Optical filters selectively pass light at a particular and specific selected slice or range of light spectrum (range of colors of light), while blocking the remainder unwanted light spectrum. The picture below shows a typical bandpass filter. Its important to note that the light inside of the large spike is in fact the ONLY light frequency the filter will let pass through the filter. The remainder of the light spectrum on the outside of the spike is therefore blocked entirely. The second hump in the graph is a secondary passband of this particular bandpass filter in the picture, but doesnot represent a part of the bandpass, but rather additional light that is passed through the filter that is unwanted. Additionally, the second hump in the graph represents out of band passband transmission, and reduces the signal to noise ratio of the desired passband frequency, known as "white noise" as can be explained, and adds to the degradation, or accuracy, of the bandpass filter represented by the order of magnatude, and subsequent logrymithic function of the instrumentation interpolating the data.
Optical filters generally belong to one of two categories; absorptive and interference. Absorptive filters (neutral density filters) are commonly used in broadband applications as a light attenuator. Absorptive filters are physically simple in construction, consisting of a coated piece of glass with the selected blocker, while interference filters can be quite complex, dependent upon the type of bandpass and desired light frequency to be blocked or passed. Filters of this type are commonly utilized in many optical instruments.
In this article, I will focus the discussion on interference filters and there utilization in optical instrumentation for chromatography.
The pharmaceutical industry applications utilize this type of filter in UV analytical instrumentation, and range from GMP protein production to research of DNA. Other industrial use optical filters are utilized to monitor a variety of materials; from the color of beer to organic contaminants in waste water.
The role of filters in determining concentration of a material is to allow only the small portion of total light spectra that is sensitive, (absorbs the light) to the concentration of the material to be measured at the detectors. Filters are typically placed prior to the detectors in the analyzer so the light must pass through the filter prior to reaching the detector.
An ideal filter eliminates over 80% of the light produced by a typical UV lamp for the majority of optical applications, the remainder 20% of available light is then utilized for the measurement of a specific substance, for example a protein molecule. Industrial applications are a different type of analyzer, and at times pass as much as 70 % of the available light. Bandpass filters are designed with the application information of the specific absorbance of any given molecule/substance, The bandpass filter is then designed specifically for that application with regard to the logarithmic function of the instrumentation to scale the concentration at the specified levels. Filters therefore are application specific, and come in a variety of bandpass characteristics along with transmission levels. This can be seen and understood by comparing the radiant flux of a UV lamp with the band pass of a filter ideal for a particular application in figure 1.
By filtering out the light that contains no light absorbsation charesteric regarding the molecule/substance material being analyzed, filters enhance the signal to noise ratio of the measurement by several orders of magnitude and supply the instrument detector with only useable light spectra in which to scale the consentration of the molecule/substance.
Optical filters are selected based on the portion of a sample’s spectra that is most sensitive to changes in the absorbzation of light when the light is directed at the the material, in most cases a liguid solution. Gases act much in the same way of absorbing the light as well. The graph shows a molecule/substance that will be analyzed, represented in figure 2. This filter bandpass indicates the molecule/substance has a light absorbation at 280nm.
Selection of the UV Lamp light range and an optical filter bandpass is important to determine the correct lamp/light supply to ensure correct light and filtering. Application of the light and filtering requirements are essential in order to determine the active ingredient during the manufacturing, for example, of medical drugs, to the presence of water in fuels. The use of the photospecramater, is a mandatory tool to use in this process of determining the light absorption of the molecule/substance desired to measure.
The most commonly used filters in the Bio Medical industry are the 254 nm filter and the 280 nm filters prospectively. Other wavelength filters include, 206nm, 214nm, 226nm, 295nm, 302nm, and 313nm filters, depending on the type process and active ingredient absorption in the light spectrum. Highly regulated industries, such as pharmaceutical manufacturers, rely on accuracy so the proper selection and performance of the filter is critical. The filter must meet a specific specification when used in these processes and predicting when a filter may fail to meet these requirements is highly desirable. Depending on the light spectrum being isolated for use, that will determine the type of blockers utilized in the filter manufacture.
Basic Optical Filter Specifications Figure 3 shows two “ideal filters” waveforms. Designing UV filters with this specific band pass is not an easy task, due to the blockers and manufacturing processes that are currently available for the market. Type A, a Standard bandwidth has an ideal bell curve. Type B. Wide band pass filter with a flat transmittance over a wide wavelength range. There are two specifications that are commonly used to characterize waveforms (light spectrum of the band pass filter) of optical filters: bandwidth, and maximum transmittance, again application specific to every molecule/substance that is analyzed.A filter is specific to where and how it is being utilized, a ham radio for example, has a significantly wider filter in its use, unrelated to optical filters, due to the voice being transmitted over the air waves. A narrow bandpass filter would make every voice sound the same with little tonal distinction from person to person.
Figure4 demonstrates how a filter’s bandwidth is determined. The bandwidth of the filter is calculated at the 50% power, or transmission point. Some manufactures refer to the mid-power point; others state the 3dB power point of the filter. The two terms are synonymous with each other. The F1 light frequency is subtracted from the F2 light frequency, and that difference is considered the band pass of the filter. The bandwidth of a filter is frequently referred to as is bandpass or passband.
The filters utilized in chromatography monitoring generally have 10nm bandwidths. Figure 4 Optical Filter Bandwidth Determination Transmittance is the fraction of light that is passed though a filter express as a ratio of the transmitted light power divided by the incident light power; where the incident light power is the intensity of the light before entering the filter and transmitted light power is the intensity of the light after passing through the filter. The terms light power and intensity are used interchangeable. The maximum transmittance of a filter is the highest transmittance observed for the filter. Maximum transmittance is frequently reported with the wavelength of the maximum transmittance (9.7% at 281.3 nm).
Figure 4 shows a normalized output power of 1.0. Here the waveform of the filter has been scaled (normalized) by dividing by its maximum transmittance. Importance of Transmittance and Bandwidth The transmittance or passing of light in unwanted wavelengths adds to the overall light reading or measuring of the photo detector. It is important to keep in mind that as the active ingredients absorbs the “correct” light, and passes the unwanted light. A broad band detector (full spectrum reading) only adds to the noise level of the reading, and if the desired light reading is not sufficient, the noise level reduces the accuracy of the instrument.
The narrow bandwidth filters are better suited to pharmaceutical manufacturing, where only a small proportion of a process sample’s spectrum is sensitive to the material of interest. For example the spectra of a protein have a maximum around 280nm. Many of the contaminates, have a UV maximum at 254nm that falls off to zero around 300nm, thus the maximum signal-to-noise is achieved using a narrow bandwidth around the protein peak at 280nm.The maximum transmittance through the filter is important for maintaining a sufficient signal-to-noise ratio, so that the instrumentation can accurately measure the active ingredients in a sample or during a process run.
The concentration of a material in a sample is proportional to the absorbance of light. The absorbance of light is not proportional to the transmittance of the light through the sample, but equal to minus the log of the transmittance. Thus if you double the amount of material in a sample with an absorbance of 1 (10% transmittance) the absorbance increases to 2, but the transmittance drops to 1%. An instrument’s electronics must scale linear over several orders in transmittance due to this logarithmic relationship, and high transmittance levels of a filter become important at high sample concentrations.
A filter with sufficient narrow bandwidth and high transmittance provide the signal-to-noise ratio need to provide the selectivity and sensitivity to make accurate measurements. In some cases, tolerances of 0.5% can be achieved.
Armed with the correct information, one can determine a “good” filter from a “bad” filter in short order. Western Test Solutions has completed extensive testing of filters to determine the correct configuration of the filter being utilized.
Tests have been conducted in the laboratory with the specific instrumentation used in the manufacturing processes. Additionally, tests utilizing the photospectrameter are performed to ensure the highest quality filter to maintain the accuracy of the instrumentation in process.
Optical Filters Explained