SEPTEMBER 2007 

Grain Moisture Meter (GMM) Series Part 3 - 

Grain Moisture Meter Measurement Technology - Near Infrared 

By G. Diane Lee 

(in collaboration with John W. Barber) 

Currently there are two technologies used in commercial grain moisture measuring 

devices in the United States—dielectric and near infrared. GMMs measure an optical or 

electrical property of the grain. When these optical or electrical properties are compared 

to known grain moisture values, a calibration for the grain can be developed, and based 

on this calibration the device can provide a moisture reading related to the measurement 

of the optical or electrical property of that grain. The September 2006 Weights & 

Measures Quarterly article, “What do Grain Moisture Meters Measure and How are They 

Calibrated,” addressed the dielectric technology (measuring the electrical property in 

grain). Part 3 of this series will review near infrared technology. Much of this article is 

based on information from a presentation by John W. Barber, “Grain Moisture Meters 

from Theory to Practice.” 

Electromagnetic Spectrum 

Near infrared grain moisture meters measure the effect of moisture on optical properties 

of grain in the near infrared portion of the electromagnetic spectrum. To begin this 

discussion let’s first discuss the electromagnetic spectrum. The electromagnetic 

spectrum is the name for types of radiation energy. Near infrared is a type of energy that 

is part of the electromagnetic spectrum. The electromagnetic spectrum consists of radio 

waves, microwaves, infrared light, visible light (light visible to our eyes), ultraviolet 

light, x-rays, and gamma rays. These energies are also described in terms of their 

wavelengths or their frequencies. The spectrum ranges from lower radiation energy, 

“radio waves” (long wavelengths, low frequency, and low energy), to higher radiation 

energy, “gamma rays” (short wavelengths, high frequency, and high energy). 

Radioactive materials can emit gamma rays. Most people may think of these two 

energies, radio waves and gamma rays, as different and in some respects they are 

different. Exposure to gamma rays is harmful, but we are exposed to radio waves on a 

daily basis through the television and radio. But, while they are different in that respect, 

they are similar in that radio waves and gamma rays are both electromagnetic radiation. 

Electromagnetic radiation can be described as a stream of photons. A photon is a 

measurement of the smallest unit of light/electromagnetic energy. Each photon contains 

an amount of energy. The difference between the various types of electromagnetic 

radiation in the electromagnetic spectrum is the amount of energy in the photon. As 

noted above, gamma rays have high energy and radio waves have low energy (see 

diagram of the electromagnetic spectrum below). 
Courtesy of the Advanced Light Source, Berkeley Laboratory 

Near Infrared Grain Moisture Meter Technology 

Infrared energy was discovered by Sir William Herschel in the 1800s. He discovered that 

there is energy from the sun that lies below the red region of visible light. He called this 

energy, “calorific rays.” This energy is now called infrared. 

 

In the early 1900’s W. W. Coblentz discovered that compounds with similar chemical 

groupings have characteristic absorption bands in the infrared region. The main 

constituents of grain (protein, oil, starch, and water) all have characteristic absorption 

bands in the near infrared region (see Diagram 1 of the electromagnetic spectrum.) 

(Diagram 2) 

Diagram courtesy of John W. Barber, Grain Moisture Meters from Theory to Practice

As depicted in Diagram 2, absorption bands can be detected when full-spectrum, collated 

light is passed through a sample, such as grain, and the transmitted light is directed 

through a slit to a prism that separates the transmitted light into spectral components with 

different wavelengths. A typical near infrared grain analyzer scans the sample of grain 

with monochromatic light over wavelengths ranging from 850 to 1050 nanometers and 

measures the transmitted energy for each wavelength scanned. In a grain analyzer a 

sample of grain is dropped through a hopper (1) to a sample chamber (6), which has 

windows on both sides of the chamber. Light from a scanning monochromator (5) is 

applied to the chamber filled with grain and a detector (7) senses the amount of near 

infrared light passing through the chamber. See Diagram 3 on the following page. 
(Diagram 3) 

Diagram courtesy of John W. Barber, Grain Moisture Meters from Theory to Practice 

Using the Beer-Lambert Law, where A is absorbance, a is a constant associated with a 

constituent at a particular wavelength, b is the path length through the sample, and C is 

the constituent concentration, a relationship of the absorbance of light to the properties of 

material through which the light is traveling is established. 

(Diagram 4) 

Diagram courtesy of John W. Barber, Grain Moisture Meters from Theory to Practice 
When an infrared radiation source (light) is applied to a grain sample, characteristic 

absorption bands can be observed. This optical characteristic can be compared to known 

moisture values to develop a calibration for the near infrared grain moisture meter. This 

absorption is affected by the amount of moisture, protein, starch and oil in the sample, the 

kernel shape and condition, kernel packing, foreign material, and temperature of the 

sample. Therefore, there are differences in the absorption for different grain types and 

classes and thus different calibrations for different grain types. 

NIST WMD greatly appreciates the contributions from John W. Barber of J B Associates 

in the development of this article. 

Look for Part 4 in this series of articles on grain moisture meters in the November edition 

of the W&M Quarterly which will address field test methods, testing apparatus and 

equipment for the evaluation of GMMs. If you have any questions or comments, you 

may contact Diane Lee by e-mail at diane.lee@nist.gov. 

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