Projection (optics)

A projection is the enlarged, reduced or otherwise modified transfer of an image onto a projection surface . A typical example is projection onto a screen with the aid of a projector . According to the type of original image, one differentiates z. B. episcopes , slide , shadow , film , video and laser projectors . With their lens, cameras usually produce a scaled-down projection of an image onto a medium (film or sensor ).

The oldest optical projection was done with the pinhole camera without a lens . The principle reversal from light to shadow is even older when the sun is depicted as a shadow point with the help of a nodus or as a shadow line with the help of a rod that casts shadows on sundials .

Basics

The projected image is generated by optical imaging . With the simple shadow projection, the image size results from the distance between the image and the point-like light source (if possible) and the distance between the object or original image and the size of the light source: ${\ displaystyle B}$ ${\ displaystyle b}$ ${\ displaystyle g}$${\ displaystyle G}$

${\ displaystyle B = {\ frac {b \ cdot G} {g}}}$

If there is a lens between the image source and the projection surface, the image is a real image . For its image size results from the ray theorem${\ displaystyle B}$

${\ displaystyle {\ frac {B} {G}} = {\ frac {bf} {f}}}$or ,${\ displaystyle B = {\ frac {G \ cdot (bf)} {f}}}$

where denotes the focal length of the lens. In addition to the imaging errors caused by the lens , the image will be distorted if the optical axis is not perpendicular to the projection surface or if it is not flat. ${\ displaystyle f}$

The laser projection is not a projection in this sense, since the image is not displayed optically, but written by the modulated laser beam .

Examples

• Wall projection in the cinema or during lectures and courses;
• Shadow : projection of an object outline through an approximately point-like light source (e.g. sun, moon, street lamp);
• Photography : making copies or enlargements of the negative;
• Photo and film cameras: projection of the object onto a chemical film or an image sensor ;
• The camera obscura or pinhole camera is an example of a scaled-down image by projection;
• Readers for microfiches : enlarged projection of archived documents onto a table or screen.