The atoms and molecules that make up a filter may seem to be solid objects, but on the scale of visible light photons, they are mostly incredibly large voids is space with unbelievably small beasties called neutron and protons (which may be even incredibly smaller other beasties, but that is another story) surrounded by very fast moving, also incredibly small, beasties called electrons. The interaction of the electrons with the electrons in adjacent atoms is what holds the material together and makes us consider it a solid chunk. The photon can either pass through the material, reflect off of one of the surfaces of the material, be deflected to a different path, or be absorbed. The light that reflects off the surfaces will be a few percent, usually less than 10%, of what hits the material. This can be reduced to less than 1%, or so, with anti-reflective coatings. Light that is deflected to a different path causes image degradation. If it is large, you need to get your money back. The light that passes through is what is absorbed in the film or CCD and eventually results in a gallery submission. The photons that get absorbed are what we buy filters for. When a photon passes through the filter, how it interacts with little beasties in the filter depends upon the wavelength (color) of the photon. The shorter the wavelength (bluer), the higher the energy in the photon. The longer the wavelength (redder), the lower the energy in the photon. All atoms and molecules have characteristic and discrete energy conditions (physicists; who can be usually identified by walking fast, waving their arms while they talk, getting very excited about subjects that generate dazed looks from the majority of the population, and writing on napkins in restaurants; call them states) in which they can exist. Only light at a wavelength that equals the energy of one of these energy states will be absorbed. Photons have a true identity problem and cannot figure out whether they are tiny little beasties or big fat waves. They look like big beasties if the photon happens to be at an energy that matches one of the energy states of the filter material. The bigger it looks the faster the beasties in the filter gobble it up. When the photon is eaten (absorbed) it ceases to exist. The beastie, which has absorbed the energy of the photon it ate, will wait for a while and then spit the photon back out. The photon will be spit out in a random direction so most of the new photons never reach the film or CCD. Sometimes the beastie spits out more than one photon. Each photon will be at longer wavelengths that the one the beastie ate, but the energy in all the photons it eventually spits out will add up to the energy of the ate. Photographers don't need to worry about this but laser people make their living because of it. Bottom line: Photons at a color that matches the characteristic absorption wavelengths of the filter don't get to the film or CCD and photons at a color that doesn't match the characteristic absorption wavelength of the filter do. Doug Stanley - long-time list lurker; - occasional (very) gallery contributor; - photography funded by funds received from laser R&D management; - past employment as researcher in obscure problems in nuclear, particle, and laser physics that most people really don't care about but were fun to solve -----Original Message----- From: owner-photoforum@listserver.isc.rit.edu [mailto:owner-photoforum@listserver.isc.rit.edu]On Behalf Of Gregory Fraser Sent: Tuesday, October 22, 2002 10:13 AM To: List for Photo/Imaging Educators - Professionals - Students Subject: Just how do filters work? Hi Alan, I never thought of that and it sounds like a creative manipulation of film. Film would actually block the image like shooting through a microscopic screen wouldn't it? Is that the way filters work? I know they prevent the passage of light but just exactly how do they do it? Is it a wavelength interference voodoo or is it just tiny bits of opaque material in the glass? Greg > -----Original Message----- > From: Alan Zinn [mailto:azinn@netbox.com] > > Greg, > I may have some tinted plex you can have. Is this > transparency film? Could > you somehow expose a sheet to get a neutral tinted material? I know, > off-the-wall...!
