In-situ imaging of plankton taxa is a helpful tool to identify the occurance of an individual in relation to the environmental parameters in its vicinity. These data contribute to the understanding of small scale effects in addition to results from net samples, which are biased by method-borne integration. To image small individuals with high magnifications and short shutter times in moving water masses the available light is a limiting factor. Here we demonstrate a new system developed for in-situ dark-field imaging of particles on small scales in optically defined volumes.With a combination of different cylindrical lenses and a linear light source a beam of high flux is projected into the water. The beam is precisely constrained in one dimension and creates a light frame along the principal axis. Particles within this frame scatter light by internal or surface refraction, recorded by a camera that aims orthogonally at this beam. With a telecentric objective the sampled volume is quantified by width and height of the image and the extension of the constrained light frame. It allows to obtain detailed information on the structure of the particles, while objects out of the lightframe are nearly invisible. With this illumination sensitive cameras allow shutter times below 1 ms to image objects on scales of 10 µm per pixel and provide images of high quality sufficient for taxonomic identification. Here we show first in-situ results of an improved video-plankton-recorder concept, based on the new developed Lightframe On-sight Keyspecies Investigation (LOKI) method.