The backscatter feature of the Aurora 3000 is achieved through the use of a backscatter shutter mounted on the light source housing inside the cell. This shutter moves periodically (approximately every second) between two locations in and out of the optical path of the light source. The position of the shutter is controlled by a servomotor with position feedback, a servo controller (mounted on the outside of the light source) and the microprocessor. An electronic spring is used to reset the shutter to its zero position at power up or in the event of an error. The shutter rotates between two pre programmed positions relative to its zero position. .
During factory calibration, the backscatter shutter is programmed to move to a position where the resulting Lambertian distribution of the light source is limited to 90° to 180°. The position of the backscatter shutter is controlled to within ±0.09°, but can be adjusted to achieve other cutoff angles if required.
Another important factor in relation to the position of the backscatter shutter is its distance from the surface of the opal glass diffuser. This is shown to the right and has been measured to be approximately 13mm. This distance will govern the rise angle of the light source ie. the angle from where the intensity goes from 90% Lambertian down to 10% at zero intensity. This has been measured to be 26° as shown below