A portable imaging apparatus and system. A sample processing device has a reaction chamber configured to receive a sample and react the sample with a fluorescent compound that emits a specified wavelength of light when excited by light within an excitation band. An illumination source can be positioned to illuminate the reaction chamber when the sample processing device is positioned for imaging. A light guide can be positioned to face the reaction chamber and transfer the emitted light to an imaging detector. The light guide has a filter that blocks the wavelength of light from the illumination source and passes the fluorescent emitted light. The light guide unit defines an angular light acceptance range and is configured to constrain angular spreading of light.