Page 154 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 147

0.20

0.15
Voltage (V) 0.10 1M

Gyrator

0.05

0.00
-0.015 -0.01 -0.005 0 0.005 0.01 0.015
Time (s)
Figure 7.7 Response of the circuit of Fig. 7.6 to 50Hz fluorescent lighting and a
10kHz modulation. The 100Hz rectified sine wave of the upper response is almost
suppressed with the active feedback circuit shown in the lower curve.

has not been affected, but the magnitude of the interference has been reduced
by a factor 15. Lower frequency offsets due to sunlight are suppressed even
further.

7.2.6 Integration
Interference from line-frequency signals (50/60Hz) and harmonics is of course
not limited to optical measurements. All handheld multimeters suffer from
similar interference, and in their design a simple form of digital signal pro-
cessing is often applied. If the instrument’s analog-to-digital converter is con-
figured to integrate the input signal for a period of one (or more) whole cycles
of the interference (16.7 or 20ms), then disturbing signals with a zero mean
are averaged close to zero. Significant suppression also occurs at all harmonics
of the disturbing frequency (limited by the sampling frequency used). High-
resolution analog-to-digital converters are often pin-switchable for 50-Hz or
60-Hz integration. While this digital filtering is useful for electrical interference
at line frequencies, it does not suppress disturbing DC photocurrents. This can
be provided by different signal processing algorithms (App. C).

7.3 Wavelength Domain: Optical Filtering
The above electronic techniques can only be considered partial fixes, when the
real issue is to stop interfering signals arriving at the photodiode in the first
place. Differences in optical wavelength characteristics can be used for this.
When the source is a semiconductor LED, organic polymer LED, laser, or even
gas discharge lamp, its optical spectrum is likely to be quite different from that
of ambient light. Hence we should consider optical filters designed to maximize
throughput of the desired signal and suppress the ambient. There are lots of
options. Laser sources are the easiest to filter. Their outputs are usually well

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