Method of measuring thickness of an opaque coating using infrared absorbance

A non-destructive method of determining an amount of opaque coating on a substrate, the method comprising:

• transmitting an infrared beam into an opaque coating on a substrate;
• collecting scattered infrared beams that are scattered by the opaque coating;
• detecting the collected infrared beams at a first wavelength and a second wavelength that is different from the first wavelength;
• comparing infrared energy Ic1 of the collected infrared beams at the first wavelength with a predetermined value of infrared energy Io1 of collected infrared beams at the first wavelength that are scattered by a reference substrate without the opaque coating to determine an absorbance value A1 for the opaque coating at the first wavelength, wherein the value A1 is a selected function of a ratio of Ic1 and Io1;
• comparing infrared energy Ic2 of the collected infrared beams at the second wavelength with a predetermined value of infrared energy Io2 of collected infrared beams at the second wavelength that are scattered by the reference substrate without the opaque coating to determine an absorbance value A2 for the opaque coating at the second wavelength, wherein the value A2 is a selected function of a ratio of Ic2 and Io2;
• and
• correlating a difference A1−A2 to an opaque coating amount.

A non-destructive method of determining an amount of opaque coating on a substrate, the method comprising:

• transmitting an infrared beam into an opaque coating on a substrate;
• collecting with a diffuse reflectance collector scattered infrared beams that are scattered by the opaque coating;
• filtering the collected infrared beams at a first wavelength and a second wavelength that is different from the first wavelength;
• detecting the filtered collected infrared beams at the first wavelength and the second wavelength;
• comparing infrared energy Ic1 of the collected infrared beams at the first wavelength with a predetermined value of infrared energy Io1 of collected infrared beams at the first wavelength that are scattered by a reference substrate without the opaque coating to determine an absorbance value A1 for the opaque coating at the first wavelength, wherein the value A1 is a selected function of a ratio of Ic1 and Io1;
• comparing infrared energy Ic2 of the collected infrared beams at the second wavelength with a predetermined value of infrared energy Io2 of collected infrared beams at the second wavelength that are scattered by the reference substrate without the opaque coating to determine an absorbance value A2 for the opaque coating at the second wavelength, wherein the value A2 is a selected function of a ratio of Ic2 and Io2;
• and
• correlating a difference A1−A2 to an opaque coating amount.