4.11.3.2. Lens and mirror 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.

Stereoscopes are essential tools used in fields like photogrammetry, which is the practice of making measurements through photographs. To see objects in three dimensions, we need at least two images taken from slightly different angles, similar to how our two eyes work. Each eye captures a different perspective of the same object, allowing our brain to combine these images into a 3D view. When using stereoscopes, any overlapping areas in the two photographs allow the viewer to see depth, making it easier to interpret the spatial relationships between objects.

Think about how you see the world with two eyes. Each eye sees a slightly different image of the same scene. Your brain merges these images, helping you perceive depth and distance. A stereoscope does the same thing with photographs, giving you a 3D view of what’s depicted, much like how you would see it in real life.

There are two types of stereoscopes used for three dimensional studies of aerial photographs; (i) Lens stereoscopes, and (ii) Lens and mirror stereoscopes.

Stereoscopes come in two main types: lens stereoscopes and lens and mirror stereoscopes. Lens stereoscopes, often referred to as pocket stereoscopes, are compact and lightweight. They consist of two lenses that allow each eye to see one photograph from the stereo-pair, producing a 3D effect. On the other hand, lens and mirror stereoscopes use mirrors and prisms to allow the user to view a larger overlap area of the photographs at once. This can be beneficial for detailed studies, although they tend to be heavier and less portable than lens stereoscopes.

You can think of lens stereoscopes like a pair of binoculars, but for photographs. They let you zoom into smaller details without needing heavy equipment. In contrast, lens and mirror stereoscopes are akin to a flat-screen TV where you can step back and see a broader picture all at once, helping you analyze the overall scene more effectively.

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.

Lens stereoscopes are compact devices that are convenient for fieldwork due to their portability. The design includes two curved lenses that magnify images from the stereo-pair of photographs. Each lens is positioned to correspond with the viewer's left and right eyes. This setup allows the user to adjust the distance between the lenses to accommodate their eyes' width, enhancing comfort during use. The small size and lightweight nature of these stereoscopes make them practical for outdoor settings.

Imagine using a small pair of binoculars that you can slip into your pocket. Lens stereoscopes work similarly; just as binoculars enhance your view of distant objects, lens stereoscopes enhance your perspective on photographs, allowing you to see things as if you were standing right there.

Mirror stereoscope is also called as Reflecting stereoscope for viewing of stereo-photographs in 3D. It consists of an arrangement of prisms and mirrors allowing almost entire overlap area to be viewed at a glance in the field of view. 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.

The mirror stereoscope, or reflecting stereoscope, enhances 3D viewing capabilities significantly. It employs mirrors placed at an angle alongside prisms to reflect light, enabling the viewer to see a large overlap area of both images simultaneously without needing to adjust the photographs much. This feature allows users to extract information effectively from vast landscapes or detailed scenes. The design not only magnifies the images but also maintains a comfortable viewing experience, expanding what you can analyze in a shorter period.

Think of a mirror stereoscope like the rearview mirror in a car that lets you see a broader view of what's behind you. Just as that mirror allows you to get a quick glance at everything behind you, the mirror stereoscope allows you to quickly view a more extensive area of the stereo-pair photographs, helping you see patterns and details you might miss when looking at individual images.

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