A biological analysis system is provided. The system comprises an interchangeable assembly configured to accommodate any one of a plurality of sample holders, each respective sample holder configured to receive a plurality of samples. The system also includes a control system configured to cycle the plurality of samples through a series of temperatures. The system further includes an optical system configured to detect fluorescent signals emitted from the plurality of samples. The optical system, in particular, can comprise a single field lens, an excitation source, an optical sensor, and a plurality of filter components. The excitation source can be one or more light emitting diodes. The field lends can be a bi-convex lens.