| Optical filters are often used to attenuate certain wavelengths of light
and produce an overall desired detector response. Interference filters rely on
harmonic interference between waves to provide very narrow passbands. Thin metal
films are spaced half the desired wavelength apart by a dielectric spacer.
Interference filters are capable of bandwidths less than 10 nm.
Absorptive
filters typically consist of glass that has been doped with a concentration of
dye that absorbs particular colors. Since spectral transmission varies
logarithmically with thickness, the band pass can be reduced by thickening the
filter. Bouger's law describes the effect of thickness on
transmission:
log10(t0) /d0 =
log10(t1) /d1
where t is the
internal transmittance, and d the thickness, for two filters made from the same
material. Internal transmittance is greater than external transmittance due to
reflection losses. Fresnel's law of reflection states that the fraction of light
reflected for normal incidence is:
r =
(n-1)2 / (n+1)2
where n is the
ratio of the refractive indexes of the two media. If n = 1.5 for air and glass,
then r = 4% for each surface. Two filters separated by air transmit 8% less than
two connected by optical cement (or even water). Multiple color filters can be
layered to match a desired function, such as the CIE photopic response, which
defines the eye's sensitivity to color. The combined spectral response is the
product of the filter's transmission and the spectral responsivity of the
detector.
International
Light optical filters are the building blocks that allow a myriad of
applications. Mounted in threaded (1 1/4-24 thread) black anodized rings, all
filters may be repeatedly interchanged on any SED, SEL, SUD, SUL, SL, SHD or PM
series detector. Unmounted filters and empty filter rings are also available.
We can also
create weighted measurement functions for applications such as the new UV
Actinic Hazard standard, where UV exposure at a critical wavelength has a
proportionally greater effect (as defined by ACGIH, NIOSH) than exposure at
nearby wavelengths. We weight the sensitivity at each wavelength using thin film
techniques or composite absorptive layering to accurately indicate exposure
levels in a single measurement. Please contact an
Applications Engineer at International Light for technical assistance.
|
Listed below
is a representative selection of the many filters that are available from stock
for custom spectral responses not found as standard configurations. These
filters allow the researcher to isolate, block or subtract specific parts of the
spectrum. Filters are available in 1/2 and 1 inch diameters.
The F Flat Response Filter
is a multilayer absorptive filter that provides uniform spectral sensitivity
over a wide spectral range for accurate radiometric and laser power
measurements.
Photopic and Scotopic Filters
are multilayer glass filters that imitate the human eye's day and night vision
response for illuminance and luminance measurements.
Broadband Color Filters
measure exclusive visible and near IR bands for primary color and plant growth
energy measurements. The PAR Photosynthetic Active Radiation Filter is a
multilayer glass filter that matches the growth response of plant
life.
The QNDS Quartz Neutral Density
Filters are multilayer thin film attenuators designed to decrease
sensitivity through uniform spectral attenuation, for intense
sources.
Narrow Band Filters are
thin film interference filters that transmit only a narrow bandpass, such as 254
nm, for measuring at mercury emission lines, laser lines, or any application
requiring an extremely narrow spectral bandpass.
Wide Band Optical Glass
Filters are required for uses involving a broader spectral range, such as
photoresist applications, or custom bandpass modifications. Available from 320
to 825 nm.
Sharp Cut Filters are
excellent "high pass" filters, blocking all light below a certain wavelength.
They are most commonly used to block short wavelength light, subtract long
wavelength light, or modify a bandpass. They can also be used, however, to
create inexpensive narrow bandpass filters. For example, a calibrated
SED033/SCS545 can be used to measure all of the light above 545 nm. Next, an
SCS562 is screwed on top of the SCS545 to measure only the light above 562 nm.
This second reading is divided by the transmission of the SCS562 (0.90) and
subtracted from the first reading. The result is the amount of light at 550 nm
(yellowish-green). |
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