| The next part of the photocontrol is the photosensor itself. At the present
time, there are two commercially viable ways to detect visible light for
outdoor lighting applications: CdS cells and silicon junction devices.
Both have their advantages and disadvantages. Currently CdS is used in
over 50% of controls sold to utilities. The appropriate choice of sensors
is a topic of considerable technical discussion within the industry. To
understand this discussion, consider the following graph showing the relative
spectral response of both CdS and Silicon. 
CdS / Cadmium Sulfide Sensors Advantages of CdS Cells are:
- Spectral response that closely approximates that of the human eye. It
sees light the way people do.
- CdS can be used with conventional controls or electronic controls.
- CdS cells were previously available with a true hermetic seal in a glass
and metal housing. Currently, most are plastic coated. Disadvantages of CdS Controls are:
- If overheated (above 70°C (158°F)), CdS cells start to drift.
Drift means that the cell suffers an irreversible change toward lower
sensitivity and higher control turn-ON and OFF. This overheating can come
from three places:
1) Self heating due to power dissipation in the cell in conventional controls
2) Heat from the fixture
3) Sun loading.
-Unsealed CdS cells in conventional controls can also deteriorate rapidly
in areas of high humidity, salt spray or acidic air pollution. This deterioration
is first seen as drift toward longer burning hours caused by earlier control
turn ON and later turn OFF.
-Possible disposal issue because of perceived cadmium hazards.
-Guilt by association. The cheapest, shortest life controls all have CdS
cells. Therefore it is tempting, but incorrect, to say that all CdS controls
drift. Silicon Sensors Advantages of Silicon Sensors are:
- No long term drift even under extreme conditions. If controls are to
be installed on high wattage (400 or 1000 watt) floodlights that run hot
(over 90°C), your engineering spec should require controls with silicon
sensors.
- Small size Disadvantages of Silicon Sensors are:
- Silicon is very sensitive to infra-red and red with little sensitivity
to the blue and green portion of the light spectrum. This causes day-to-day
wandering of turn ON and turn OFF corresponding to variations in the red
content of light at both sunset and sunrise. A silicon control that turns
ON at 1 footcandles (10.8 lux) on a clear night may turn ON at 3 or 4
footcandles (32.3-43.0 lux) the next night as the weather changes to cloudy.
The same is true for turn OFF.
- Electronic circuit is required. A conventional control cannot be made
with a silicon sensor. Filtered silicon sensors For years it has been known that, if a glass infrared blocking optical
filter is placed inside the control in front of a silicon sensor it will
eliminate sensitivity to infra-red. The result is a control that has the
advantages of both CdS and silicon with none of the disadvantages, except
cost. Glass filters are expensive. However, recent advances in polymer technology have made filtered silicon
a real possibility. Several utilities have added filtered silicon to their
requirements. It adds slightly to initial cost, but it will pays back
by eliminating switch point drift and improving real and apparent turn
ON uniformity. Long and short term stability is excellent. DTL makes controls with CdS cells (D series), unfiltered silicon sensors
(DP series) and filtered silicon sensors (DE series). | 
