FAQ: “Do we have to keep the white tile at the reflectance port during transmittance readings?”
The answer is yes, you need to keep the calibrated white tile at the reflectance port of HunterLab sphere instruments whenever you perform transmittance measurements in TTRAN Total Transmission or RTRAN Regular Transmittance modes.
In colorimetric sphere instruments, the same fixed light path is used for color measurements in both reflectance and transmittance modes.
When you measure in transmittance, the lens is viewing where the sample is (TTRAN port in this case) but also continues to view through the sphere to the reflectance port so the sphere must be filled in with the calibrated white tile.
The calibrated white tile is not functioning as an instrument standard in this situation, but as part of the optical path during transmittance measurements. This calibrated white tile standard needs must be left at the reflectance port for all subsequent sample readings because it is effectively part of the optical path when you standardize the instrument to air or the transmittance cell + DI water as top-of-scale.
FAQ: "Does the procedure need to be performed with or without the white tile attached?"
When you standardize on the cell + DI water serving as a reference for 100% transmittance (equivalent to 0% absorbance), the calibrated white tile must be at the reflectance port filling in the reflectance port on the sphere. As the calibrated white tile is effectively part of the sphere wall, it is left there for all subsequent transmittance measurements.
FAQ: “We only measure samples in transmittance. Should we put the white calibrated tile back in the standards box when the instrument is not in use?"
The calibrated white tile is well protected at the reflectance port and could be left there all the time if all you are doing is transmittance color measurements. Pressed flush against the reflectance port, the tile is as well protected here as if it were in the standards box. It is only exposed to light during the brief interval a flash measurement is taken, so any fading over years of operation is not a problem. And, you will not have to remember to put it back for measurements… :)
Similarly, I suggest you leave the instrument power on. With a pulsed xenon lamp, the only power that is consumed is to power the green LED light and minimal electronics. The only time power is consumed is when a measurement is being taken. Powered up, the unit is ready to take measurements at any time.
FAQ: "We have a ColorQuest XE that we use to measure transparent PET plaques for yellowness (YI E313 C/2). Both the calibrated white tile in the black holder and the round disk we have on the sample clamp made of same TiO2 porcelain-on-steel. Yet you always recommend using the calibrated white porcelain instrument standard - why is this?"
When you are measuring transparent materials, particularly ones that have very little color, any difference in the optical path can make a difference in the measurement. The white tile at the reflectance port is part of the measurement.
There is some variation in the manufacture of these TiO2 porcelain-on-steel materials (less so with Japanese opal which is highly consistent). HunterLab has a tighter tolerance for the inter-piece agreement used for an instrument standard than the disk used to back samples. It is best practice to promote inter-instrument agreement at multiple sites to always use the calibrated white tile at the reflectance port when measuring the color of transparent samples.
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