VT33CT datasheet(34/76 Pages) PERKINELMER

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What Are Analog Optical Isolators?

Light History Considerations

Photoconductive cells exhibit a phenomenon knows as hysteresis, light

memory, or light history effect. Special consideration must be given to

this characteristic in the analog optoisolator because the

photoconductive element is normally in the dark. This will lead to

having the photocell initially in a “dark adapted” state in many

conditions.

The light levels that are seen by the photocell in many analog

optoisolator applications are quite low, ranging from 0.1 to 1.0 fc. The

effect of this combination of dark adapt and low light levels will be seen

in the following table.

The table shows the relationship between light history and light

resistance at various light levels for different material types. The values

shown were determined by dividing the resistance of a given cell,

following “infinite” light history (RLH), by the resistance of the same cell

following infinite dark history (RDH). For practical purposes, 24 hours in

the dark will achieve RDH or 24 at approximately 30 fc will achieve RLH.

Variation of Resistance with Light History Expressed as a

Ratio RLH/RDH at Various Test Illumination Levels

The table illustrates the fact that the resistance of a photocell can

increase substantially as it transitions from dark adapted state to a light

adapted state. The table shows that the Type 1 photocell can increase

resistance by a factor of more than three times as it light adapts up to

0.1 fc. In some applications, this can be an important consideration. In

general, the magnitude of this effect is larger for types 1, 4, and 7 than

for types Ø, 2, and 3.

Each specific material type represents a tradeoff between several

characteristics. Selecting the best material is a process of determining

what characteristics are most important in the application. The chart

gives some appreciation for the general interrelationships between the

material types and their properties.

Material

Type

Illumination (fc)

0.01

0.1

1.0

100

Type Ø

1.60

1.40

1.20

1.10

Type 1

5.50

3.10

1.50

1.10

1.05

Type 2

1.50

1.30

1.20

1.10

Type 3

1.50

1.30

1.20

1.10

Type 4

4.50

3.00

1.70

1.10

Type 7

1.87

1.50

1.25

1.15

1.08

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