A Fast Double Shutter for CCD-Based Metrology

Abstract

Image based metrology such as Particle Image Velocimetry (PIY) depends on the comparison of two or more images of an object taken in fast succession. Cameras designed for these applications provide the so-called 'dou­ ble shutter' mode: One frame is captured with a short exposure time and in direct succession a second frame with a long exposure time can be recorded. The difference in the exposure times is typically no problem since illum i­ nation is provided by a pulsed light source such as a laser and the measurements are performed in a darkened environment to prevent ambient light from accumulating in the long second exposure time. However, measure­ ments of self-luminous processes (e.g. plasma, combustion „.) as weil as experiments in ambient light are diffi­ cult to perform and req uire special equipment (external shutters, high-speed image sensors, multi-camera sys­ tems „.). Unfortunately , all these methods incorporate different drawbacks such as reduced resolution , degraded image quality, decreased light sensitivity or increased susceptibility to decalibration. In the solution presented here, off-the-shelf CCD sensors are used with a special timing to combi ne neighbouring pixels in a binning- 1ike way. As a result, two frames of short exposure time (-10 µs each) can be captured in fast succession (- 200 ns interframe time). They are stored in the on-chip vertical register in a line-interleaved pat­ tern, read out in the common way and separated again by software. The two resultant frames are completely congruent; they expose no insensitive l ines or line shi fts (interlace artefacts) and thus enable sub-pixel accurate measurements. A third frame can be captured at the füll resolution analogue to the double shutter tech niq ue. Image based measurement techniques such as Particle Image Yelocimetry (PJV) can benefit from th is mode when applied in bright environments. The third frame is useful e.g. for acceleration measurements or for particle tracking applicati ons.