HunterLab does not supply glass or plastic filter transmission standards but there are some sources below.
HunterLab would always encourage a client to regularly use the diagnostic options that come with our sphere instruments to verify instrument performance over time.
- Short Term Repeatability on the White Tile to verify the instrument electronics are in good health.
- Green Tile Test for verification that the instrument has not changed in long term photometric performance.
- Didymium Filter Test for verification that the instrument has not changed in long term wavelength performance.
If your sphere instrument gets a PASS on these three OQ (Operation Qualification) diagnostics tests, you can have a high degree of confidence that the instrument is operating correctly.
Additional transmission filters can provide an extra level of validation, particularly related to the measurement of transparent solid samples. Here are three situations where having one or more additional transmission filters could be helpful:
- When your product is a transparent solid, having a more stable; durable, cleanable and uniform glass filter to serve as a PQ Performance Qualification check filter can help validate the measurement process. Each day, after standardization, operators can read the check filter and compare to baseline values measured in the same color scales as the product. If the read values match the baseline values closely, the operator and instrument are performance qualified to measure production samples.
- Additional confidence in the measurement process can be gained by measuring the color of a uniform glass filter, similar in color to the product, over time. It does not have to be the exact same color as the product, just in the same area of color space. Documenting the filter results over time helps verify that the product color you measure today is the same as measured weeks, months or years ago.
- When you are conducting an inter-instrument agreement study of transmission color measurements taken on a transparent solid product at multiple sites, inclusion of a glass filter can allow outlier sites to be easily spotted. The glass (best) or plastic filters will be more uniform and stable than the product. All sites should agree closely on a uniform glass filter of similar color as the product. If not, the usual problem is that outlier sites do not have their instrument configured correctly for the measurement. If a site is an outlier on the check filter, their product measurements are also suspect. Stopping the test at this point and figuring out what is wrong is the best path forward.
Considerations in selecting Transmission Standards
The size of the filter must be at least 35 mm square to cover the TTRAN or RTRAN port of any of HunterLab’s sphere sensors.
A Transmission Clamp Accessory (HL# C02-1005-444) is useful to position these filters consistently at the port.
Filter standards can be purchased with traceable, assigned spectral values (calibrated) for the filters, or purchased uncalibrated and the client can baseline the filters themselves (uncalibrated) on an instrument designated as the reference sensor. HunterLab expects our instruments to read closely to third party standards but not necessarily in perfect agreement to the assigned values.
Also note that when you receive assigned values from a third-party site, they may not be in the same metrics as you measure in. For example, you may purchase calibrated filter standards that are assigned in spectral data and CIE L*, a*, b* D65/10 color values. If you are measuring your product in Hunter L, a, b D65/10 and Yellowness Index per ASTM E313, you will have to convert the assigned spectral data to the metrics your use.
Primary Sources for Transmission Filters
Primary sources for transmission filters (calibrated and uncalibrated) are:
NRC - National Research Council of Canada
Ottawa, Ontario K1A 0R6 Canada
NPL - National Physical Laboratory
Teddington, Middlesex TW11 0L United Kingdom
+020 8977 3222
Secondary Source for Transmission Filters
Secondary sources for calibrated and uncalibrated glass and plastic filters for industry are:
Avian Technologies LLC
Sunapee, NH 03782 USA
Avian offers calibrated and uncalibrated filters, as well as other co lorimetric tile standards.
Stamford, CT 06907 USA
Hoffman offers a Chromaticity Reference Filter Set of 20 – 2×2 inch acrylic filters used for providing the secondary Chromaticity and transmission standards needed in photometric and colorimetric laboratories. Hoffman Engineering is a NIST-traceable lab.
Alternate Sources for Transmission Filters
Other sources for uncalibrated 50 mm x 50 mm (2 in x 2-in) transmission filters – long band pass, chromatic, neutral density are:
Schott North America, Inc.
Duryea, PA 18642 USA
Edmund Optics, Inc.
Barrington, NJ 08007 USA
Color Measurement Method for Transmission Filters on HunterLab Sphere Instruments
- Position Transmission Clamp Accessory (HL # C02-1005-444) at TTRAN transmission port.
- Standardize in TTRAN LAV using Air as the top-of-scale standard to set 100% transmission.
- As a PQ Performance Qualification step, read back Air before beginning measurements. You should read L* = 100.0, a* = 0.0, b* = 0.0, Y transmission = 100.0 D65/10 closely.
- Check your filters for cleanliness.
- Read the transmission filters and compare them to the assigned or baseline values.