What Are the Technical Terms for Optical Filters?

Introduction of optical filters

The optical filter is made of plastic or glass, added with special dyes. The red filter can only pass the red light and so on. The transmittance of the glass sheet is originally similar to the air, and all colored lights can pass through, so it is transparent. While after adding dye to the glass sheet, the molecular structure and the refractive index also change, and the passage of certain colored lights is changed.

For example, a beam of white light passes through a blue filter and emits a beam of blue light, while green light and red light are really weak, and most of them are absorbed by the filter. Optical filter films are optical instruments used to select the required wavelength band. One common feature of filters is that no optical filter can make the image of celestial bodies brighter because all optical filters absorb some of wavelength, which makes the object darker.

Technical terms of optical filters

1. Incident angle:

The angle between the incident light and the normal of the optical filter surface. When the light is normal incident, its incident angle is 0°.

2. Spectral characteristics:

The spectral parameters of the filter: transmittance (T), reflectance (R), optical density (OD), phase, polarization state (s), p and other characteristics relative to the wavelength change.

3. Center wavelength:

The center of the bandpass filter is called the center wavelength (CWL). The width of the passband uses a larger transmittance. The full width at half maximum (FWHM) is usually called half-width.

4. Effective aperture:

Physical area effectively used in the optical system, usually similar to the external dimensions of the filter, concentric, and slightly smaller in size.

5. Cut-on and cut-off:

Cut-on corresponds to the 50% point of the spectral characteristic from attenuation to transmission, and cut-off corresponds to the 50% point of the spectral characteristic from transmission to attenuation. Sometimes it can be defined as the 5% or 10% point of the peak transmittance.

6. Tolerance:

Any product has manufacturing tolerances. Take the optical filter as an example, the center wavelength must have a tolerance, and the half-width must have a tolerance, so you must indicate the tolerance range when ordering products. In the actual use of optical filters, the smaller the tolerance, the better, the smaller the tolerance, the greater the manufacturing difficulty and the higher the cost. The user can propose a reasonable tolerance range according to actual needs.