A field deployable optical assembly for use in testing a light-responsive sample is disclosed. The assembly includes a microfluidic device, a first optical package, and a second optical package. The first optical package includes a light emitting diode (LED), a first optical device, and a first light-path control, the first optical package configured to guide and focus light from the LED onto the sample. The microfluidic device includes a tethered control substance. In response to a substance within the sample being associated with, and attaching to, the tethered control, the sample emits light. The second optical package includes a photo sensor, a second optical device, and a second light-path control, the second optical package configured to guide and focus the light emitted from the sample onto the photo sensor.