The DITABIS micron is an Imaging Plate readout system dedicated for TEM use. Unlike other applications where Imaging Plates are used, the TEM requires small pixels, highest image quality and a high dynamic range.
The instrument employs a drum scanner layout. The Imaging Plates are fixed to the surface of a rotating drum and read out by a laser beam that slowly moves alongside the drum and scans the whole plates in a continuos line. Up to 5 plates are scanned simultaneously along one slot around the drum surface. (see figure top)
The released luminescence signal from the plates is collected with special high aperture mirror optics and detected with a Photo Multiplier Tube (PMT). The signal is amplified and converted to digital values with an analog to digital converter (ADC). The data is sent to a hard disk within the reader that collects the complete data of one scan. After that the image data is transmitted to the computer system and displayed on the monitor (click figure beside).
The collected luminescence signals are converted to digital signals of 16 bits. This allows for 65000 shades of grey which is quite enough for normal TEM images. Optionally a second data channel can be used to expand the dynamic range to 1 million shades of grey.
The instrument is built to meet the demanding needs of TEM imaging:
- The Laser is highly focused to a spot diameter of about 5µm. The pixel size is 15µm to 50µm (Variable Pixel Size). The high focus reduces the cross-talk between pixels to get maximum sharpness and a good MTF (Modulation Transfer Function, up to 38% at Nyquist for 25.0µm pixel).
- The collecting mirror has a high aperture so it is collecting a great portion of the light emitted. In combination with the sensitive Photo Multiplier detector an enormous sensitivity and DQE (Detection Quantum Efficiency, about 80%) is reached.
- The instrument has two synchronous data channels of 16 bit. For higher dynamics the second channel converts an amplified version of the PMT signal. So two images are obtained one with high and one with lower gain (factor is about 32). The two images can be numerically combined to form an image with at least 20 bit data dynamic range (1:1 Million).