A Liquid Crystal Multifilter System for Diascopic Microscopy

An electrically addressable, transmissive liquid crystal display device is placed in the diascopic illumination path of a microscope. The display emulates the disks, orifices, and color filters used in well-known contrast-enhancing darkfield, brightfield, Rheinberg, and oblique illuminations. Each illumination modality is selectable, sizable, and locatable by software in a computing device that is connected to the display. Tedious construction and placement of pre-cut disks, orifices, and filters are no longer needed. Light leaving the liquid crystal display is polarized so with the addition of an analyzer a subject may be viewed in polarized light. Introduction Various subjects such as protozoa, plant and animal cells, and the like are nearly transparent when viewed with ordinary brightfield diascopic illumination. Light is only weakly absorbed in these nearly transparent subjects so that they are not differentiated from the background light that illuminates them. In the past, various image contrast-enhancing techniques have been used to render these subjects visible and more easily studied. Dyes such as hematoxylin and eosin have been used to respectively stain the nuclei and cytoplasm of cells for improved visualization. A number of illuminationand light-processing techniques are also used to improve visualization with or without staining of subjects. These techniques include darkfield, Rheinberg, oblique, monochrome, polarized, phase-contrast, differential-interference-contrast, and intensity-modulationcontrast illumination. Each of these techniques requires its own add-on microscope accessory, and many accessories must be adjusted to match the numerical aperture of the objective lens being used. Because of these encumbrances, microscopists often pass up an opportunity to quickly view a subject in a variety of perspectives. This article describes the DiablocTM Optic Multifilter (DOM), a digital illumination filter for diascopic microscopy. It switches between brightfield, darkfield, Rheinberg, oblique, and monochrome illuminations by the simple rotation of a knob. Adjustments for different colors and different numerical apertures of objective lenses in darkfield and Rheinberg illuminations are also done by rotating a knob. With our apparatus, a microscopist can quickly move among these contrastenhancing methods and see what they would otherwise miss. The following paragraphs describe the physical setup for the diascopic contrast-enhancing methods encompassed in the DOM. Types of Illumination Brightfield illumination. Brightfield illumination is the simplest technique used in microscopy. A subject is illuminated from one side, and light transmitted through the subject is viewed from the opposite side. Although this technique is widely used, it has limitations in the viewing of transparent or very translucent subjects because there is little contrast between the subject and the background light. Darkfield illumination. Darkfield illumination enhances contrast by obliquely illuminating a subject. A central light block is inserted between a microscope’s light source and condenser so that only light that is reflected or refracted in a subject reaches the microscope’s objective lens. There is no direct light path from the microscope’s light source, through or around a subject, so the subject is revealed in a bright image on a dark background. In the past, numerous devices have been used for darkfield illumination. These range from a simple “spider light stop,” an opaque disk that is positioned at the light entrance of a condenser, to specialized devices such as a cardioid darkfield condenser [1]. The latter has a number of internal components and is more expensive than the simple opaque disc light stop. In order to optimize a darkfield opaque disk setup, the diameter of the disk must be tailored to the numeric aperture of the microscope’s objective lens. If the disk is too small, stray light will enter the microscope’s objective lens and reduce the contrast of the resultant image. If the disk is too large, the amount of light available for viewing an image is less than optimum. In the past, disks of varying sizes were used. When an objective lens was changed, it was necessary to change disks [1]. Rheinberg illumination. Rheinberg illumination is an extension of darkfield illumination. An optical filter material of a first color replaces the central light block of a darkfield opaque disk, and another optical filter of a second color covers the outer light path that obliquely illuminates a subject [2]. The filters are located in the same position as a darkfield light block. In this technique, the first color of the central light path forms the background of an image, while the second color of the outer light path illuminates the subject. In the past, the filters were laboriously cut from gelatin color filter materials. As with conventional darkfield, for optimum results the diameter of the central filter must be tailored to the numerical aperture of the objective lens. Figure 1: Top, perspective, and side views of the DOM. The Multifilter is 15 × 7.5 × 2.8 cm in length, width, and height.