Optical Properties & Specifications
In order to help you better understand how our products are created and used, this section introduces some of the terms used to specify filter performance, offers suggestions for filter use and handling, and describes the characteristics of available products. For a more detailed explanation of these concepts, see our filter handbook.
Optical Quality Parameters and Properties
The following is a list of important parameters used to define the overall image quality of a filter or beamsplitter. The first three parameters are illustrated in Figure 18.
A measure of the deviation of the surface of an optical element from a perfect plane, usually measured in fractions or multiples of a wavelength of visible light (usually 550 or 630 nm, but sometimes using the CWL for a bandpass filter). The actual wavefront distortion that a plane wave of light undergoes when reflected from the surface is twice the value of the surface flatness.* The wavefront distortion of light reflected off a beamsplitter or mirror is solely determined by the surface flatness of the reflecting surface, usually the front surface.
*This is strictly true only for light reflected at normal incidence. The value for light reflected at non-normal incidence is modified by a cosine factor. For example, the reflected wavefront distortion at 45 degrees angle of incidence is approximately .4 times the surface flatness.
Transmitted Wavefront Distortion (TWD)
A measure of the distortion a plane wave of light undergoes when transmitted through an optical element is also measured in fractions or multiples of a wavelength of light, the same as for surface flatness described above. The surface flatness of the outer surfaces of the element and, to a lesser degree, internal structures that cause inhomogeneity of the refractive index, combine to make up the overall TWD of the optical element.
Sometimes called parallelism, wedge is a measure of how parallel are the outer surfaces of an optical element. Wedge is usually measured in arc-minutes or arc-seconds of angle. The main effect of wedge is to induce an angular deviation in the direction of a light beam, causing, for example, image shifting. The amount of angular deviation is about one-half the wedge angle for a typical filter.* The wedge of internal coating surfaces, while not contributing greatly to beam deviation, can cause noticeable ghost images as a result of off-axis internal reflections.
*For small angles of incidence, the angular deviation = (N – α), where N is the refractive index of the glass in the filter, and α is the wedge angle. Most filters use optical glass with a refractive index of approximately .5.
Figure 18: Illustrations of image quality parameters when working with filters
Scratches & Digs
A set of specifications exist for defining the maximum allowable size and number of scratches and digs on an optical surface. The term digs includes such things as particles and small bubbles embedded inside a filter and macroscopic inclusions in exposed coatings. The scratch/dig values (e.g., 80/50) specify the scratch width in microns and the dig diameter in tens of microns, respectively. Although extensive evaluation procedures exist if rigorous standards must be maintained (military specification MIL-F-48616, for example), a qualitative visual assessment of the scratches and digs usually suffices.
Small breaks in the coating of an interference filter, pinholes are usually caused by the presence of dust particles on the substrate during coating. Pinhole size must be measured against standard maximum-tolerance pinholes under specific conditions using a high-intensity illuminator.
The surface area of an optical filter which is free of any defects or obstructions. On interference filters the clear aperture is often delimited by an annulus of metal or opaque material around the outside edge of the filter. The clear aperture of optical filters should not restrict the overall aperture of the microscope. It is also of critical importance that there is no leakage of unfiltered light around the edge of the clear aperture.
Polarization indicates the vibrational orientation and phase of the electric and magnetic components of a light wave as it propagates space. These vibrations are transverse to the direction of propagation of the light, and can be oriented at some angle around the axis of propagation. When the orientation and the phase of the vibrations change rapidly and randomly in time, the light is said to be unpolarized. When the vibrations are restricted to one particular orientation angle over an extended length of time, the light is said to be plane-polarized. Light can be partially as well as totally plane-polarized. When light strikes a specular surface at non-normal incidence, the component of the electric field vibrations parallel to the plane of incidence of the surface (P-plane) behaves differently than the component perpendicular to the plane of incidence (S-plane). This causes a polarizing effect that is aligned orthogonally to the orientation of the surface. Dichroic beamsplitters (and, in fact, any thin-film interference coating that is used at non-normal angles-of-incidence) will cause some amount of polarization, the precise effect varying greatly with wavelength and with the particular coating design.
Specifications for Proper Use
Filter dimension varies greatly from microscope to microscope. Chroma’s most common filter size is 25mm diameter (~1”), whereas our most common dichroics are 25.5x36mm or 26x38mm. We can, however, produce optics of almost any physical specification that is required. Please contact us for more information.
Angle of Incidence (AOI)
The angle of incidence (AOI) is the angle between the optical axis of the incident light and the axis normal to the surface of the filter. Most filters are designed to be used at zero degrees angle of incidence, called normal incidence, but for beamsplitter coatings the usual angle is 45 degrees. It should be noted that most types of filters, including thin-film interference coatings, are “angle-sensitive,” which means that the characteristic performance changes with angle. If a filter or beamsplitter is to be used at any angle other than the usual zero- or 45-degree angle, it must be specified explicitly.
The performance of a filter is highly dependent upon its orientation in a microscope. In addition to being angle sensitive, fluorescence filters must be aligned and oriented correctly to be effective. Proper orientation of the filter is necessary in order to minimize autofluorescence and maximize performance. There is a caret (arrow) located on the edge of each filter in order to aid orientation. Excitation (x) filters should be positioned with the arrow pointing toward the specimen, toward the inside of the cube, and away from the light source. Emission (m) filters should be placed with the arrow pointing toward the specimen, toward the inside of the cube, and away from the detector/eye. Dichroic mirrors should be mounted with the coated surface toward the light source, excitation filters, and the specimen. The dichroics either have an arrow on the side pointing to the coated side, or they are beveled on the coated side. The beveled side is the smaller surface.
Cleaning & Handling
Handle coated pieces by the edges only. Clean gently only if necessary. Loose particles should be removed with a bulb puffer or filtered, pressurized air cleaner. If necessary, gently wipe surface using anhydrous alcohol and lint-free lab towels. Use new surface of towel with each wipe.
AVOID TOUCHING OR WIPING A/R COATED OR METAL MIRROR SURFACES
AVOID HANDLING EXPOSED COATINGS WITH BARE FINGERS
The two most important environmental factors to consider when dealing with optical filters are temperature and humidity. Sharp-cut longpass filter glasses have a shift in cut-on of approximately 0.1 to 0.5 nm per °C temperature change and some types of filter glass can be affected by high humidity or even unusual environments such as intense UV radiation (“solarization”). (Schott Glasswerke catalogue.)
The information regarding orientation and cleaning/handling can also be found on our Customer Support page.