Optical Path & Resolution
ISIIS plankton imagers are based on back-illumination photography. It is a shadowgraphic system that captures silhouettes of plankton. It uses a pseudo collimated beam of light ensuring all light rays are parallel throughout the imaging area. The result are projections/shadows of organisms that are telecentric: the size of an object is not dependent of its position within the field of view, there is no image magnification, objects are displayed in real size.
ISIIS imaging systems use industrial grade cameras. On towed sleds, they use a line-scan camera creating one single continuous image representing a real slice of the ocean. However they can also be fitted with a classic area-scan camera if a system is to be used still (underwater monitoring station) or do slow vertical profiles.
Folding the optical path with mirrors
With line-scan cameras, objects are scanned as they flow passed the camera imaging area
The image resolution is dependent upon the size of the imaging area (i.e the size of the optical lenses) and the camera resolution. For example, an image resulting from the use of a 13cm field-of-view optical lens and a 2048 pixels camera will have a pixel-resolution of approximately 63 microns: 13cm / 2048 pixels. In most cases, one needs at least 10 to 15 pixels to be able to identify an object or organism. Therefore an imaging system with a 63 microns pixel-resolution is good for organisms that are about 1 millimeter in size.
For an effective depth-of-field of 50cm and a field-of-view of 13cm, towing the ISIIS imager at towed at 5 knots results in a sampling rate of 167 liters of water per second: 2.57 m/sec x .13m x .50m = 167 liters/sec
When using a classic camera (area-scan), the pixel resolution of the images are still defined the same way. However the imaging system acquire images in time-lapse mode at regular intervals.
Examples of possible combinations for a line-scan system
Examples of possible combinations for an area-scan system
ISIIS systems image organisms/particles in-situ as they swim/flow in between its two pods (fitted with a camera and illumination system). The instrument captures images of mostly undisturbed organisms, at a fine spacial and temporal resolution.
Typical frames from our ISIIS imagers
*These frames are not original images: they are not necessarily to scale and data has been compressed
Regions of Interests or cleaned-up images showing different taxa
A montage of frames to quickly show a sped-up profile as seen by ISIIS (San Diego, CA) as it is going from shallow depth to 150m deep.