VTL5C2SLASH2 datasheet(10/76 Pages) PERKINELMER

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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.

When selecting a photocell the design engineer must ask two basic

questions:

What kind of performance is required from the cell?

What kind of environment must the cell work in?

Performance Criteria

Sensitivity

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.

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.

Over the years PerkinElmer has developed different “types” of

photoconductive materials through modifications made to the chemical

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.

RH = ρH (w / l )

where:

RH = resistance of cell at light

level H

sheet

resistivity

photoconductive film at light level

w = width of electrode gap

l = length of electrode gap

Sheet

sensitivity

(

for

photoconductive films at 2 fc are in the range of 20 M

Ω per square.

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.

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.

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