4.11.3. Stereoscopes

Stereoscopic vision is the basic pre-requisite for photogrammetry and photo-interpretation in 3D environment. Many applications need information extraction with stereo-images rather than mono-images. This requires two views of a single object from two slightly different positions of camera. The photographs are taken from two different positions with overlap in order to reproduce the objects in a manner as they are individually seen by the eyes.

Stereoscopic vision allows us to perceive depth and three-dimensional shapes by using two slightly different perspectives of the same object, similar to how our two eyes work together. When photographs are taken from two different positions, they provide overlapping views, enabling the brain to combine these images into a single 3D model. This process is crucial in fields like photogrammetry, where accurate visual representations are needed.

Think about how you look at a sculpture from two sides. Each side gives you a different perspective of the shape, and your brain combines these views to understand the sculpture's full depth and form. This is similar to how stereo-images provide depth perception.

Stereoscope is an optical device for 3D viewing of landscapes or objects. A stereoscope helps viewing a stereo-pair; left image with the left-eye and right image with the right-eye, to create a 3D model. To do so, the photographs are to be properly oriented below the stereoscope in the same manner as they were taken at the time of photography. When each eye views the respective image, these two images help creating a 3D view in overlap region. The fusion of common area of these two images in the brain will allow the judgement of depth or distance.

A stereoscope enables 3D visualization by presenting each eye with one of the two images taken from slightly different angles. By aligning the stereo-pair correctly, each eye sees its respective image, and the brain fuses them into one cohesive image with depth. The overlap area of the images is crucial as it provides a direct comparison that the brain uses to assess the distance and depth between objects.

Imagine looking through a pair of binoculars. Each barrel gives you a slightly different view of a distant landscape. Your brain works to combine these views into one image, allowing you to perceive distances accurately—a similar concept occurs when using a stereoscope.

There are two type of stereoscopes used for three dimensional studies of aerial photographs; (i) Lens stereoscopes, and (ii) Lens and mirror stereoscopes. (i) Lens stereoscopes: Lens stereoscope is also called as pocket stereoscope, as it can be kept in pocket owing to its small size. Being light-weight, it is easy to transport in the field, if required. It consists of two plano-convex lenses with magnifying capability, which are mounted on a metallic frame. The distance between these two lenses is adjustable as per the comfort of users’ eyes. The eye base average distance is approximately 65 mm for a human-being.

Stereoscopes come in two main types: lens stereoscopes, which are compact and portable, using two convex lenses to magnify images for 3D viewing, and mirror stereoscopes, which utilize prisms and mirrors to allow for a wider, simultaneous view of stereo-images. Lens stereoscopes, being small and lightweight, are ideal for fieldwork, whereas mirror stereoscopes enable viewing more extensive image areas, making them suitable for detailed analysis.

Think of a pair of glasses compared to a wide-screen display. Just like glasses enhance your vision of close items, lens stereoscopes allow for focused 3D views of smaller images. In contrast, a mirror stereoscope is like a large TV that lets you see many details at once in a broader perspective.

The main advantage of a mirror stereoscope is that the observer can see the entire overlap area from both the images in a magnified form, utilising the property of optics, even-though these photographs may not necessarily be located just below the lens. They are however heavier to carry in the field. The mirror stereoscopes are most widely used in stereo-photogrammetry and 3D measurements, in combination with an instrument called, parallax bar.

The mirror stereoscope's primary advantage is its broad field of view, allowing users to observe a larger area of the stereo-pair at once. This enhances detailed analysis in stereo-photogrammetry, though it sacrifices transportability due to its heavier construction compared to lens stereoscopes. Parallax bars are often used in tandem for accurately measuring differences in elevation and distance in 3D projects.

Consider using a professional-grade camera (mirror stereoscope) versus a smartphone camera (lens stereoscope). The professional camera offers more advanced features and a larger lens for a broader view but can be bulkier and heavier to carry around. Meanwhile, the smartphone is easier to transport but may not offer the same depth in images.

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