Gated Optical Imager Tutorial

Gated Optical Imager Tutorial 

Kentech has built these systems for many years. Although our standard system uses only 18mm Generation 2 wafer tubes, larger tubes may be used with some loss of temporal resolution.

Gating Technology
To achieve fast gating, a gate signal is capacitively coupled to the cathode. Conventional gating schemes give rise to irising problems caused by the RC time constant of the cathode to MCP capacitance and the cathode resistivity.

This RC time constant gives rise to a diffusion of charge towards the tube axis and the diffusion rate cannot be increased by external drive circuitry, consequently the tube gate time achievable with conventional gating is limited. This limitation is avoided with the Kentech gating scheme. A highly conducting mesh is placed in front of the input window of the tube and this is pulsed to a high voltage. Most of the voltage appears between the mesh and the cathode but sufficient also appears between the cathode and the MCP input surface to gate the tube quickly.

More recently we have employed gating rings rather than meshes. These require even more voltage to drive them but are useful when imaging coherent light that may diffract badly through a mesh.

Output Recording
The output image from a GOI system may be recorded either on film or with some kind of image grabbing system. CCD cameras and computer controlled framestores make an ideal solution. Kentech has supplied both 4 frame and 8 frame systems with CCD readouts on each channel. Each channel may be completely separate.

When considering the input to an array of these devices one may choose either wavefront or intensity division. Wavefront division will effectively produce a picture from a different angle of view onto each channel. Often this may be satisfactory but some events are significantly anisotropic and intensity division of the incident light may be necessary. In such a case there must be sufficient light to produce an image on each channel. Kentech is happy to advise on such schemes.

Short gates
Ultra short gates can be achieved by using smaller image tubes. Indeed some of these tube have a higher spatial resolution than the conventional 18mm tubes and it is not clear that the number of resolvable elements actually falls. 50ps gate width when deconvolved from the test equipment response has been observed in 12mm tubes.

Larger Apertures
With larger aperture tubes the capacitive gating scheme also works but with reduced temporal response. around 200ps is achievable with a 40mm tube.

With such short gates one can do many things otherwise not possible.
Much work has recently been done of fluorescence lifetime imaging both for tissue and for other chemical systems.

Lidar is also a possibility. imaging through scattering media, partiucularly fog or possibly underwater if the scattering is not too serious can be improved greatly by the added contrast that time range gating can bring to the imaging problem. In addition one may image through self luminous media. e.g. fires, as the light source can have a much greter peak brightness than the medium and the gated detector can use this peak brightness effectively.