SUB-DIFFRACTION LIMIT IMAGE RESOLUTION AND OTHER IMAGING TECHNIQUES

The present invention generally relates to sub-diffraction limit image resolution and other imaging techniques. In one aspect, the invention is directed to determining and/or imaging light from two or more entities separated by a distance less than the diffraction limit of the incident light. For example, the entities may be separated by a distance of less than about 1000 nm, or less than about 300 nm for visible light. In one set of embodiments, the entities may be selectively activatable, i.e., one entity can be activated to produce light, without activating other entities. A first entity may be activated and determined (e.g., by determining light emitted by the entity), then a second entity may be activated and determined. The entities may be immobilized relative to each other and/or to a common entity. The emitted light may be used to determine the positions of the first and second entities, for example, using Gaussian fitting or other mathematical techniques, and in some cases, with sub-diffraction limit resolution. The methods may thus be used, for example, to determine the locations of two or more entities immobilized relative to a common entity, for example, a surface, or a biological entity such as DNA, a protein, a cell, a tissue, etc. The entities may also be determined with respect to time, for example, to determine a time-varying reaction. Other aspects of the invention relate to systems for sub-diffraction limit image resolution, computer programs and techniques for sub-diffraction limit image resolution, methods for promoting sub-diffraction limit image resolution, methods for producing photoswitchable entities, and the like.