Optical Fourier-plane analysis may prove useful for obtaining statistical data on the densities, sizes, shapes, indices of refraction, and perhaps other properties of particles (particularly, biological cells) suspended in liquids. This concept could potentially be the basis of a new class of simple, portable, relatively inexpensive instruments for diagnosis of samples of blood and other biological materials.

Figure 1. A Liquid Containing Suspended Particles is placed in a container, one of the inner faces of which is grooved.
Figure 2. The Fourier-Transform Image of a filled container like that of Figure 1 is scanned to obtain statistical data on the suspended particles.
As currently envisioned, the concept involves placement of the sample of liquid/ particle suspension in a specially designed transparent two-piece acrylic container. One of the pieces is said to be lenticular because its inner face features a linear array of grooves like the grooves of a diffraction grating or the Fresnel analog of a cylindrical lens (see Figure 1). The sizes and shapes of the grooves may or may not change incrementally along the array, depending on the specific application. Typically, the lineal density of channels is 100 to 200 per inch (about 40 to 80 per centimeter).

The other piece of the container includes a flat plate that covers the grooves, leaving the ends of the grooves open. This other piece can also include a handle for holding the container and an area for placing a drop of the liquid/particle suspension to be analyzed. Once placed, the liquid moves to and fills the grooves by capillary action.

The filled container is positioned in the apparatus shown in Figure 2, at the focal point of the Fourier-transform lens. The container is illuminated with light from the laser. The intensity in the resulting Fourier-transform image is measured as a function of position along an axis perpendicular to the grooves. This measurement is performed by use of a photodetector with a slit aperture in the Fourier-transform plane; the photodetector is moved along this axis by use of a translation stage. The relative intensity measured as a function of position constitutes a Fourier signature that can be analyzed to determine the relative concentrations of particles (cells) of various sizes.

The feasibility of this concept was demonstrated in preliminary experiments on suspensions of microspheres with diameters of 2, 4.5, and 15 μm. The best results were obtained with a lenticular piece featuring a prismatic groove pattern — the sawtooth pattern shown in Figure 1. Each suspension was found to produce a unique and repeatable Fourier-transform signature. Similar results were obtained in preliminary experiments on samples of blood.

This work was done by Steven H. Mersch of Point Source, Inc., for Johnson Space Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Mr. Steven Mersch
Point Source
1864 Dayton Pike
Germantown, OH 45327
Telephone No: (513) 855-6020

Refer to MSC-22575, volume and number of this NASA Tech Briefs issue, and the page number.