Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Taking Measure

Just a Standard Blog

Lighting the Way Toward Better Measurements

Howard Yoon with the NIST standard of spectral irradiance, a bright light, well-calibrated light bulb..

Here I am with the NIST standard of spectral irradiance. It's not turned up all the way. 

Credit: N. Hanacek/NIST

As a physicist who explores ways to measure light more accurately, it should come as no surprise that I’m fascinated by even common optical phenomena that we see all around us. For instance, rainbows occur because light travels through water and air at different speeds and because different colors of light travel at different speeds through water. Oil slicks on water, opal and butterfly wings look the way they do because of the way light waves bouncing off of different surfaces or facets interfere with one another.

But while these effects are beautiful, they have practical aspects as well. For instance, the same effect that causes rainbows also allows fiber optic cables to faithfully carry information thousands of miles.

Our understanding of light—and our ability to manipulate it—is helping us to improve our understanding of our world and how we affect it. Much of my own work focuses on ensuring that the instruments on remote sensors, such as satellites that measure Earth’s vegetation cover and land use, are working properly by comparing them against some known quantity or standard, a process called calibration.

My group is also working to improve temperature sensing using techniques based on light. More accurate temperature measurements with a lower margin of error will help climate scientists to make better models of climate change and better evaluate our strategies to address it.

Future technologies will use light-sensing methods. Self-driving cars rely on light-based technologies like radar and light detection and ranging (LIDAR) and get where they need to go using the global positioning system, or GPS. GPS only works because we are able to measure time and the speed of light with incredible accuracy.

In fact, the second itself is defined as the time it takes for specially energized cesium atoms to emit a little more than 9 billion waves of radiation.

Many of the base units in the International System of Units (SI) rely, in one way or another, on different wavelengths of light, from nanometer-long X-rays to kilometers-long radio waves.

The meter is defined as the distance light travels in a little less than one three-hundred millionth of a second. The candela is a measure of the intensity, or brightness, of a specific frequency of green light that most excites the human eye. The mole, used for counting atoms and molecules, is measured using a method called X-ray crystallography, and the practical realization of the ampere, the measure of electric current, relies on microwave irradiation.

The kilogram is the last SI unit based on a physical artifact, a cylinder of platinum and iridium. NIST is in the process of redefining the kilogram using a device called the watt balance. This machine will define the kilogram based on the amount of electromagnetic power it takes to hold a kilogram against the Earth’s gravity. Key to the watt balance is a laser interferometer, a technology that uses the wavelengths of light like the gradations on a ruler to measure distance with great accuracy and precision.

The proposed new definition of the kelvin, the SI unit of temperature, will use acoustic thermometers that measure the ratio between sound and light waves to give us measurements with the highest accuracies.

Light has other physical properties that are yet to be used to realize SI units. Here at NIST, researchers recently theorized it is possible to build “molecules of light.” We used photons to break distance records for quantum teleportation and to prove that “spooky action at a distance” really occurs. I’m excited to see what new innovations the use of quantum effects might bring. Will we one day use them to again redefine the SI units? Will that redefinition open up new technological areas that we have not already envisioned?

If history is any guide, I think they probably will.

So, I just want to say that light is amazing. Light lets us see things that our eyes didn’t evolve to see and study things that further the frontiers of knowledge. The power of light to help us make better measurements, and thus new and better technologies, is truly illuminating.

About the author

Howard Yoon

Howard Yoon has spent the past 25 years thinking about how to measure light more accurately, 19 of them as a researcher in NIST’s Optical Radiation Group. His work improves standards that make it possible to use light as a measurement tool. His favorite wavelength of light is 650 nanometers, the wavelength historically used for optically determining the temperatures of metals.

Related posts


Dear Howard Yoon , Your blog on better measurement by light is very informative , innovative, upto what accuracy measured unit can be achive . There must be some comparison with conventional and with light you may have arrived.
The realization of the derived units of pressure using the index of refraction of elemental gasses is yet another example where NIST and optical properties are defining the SI in ways that could be disseminated to field labs and locations worldwide with better uncertainties than ever before.
It was very informative and good article explaining SI units in terms of light. Thank you.
Having seen a candle burning on the floor have its flame lensed onto the ceiling by a second candle burning on the coffee table almost directly over it, I can find only a few articles on flames as lenses (laser attenuation in tubes of gas, for instance). Since flame use is so much older than electric lighting & etc, and since there is potential impact on the displacement of star images by the sun, on gravitational lensing, and on the usefulness of variable lenses, I wonder if you are familiar with more research on the subject. Thank you for your time.

Add new comment

Enter the characters shown in the image.
This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.
Please be respectful when posting comments. We will post all comments without editing as long as they are appropriate for a public, family friendly website, are on topic and do not contain profanity, personal attacks, misleading or false information/accusations or promote specific commercial products, services or organizations. Comments that violate our comment policy or include links to non-government organizations/web pages will not be posted.