How do we describe the physical world around us? One of the most important ways is with numbers—numbers that tell us the length, breadth, height, shape, and texture of things, how far away they are, and in which direction. Even for those of us in photonics, it is astonishing how extensively the quantitative description of our environment depends on optical instruments.
The foundational unit of distance—the meter—has always been intimately linked to optical metrology. In 1792, amidst the chaos of the French Revolution, visionary scientists gathered to define a common measure of length for all mankind, based on a natural constant. They chose the size of the Earth itself, defining the meter as one ten-millionth of the distance from the North Pole to the equator along the Paris meridian. But how to measure the Earth?
In ancient Greece, Eratosthenes hired a professional “pacer” to measure the distance from Aswan to Alexandria by carefully walking the 800 km distance. This value, together with the seven-degree difference in latitude between these cities, provided a remarkably good estimate for the Earth’s circumference. Two thousand years later, the scientists who invented the metric system sought a more precise value, using better tools and accounting for the Earth’s imperfect shape. The key was an optical instrument called the cercle répéteur. Invented by Étienne Lenoir and perfected by Jean-Charles de Borda, this instrument measured angles with two telescopes affixed to graduated brass circles. Using error-canceling procedures, it achieved an astonishing accuracy of one second of arc, or 30 m positioning accuracy along a global meridian line. The same instrument could measure distances between cities and hence the Earth’s size using a mesh of triangular surveys, according to a method demonstrated in 1615 by Dutch scientist Willebrord Snell (yes, the same fellow who gifted us with the law of refraction).
Astronomers Jean-Baptiste Delambre and Pierre Méchain traveled for years across the countryside from Barcelona to Dunkirk, completing a network of 115 triangles in 1799. The task required compensating for uneven terrain, calculating the effects of refraction, and convincing local authorities they were not counterrevolutionary spies. They faced invading Prussian armies and imprisonment in Spain, while making the most precise geodesic measurements in history up to that time. The adventurous story, including how the final platinum Mètre des Archives included an accidental, permanent offset of 0.2 mm, is told in Ken Alder’s book The Measure of All Things.
Over the two centuries following the original determination of the meter, a succession of refinements explored the full range of optical methods of measurement. These included using microscopy to compare a new platinum-iridium universal standard meter to each of the national standards around the world in 1889; linking the meter to the wavelength of light in 1960; and finally, in 1983, redefining the meter as the distance traveled by light in a fixed fraction of a second. At every step, optics has been essential to the definition of the meter.
From this history and from all that we know about modern methods of measurement for science and industry, it is no surprise that a major branch of optics and photonics is metrology. This is reflected in SPIE conferences, from Photonics West in January to Optifab in October. One of my favorite events is the biannual SPIE Optical Metrology, the premier European celebration of optical measurements, co-located with the Laser World of Photonics trade show from 23-26 June 2025. Spanning several subdisciplines from modeling methods to industrial applications, with new applications in digital optical technologies and quantum sensing, this conference reaffirms what we already know: Optics and photonics are fundamental to measuring and understanding the world around us.
Peter de Groot
2025 SPIE President
