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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