From: "Keith Davison" To: infrared@a1.nl Date: Thu, 24 Jun 2004 09:04:14 +0100 Subject: Floppy disk IR filter Reply-To: infrared@a1.nl I've been following an IR releated thread on a digital camera forum regarding the use of floppy disks as IR filters and they really do seem to work! The floppy casing is opened and a circle is cut from the magnetic disk media, this is then used as a normal before the lens filter. The images posted are clear and show obvious Wood effect, so some IR filtration is occurring. The material may be even more useful than the digicam users realise as they are using cameras with internal IR blocking filters still in place and will only be seeing the light falling between the cutoffs of the two filters. I have heard about using layers of exposed E6 film as a rough and ready IR filter, but never magnetic media! I'll run a spectrum on a floppy and post the results, but in the meantime, does anyone know why floppy disks should work? Keith Davison University of Newcastle upon Tyne To: infrared@a1.nl Subject: Re: Floppy disk IR filter From: "Nic Hume" Date: Thu, 24 Jun 2004 20:01:00 -0700 Reply-To: infrared@a1.nl On 24 Jun 2004 at 9:04, Keith Davison wrote: > I've been following an IR releated thread on a digital camera forum > regarding the use of floppy disks as IR filters and they really do > seem to work! > > The floppy casing is opened and a circle is cut from the magnetic disk > media, this is then used as a normal before the lens filter. The > images posted are clear and show obvious Wood effect, so some IR > filtration is occurring. The material may be even more useful than the > digicam users realise as they are using cameras with internal IR > blocking filters still in place and will only be seeing the light > falling between the cutoffs of the two filters. > > I have heard about using layers of exposed E6 film as a rough and > ready IR filter, but never magnetic media! > > I'll run a spectrum on a floppy and post the results, but in the > meantime, does anyone know why floppy disks should work? I've done a little bit of digging and chatted to a few chemistry geek friends and have found a few things that may explain what you're seeing: 1) Most types of film, as well as most types of floppy disks are basically transparent (visibly clear) mylar plastic, coated with either a light-sensitive emulsion (in the case of film) or a ferro-magnetic emulsion, sensitive to electrical and magnetic impulses (in the case of floppy media.) 2) A quick web search has indicated that quite often spectral sensitivity tests involve placing a sample on a mylar sheet and then running it through a spectrometer. This seems to indicate that mylar is likely transparent to most relelvant portions of the spectrum. If it was subject to peaks or dips in transmission it would likely not be used for general scientific testing purposes. 3) As best I can tell, the ferro-magnetic emulsion on floppy disks is comprised largely of Ferric Oxide (aka Hematite, Fe2O3) http://en.wikipedia.org/wiki/Ferric_oxide 4) I have not been able to find information on the chemical structure of exposed E6 film. I believe it is logical to assume that exposed E6 is *NOT* a ferric-oxide structure. I may be wrong in this assumption, but I'm under the impression that colour film is primarily chrome and silver based. 5) According to Emporia State University, "http://www.emporia.edu/earthsci/amber/go340/visual.htm" "Different oxidation states of elements can also influence absorption. Ferrous iron (Fe2+) creates green, but ferric iron (Fe3+), found in a similar crystal structural site, can produce the yellow. Oxidation state is the reason heat-induced gem enhancement can intensify a color or create more desirable colors (Hurlbut & Kammerling, 1991, p. 70)." This would seem to indicate that the regardless of the actual chemical compound that may be present on a mylar sheet, the actual bonding properties resulting from treatment of that chemical play a very significant role in the transmission properties of the emulsion. 6) Basically I *think*... Key word here, *THINK*, that the plastic used in film and floppies is the same, it's just a bit of a fluke that the emulsions used to coat the plastic on floppies happens to have a transmission cutoff that is useful to IR photographers... Hope this helps, or at least gives people a place to start poking around themselves. I'm very VERY bored, and it's 3:30 AM on Thursday morning... sorry for the long rant... --Nic P.S. Keith, I would be VERY VERY interested in seeing any data you get back from running your spec on the floppy material. If you are able, I'd also love to see data from exposed E6 film for comparison. Not sure if that's something that's feasible for you or not, but it would be interesting nonetheless. Nic Hume nhume@irk.ca (250)-383-3584 Journalism, as everyone knows, is a nasty business staffed by ribald scribblers. Nobody takes it seriously. From: "Keith Davison" To: infrared@a1.nl Date: Fri, 25 Jun 2004 09:10:59 +0100 Subject: Floppy disk IR filter - results Reply-To: infrared@a1.nl I ran material from a 3.5" HDD floppy (brand unknown), through the spectrophotometer, and sure enough there is an IR pass effect. First, I have to say that the instrument that I used is not designed for high Optical Density (OD), measurements and bottoms out at 2.0 OD. Having said that, it is perfectly OK for measuring the absorbance spectra of standard infrared Wratten filters. The floppy material was >2OD from 300 - 700nm. At 750nm the absorbance dropped to measurable levels for my instrument smoothly reducing from 1.93OD at 750nm to 1.74OD at 900nm. So, there is a slight bias to IR pass in this material, but it is nowhere near as discriminatory as a "proper" IR filter, perhaps the absorbance varies with manufacturer. Although I have not tried it myself, posted examples using the material seems to allow some Wood effect and the usual "everyone wears white clothing" effect (the Persil effect?). http://gehu.free.fr/IR%20samples/index.html While the results are not startling, compare the cost of a Pukka Schott or Hoya IR filter to a trashed floppy. Incidently, for large format and ultrawide fans, 5.25" floppies are said to show the same effect, don't know about 8"s though. Keith Davison University of Newcastle upon Tyne