How are filters made?

[jerevan]

Hello,

 

I got an old Kodak filter today in a lot bought on an auction site.

It's a blue photoflood, 27 mm screw-in. I thought filters were just

coloured glass. Until now, that is.

 

The one I got look like it has two glasses, with some blue film

sandwiched in-between. The film has separated and seems to lie

inbetween the glass partss. I guess there's no way to make it useable.

 

Is this the common way to make filters?

[pdumais]

Colored glass is one way to make filters. Another way is to make a multilayer of thin films a few hundred nanometers thick. There are 'recipes' to make optical bandpass, lowpass and highpass filters. But this is a fairly high tech and expensive approach.

 

The other possibility is that you have a blue gel sandwiched in glass for support.

[alex_lofquist]

You probably have one of the Kodak "Wratten" filters which is a gel sandwiched between two pieces of optical glass. It is not worthwhile trying to repair them.

[jerevan]

Thanks, I figured that it wasn't very worthwhile to try and repair. But asking is cheap :)

 

Maybe I can use the filter ring which is well-made, for something else.

[garrylewis747]

There was three ways a filter was made. Gell sandwiched between 2 glasses, dye glass, gel laminated to a single piece of glass. Then came multi-coating. If it is separated, unless you like experimenting, then don't use it.

[winfried_buechsenschuetz1]

As mentioned by others, most photographic filters today are made of coloured (or rather tinted) glass. In most cases metal oxides are added to get the desired color characteristics. I have seen an old yellow filter where the yellow color had started to come off in tiny particles, this one obviously just had a coloured layer on the glass.

 

Another method to make filters (for technical purposes) is to coat the glass with thin layer(s) of a clear material with a refraction index different from that of the base glass, the thickness of the layers is less than one wavelength of the desired cut off wavelength (or colour). You can make very sharp cut off filters (also referred to as interference filters) this way but this feature is hardly used in photography.

[glenn_thoreson]

To help keep laminated filters (or lenses) from separating, NEVER put cleaning solution directly on them. It can seep between the glass and the mount and hasten, or cause delamination. Always put just enough solution on a tissue or cloth to moisten it. Then, use the tissue to clean the filter. Many good lenses and filters get ruined through careless "cleaning".

[david_goldfarb]

I believe that Tiffen filters are generally gel sandwiches.

 

Heliopan and B+W are usually dyed in the mass.

 

Polarizers are sandwiches in general, from any manufacturer.

[alex_lofquist]

I bought a small, sharp peak interference filter some years ago. It cost over $250 then. I didn't like the results for my specific application (much too "slow" for one thing), and went back to another method to discriminate among wavelengths. For solar photography, an interference filter can make a lot of sense, such as photographing the sun's disk in hydrogen alpha light.

[lex_jenkins]

A Tiffen ad in a recent photo mag graphically shows that at least some of their filters are laminated of optical glass and colored resin or other material.