![]() |
Electronic Components Datasheet Search |
|
VTL5C1 Datasheet(PDF) 12 Page - PerkinElmer Optoelectronics |
|
|
VTL5C1 Datasheet(HTML) 12 Page - PerkinElmer Optoelectronics |
12 / 76 page ![]() 7 Selecting a Photocell The decay or fall time is defined as the time necessary for the light conductance of the photocell to decay to 1/e (or about 73%) of its illuminated state. At 1 fc of illumination the response times are typically in the range of 5 msec to 100 msec. The speed of response depends on a number of factors including light level, light history, and ambient temperature. All material types show faster speed at higher light levels and slower speed at lower light levels. Storage in the dark will cause slower response than if the cells are kept in the light. The longer the photocells are kept in the dark the more pronounced this effect will be. In addition, photocells tend to respond slower in colder temperatures. Light History All photoconductive cells exhibit a phenomenon known as hysteresis, light memory, or light history effect. Simply stated, a photocell tends to remember its most recent storage condition (light or dark) and its instantaneous conductance is a function of its previous condition. The magnitude of the light history effect depends upon the new light level, and upon the time spent at each of these light levels. this effect is reversible. To understand the light history effect, it is often convenient to make an analogy between the response of a photocell and that of a human eye. Like the cell, the human eye’s sensitivity to light depends on what level of light it was recently exposed to. Most people have had the experience of coming in from the outdoors on a bright summer’s day and being temporarily unable to see under normal room levels of illumination. your eyes will adjust but a certain amount of time must elapse first. how quickly one’s eyes adjust depends on how bright it was outside and how long you remained outdoors. The following guide shows the general relationship between light history and light resistance at various light levels. 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 hours at approximately 30 fc will achieve RLH. Typical Variation of Resistance with Light History Expressed as a Ratio RLH / RDH at Various Test Illumination Levels. This guide illustrates the fact that a photocell which has been stored for a long time in the light will have a considerably higher light resistance than if it was stored for a long time in the dark. Also, if a cell is stored for a long period of time at a light level higher than the test level, it will have a higher light resistance than if it was stored at a light level closer to the test light level. This effect can be minimized significantly by keeping the photocell exposed to some constant low level of illumination (as opposed to having it sit in the dark). This is the reason resistance specifications are characterized after 16 hours light adept. Environmental/Circuitry Considerations Packaging In order to be protected from potentially hostile environments photocells are encapsulated in either glass/metal (hermetic) package or are covered with a clear plastic coating. While the hermetic packages provide the greatest degree of protection, a plastic coating represents a lower cost approach. The disadvantage of plastic coatings is that they are not an absolute barrier to eventual penetration by moisture. This can have an adverse effect on cell life. However, plastic coated photocells have been used successfully for many years in such hostile environments as street light controls. Temperature Range The chemistry of the photoconductive materials dictates an operating and storage temperature range of –40°C to 75°C. It should be noted that operation of the cell above 75°C does not usually lead to catastrophic failure but the photoconductive surface may be damaged leading to irreversible changes in sensitivity. The amount of resistance change is a function of time as well as temperature. While changes of several hundred percent will occur in a matter of a few minutes at 150°C, it will take years at 50°C to produce that much change. Power Dissipation During operation, a cell must remain within its maximum internal temperature rating of 75°C. Any applied power will raise the cell’s temperature above ambient and must be considered. Illumination RLH / RDH Ratio 0.01 fc 0.1 fc 1.0 fc 10 fc 100 fc 1.55 1.35 1.20 1.10 1.10 |
Similar Part No. - VTL5C1 |
|
Similar Description - VTL5C1 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |