Abstract: Method for detecting biological or chemical components in liquid or gas is based on measuring changes of the sensor layer thickness due to binding reactions. A plate or a gap with two surfaces of a solid optical material is used as the sensor layer. The surfaces are located at a distance of more than 10 ?m, which allows pumping liquids through the gap at moderate pressure drops and investigating large biological objects (e.g., cells), or employment of affordable plates that are rigid enough without any substrate. The indicated thickness of the plate or the gap permits using of ultra bright superluminescent diodes as light sources, because it allows recording within their narrow spectrum a sufficient number of interference maxima and minima for precise registration of molecular binding reactions, which lead to much higher sensitivity of the method as compared with thin-film methods.