Selecting a Photocell
Specifying the best photoconductive cell for your application requires
an understanding of its principles of operation. This section reviews
some fundamentals of photocell technology to help you get the best
blend of parameters for your application.
composition of the detector. For a given type of photoconductor
material, at a given level of illumination, the photoconductive film will;
have a certain sheet resistivity. The resistance of the photocell at this
light level is determined by the electrode geometry.
R =
ρ (w / l )
H
H
When selecting a photocell the design engineer must ask two basic
questions:
where:
1. What kind of performance is required from the cell?
2. What kind of environment must the cell work in?
R = resistance of cell at light
level H
H
ρ
=
sheet resistivity of
H
Performance Criteria
photoconductive film at light level
H
Sensitivity
w = width of electrode gap
l = length of electrode gap
The sensitivity of a photodetector is the relationship between the light
falling on the device and the resulting output signal. In the case of a
photocell, one is dealing with the relationship between the incident light
and the corresponding resistance of the cell.
Sheet sensitivity
(ρ
)
for
H
photoconductive films at 2 fc are in the range of 20 M per square.
Ω
The ratio w / l can be varied over a wide range in order to achieve
design goals. Typical values for w / l run from 0.002 to 0.5, providing
flexibility for terminal resistance and maximum cell voltage.
Spectral Response
Like the human eye, the relative sensitivity of a photoconductive cell is
dependent on the wavelength (color) of the incident light. Each
photoconductor material type has its own unique spectral response
curve or plot of the relative response of the photocell versus
wavelength of light.
Defining the sensitivity required for a specific application can prove to
be one of the more difficult aspects in specifying a photoconductor. In
order to specify the sensitivity one must, to some degree, characterize
the light source in terms of its intensity and its spectral content.
Within this handbook you will find curves of resistance versus light
intensity or illumination for many of PerkinElmer’s stock photocells. The
illumination is expressed in units of fc (foot candles) and lux. The light
source is an incandescent lamp. This lamp is special only in that the
spectral composition of the light it generates matches that of a black
body at a color temperature of 2850 K. This type of light source is an
industry agreed to standard.
The spectral response curves for PerkinElmer’s material types are
given in the handbook and should be considered in selecting a
photocell for a particular application.
Over the years PerkinElmer has developed different “types” of
photoconductive materials through modifications made to the chemical
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