The Mystery of the Siemens Star: A Simple Pattern that Measures Sharpness of Eyes and Machines

The Mystery of the Siemens Star: A Simple Pattern that Measures Sharpness of Eyes and Machines. Have you ever seen a star-like pattern and wondered what it's for? This pattern is not just a decoration, it's a testing tool used from printers to space cameras. This article will reveal the secret behind the black and white circle that can measure the optical resolution limit of humans and machines.. Initially, I thought it was just a normal pattern Several years ago, when I was checking a laser printer in the office, I saw a piece of paper printed with a strange pattern. It looked like a black and white fan radiating from the same center, rotating like a star. My coworker, a graphic engineer, saw me staring and said, "That's a Siemens star. It's not a decoration, but a resolution test tool." I was amazed. All this time, I thought the pattern was just a retro decoration or maybe an old company logo. But apparently, the simple-looking circle is a scientific tool used to measure how far a device — a camera, printer, or screen — can see fine details. Without it, many optical technologies we use today might not be as accurate. From center to edge: a game of distance and sharpness The Siemens star or called spoke target among engineers consists of bright lines radiating from a central point. The closer to the center, the narrower the distance between the lines. In theory, at the exact center, the lines meet — but only in an ideal world. In the real world, every optical device has a limit. When the lines get too close, the device can no longer distinguish between black and white lines. That's where the blur point appears. A simple example: if you look at a Siemens star on a computer screen, at a certain distance you see a clear pattern. But when you zoom in to the center, you see a gray circle — a sign that the screen pixels are not fine enough to separate the lines. This phenomenon is called aliasing or blurring . The closer the blurry center, the lower the device's resolution. This is why the Siemens star becomes a benchmark for measuring sharpness in the printing and camera manufacturing industries. How engineers use this star to test printers and cameras In the printing world, the Siemens star is a simple yet powerful tool. For example, when a laser or inkjet printer prints this pattern, engineers can see with the naked eye how much of the pattern's center remains clear. If the center is blurry or disappears, it means the printer's resolution cannot print details finer than the size of the ink dots it produces. By comparing prints from various printers, they can determine the actual DPI dots per inch of each device. For cameras, the usage is more interesting. Engineers will print the Siemens star on high-quality paper, then take a picture from a fixed distance. By analyzing the image, they can see where the pattern's center starts to blur. This allows them to measure the optical resolution of the camera lens and sensor. In fact, in smartphone camera tests, the Siemens star is often used to compare sharpness between models — something that cannot be done with regular landscape pictures. The secret behind the moiré phenomenon and blurry circles One of the most interesting aspects of the Siemens star is how it reveals moiré patterns — interference patterns that appear when grids or repeating patterns are printed or displayed out of sync. When the Siemens star is printed at low resolution, moiré circles will appear, forming fake concentric circles or rings. This phenomenon is not only beautiful but also helps engineers detect sampling problems that may occur in digital imaging systems. Moreover, the Siemens star is also used in the video field — specifically for setting the back focus of interchangeable lenses on video cameras. This technique is called back focus chart . By recording a Siemens star mounted on a wall, technicians can adjust the lens until the pattern's center appears sharpest. This is crucial so that the image does not blur when the lens is changed from a close to a distant view. Without the Siemens star, this process would become more complicated and reliant on human interpretation. Surprise: this star is also used in astrophotography Perhaps many are unaware that the Siemens star is also used in astronomy to test telescopes. Although real stars in the sky are too far away to be used as test targets, astronomers often use printed or digital Siemens stars to measure the resolution of telescopes and CCD cameras. By observing how the pattern's center responds to starlight, they can evaluate the optical quality of the telescope — whether there is spherical aberration or astigmatism. Another surprising application: in 3D printing, the Siemens star is used to measure layer resolution. By printing this pattern on an object's surface, users can see whether fine details can be produced or are lost. This is particularly useful for those printing miniature models or mechanical parts that require high precision. Conclusion: a small circle that holds big secrets What started as a simple pattern resembling a star or a fan has become an essential tool in various industries. From printing offices to astronomy labs, the Siemens star proves that even the simplest designs can have a significant impact. It reminds us that sometimes, the answers to complex problems can be found in ordinary patterns. Now, every time I see a circular pattern on a screen or paper, I no longer consider it just a decoration. I understand that behind the blurry or clear center, data about the limits of the technology we create is stored. Perhaps, without realizing it, we have long been living in a world measured with the precision of the Siemens star. --- Reference: Siemens star — Wikipedia https://en.wikipedia.org/wiki/Siemens star