EP 2.2.1 Defines three instrumental methods
1. Nephelometry - View the specimen at right angles ( 90 degrees ) to the direction of incident light. Use for NTU values less than 1750 to 2000 units.
2. Turbidimetry - A property of the specimen's ability to scatter or absorb light as opposed to transmitting it in a straight line through the sample.
3. Ratio Turbidimetry - ratio of the transmittance measurement to the 90 degree measurement
HunterLab does not offer 90 degree detection, so technically we only fully conform to a Turbidimetry measurement, but EP 2.2.1 says "Instruments with range or resolution, accuracy and repeatability capabilities other than those mentioned above may be used provided they are sufficiently validated and are capable for the intended use." So any instrument may be used as long as it is validated.
There are some samples that can only be correctly measured using Nephelometry, but HunterLab has been able to show valid correlations to Opalescence and lower NTU units when creating calibration curves using our benchtop transmittance capable instruments.
See an05_07 for a method to validate HunterLab instruments.
[Opalescence - an05_07](https://support.hunterlab.com/hc/en-us/articles/203995865-Opalescence-an05-07)