How to Measure the Color of Dark Amber, Green or Blue Transparent Preforms

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FAQ: “Our customer has an accessory to do measurements with transparent preforms. My question is how I must to measure dark transparent preforms? On clear preforms, measurement data looks good. However, we could use some advice on darker colors, particularly amber.”

 

The images you sent show that the amber preform is held in our D01-1011-833 Adjustable Preform or Cylinder Holder accessory.

Measuring clear, lighter preforms is fine but amber preforms offer a special challenge as the intention of the amber colorant is to absorb all light in order to protect the blown bottle contents. The same statement can apply to very dark green and blue preforms.

While the pulsed xenon lamp in our HunterLab sphere instruments is very powerful and will blast through most materials, the amber preform sample is a challenge.

In general if you can hold a preform up to a light and see detail on the other side of the preform, HunterLab sphere instruments should be able to measure usable color signal from the whole transparent preform measured in the holder.

For amber preforms, here is how we approach measurement of amber preforms:

  • You first mount the D02-1011-833 Adjustable Transmission Preform or Cylinder Holder in the transmission compartment in one of our HunterLab sphere sensors.
  • You then standardize the instrument with the first step being to set 0% transmission as the bottom-of-scale using the Light Blocker.
  • Then you set top-of-scale with the white tile used to fill in the reflectance port and Air as a reference for 100% transmission.
  • If you insert a whole preform into the holder and take a reading, the Y D65/10 transmission value should be > 10% to have a useable color signal. A Y transmission value is equivalent to L* D65/10 value of approximately 40 (L* is calculated directly from Y). It's not that you can't measure less than this, but remembering that a blocked detector or perfect black is 0 if your whole preform is measuring below Y = 10/L* = 40, your signal is very low down in the dark and murky, and it is difficult to separate lot differences when you are measuring so low.
  • If Y < 10/L* < 40, one alternative to raise the signal is to cut the preform in half resulting in a thinner sample path length. Just cut down the tube to above the threads and measure a single wall cut-out of the preform. See if the signal is high enough in this form.
  • A further alternative is to blow the preform into a test bottle and measure through the bottle or make a single wall cut-out of the bottle and measure that. This form, will have high enough signal strength to differentiate lots. However, it requires more sample preparation time.
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