Filters

Filters are used to select wavelengths (colors) of light. A filter works by excluding all but a limited set of wavelengths. A filter is rated by its type and a wavelength number in nanometers. There are three general types of filters, long pass, short pass and band pass. The wavelength number is were the filter action begins. The wavelengths that are allowed through a filter are called the pass band.

Long pass filters lets all the wavelengths longer than the wavelength number through. For instance a 510 nm long pass filter would let all the wavelengths greater than 510 nm through the filter. These types of filters are used as barrier filters in fluorescence. They will let the fluorescent emitted light through and not the exciter light.

Short pass filters let light that is lower in wavelength through. A 480 nm short pass filter would let light that is shorter in wave length 480 nm (less than 480nm) through the filter. Short pass filters are sometimes used as exciter filters for fluorescence.

The band pass filter allows only a set of wavelengths through. The wavelength number is referred to as the center wavelength of a band pass filter. The center wavelength is the wavelength that allows the most light through within the range of wavelengths that will be passed through. Frequently this will be the center wavelength of the filter. A band pass filter is rated by center wavelength and the pass band or width.

A band pass filter might have a rating of 480 nm center and a pass band of + or - 20 nm. This means that this filter will pass light from 460nm to 500nm. The pass band can be quite narrow and can be uneven. An uneven pass band would have more width on one side than the other. A 480 nm filter with a width of plus 5 and minus 10 is an example.

Commercially filters are made of either stained glass or interference materials. Stained glass is the older material, it is glass that has some impurities added to it when it is made to give it a given color. These filters are cheap and in some applications work well.

Interference filters use very thin glass and coatings to set up constructive and destructive interference in the filter. This interference creates the filter effect. These filter can be made to just about any specification you want. Most modern microscopes use interference filters for fluorescence.

An interference filter is usually a band pass filter. It is specified by the center wavelength and the width. Manufacturers of interference filters provide charts of the intensity versus wavelength. A chart is the best way to understand a band pass filter.

A wavelength chart shows two other filter phenomenon, filter factor and filter inefficiency. Filter factor is the amount of light that the filter eats, that is the amount of light in the pass band that the filter converts to heat. If a filter is 80% efficient then it allows 80% of the pass band through. No filter passes all the light in the pass band the higher the efficiency the better. Filter inefficiency is seen as the slope of the starting and ending curves of the filter. Theoretically when the pass band starts the line on the chart should just go straight up. It doesn't, there is always a curve at the beginning. In a fluorescence system you will see inefficiency as "bleed through" or a light background. This inefficient area can cause problems if you want a very narrow pass band. To get around this sometimes two filters are used, the second filter helps the first one to be more efficient. Using two filters usually increases the filter factor, however.

The reason that interference filters are used is that they can be made to such demanding specifications. If you want a 5 nm band pass filter centered at 490 nm just tell the manufacturer and they will quote a price. Costs for interference filters have come down in the last few years. They are very affordable.

Interference filters have some draw backs. They can't be cut down for size. Since they are made up of thin layers of glass with small chambers in them if you try to cut them down the just come apart. They also like to be cleaned very gently.

Stained glass filters are still used in a lot of applications such as near-UV filters and routine use blue filters. Even here the interference filters are making inroads. Nikon makes an interference blue filter that is a real joy to use and Olympus makes an interference green filter that I have always used for black and white film and tv.

Over the long haul just about any filter will age and change. Usually this takes the form of lightening. Some interference filters will start to show deteriorated coating. If your fluorescence is starting to look spotty this may be the cause.