A Reseau plate is a transparent plate with fiduciary marker

Fiduciary marker

A fiducial marker or fiducial is an object used in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure...

s placed at the focal plane of a camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

 just in front of the film to provide a means of correcting images to enable them to be used for precision measurement. The crosshairs visible in photos taken on the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 are an example of this usage.

Description

Potential distortions include the incorrect position of the film in the camera, the distortions of the film by the development process, and (if digitally scanned) the nonlinearity of the image scanner. These errors may be visually imperceptible but limit the precisions of measurements. Because the position of the marks is very precisely known, the image can easily be corrected to account for these effects. When used for photogrammetry

Photogrammetry

Photogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century....

, multiple images can be combined using the fiducial marks and knowledge of the camera geometry to enable measurement of distances between objects in the images.

Use on lunar cameras

The Hasselblad Lunar Surface Data Camera was fitted with a Reseau plate. The Reseau plate was made of glass and was fitted to the back of the camera body, extremely close to the film plane. The plate was 5.4 x 5.4 cm in the film plane, which was the useful exposure area on the 70 mm film.

The Reseau plate was engraved with a 5 x 5 grid of crosses. The intersections of the crosses were 10 mm apart and accurately calibrated to a tolerance of 0.002 mm. Except for the double-sized central cross, each of the four arms on a cross was 1 mm long and 0.02 mm wide. The crosses (also known as fiducial

Fiducial

* In law the term "fiducial" means "of or pertaining to a fiduciary".* In imaging technology, a fiduciary marker or fiducial is an object used in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure.* "Fiducial" is also used for...

s) were recorded on every exposed frame and provided a means of determining angular distance

Angular distance

In mathematics and all natural sciences , the angular distance between two point objects, as observed from a location different from either of these objects, is the size of the angle between the two directions originating from...

s between objects in the field-of-view.

When the Hasselblad Lunar Surface Data Camera was fitted with a 60 mm lens, the images of the reseau crosses on the film have an apparent separation of 10.3 degrees. With a 500 mm lens fitted, the apparent separation is 1.24 degrees.