# Light Measurement Optical Filters

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 10nm.

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.

Bouguer's law describes the effect of thickness on transmission: log101) / d1 = log102) / d2, where t is the internal transmittance, and 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 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.

ILT's optical filters are the building blocks that enable our meters to be used in a vast array of applications. Mounted in threaded (1 1/4-24 thread) black anodized rings, all filters may be repeatedly interchanged on any SED, SEL, and SHD detector. Unmounted filters and empty filter rings are also available.

ILT 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. ILT weights the sensitivity at each wavelength using thin film techniques or composite absorptive layering to accurately indicate exposure levels in a single measurement. Please contact us using the form below for technical assistance.

Part Number, Description Specifications
A, Photoresist Filter Tailored filter designed to provide 'A' response (320-475 nm) with silicon sensors.
ACT5, Actinic/Germicidal Filter Multipurpose filter produces expanded actinic response or effective germicidal/bactericidal response when used with SED(SEL)240 Detector as well as UV Actinic Hazard response curve per ACGIH.
T2ACT5, Actinic Germicidal Filter / Diffuser ACT5 combined with T2X Diffuser. Provides extra sensitivity with good spatial response required for use with ILT1400 for optical hazard measurements.
B, Photoresist Filter Filter designed to provide 'B' response (326-401 nm) with silicon detectors.
BAS, Bilirubin Action Spectrum Blue light filter designed to modify the spectral sensitivity of silicon sensors to match the Bilirubin Action Spectrum curve
FUVC Far UV Filter Interference filters with center wavelength at 230 nm nanometers. Used to block longer wavelengths for testing excimer 222 nm lamps.
FFlat Response Filter Filter to be combined with silicon detectors to produce a flat spectral response.
HMR-1, Hot Mirror IR rejecting hot mirror. Combined with F filter for flat VIS response.
NS185, Narrow Band Filter, UV Interference filter with center wavelength 185 nm.
NSXXX, UV Narrow Band filters, Ultraviolet Interference filter with center wavelength at XXX nanometers, FWHM 10 nm. Standard filters available are NS254, NS280, NS313, NS335, NS350, NS365.
NSXXX, Visible Narrow Band Filter, Visible Interference filters with center wavelength at XXX nanometers, FWHM 10 nm. Standard filters available are NS400, NS405, NS436, NS500, NS546, NS600, NS633 and NS700.
NSXXX, custom Interference filter with center wavelength at XXX nanometers, for ranges not noted above. CUSTOM NSUV, NSVIS and NSIR, with many FWHM and CW ranges available, contact technical support for assistance.
NDFSS102, Neutral Density Filter Set UV-IR: filters mounted to fit SED(SEL)(SHD)(SL) detectors. QNDS1, QNDS2 and QNDS3 included.
PAR-1Photosynthetically Active Radiation  Tailored filter to be combined with silicon detectors. Combined quantum response is spectrally flat in the visible range.
QNDS1, Neutral Density Filter Quartz filters with 10 times attenuation.
QNDS2, Neutral Density Filter Quartz filters with 100 times attenuation.
QNDS3, Neutral Density Filter Quartz filters with 1000 times attenuation.
SCSXXX, Sharp Cut Filters Glass filters with a wavelength of XXX at 60% transmission. They pass wavelengths above XXX and absorb wavelengths below XXX. Standard filters available are SCS280, SCS365 (LIMITED AVAILABILITY), SCS395, SCS440, SCS465, SCS490, SCS520, SCS530, SCS545, SCS590, SCS605, SCS622, SCS650SCS695, SCS740, SCS780, SCS810, SCS830.
BLUE, Blue Color and Hazard Filter Filter mounted for use with silicon detectors.
TFRD, Far Red Color Filter Filter mounted for use with silicon detectors.
TRED, Red Color Filter Filter mounted for use with silicon detectors.
TLS312, Narrowband UVB Phototherapy Filter Narrow band filter for use with GaAsP detector, for TL-01 lamp measurement.
UVA, Phototherapy Filter Filter, when combined with silicon detector, produces UVA response from 315 to 390 nm (10%).
UVB-1, Phototherapy Filter Filter, when combined with solar blind vacuum photodiode, produces response from 265 to 332 nm.
UVB-2 Includes both a short pass and long pass filters to block UVC and UVA. Passes light from 280-310 nm.
UVF Flattens response of the 005 series UV-Visible GaAsP Detector over a range of ~350-405 nm (full range 275-450 nm)
WBSXXX, Wide Band Filters, UV-IR Mounted glass filters with center wavelength at XXX nanometers. (note, many WBS filters cannot be used with silicon sensors due to secondary response in the IR, so they are typically sold with the SED(SEL)005. Standard filters available are WBS320, WBS350, WBS375WBS400, WBS405, WBS440, WBS460, WBS465, WBS480, WBS520.
Y4 Photopic Correction Filter Proprietary blend of filter glasses combined to modify the spectral sensitivity range of ILT's silicon sensors to create and excellent match to the CIE photopic curve (with F' of 1.56). Works with ILT's SED033 and SED100 sensors to provide accurate lux, lumen, and cd/m2 readings when combined with optics and calibrations.
ZCIE, Scotopic Filter Filter used with silicon detectors to produce a match to the CIE Scotopic Response.
ZPM, Scotopic Filter Scotopic correction filter corrects photomultiplier SPM068-1 to CIE eye response to allow low light level cd/m2, lux and lumens measurements.