5.6 Optical Filters
Opticalfiltersare devices that selectively transmit light in one or more specific
bands of wavelengths while absorbing or reflecting the remainder [ 12 – 16 ]. The
absorption or reflection properties give rise to the two broad categories of
absorptivefilters and dichroic filters, respectively. In an absorptivefilter, the
unwanted light spectrum is blocked by being absorbed and contained within the
filter. In adichroicfilterone spectral slice of light is reflected and another spectral
range is transmitted through the device. A characteristic of an absorptivefilter is
that light can impinge on thefilter from a wide range of angles and thefilter will
maintain its transmission and absorption properties. In contrast, a dichroicfilter is
highly angle sensitive. In general, an absorptivefilter is placed perpendicular to the
optical beam, whereas a dichroicfilter is placed at a 45° angle relative to the optical
beam.
Historically opticalfilters were designed to pass a single designated wavelength
or a single narrow spectral band. This characteristic imposed a time-consuming
limitation in areas such asfluorescence imaging when trying to analyze events
occurring at multiple wavelengths. In that case, different sets of emission and
excitationfilters had to be inserted into the optical path one at a time based on the
specific wavelength-dependent process that was being examined. Thus the ability to
evaluate live specimens or time-sensitive events was difficult.
The implementation of the worldwide telecom network brought a demand for
highly reliable dichroic opticalfilters with stringent spectral performance that could
be reproduced and controlled precisely during manufacture. This fabrication tech-
nology involves depositing a series of thin optical interference coatings on aflat
glass surface to create durable dichroicfilters that are resilient to temperature and
humidity effects. This durability is important for both telecom and biomedical
applications. These devices often are calledthin filmfilters(TFF). Such TFF
devices can be made to pass several different wavelength bands simultaneously and
thus allow the use of a series of color-dependent biomarkers to label and analyze
multiple time-sensitive events in the same specimen.
Some key parameters of dichroicfilters are illustrated in Fig.5.11. Several
categories of opticalfilters are of interest in biophotonics applications.Longpass
filtersallow only light above a specified wavelength to be transmitted, with all other
wavelengths being reflected or absorbed. Thecut-on wavelengthor central wave-
length of a longpassfilter is specified at 50 % of the peak transmission. Conversely,
shortpassfiltersallow only light below a specified wavelength to be transmitted.
Thecut-off wavelengthof a shortpassfilter is specified at 50 % of the peak trans-
mission. Longpass and shortpass optical filters can have a very sharp
wavelength-separation slope, in which case they sometimes are referred to asedge
filters.Bandpassfiltersare used to transmit one or more extremely narrow (<2–
10 nm) or broad (e.g., 50 nm or 80 nm) bands of wavelengths while blocking (or
138 5 Fundamentals of Optical Detectors