Neutral density filters
Midwest Optical Systems has developed a new line of neutral density filters for imaging at visible, near-infrared and near-UV wavelengths (350-1,100nm). These NI filters are available from stock in four optical densities that range from 50 per cent to 6.25 per cent overall transmission in all popular filter thread sizes.
Neutral density filters are used to reduce the amount of light reaching the sensor. This allows for longer exposure times and reduced depth of field by using a wider iris opening, thus improving the separation of subject matter from the background.
Unlike absorptive filter glass and non-reflective neutral density filters, the low-reflectivity NI filters from MidOpt feature uniform attenuation simultaneously over the entire visible and near-IR range imaged by most industrial CCD/CMOS cameras, with the actual per cent transmission (or optical density) remaining constant.
These filters also feature a reflection-inhibiting vacuum-evaporated protective coating that is designed to reduce undesirable back reflections while maintaining uniform transmission over an extremely broad spectral range. Compared to absorptive neutral density filters, MidOpt's NI filters use a low expansion glass substrate material to enhance durability and resistance to temperature extremes and thermal shock.
Optical densities in the NI filter line include NI030 (OD of 0.3, 50 per cent transmission), NI060 (OD of 0.6, 25 per cent transmission), NI090 (OD of 0.9, 12.5 per cent transmission) and NI120 (OD of 1.2, 6.25 per cent transmission).
NI filters are available off-the-shelf in standard threaded mounts, in thicknesses of 1, 2 and 3mm, and in custom shapes and sizes. Typical custom applications include reducing transmission from broadband light sources, low power laser attenuation and partial shrouding of inspection applications.