Nomenclature and Terminology Related to Optical Filter Film

Optical filter film is an optical component that allows only a certain band of light to pass through (with high transmittance).

Peak transmittance of optical filter film

If the initial light value is 100%, and after passing through the filter, it is measured spectrally and found to be only 85%, then the optical transmittance of the filter can be defined as (Tp)>80%.

Center wavelength of optical filter film

The wavelength used in practical applications of optical filter film, for example, if the peak wavelength of a light source is 850nm LED, then the required center wavelength is 850nm.

Half-bandwidth of optical filter film

This is the wavelength corresponding to half of the highest transmittance, obtained by subtracting the left and right wavelengths with a peak value of 90% and a half value of 45%. If the corresponding wavelengths are 875nm and 825nm, then the half-bandwidth is 50nm.

Cut-off range of optical filter film

It is used to indicate the wavelength interval of the spectral region where the energy of light passing through the filter is attenuated. The intensity of attenuation is also referred to as the cut-off depth and is usually expressed using optical density or transmittance.

Optical density of optical filter film

Optical density (OD) describes the degree of cut-off of the filter and is related to the amount of energy transmitted. The higher the optical density value, the lower the transmittance.

Incident angle of optical filter film

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

Cut-off rate of optical filter film

The transmittance corresponding to the cut-off region. It is very difficult to achieve a transmittance of zero, and therefore, the transmittance close to zero is usually selected. However, a transmittance of 10^-5 or higher can meet most requirements and is typically converted to an optical density value of OD>5.

Cut-off bandwidth of optical filter film

The minimum range of unwanted wavelengths that can be accepted. Since the response range of most electronic imaging sensors is 350-950nm, the actual range can be slightly wider than this interval. The determination of the cut-off range for ultraviolet and infrared is more complex and depends on the response range of the sensor used.

Starting wavelength of optical filter film

It refers to the wavelength at which the transmittance of a long-pass filter is increased to 50%.

Cut-off wavelength of optical filter film

It refers to the wavelength at which the transmittance of a short-pass filter is reduced to 50%.

Effective aperture of optical filter film

The physical region that is effectively utilized in an optical system. It is usually similar in size to the appearance of the filter, concentric, and slightly smaller in size.