Spectroscopic Measurements of Oxygen Saturation in the Retina

We introduce a new type of retinal oximeter able to capture multiple spectroscopic sensitive images of the retina in a single snapshot. The core of our apparatus is a multiaperture camera that can be easily interfaced with a fundus camera system. A lenslet array divides the image collected by a fundus camera into identical subimages. A bandpass filter array in front of the lens system is used to collect the subimages only at the wavelengths of interest. Two different instruments were designed based on this simple principle. The first system can collect six different images, while the second system can collect up to 18 different images in a single snapshot.     

Evaluation of spherical particle sizes with an asymmetric illumination microscope

A polarized microscope system is used to perform goniometric measurements of light scattered by small particles. The light incident angle on a sample of monodispersed latex microspheres is increased sequentially and a microscope objective lens collects scattered light from the samples. Light is only collected at angles greater than the objective lens numerical aperture (NA) so that only light scattered by the spheres is collected. The experimental results were modeled with a Mie theory-based algorithm. Experiments conducted with microspheres of diameter 1.03, 2.03, and 6.4 /spl mu/m show that, by decreasing the objective lens NA from NA=0.55 to NA=0.0548, a more distinguishable scattering pattern is detectable. From these highly shaped curves, we found that the size of a sphere of nominal diameter 2.03 /spl mu/m was 2.11 /spl plusmn/0.06 /spl mu/m and a 6.4 /spl mu/m sphere was 6.34 /spl plusmn/0.07 /spl mu/m.     

Improved method for calibrating a Stokes polarimeter

We present a method for calibrating a polarization state analyzer that uses a set of well- characterized reference polarization states and makes no assumptions about the optics contained in the polarimeter other than their linearity. The method requires that a matrix be constructed that contains the data acquired for each of the reference polarization states and that this matrix be pseudoinverted. Since this matrix is usually singular, we improve the method by performing the pseudoinversion by singular value decomposition, keeping only the four largest singular values. We demonstrate the calibration technique using an imaging polarimeter based upon liquid crystal variable retarders and with light emitting diode (LED) illumination centered at 472 nm, 525 nm, and 630 nm. We generate the reference polarization states by using an unpolarized source, a single polarizer, and a Fresnel rhomb. This method is particularly useful when calibrations are performed on field-grade instruments at a centrally maintained facility and when a traceability chain needs to be maintained.