Refractive index is an effective and important physical property that can give insight into the quality of a firefighting foam, premixed solution, or bladder water sample. The refractive index is a measurement of the angle in which light bends as it passes through a substance and is proportional to the amount of solvent present in a sample.

What is Refractive Index?
Light can be thought as a wave of energy that moves through space. The speed of light in a vacuum is constant, but will vary when passing through a substance. Refractive index of a substance is the ratio of the speed of a light beam in a vacuum to its speed in the substance. The velocity of the light passing through a liquid can be calculated by the deflection of the beam of light. The refractive index is most accurate using a monochromatic light, so the yellow D line of sodium (wavelength 589 nm) is most widely used. Additionally, the refractive index is also very temperature dependent, the standard temperature is 20°C, which is good to keep in mind when using a refractometer in the field.

The refractive index is characteristic of a material and will depend on the chemical concentration and composition of a solution since different solutions will bend the light in different ways. This method is very useful in analytical chemistry and is a tool used in both chemical measurement and identification.

What does an “out of spec” Refractive Index mean on a Dyne Test Report?
Different firefighting foam concentrates have a range of different refractive indexes due to the various formulations that manufacturers use. In general, a foam used at a lower concentration has a higher refractive index. For example, a 1% AFFF will have a higher refractive index than a 3% AFFF and a 3% AFFF will have a higher refractive index than a 6% AFFF. Also, foam that is freeze protected tends to have a higher refractive index because of the glycols added. Typically, a range is created from the specifications noted on the product’s data sheet. When a refractive index for a product is not available, a generic refractive index range is used. Note, the physical properties are not a measurement of performance and as such will not cause the sample to fail. If the value is outside of the specification, it is simply noted as “out of spec”.

A refractive index can be “out of spec” on a Dyne test report for a few different reasons. It could indicate that the sample is a different type or brand of foam than indicated on the report. A refractive index that is lower than the specification can indicate a material with a lower refractive index has been added to the sample such as a different type or brand of foam concentrate, but typically indicates dilution with water. This is because the refractive index of water is typically lower than that of foam – the refractive index of tap and sea water (wavelength 589.3 nm) is approximately 1.3330 and 1.3390, at 20 degrees centigrade, respectively. Dilution can happen for some the following reasons:
1. Improper sampling such as contamination of water while sampling
2. Inadvertent dilution with water from systems failures such as a bladder rip,
3. The sample is a premixed solution or bladder water

A refractive index that is greater than the specification can indicate that a material with a higher refractive index has been added to the sample such as a different type or brand of foam concentrate.

For an explanation on all physical properties and performance properties tested on firefighting foam samples, refer to Foam Testing Explanation LBTR-4054. If you have any questions about appearance, polymer separation, sediment, mineral oil, or any other test, our chemists are ready to answer questions. Call Dyne at (800) 632-2304 or email,, for assistance with understanding your test results.