2.4. The Detector's Dynamic Range
A detector generally meets the linearity condition for only a limited range of the input signal level. There are two effects that define the boundaries of this dynamic range:
- At very low levels of the input signal, the detector’s output is largely dominated by noise. Noise is a random temporal fluctuation of the output signal that occurs even when the input signal is constant. The absolute level and the frequency distribution of these variations depend on the physical properties of the detector and the subsequent electronics. For many detectors, noise is largely independent from the absolute level of the input signal and can be neglected for input signals above a certain minimum level. However, for very low input signals, the output signal is dominated by noise and no longer quantifies the physical quantity to be determined. The lower limit of the measurement range, which is posed by noise, is quantified by the noise equivalent input. The CIE defines the noise equivalent input as the value of the respective physical quantity (radiant power or luminous flux, irradiance or illuminance, …) that produces an output signal equal to the root mean square noise output. Since the shape of the noise signal depends on the temporal resolution that can be achieved of the recording electronics (often characterized by the electronics’ time constant), the noise equivalent input is defined for a specific frequency and bandwidth. Unless otherwise stated, a 1 Hz bandwidth is usually considered. Depending on the detector’s characteristics, its noise level can be reduced by longer detector integration times or by averaging subsequent measurements of the same input signal.
- At high levels of the input signal, the detector’s output signal no longer increases proportional to its input signal, and the detector therefore does not meet the linearity condition. Instead, physical limits of the light sensitive element and / or the electronics cause saturation of the output signal. This saturation increases disproportionately in relation to the input signal before reaching a constant level. To a certain extent, subsequent correction of the detector’s output signal can account for the effects of saturation and thus extend the detector’s dynamic range. This correction has to be based on a thorough laboratory investigation of the detector’s dynamic behavior and at the same time cause higher measurement uncertainties if the input signal levels are high.
The detector’s dynamic range depends on the type of the photodiode. The dynamic range of the overall measurement system depends on both the detector and electronic meter’s range capabilities. For example, a typical silicon photodiode can measure over 2 mA of current before saturating. However, the upper current measurement range of the meter may be limited to 200 μA.
This range covers extremely low intensity levels, for instance the quantification of erythemally active UV radiation, or very high intensity levels that are used for industrial UV curing processes.