Neutral density (ND) filters available from Omega Optical can evenly attenuate the light’s intensity across a wide spectral range.
Attenuation, or optical density (OD), is achieved in two ways: by a combination of reflection and absorption using a thin-film metal coating, or by absorption using a light-absorbing glass.
ND filters are capable of attenuating spectral regions chosen from 250 to 2500 nm. OD can be defined as −log10 (T) and can be specified between 0.04 and 4. Since the overall attenuation is additive, ND filters can be arranged on top of each other to get the required OD level.
Absorption Glass Neutral Density (ND)
Gray-tinted and uncoated glass attenuates completely by absorption. The extent of attenuation is governed by the width of the glass. The absorption glass neutral density filters are usually effective only in the visible region of the spectrum and are not transmissive in the UV region. The filters’ surface can be anti-reflected to offer the required OD with absorbance, as well as very low reflectance.
Metallic-Coated Neutral Density (ND)
Metallic-coated neutral density filters perform attenuation by a combination of absorption and reflection. Therefore, this kind of neutral density filter is relatively more neutral or uniform when compared to the absorption glass filter. Metallic-coated neutral density filters can be produced for a broader spectral range (refer to Figure 1 for a comparison).
Figure 1. Neutral density comparison.
A comparison of a metal-coated neutral density filter and an absorption glass is shown above.