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VTL5C1 Datasheet(PDF) 34 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 34 Page - PerkinElmer Optoelectronics |
34 / 76 page ![]() 29 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 10 100 Type Ø 1.60 1.40 1.20 1.10 1.10 Type 1 5.50 3.10 1.50 1.10 1.05 Type 2 1.50 1.30 1.20 1.10 1.10 Type 3 1.50 1.30 1.20 1.10 1.10 Type 4 4.50 3.00 1.70 1.10 1.10 Type 7 1.87 1.50 1.25 1.15 1.08 |
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