Filter Terminology | Fabrication Techniques | Selection Guide| Application Examples

Optical filters can be used to attenuate or enhance an image, transmit or reflect specific wavelengths, and/or split an image into two identical images with controlled brightness levels relative to each other. To understand the importance of choosing the correct optical filter for any application, consider key terminology, fabrication techniques, and the various types of filters available today.

Key Optical Filter Terminology

Before delving into fabrication techniques and the types of optical filters available in the industry today, it is first important to review key terminology associated with them. Since all filters, independent of how they are manufactured, pass, absorb, and/or reflect some portion of incident light, they share common optical parameters.

Central Wavelength

Center Wavelength (CWL) is often used to denote the peak transmittance of a bandpass filter or, in the case of a notch filter, the peak reflectance. However, the term is commonly misused - CWL is actually defined as the midpoint between the wavelengths where transmittance is 50% of the peak, referred to as the Full Width at Half Maximum (FWHM). For interference filters, the peak is typically not at the midpoint wavelength. Refer to Figure 1 for an illustration of CWL and FWHM.

Bandwidth

Bandwidth is a wavelength range used to denote a specific part of the spectrum that passes incident energy through a filter. Bandwidth is also referred to as FWHM (Figure 1).

Blocking Range

Blocking Range is a wavelength interval used to denote a spectral region of energy that is attenuated by the filter (Figure 2). The degree of its blocking is typically specified in terms of optical density.

Figure 1: Illustration of Center Wavelength and Full Width at Half Maximum [View Larger Image]

Figure 2: Illustration of Blocking Range [View Larger Image]

Optical Density

Optical Density (OD) describes the blocking specification of a filter and is related to the amount of energy transmitted through it (Equations 1 – 2). A high optical density value indicates very low transmission, and low optical density indicates high transmission. Figure 3 depicts three different optical densities: OD 1.0, OD 1.3 and OD 1.5 that show the higher the OD value, the lower the transmission.

(1)

(2)

Dichroic Filter

A Dichroic Filter is a type of filter used to transmit or reflect light, depending on the wavelength; light of a specific wavelength range is transmitted, while light of a different range is reflected or absorbed (Figure 4). Dichroic filters are commonly used for longpass and shortpass applications.

Figure 3: Illustration of Optical Density [View Larger Image]

Figure 4: Illustration of a Dichroic Filter Coating [View Larger Image]

Cut-On Wavelength

Cut-On Wavelength is a term used to denote the wavelength at which the transmission increases to 50% throughput in a longpass filter. Cut-on wavelength is indicated by λ_cut-on in Figure 5.

Cut-Off Wavelength

Cut-Off Wavelength is a term used to denote the wavelength at which the transmission decreases to 50% throughput in a shortpass filter. Cut-off wavelength is indicated by λ_cut-off in Figure 6.

Figure 5: Illustration of Cut-On Wavelength [View Larger Image]

Figure 6: Illustration of Cut-Off Wavelength [View Larger Image]

  1 , 2 , 3 , 4 | Next View All