A modular testing device comprising: a base unit; and an expansion unit that communicates with the base unit, wherein the expansion unit comprises: a housing; a receptacle in the housing configured to receive a tube array containing biological sample and reagent mixtures; and an optical assembly positioned in the housing and adapted to receive the tube array in a portion housing (174: 220, 222), wherein the optical assembly is configured to amplify the biological sample using heat, to optically excite the biological sample and reagent mixtures and to detect a signal from the biological sample and reagent mixtures in the tube array, wherein data that is collected in the optical assembly is communicated to the base unit, characterized in that the optical assembly of the expansion unit includes an excitation filter (248, 278) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array and an emission filter (244, 274) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array, wherein the optical assembly is configured to emit radiation through the excitation filter (248, 278) into each biological sample and reagent mixture in the tube array, and wherein the optical assembly is further configured to receive emissions from each biological sample and reagent mixture in the tube array through the emission filter (244, 274). a base unit; and an expansion unit that communicates with the base unit, wherein the expansion unit comprises: a housing; a receptacle in the housing configured to receive a tube array containing biological sample and reagent mixtures; and an optical assembly positioned in the housing and adapted to receive the tube array in a portion housing (174: 220, 222), wherein the optical assembly is configured to amplify the biological sample using heat, to optically excite the biological sample and reagent mixtures and to detect a signal from the biological sample and reagent mixtures in the tube array, wherein data that is collected in the optical assembly is communicated to the base unit, characterized in that the optical assembly of the expansion unit includes an excitation filter (248, 278) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array and an emission filter (244, 274) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array, wherein the optical assembly is configured to emit radiation through the excitation filter (248, 278) into each biological sample and reagent mixture in the tube array, and wherein the optical assembly is further configured to receive emissions from each biological sample and reagent mixture in the tube array through the emission filter (244, 274). a housing; a receptacle in the housing configured to receive a tube array containing biological sample and reagent mixtures; and an optical assembly positioned in the housing and adapted to receive the tube array in a portion housing (174: 220, 222), wherein the optical assembly is configured to amplify the biological sample using heat, to optically excite the biological sample and reagent mixtures and to detect a signal from the biological sample and reagent mixtures in the tube array, wherein data that is collected in the optical assembly is communicated to the base unit, characterized in that the optical assembly of the expansion unit includes an excitation filter (248, 278) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array and an emission filter (244, 274) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array, wherein the optical assembly is configured to emit radiation through the excitation filter (248, 278) into each biological sample and reagent mixture in the tube array, and wherein the optical assembly is further configured to receive emissions from each biological sample and reagent mixture in the tube array through the emission filter (244, 274). the optical assembly is configured to amplify the biological sample using heat, to optically excite the biological sample and reagent mixtures and to detect a signal from the biological sample and reagent mixtures in the tube array, wherein data that is collected in the optical assembly is communicated to the base unit, characterized in that the optical assembly of the expansion unit includes an excitation filter (248, 278) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array and an emission filter (244, 274) positioned adjacent the portion housing (174: 220, 222) receiving the tube array that extends across the entire length of the tube array, wherein the optical assembly is configured to emit radiation through the excitation filter (248, 278) into each biological sample and reagent mixture in the tube array, and wherein the optical assembly is further configured to receive emissions from each biological sample and reagent mixture in the tube array through the emission filter (244, 274).