Specifying Bandpass Filters

The first step toward developing a successful design of any optical interference filter having is an understanding of the operating environment of the filter. Parameters such as angle-of-incidence, operating temperature, illumination source, and detector have a significant effect upon filter performance. The following is a description of all the important parameters associated with the specifications of Bandpass Filters:

Half-power wavelengths

Two wavelengths in the bandpass where the transmittance is equal to 50% of peak transmittance (50%Tpk). In some cases, a bandpass can have more than two wavelengths where the transmittance is equal to 50%Tpk. In these cases, the shortest wavelength of 50%Tpk encountered on the transition slope (from rejection to transmittance), and the longest wavelength of 50%Tpk encountered on the transition slope (from transmittance to rejection), would be chosen as the half-power wavelengths. This condition appears most often on fluorescence filters, where bandshapes often contain considerable (and unobjectionable from the performance standpoint) occilation in the transmittance band.

Bandwidth

This is the difference between the half-power wavelengths. The value that is calculated is the full bandwidth at the 50%Tpk point. It is referred to as "full width at half maximum FWHM). In common usage, full-width-at-half-maximim has been shortened to "half bandwidth".

Center Wavelength

A wavelength defined by calculating the average of the two half-power wavelengths.

Peak Transmittance

The maximum transmittance within the bandpass referred to as Tpk.

Band shape

This is usually defined as a multiple of the BW or FWHM, and it defines the rate of attenuation with respect to wavelength change along the slopes of the bandpass. Some manufacturers specify the number of cavities (or periods) of a bandpass filter to describe this rate of attenuation (the higher the number, the greater the rate). This is, however, an inadequate definition since it merely implies but does not state a certain rate of attenuation.

Rejection, Spectral

This is the maximum transmittance allowed outside the bandpass at wavelengths within a defined spectrum. It is expressed in either percent transmittance or optical density. When expressed in transmittance, the specification indicates the maximum level of light allowed at any wavelength within the rejection band. The specification uses the "less than" inequality symbol; for example %T≤ 1.0 x 10e-4. When expressed in optical density, the specification indicates the minimum level of rejection of light at any wavelength within the rejection band. The specification uses the "greater than" inequality symbol; for example: Blocking ≥ OD4. Both of these specifications are saying the exact same thing.

Rejection, Signal-to-Noise Ratio

This is a very important specification for a bandpass filter since it is a measure of the ultimate linearity achievable by a photometric system. It is the ratio of band pass energy to the "total energy" outside the bandpass. Commonly referred to as signal-to-noise ratio (S/N) or integrated blocking, it is usually expressed in optical density (OD). This parameter must be specified along with the radiant source of energy and photo detector that are used in the system. For example, a system utilizing a 75 watt Xenon Lamp and an S-20 photomultiplier with a certain bandpass filter may yield a signal-to-noise ratio of 4 OD (or 10,000/1 ), whereas that same filter in a system utilizing a tungsten lamp with a color temperature of 2800K and a silicon detector may yield a signal-to-noise ratio of only 3 OD (or 1000/1 ).

Simple tilts in collimated incident light will cause the center wavelength (λ₀) of a thin film band pass filter to shift to a shorter wavelength. Because of this characteristic, the user can tune around a particular wavelength merely by tilting the filter. Most bandpass filters can be used at angles of incident light up to 15°. Certain types of bandpass filters can be tilted to greater angles, but it is not commonly recommended to do so.

Bandpass interference filters exhibit a positive temperature coefficient (i.e., the center wavelength (λ₀) will shift to a longer wavelength with increasing temperature). This shift is expressed in terms of nm/1°. For example, consider the temperature shifted wavelength from 22°C to 50°C of a filter with a center wavelength at 500nm and a temperature coefficient of 0.01 nm/1 °C.

Shifted wavelength is given by:

λ(°C) = (λ₀) +ΔT (Δλ₀/1°C) =

500 nm + (50°-22°)(0.01 nm/1°C) = 500.28 nm

We specify a temperature range in which the bandpass filter can operate safely. However, this does assume a gradual change of approximately 5°C per minute. An abrupt change could cause physical and spectral damage to the component due to the possible differences in expansion coefficients of the many elements in the filter assembly.

For applications where a severe environment is anticipated, or where design constraints require film-to-the-edge construction, our Stabilife® coating technology provides the most effective solution. However, for many applications, soft coating technology is the appropriate choice. Soft coatings require protection from a humid environment. We utilize several methods of providing this protection in the construction of our soft coated filters. We incorporate a moisture barrier between the humid environment and the coating. Although this barrier will prolong the life of the component, we do not guarantee against humidity in terms of longevity; we guarantee the product will meet well-defined Military Specifications. The user should not be misled by claims of longevity since these claims cannot be substantiated by normal inspection procedures and therefore must be accepted on faith that the component will survive in operation.

We specify, certify and regularly test all of our products to the humidity requirements of MIL-STD-81O, MIL-F-48616, or MIL-C-48497. These specifications, which have been established by the U.S. Government to determine the effects of moisture on components and equipment, can be performed by the customer to confirm compliance

This list of parameters includes the most common items used to specify a bandpass filter. Many of the parameters are general enough to apply to any optical coating. Care should always be take to avoid overspecification. For example, specifying some of the specifications in the optical category such as transmitted wavefront error, can have a significant impact on cost (depending on the chosen value)

Specifications are basically elements of a recipe developed to describe a product that will deliver a certain level of performance. In many cases, it is quite difficult to define thresholds for certain parameters which will truly effect whether a product has acceptable or unacceptable performance. The more our design engineers know about the application including performance goals and challenges, the better able they are to assist in developing a set of specifications the will deliver the desired performance.