jgd@rsiatl.UUCP (John G. De Armond) (04/11/90)
phil@ingr.com (Phil Johnson) writes: >As a health-physicist you SHOULD BE AWARE that UV sources come in two flavors >(1) Short wave length - such as germicidal lamps > These are fairly safe to view without a filter, such as eyeglasses. >(2) Long wave length - such as EPROM erasers > These can cause severe damage to eyes if view directly without a filter. >This is one of the first safety lessons taught when dealing with UV-based >analytical instruments, such as UV spectrophotometers. Actually, you have it backwards. The quartz eveloped germicidal lamps transmit all the spectral lines of mercury and argon - the two emitting components. On the other hand, the long wavelenght light emitted by black lights and the like are safe. Most EPROM erasers, germicidal lamps and spectrophometers share one thing in common - they all use a quartz-windowed mercury vapor light as the excitation source. I know for a fact that my old L&N unit does. As I mentioned in my original posting a year or so ago, the intensity of light emitted by the low power tubes is not much greater than noonday sunlight. Not a good idea to stare at it but not sufficient of a hazard to panic. To put this in perspective, the only person I personally know who has been damaged by exposure to low pressure mercury discharge lamps is a relative of mine who worked ALL DAY in a meat locker that contained a pair of exposed 40 watt germicidal lamps. At the end of the day, he noticed that his eyes felt like they had sand in them. By the next morning, he had some pain and drainage from the eyes. In 2 weeks, all symptoms were gone and his vision was back to 20-20. Now over 20 years later, he is approaching 80 years of age and still does not use glasses :-) The above is not, of course, an advocation of pushing the envelope by needless exposure. ALARA dictates that all reasonable care be taken to avoid radiation exposure. Just be aware that if you <gasp> actually glance at an energized tube, your eyes won't fall out or rot away. Now let me describe 2 high speed EPROM erasers I have in my lab. The first consists of a 12 watt germicidal lamp in a Lampi (brand) fixture mounted in a short piece of rain guttering. I use the device with it facing the ceiling (and covered if someone else is in the room, mainly for psychological (sp) reasons.) The EPROMS are propped against the surface of the tube. 2764s are done in about 30 seconds. Higher density ones generally go faster. The second unit is a 250 watt high pressure mercury vapor lamp I made from a streetlight and which is used to expose PCB resist. I removed the outer glass envelope from the quartz tube and mounted it in the focus of the reflector from a quartz-halogen outdoor lamp. The ballast is immersed in a tank of transformer oil for cooling. This is necessary because the tube is now exposed to air and never really reaches operating temperature. It draws more current than originally designed. At 18", this unit will burn a PCB in perhaps a minute and will nuke an EPROM in oh, 5 seconds or so :-) I find this lamp very useful for erasing EPROMs in bulk, say a couple hundred at a time. I simply mount the lamp a few feet above the tabletop and increase the exposure to 10 minutes or so. As one might expect, welding goggles are apropos here. :-) BTW, does anyone know of an inexpensive (key word here) UV-transparent glass or plastic that I could use in a vacuum frame for PCB work? The thin Plexiglas I now use cuts the UV radiation by over 6 db. John -- John De Armond, WD4OQC | We can no more blame our loss of freedom on congress- Radiation Systems, Inc. | men than we can prostitution on pimps. Both simply Atlanta, Ga | provide broker services for their customers. emory!rsiatl!jgd | - Dr. W Williams | **I am the NRA**