Methods for discriminating signal from noise

Optical density is a measure of the transmittance of a material. It is given by OD = logj (l/T), where T is the transmission of a component expressed as a fraction. Thus for a component with 1% transmission efficiency T = 0.01 and OD = 2. 

Furthermore, thin-film interference filters often suffer from a polarization sensitivity, which can produce unwanted selectivity in applications where polarization sensitivity is desired (see Section 3.4). Fortunately, a number of suppliers (Chroma Technology Corp., VT, USA for example) are now becoming familiar with the requirements of filters for single molecule applications and are constantly developing the technology. Compared to other types of filters the interference type cannot be beaten on cost good quality filters suitable for single molecule work are available for around US 150. 

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