A device and method for imaging fluorescently labeled molecules (e.g., nucleic acids) includes securing a modular attachment device to the mobile phone with a sample containing stretched, fluorescently labeled nucleic acid molecules and illuminating the sample with excitation light to cause the fluorescently labeled nucleic acid molecules to emit fluorescent light. Images of the nucleic acids are captured using a camera of the mobile phone. The images from the mobile phone are transferred to a remote computer for image processing and analysis. The images are processed by the remote computer to generate analysis data of sample, wherein the analysis data includes the length of nucleic acid molecules contained in the sample or the length of molecular sub-region(s). The mobile phone or another computing device receives from the remote computer the analysis data and displays at least some of the analysis data thereon.