Selecting a Photocell
Slope Characteristics
Plots of the resistance for the photocells listed in this catalog versus
light intensity result in a series of curves with characteristically different
slopes. This is an important characteristic of photocells because in
many applications not only is the absolute value of resistance at a
given light level of concern but also the value of the resistance as the
light source is varied. One way to specify this relationship is by the use
of parameter (gamma) which is defined as a straight line passing
through two specific points on the resistance curve. The two points
Likewise, for dual element photocells the matching factor, which is
defined as the ratio of the resistance of between elements, will
increase with decreasing light level.
used by PerkinElmer to define
fc).
γ
are 10 lux (0.93 fc) and 100 lux (9.3
Log Ra – Log Rb
γ = -------------------------------------
Log a – Lob b
Dual Element Photocell Typical Matching Ratios
0.01 fc
0.1 fc
1.0 fc
10 fc
100 fc
Log(Ra ⁄ Rb)
= ------------------------------
Log(b ⁄ a)
0.63 – 1.39 0.74 – 1.27 0.75 – 1.25 0.76 – 1.20 0.77 – 1.23
Dark Resistance
As the name implies, the dark resistance is the resistance of the cell
under zero illumination lighting conditions. In some applications this
can be very important since the dark resistance defines what
maximum “leakage current” can be expected when a given voltage is
applied across the cell. Too high a leakage current could lead to false
triggering in some applications.
Applications for photocells are of one of two categories: digital or
analog. For the digital or ON-OFF types of applications such as flame
detectors, cells with steep slopes to their resistance versus light
intensity curves are appropriate. For analog or measurement types of
applications such as exposure controls for cameras, cells with shallow
slopes might be better suited.
The dark resistance is often defined as the minimum resistance that
can be expected 5 seconds after the cell has been removed from a
light intensity of 2 fc. Typical values for dark resistance tend to be in the
500k ohm to 20M ohm range.
Temperature Coefficient of Resistance.
Resistance Tolerance
Each type of photoconductive material has its own resistance versus
temperature characteristic. Additionally, the temperature coefficients of
photoconductors are also dependent on the light level the cells are
operating at.
The sensitivity of a photocell is defined as its resistance at a specific
level of illumination. Since no two photocells are exactly alike,
sensitivity is stated as a typical resistance value plus an allowable
tolerance. Both the value of resistance and its tolerance are specified
for only one light level. For moderate excursions from this specified
light level the tolerance level remain more or less constant. However,
when the light level the tolerance level remain more or less constant.
However, when the light level is decades larger or smaller than the
reference level the tolerance can differ considerably.
From the curves of the various types of materials it is apparent that the
temperature coefficient is an inverse funstin of light level. Thus, in order
to minimize temperature problems it is desirable to have the cell
operating at the highest light level possible.
Speed of Response
As the light level decreases, the spread in the tolerance level
increases. For increasing light levels the resistance tolerance will
tighten.
Speed of response is a measure of the speed at which a photocell
responds to a change from light-to-dark or from dark-to-light. The rise
time is defined as the time necessary for the light conductance of the
photocell to reach 1-1/e (or about 63%) of its final value.
6
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
