the following three Neutral Density Filter Sets:
Each set consists of three filters with nominal optical densities
(OD) of 0.3, 0.6, and 1.1 to 1.4. Actual ODs 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 readers
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 users overall requirements for uncertainties
in the dosimetry system. A typical recertification interval might
be between one and five years.