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Posted: 1/18/2011 9:19:27 AM EDT
| Well, I figured I'd start a little discussion about ebi. I really never hear very much about it, and how it affects the ability to see in very dark conditions. It seems signal to noise is always talked about in seeing in dark conditions, but its not the only factor in overall performance in those conditions. I'd like to hear ya'lls opinions on where you rate ebi in overall tube performance. |
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It's very temperature dependent but in Australia where the nighttime temps can be around 30C (86F) sometimes, it's quite noticeable on my Gen3. I've even chilled my tube once so that I could reduce the EBI component and it's quite noticeable to the eye. One of the members here did a LOT of research and discovered that EBI is not consistent across tubes. He discovered that modern thin-filmed tubes suffer more significantly from EBI effects than older tubes and that the EBI is not linear, leading us to speculate on the cause. We never did find out why thin-film tubes are so badly affected. There has been some military research into this as well, with temperature playing a significant part in the detection and recognition range under very dark conditions due to EBI effects. Finally, not all EBI effects that are measured can be attributed to thermionic emission. Electrostatic forces within the tube will also contribute to electron emission and other factors can play a part as well. Monitoring EBI is an excellent method for determining the positive ion generation levels of a tube however. It's quite easy to tell the difference between a thermionic electron and a sudden burst of electrons caused by a positive ion striking the photocathode. Anyway, I'll leave the rest of the detail to the guy who actually conducted the research. Hopefully he'll spot this and respond with some incredible videos that he took of the variation IN EBI with the change in temperature. But overall, my rating on the significant of EBI in tube evaluation is that it is a very important factor. A low EBI is something to value. David
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EBI levels are also a very important in my book, very very important. I might even go so far to say it's the most important parameter, to me but one would also be wise to consider other things and not just EBI values. For me, an EBI value of less than 1 is good.
As David said, field emissions caused by extremely high electrostatic forces play a role as well in the levels of dark current, although on a smaller scale than thermionic emissions. High dark current will impart a hazy appearance to the viewed scene and is most noticeable when a scene has low levels of ambient light. There's a number of posts regarding EBI over on another night vision forum http://www.nightvisionforums.com/phpBB3/viewtopic.php?f=9&t=6224 Here's a couple of video's that show the affect temperature has on EBI/Dark Current; The long version http://www.youtube.com/watch?v=U7qZd2dG8uI |
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Thanks for the links. A lot of great info in that thread. Any ideas why the thermionic emissions seem to affect the thin filmed tubes greater than the filmed?
Is it pretty common for thin filmed tubes to have ebi levels around 2.5? I noticed on my data sheet on a thin filmed tube the ebi is 2.3. Tube works very well in very dark conditions on a 6x scope. |
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Quoted:
Any ideas why the thermionic emissions seem to affect the thin filmed tubes greater than the filmed? I have an idea but am not completely sure. I thought I'd add some more to my first post which seemed a little light on information, so here's a bit more that hopefully doesn't make it more confusing. It can be a bit repetitive but here it is anyway; Equivalent Background Illumination (EBI) is a measure of the dark current, the flux electrons generated by the components and forces within an image intensifier in the absence of photoelectrons. The measure of EBI corresponds to the input illumination necessary to double the background output brightness. Simply stated as the level of input illumination needed to overcome the internal background illumination/dark current to produce an image. I like to think of it in terms of an internally lit house. From the inside the house you can't see much but when you turn your external floodlights on the whole exterior is visible. How bright do the floodlights need to be in order to overcome the interior lighting? The interior lighting represents the dark current, while the exterior lighting represents photons that generate photoelectrons. This is a simplified analogy that doesn’t work for everyone. EBI is severely affected by temperature; the warmer the intensifier and internal components, the higher the dark current will be i.e. the brighter the background illumination. When the factory measures EBI, it is generally at an ambient temperature of 21 degrees Celsius. As the temperature rises the EBI values have been shown to increase exponentially, nearly doubling for every 3-5 degrees of temperature increase. This means that your tube may have an EBI value of 3.99 at 21 degrees Celsius but when the temperature reaches 28 degrees Celsius the EBI value may be 8.00 or higher. This is why EBI can be real problem when ambient temperatures are relatively high. Dark Current, as mentioned above, is the flux of electrons from a cold cathode. In this case cold cathode has nothing to do with temperature but simply means that the cathode is not exposed to a source of electrons and so is not generating photoelectrons. A cold cathode could be thought of as a dark cathode, one that is not generating photoelectrons. This cold cathode is still emitting electrons, though thermal electrons instead of photoelectrons. The MCP multiplies these thermal electrons, just as with photoelectrons and just as with photoelectrons the multiplied thermal electrons imping on the phosphor screen causing it to decay and emit photons. An image intensifier cares not, what generated the electron and will react the same way for all types whether their origin is photo, thermal or electrostatic. So in the absence of any input illumination on the photo cathode, the phosphor screen will still show a definite background illumination. This is mainly to dark current, mainly the flow of thermal electron from the photocathode. Other sources that can contribute to background illumination are ion events and field emissions wherein emissions of electrons are induced by intense electromagnetic fields. The electromagnetic fields may also affect thermal electron emissions by raising the temperature of components within the intensifier. Just as with parent photoelectrons, EBI is independent of the gain. Regardless of the gain, the flux of thermal and/or photoelectrons emitted by the PC will remain relatively constant as long as the temperature remains constant, however increasing voltage to the MCP may increase the probability of electrostatic generated electrons. There are a lot of forces at play in a image intensifier, not all of which are easily explained. Observing a scene with low luminance, EBI i.e. thermal and electrostatic generated electrons can cause the scene to appear hazy. In other words one can say that higher EBI means lower image contrast at low luminance, as well as, a significant reduction in range performance. The affect of this are most notable when viewing deep sky objects which have very weak luminosity and in environments where the ambient light levels are at a minimum.. High EBI can entirely mask an object making it impossible to view as where a tube with lower dark current could easily make the object visible. In my view, EBI value is one of the most important characteristics of an image tube and trumps resolution, halo, and PC Luminous Sensitivity and PC Radiant Sensitivity. But I mainly use my equipment for observing deep sky object in tropical and subtropical temperatures. The indications from prior research suggest that AIO film thickness may have a direct affect on EBI values. Whether this relation is related to the film thickness alone or a combination of voltage and thickness is not clear. It has been observed that thin film tubes suffer from higher EBI values more often than tubes with films closer to the standard filmed thicknesses. Cathode voltage for standard film tubes is -800v, -600v for thin filmed and -200 for un-filmed. With the voltage and film information, a possible theory is the relation of the relatively high cathode voltage to film thickness in a thin filmed tube. The voltage may just impart enough energy to the relatively weak thermal electrons for them to make it past the thin film in greater numbers than those in a standard film tube. Overall, un-filmed and standard filmed tubes seem to have generally lower EBI values than thin filmed tubes, so it would seem plausible that there is a relationship with the voltage and film thickness and an increase in dark current. |
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I've seen documentation that suggests the strength of the electrostatic field does affect the rate of thermionic emission. It does, after all, penetrate into the photocathode and is responsible for the band-bending that occurs near the surface of the material. This is critical in Gen2 but is still important in Gen3. What has us confused is the non-consistent rate of emission from across the tube on thin-filmed devices ( it's strongest near the PC input lead ). Best we can come up with is some kind of standing wave in the electrostatic field around the PC or perhaps a very slight variation due to resistance or other losses. But both theories are just guesswork. There's nothing to base them on. The variation in emissions across the photocathode are still a mystery to us. Keep in mind though that when a tube is emitting only thermal electrons, it's possibly going to have a higher PC voltage than when it's in normal use as pure capacitance alone will increase the voltage to the theoretical maximum that the power supply can deliver. David
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