Betreff: CVNet - Answers to: Linear Density Filter Von: wolf Datum: Mon, 11 Jul 2005 17:16:23 +0200 An: cvnet@mail.ewind.com hi all, a big 'thank you' to all of you for your great input. To provide everybody with your ideas, i post your answers at the end of this mail. thanks again, wolf harmening -- Dear Wolf, A Beckamn Instruments (California) 'duo-dial' might work. this consists of two counter-rotating circular neutral density (ND) wedges, mounted on the same axis. Each reduces light from 0 to 2 log units, so together they span from 0 to 4 log units. The counter-rotation ensures that the area of view has a spatially uniform loss of light. the duo-dial offers a small viewing area, so you would have to mount it between the head and the LCD screen, but closer to the subject's eye. You would also have to use a black patch to cover the other eye. Alternatively, and cheaper, you can buy vey large neutral density sheets of gelatin filter, each sheet providing 0.3 ND. To vary the light level you have to interpose more sheets. If you do this, measure the transmittance when the LCD monitor is blue, green, or red, because the 0.3 only refers to the average across the spectrum - gelatin is not truly neutral, and may let through slightly more blue than red. Any errors will be small but will add up if you use (say) 10 sheets. This way you cannot get steps smaller than 0.3 ND, but of course you can vary the LCD monitor instead, so this doesn't mater as long as you have calibrated the sheets. Kodack makes these sheets ('Wratten'), as does an English company, Lee. They are much cheaper from a theatrical supply company (you can get rolls of the stuff and cut it up as you please), and since you have to calibrate the sheets anyway, buying from an optical house doesn't really save anything. Adam Reeves -- Hi, We bought filters from LEE filters (http://www.leefilters.com/home.asp). Don't know whether they also sell in Germany, but give it a try. We paid 3 pounds for a A1 or A2 sheet; you then cut it to size. In general, look for suppliers to the lighting industry and the theatre. In the Lee filter catalogue each filter comes with a spectrogram and a transmission factor (%). Hope that helps, cheers, alexa -- Hello, I recently spent several hours trying to track down some large neutral density filters. Eventually I found a company in the UK called Lee filters (www.leefilters.com/home.asp) who produce large (1.2 x 0.5 metres) sheets of neutral density acetate, which is suitable for use in vision research. They offer a range of densities (0.15, 0.3, 0.6, 0.9 and 1.2 log unit) which, because they are so thin, are easy to combine to get different levels of filter density. The filter can also be cut into smaller sections. Each sheet is also rather cheap, at under €10. See this page for product codes: http://www.leefilters.com/LP1.asp?PageID=34 The company is based in England, but you have to buy their products through an authorised reseller. There is a section of their website you can use to work out where the nearest reseller is, but I'm not sure if they have any in Germany. Good luck! - Daniel Baker -- Spindler und Hoyer? -- Hans -- I presume you do not mean a linear variable density filter but a large-area uniform neutral density filter. Very many years ago we bought neutral density filters of large size (200 x 250 mm) from ILFORD for use in essentially the same application as yours. These were made in the same way as normal (coloured) darkroom safelight filters (e.g. see www.ilford .com/html/us_english/pdf/safelight .pdf ) and were not very expensive. Although Ilford do not currently list neutral safelight filters, it would probably be worth asking if they could supply them. If Ilford can do this, make sure that you get filters without a diffusing layer. It is possible to get the full range of Wratten neutral density filters in large sizes (350 x 450 mm; up to 4.0 D) but they are ridiculously expensive (e.g. see http://www.bhphotovideo.com/bnh/controller/home?A=details&kw=KOND.61418&is=REG&Q=&O=productlist&sku=79875 ). It might be possible to use neutral acrylic sheet (e.g. Perspex) which is available with densities up to about 1 and quite cheap. -- Dear Wolf Harmening, The place to go is a movie lighting store. When movies are being shot, the brightness of the lights are adjusted with enormous neutral density filters. At shops that cater to that business, they sell N.D. filters from large rolls. Often places that rent lights for film-making also sell neutral density filters. Josh Wallman Josh Wallman Department of Biology City College, City Univ. of New York New York, NY 10031 phone: (212) 650-8541 fax: (212) 650-8451 -- Dear Wolf, Göttinger Farbfilter has large sheets of neutral density filters that you can cut to your needs. Firma Göttinger Farbfilter GmbH Straße Brauweg 40 Ort 37073 Göttingen Ansprechpartner Frau Gerhardy Telefon 0559392980 Email 320013396674-0001@t-online.de Produktseite www.goettingerFarbfilter.de Best wishes, Karl -- Prof. Karl Gegenfurtner, Abteilung Allgemeine Psychologie Justus-Liebig-Universität, Otto-Behaghel-Str. 10, 35394 Giessen phone: +49 641 9926100 mailto:gegenfurtner@uni-giessen.de fax: +49 641 9926119 http://www.allpsych.uni-giessen.de/karl -- Hello, You can get linear density filter foils from companies who sell technical equipment for theatre lighting. Unfortunately i can't recommend any company since i made a bad experience with the company where i ordered the foils. The foils are delivered in rolls and are very cheap. Perhaps lee filters has the product you are looking for. http://www.leefilters.com/home.asp Best, Simon -- Hello Wolf, I buy for folks at the Center for Visual Science and Walt Makous forwarded your query to me. I have an answer that is very inexpensive but I don't know if there is sufficient precision or choice of densities for your research. When someone here wants a BIG filter, I pull out my sample book of Roscolux filters. These are used by theatres for coloring spotlights. A sheet 20 x24inches is less than $10 here. The website says its called 'Supergel' "overseas". There are only four neutral grays 25% to 70% transmission. Each sheet comes with its own transmission curve, presumably the curve is generic rather than individually measured. The site has lots of info, but is concerned more with durability than precision. http://www.rosco.com/us/filters/roscolux.asp#Colors Happy hunting, Teresa Williams -- You can buy perfectly good optical quality ND filters in A4 sheets and rolls from theatrical supply houses who sell lighting. You can get 0.1, 0.3 and 0.9ND. Theatre people cut them up like paper and they are cheap. If you can't find any write back to me and I will tell you the precise brand I used. Yrs truly Vaegan, VisionTest Australia, ph +612 91306175 CRICOS Provider Code: 00098G -- Try a theatrical light supply house. Many of them frequently stock large gelatin filters (including netral density) that are quite cheap, a few dollars instead of a few hundred dollars. Lee is one of the companies that makes them. Just go to google and search for ?lee filters?. Kent Higgins -- Wolf Harmening RWTH-Aachen, Biologie II Biologische Informationsverarbeitung Kopernikusstr. 16 52074 Aachen voice: +49-(0)241-8025859 mail: wolf@bio2.rwth-aachen.de ------------------------------------------------------------------- To get information on using CVNet, send a note to: majordomo@mail.ewind.com In the body of the message, enter: info cvnet