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FWT offers the following three Neutral Density Filter Sets:

Model NIST Traceable Use in Reader
FWT-82S No FWT-91R
FWT-82C Yes FWT-91R
FWT-160 Yes FWT-100
Each set consists of three filters with nominal optical densities (OD) of 0.3, 0.6, and 1.1 to 1.4. Actual OD’s may vary.

The FWT 82C and FWT-160 are supplied with certified transmission and optical densities. This certification is performed at Far West Technology using a spectrophotometer with measurements traceable to NIST (National Institute of Standards and Technology) through a set of reference materials. The certified values represent transmission and optical density values traceable to standards kept at NIST. These filters may be recertified to verify whether the transmission value of the filter has changed over time.

The FWT-82S, FWT-82C and FWT-160 filter sets are used in similar ways to verify the optical density readings of a Radiachromic or Opti-Chromic reader. Far West Technology recommends the following procedures for a routine periodic check:

1. Turn on the reader, allow it to warm up, and zero as usual.
2. (a) For the FWT-100 and FWT-200 readers run the Neutral Density Filter Check menu option and follow the instructions.
(b) For the FWT-91R and FWT-92(D) readers remove the film holder and insert each neutral density filter in the reader. Record values of each filter for the appropriate wavelength(s).
3. Verify that each reading is within expected tolerances.

Certified Neutral Density Filters read on a Radiachromic or Opti-Chromic Reader may exhibit different OD values than the certified values. This is to be expected and is due to such factors as wavelength and bandwidth of an interference filter versus those of a grating, as well as IR filtering and electronics. The difference may be viewed as a bias. The filters are used to verify that this bias is consistent over time. The certified filters relate the bias back to the original readings of the filters. It is recommended that these readings be established each time a lot of dosimeters is calibrated. Periodic readings afterward demonstrate consistency of the current readings to the readings with which the calibration curve was generated.

The acceptable tolerance of the filter readings will depend on the user's applications. Far West Technology recommends a maximum tolerance of ±5%, based on the values of a reader when that reader’s bias was determined. For example, a filter with a certified OD of 0.300 might read 0.288 in a reader when establishing the bias. The tolerance of the filter in that reader would then be 0.288 ±5%.

The FWT-82C and FWT-160 filters may be recertified. The FWT-82S filters may be certified, becoming an FWT-82C. Aging of the glass may cause some filters to change transmittance by about ±1% over a period of approximately one year. The frequency of recertification depends on the user’s overall requirements for uncertainties in the dosimetry system. A typical recertification interval might be between one and five years.
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Specifications subject to change without notice.
Far West Technology, Inc. © 2003 Fax: +1.805.964.3162 Tel: +1.805.964.3615 Email: info@fwt.com