absorbers to the scattering medium.
Our experiment consists of a simple optical imaging geometry where the image of an object is transmitted through aqueous suspensions of nanoparticles (SI Fig. 1). A fiber-optic white light source (Edmund Optics, MI-150) with a 650 nm bandpass filter (Thorlabs, FB series, FWHM = ±10 nm) was used for illumination. A CCD camera (Princeton Instruments, PIXIS 400) sensitive in the 400-1100 nm wavelength range was used for imaging. The object was a 1951 US Air Force (USAF) resolution target conforming to the military standard (MIL-STD-150A) (see SI Fig. 2). An aqueous nanoparticle suspension in a 30 mm × 24 mm × 5 mm (interior dimensions) glass cuvette (Starna, model 96-G-5) was placed directly in front of the camera so that the suspension influenced only the target image and not the illumination source.
ACS Paragon Plus Environment
Page 5 of 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
Figure 2: Imaging through scattering media with varying scattering and absorption coefficients. (A) TEM image of the ZnO particles in the solutions. (B) Top to Bottom: Images obtained through ZnO solutions with increasing scattering coefficient. Left to right: Increasing carbon black concentration or absorption coefficient in the ZnO solutions. (C) Calculated structural similarity index (SSIM) and (D) cut-off spatial frequency of the images shown in (B).
To study how absorption-induced image resolution enhancement is affected by the scattering properties of the medium, we used a solution of ZnO nanoparticles with a size distribution with maximum particle size
