I used to wonder if calling a target RETRO-reflective was redundant. In short, it’s not. In long-form, a reflective object is something like a mirror, or a chrome bumper, where the light that hits the surface is reflected in every direction. A retro-reflective object is a surface that sends light back to the original source. In the case of photography, we can provide a strong light source with every click of the shutter using the flash. Using the flash with retro-reflective targets makes for a strong combination in the field of photogrammetry.
As you likely know, photogrammetry programs commonly identify contrasting circles for target marking. Given that fact, a white circle printed on a black background would be a very good target. However, a better target would be a white retro-reflective material on a black background, as the contrast is increased. See the photo below, and try to find the two non-retro-reflective targets…not too hard, huh?
High contrast makes target selection easier and more accurate. For instance, Photomodeler indicates accuracy of 1 part in 30,000 can be achieved using an 11+ MegaPixel, field calibrated camera, with retro-reflective targets. 1 part in 30,000 means that an object with a largest dimension of 25 feet, say like a car, can be modeled with 0.01 inch accuracy at 68% probability (more on that in another article).
On top of that, since retro-reflective targets create such good contrast, they can be much smaller (or farther away) and still create a consistently selectable target. For a project with a target up to 22 feet away (which is a typical distance when photographing a sedan), Photomodeler recommends using a target diameter of about 1 inch (with a 14 MP camera). However, if you’re using retro-reflective targets, the target can be half that size, or less. Put another way, if you were not using retro-reflective targets, the farthest you should be from a 1 inch target is 22 feet, but if that target was retro-reflective, it could be 44 feet away and still allow for consistent, accurate selection. In addition, if a vehicle or scene has to be photographed at night, retro-reflective targets will offer optimum contrast, making nighttime documentation possible and accurate.
In short, there are three major benefits to using retro-reflective targets:
- Allows for smaller targets, or longer distance targeting
- Makes nighttime documentation possible and accurate
Hopefully this article helps you understand how retro-reflective targets can help with your future projects, and shows why Lightpoint chooses to use them in all of our projects.
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