CCD Processing Engine
Linear CCD's are widely used in mass market applications such as bar code readers and scanners but they also make excellent detectors for spectroscopy. The linear CCD processing engine shown in the image below is a USB-enabled Propeller board that generates clocking signals for a CCD chip and also incorporates a correlated doubling sampling (CDS) processor chip (an Analog Devices AD9826) for video sampling (see below for a description of CDS). The connector on the left edge of the PCB brings out digital and analog signals from/to a daughter board on which the detector chip is mounted.
What is Correlated Double Sampling ?
During readout of an image from a CCD chip, pixel data must be moved into, and clocked out from a shift register to a readout amplifier. Before each new pixel is read out, a reset pulse is used to clear the previous pixel data. In order to accurately measure light intensities, it is usual practice to sample the readout amplifier both at “reset” and again at “video” levels and take the difference between these levels as the true measured pixel value. This process is known as correlated double sampling (CDS).
The Analog Devices AD9826 is a complete analog signal processor for imaging applications. It features a 3-channel architecture designed to sample and condition the outputs of tri-linear color CCD arrays. Each channel consists of an input clamp, Correlated Double Sampler (CDS), offset DAC, and Programmable Gain Amplifier (PGA), multiplexed to a high-performance 16-bit A/D converter. The AD9826 can operate at speeds greater than 15 MSPS with reduced performance. Single channel operation is also possible and this mode of operation is used here.
The AD9826’s 16-bit digital output is multiplexed into two 8-bit output bytes, that are accessed using two read cycles. There is an optional single byte output mode. The internal registers of the AD9826 are programmed through a 3-wire serial interface; this allows setting the gain, offset, and operating mode. The AD9826 operates from a single 5 V power supply, typically consumes 400 mW of power, and is packaged in a 28-lead SSOP.
CCD Spectrometer LabVIEWTM Front Panel
At right is the LabVIEWTM front panel that controls the CCD processing engine described above. The user can set the AD9826 gain and offset, the number of clears of the serial register prior to data acquisition, the exposure time and the number of spectral scans to average.
The vi also loads in 3 wavelength calibration parameters that relate pixel number on the CCD to the actual wavelength in nm, via a quadratic polynomial. A description of the wavelength calibration procedure can be found here.
The spectrum of a compact fluorescent lamp can be seen in the graph window of this panel. This spectrum reveals a number of mercury emission lines as well as some rare earth emission lines that are useful for wavelength calibration purposes.
XMOS ILX511 Image Sensor Shield
An XMOS Startkit shield that is compatible with the ILX511 CCD sensor has also been developed, again using a LabVIEWTM vi to operate the instrumentation. Communication with the XMOS Startkit board is via a 3.3V TTL UART cable (FTDI, part # TTL-232RG-VSW3V3-WE) that connects to the Startkit using two I/O pins (Rx and Tx) and operates at 921600 baud.
ELIS-1024 CMOS Image Sensor
The ELIS-1024 is a 1024 pixel linear image sensor with low dark current photodiode pixels (pixel format is 7.8 x 125 um). Unlike a CCD, this detector can be read out non-destructively and the device also has a shutter control that can be used to obtain exposure times less than a microsecond.
The quantum efficiency is flat (to within about +- 10%) at ~ 0.85 from 340 nm to 820 nm, falling to 0.4 @ 300 nm. The data sheet claims the sensor is “sensitive to 200 nm”.
At left is a small sensor PCB for the ELIS-1024 that connects to an XMOS shield developed to evaluate this sensor in a small transmission grating spectrometer.
ELIS-1024 XMOS Starkit Shield
The ELIS-1024 sensor PCB plugs into this shield via the 8-pin IDC header at bottom left. On-board the shield are an AD623 instrumentation amplifier that performs level shifting to remove the ELIS-1024 pedestal of ~2V and also provides some gain to fully use the input range of the LTC1865 analog-to-digital converter.
CWFL Spectrum Using ELIS-1024 Sensor
The spectrum opposite was taken by coupling light from a cool white fluorescent lamp (CWFL) via an optical fibre into a transmission grating spectrometer fitted with a 600 line/mm grating (shown in the photo below).
For a discussion of how this spectrum was calibrated see here.
600 line per mm Transmission Grating Spectrometer
A breadboard transmission grating spectrometer assembled using components from ThorLabs. Light enters via a fiber at the left hand side (not shown). The input beam is collimated by a short focal length lens and then passes through the transmission grating. The diffracted first order beam is focussed by a second lens onto the ELIS-1024 detector which is mounted on a cage plate. By moving the cage plate on the four rods one can focus the light onto the image sensor so as to obtain the sharpest spectral peaks.