Absorptive filters, which are typically made from pigmented gelatin resins or dyed glass, are among the most commonly used types of filters for bright field and fluorescence microscopy. These filters work by attenuation of light through the absorption of certain wavelengths. Essentially, spectral performance is dependent on the filter’s physical thickness and the amount of dye that exists in the glass or gelatin matrix.
Until about a century ago, large blocks of dyed glass and liquid filters were the main methods of filtering light. Various organic chemicals generate bold colored solutions when they are dissolved in water or alcohol, and these offered numerous absorption filters for early scientists and photographers.
In the mid-1800’s, an English chemist by the name of William Perkin accidentally stumbled upon a natural substance, referred to as mauveine, while trying to synthesize the drug quinine. He discovered that the chemical was able to create a wonderful purple-colored solution when dissolved in alcohol and the amazing potential it had for creating dye products. Perkin’s findings led to the development of a number of synthetic dyes that are used today.
The modern absorption filter of today is made up predominantly from synthetic gels or colored filter glass, and constitutes the largest classification of filters for applications that don’t need an exact definition of transmitted wavelengths. Absorptive filters are also useful in blocking short wavelengths and transmitting longer ones at the same time.
They are widely available as glass, acetate, plastic-coated glass, and gelatin bases that are coated or mixed with organic and inorganic dyes. Various materials are used in polymer and glass filters, including earth transition elements, colloidal dyes, and other molecules that produce sharp absorption transitions.
The main advantages of polymer and glass absorption filters are their low cost and stability under various climates and conditions. The filters are also made with light-absorbing chemical species that are mixed throughout the filter material instead of being deposited on the surface. This reduces the odds of destruction as a result of any scratches or abrasions.
Glass absorptive filters can also resist damage from chemicals or oils in fingerprints, as opposed to polymer-based filters that typically don’t have this advantage. Both glass and polymer filters are not sensitive to the incident illumination angle and offer uniform spectral traits, with the exception of minor changes in absorption as a result of heightened thickness when the filters are placed away from the perpendicular.