Selecting a Photocell
The decay or fall time is defined as the time necessary for the light
conductance of the photocell to decay to 1/e (or about 73%) of its
illuminated state. At 1 fc of illumination the response times are typically
in the range of 5 msec to 100 msec.
The speed of response depends on a number of factors including light
level, light history, and ambient temperature. All material types show
faster speed at higher light levels and slower speed at lower light
levels. Storage in the dark will cause slower response than if the cells
are kept in the light. The longer the photocells are kept in the dark the
more pronounced this effect will be. In addition, photocells tend to
respond slower in colder temperatures.
Light History
All photoconductive cells exhibit a phenomenon known as hysteresis,
light memory, or light history effect. Simply stated, a photocell tends to
remember its most recent storage condition (light or dark) and its
instantaneous conductance is a function of its previous condition. The
magnitude of the light history effect depends upon the new light level,
and upon the time spent at each of these light levels. this effect is
reversible.
To understand the light history effect, it is often convenient to make an
analogy between the response of a photocell and that of a human eye.
Like the cell, the human eye’s sensitivity to light depends on what level
of light it was recently exposed to. Most people have had the
experience of coming in from the outdoors on a bright summer’s day
and being temporarily unable to see under normal room levels of
illumination. your eyes will adjust but a certain amount of time must
elapse first. how quickly one’s eyes adjust depends on how bright it
was outside and how long you remained outdoors.
The following guide shows the general relationship between light
history and light resistance at various light levels. The values shown
were determined by dividing the resistance of a given cell, following
infinite light history (R
LH
), by the resistance of the same cell following
“infinite” dark history (R
DH
). For practical purposes, 24 hours in the
dark will achieve R
DH
or 24 hours at approximately 30 fc will achieve
R
LH
.
Typical Variation of Resistance with Light History Expressed as a Ratio R
LH
/
R
DH
at Various Test Illumination Levels.
Illumination
R
LH
/ R
DH
Ratio
0.01 fc
1.55
0.1 fc
1.35
1.0 fc
1.20
10 fc
1.10
100 fc
1.10
This guide illustrates the fact that a photocell which has been stored for
a long time in the light will have a considerably higher light resistance
than if it was stored for a long time in the dark. Also, if a cell is stored
for a long period of time at a light level higher than the test level, it will
have a higher light resistance than if it was stored at a light level closer
to the test light level.
This effect can be minimized significantly by keeping the photocell
exposed to some constant low level of illumination (as opposed to
having it sit in the dark). This is the reason resistance specifications
are characterized after 16 hours light adept.
Environmental/Circuitry Considerations
Packaging
In order to be protected from potentially hostile environments
photocells are encapsulated in either glass/metal (hermetic) package
or are covered with a clear plastic coating. While the hermetic
packages provide the greatest degree of protection, a plastic coating
represents a lower cost approach.
The disadvantage of plastic coatings is that they are not an absolute
barrier to eventual penetration by moisture. This can have an adverse
effect on cell life. However, plastic coated photocells have been used
successfully for many years in such hostile environments as street light
controls.
Temperature Range
The chemistry of the photoconductive materials dictates an operating
and storage temperature range of –40°C to 75°C. It should be noted
that operation of the cell above 75°C does not usually lead to
catastrophic failure but the photoconductive surface may be damaged
leading to irreversible changes in sensitivity.
The amount of resistance change is a function of time as well as
temperature. While changes of several hundred percent will occur in a
matter of a few minutes at 150°C, it will take years at 50°C to produce
that much change.
Power Dissipation
During operation, a cell must remain within its maximum internal
temperature rating of 75°C. Any applied power will raise the cell’s
temperature above ambient and must be considered.
7
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
