In photonics since 1999. Benelux technical support, answer within one working day.
Colour glass or interference filter?
A colour glass filter absorbs unwanted light in the bulk of the glass; an interference filter reflects it from a coating. Absorption is insensitive to angle and cheap, but heats up and has gentle edges. Interference gives steep edges and narrow bands, but the band shifts as you tilt it.
THE SHORT ANSWER
A colour glass filter absorbs unwanted light in the bulk of the glass; an interference filter reflects it from a coating. Absorption is insensitive to angle and cheap, but heats up and has gentle edges. Interference gives steep edges and narrow bands, but the band shifts as you tilt it.
| Colour glass mechanism | Bulk absorption |
|---|---|
| Interference mechanism | Thin-film reflection |
| Colour glass angle sensitivity | Very low |
| Interference angle sensitivity | High, band shifts blue with tilt |
| Typical colour glass thickness | 3 mm |
| Example | GG 475 long-pass, cut-on 475 nm |
Both filters remove light you do not want. Where the light goes is the whole difference.
Absorption
Colour glass is doped so the unwanted wavelengths are absorbed as the light passes through. Because the mechanism is a property of the material, it does not care about the angle of incidence, which makes colour glass the honest choice in a converging beam or a wide-angle system. The transitions are gradual, and the absorbed energy stays in the glass as heat.
Interference
An interference filter is a thin-film stack that reflects the unwanted band. Edges can be very steep and passbands very narrow, which is what makes fluorescence work possible. But the layer thicknesses are only correct for one path length through the stack, so as you tilt the filter the effective thickness changes and the band moves toward shorter wavelengths.
Practical selection
Ask what the ray angles are before anything else. In a collimated beam at normal incidence, an interference filter will do everything you want. In an f/2 cone, the rays at the edge of the pupil see a meaningfully different filter from the rays at the centre, and a colour glass filter that ignores the issue is often the better engineering answer even though its curve looks less impressive.
Fluorescence and stray light
If you are measuring weak signals, ask whether the filter glass itself fluoresces under your excitation. It is not on most datasheets and it has ruined more than one measurement.
When to use it
- Blocking a broad region cheaply and without angle sensitivity: colour glass
- Isolating a narrow line or a steep edge: interference
- A converging or diverging beam, where rays hit at a spread of angles: colour glass
- Fluorescence separation where excitation and emission are close together: interference
What to watch out for
- An absorbing filter converts blocked light into heat; in a strong beam it can crack or fluoresce
- Interference filters shift toward the blue as the angle of incidence increases, so a tilted filter is a different filter
- Colour glass out-of-band blocking is rarely as complete as the datasheet curve suggests at high dynamic range
- Some absorbing glasses fluoresce, which is a problem if you are measuring weak emission
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.