Optical Density (OD) is defined as the logarithm to the base ten of the ratio of the power of the incident beam to that of the exiting beam. The optical density can be convert to transmittance using the following equations:

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“The Scientific side of the business needed the leadership and entrepreneurial spirit to take it to the next level,” Edmund says. “That’s why we think VWR was a good fit.”

Edmund Scientific, which targets some educators in addition to hobbyists, will retain its name under the $50 million SK. “The Edmund Scientific name has a lot of history,” says Randy Burkard, president of Tonawanda, NY-based SK.

Burkard says his company plans to share an as yet undetermined number of Edmund products with three SK catalogs that mail to teachers. At the same time, some items from the existing SK catalogs will be added to the Edmund book. But the company does not plan to mail Edmund books to the SK lists or SK books to the Edmund house file.

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Solid glass absorptive ND filters are relatively neutral in the 400 to 700 nm range. They are usable from 350 nm to 2500 nm, but the transmittance is different from that expected from the density value. Since most of the incident energy is absorbed, there filters produce fewer problems from multiple reflected beams comparing to the metallic type, but they are intended only for use with lower power sources.

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B-TO-B A BETTER BET Until the early 1980s, Edmund Scientific had been the cataloger’s flagship book. But during the past 20 years, Edmund Optics de-emphasized it in favor of its Edmund Industrial Optics catalog, which sells optical and imaging component solutions to semiconductor, telecommunication, and medical professionals. Whereas Edmund Scientific accounted for 75%-80% of the company’s sales in 1980-81, by this year it accounted for only 20% of overall sales.

These ND filters attenuate by absorption (and Fresnel reflection, the constant reflection from the air-glass interfaces). The absorption is iconic in anature so this type of density filter follows the Beer and Bouguer"s Laws.

Four years after putting Edmund Scientific on the selling block, Edmund Optics has sold the $10 million science hobbyists catalog to Science Kit & Boreal Laboratories (SK), a division of $1.46 billion laboratory supplies cataloger VWR Scientific Products. Terms were not disclosed.

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Neutral density (ND) filters are used to equally attenuate the intensity of a light beam over a wide wavelength range. ND filters range from colorless to gray in appearance. Metallic neutral density filters provide a relatively consistent degree of attenuation over a wide spectral band. We offer a variety of catalog ND filters for UV/VIS applications including absorptive, metallic on glass or fused-silica, and variable linear or circular, as well as filters for infrared applications.

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Shop and browse all of our standard Neutral Density Filter models, or select a product series below for more information on our products and capabilities.

The metallic ND filters use a thin coating of inconel on a glass or fused silica substrate. The inconel material contains a number of different elements such as nickel (Ni), chromium (Cr), cobalt (Co), and iron (Fe). Careful control of the alloy composition and vacuum deposition produces coatings which are spectral neutral over a wide wavelength range. The metallic ND filters are generally more neutral comparing to the absorptive type of ND filters. UV Fused silica substrate are useful in the ultraviolet spectrum, while the cost effective B 270 optical glass substrate provides excellent performance in the visible and Near-IR spectrum range. Always use these filters with the reflective side facing the radiation source. The metallic ND filters withstand higher power and thermal shock better comparing to the absorptive type. These filters are spectrally neutral beyond 2 µm.

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NOTE: Do not place ND filters in series with their surfaces parallel. Parallel surfaces may cause multiple internal surface reflections which can combine to cause an increase in transmittance; this is especially true of the metallic type filters.

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Neutral Density filters can be stacked in series to produce higher densities. The total optical density is the sum of the individual densities. The total transmittance of a stack of neutral density filters is the product of the transmittance of each filter.

The $48 million Barrington, NJ-based Edmund Optics is retaining its core title, business-to-business catalog Edmund Industrial Optics. “The growth rates were faster on our optical b-to-b business,” says Nicole Edmund, vice president of marketing. “So with a limited amount of resources, we wanted to stick with what was giving us our greatest return.”