Ground glass is

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

whose surface has been ground to produce a flat but rough ( matte) finish, in which the glass is in small sharp fragments. Ground glass surfaces have many applications, ranging from ornamentation on windows and table glassware to scientific uses in optics and

laboratory glassware Laboratory glassware is a variety of equipment used in science, scientific work, traditionally made of glass. Glass may be blown, bent, cut, molded, or formed into many sizes and shapes. It is commonly used in chemistry, biology, and analytical ...

Uses

Photography

photography Photography is the visual arts, art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is empl ...

, a sheet of ground glass is used for the manual focusing in some

still A still is an apparatus used to distillation, distill liquid mixtures by heating to selectively Boiling, boil and then cooling to Condensation, condense the vapor. A still uses the same concepts as a basic Distillation#Laboratory_procedures, ...

and

movie camera A movie camera (also known as a film camera and cine-camera) is a type of photographic camera that rapidly takes a sequence of photographs, either onto film stock or an image sensor, in order to produce a moving image to display on a screen. In c ...

s; the ground-glass viewer is inserted in the back of the camera, and the lens opened to its widest

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

. This projects the scene on the ground glass upside down. The photographer focuses and composes using this projected image, sometimes with the aid of a

magnifying glass A magnifying glass is a convex lens—usually mounted in a frame with a handle—that is used to produce a magnified image of an object. A magnifying glass can also be used to focus light, such as to concentrate the Sun's radiation to create ...

(or loupe). In order to see the image better, a dark cloth is used to block out light, whence came the image of the old-time photographer with his head stuck under a large black cloth. A ground glass is also used in the reflex finder of an SLR or TLR camera. In motion-picture cameras, the ground glass is a small, usually removable piece of transparent glass that sits between the rotary disc shutter and the viewfinder. The ground glass usually contains precise markings to show the camera operator the boundaries of the frame or the center

reticle A reticle or reticule, also known as a graticule or crosshair, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the electronic v ...

, or any other important information. Because the ground glass is positioned between the mirror shutter and the viewfinder, it does not interfere with the image reaching the film and is therefore not recorded over the final image, but rather serves as a reference for the camera operator. Ground glasses commonly serve as a framing reference for a desired

aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...

. Because most films shot with spherical lenses are shot full-frame and later masked during projection to a more

widescreen Widescreen images are displayed within a set of aspect ratio (image), aspect ratios (relationship of image width to height) used in film, television and computer screens. In film, a widescreen film is any film image with a width-to-height aspect ...

aspect ratio, it is important not only for the operator to be able to see the boundaries of that aspect ratio, but also for the ground glass to be properly aligned in the camera so that the markings are an exact representation of the boundaries of the image recorded on film.

Lighting

Ground or frosted glass is widely used as a weather- and heat-proof light diffuser in ambient

lighting Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. ...

, namely on glass covers or enclosures for lamp fixtures, and sometimes on incandescent bulbs. Its functions include reducing glare and preventing

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

l damage by direct sight of the lamp filament. This hides unsightly details of the lamp and fixture without blocking its light, yielding a softer illumination without giving hard

shadow A shadow is a dark area on a surface where light from a light source is blocked by an object. In contrast, shade occupies the three-dimensional volume behind an object with light in front of it. The cross-section of a shadow is a two-dimensio ...

Chemistry

Ground glass surfaces are often found on the glass equipment of

chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...

laboratories A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which science, scientific or technological research, experiments, and measurement may be performed. Laboratories are found in a variety of settings such as s ...

. Glass flasks, stoppers, valves, funnels, and tubing are often connected together by

ground glass joint Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from interchangeable commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints may ...

s, matching pairs of conical or spherical surfaces that have been ground to a precise shape. Flasks and

test tube A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top and closed at the bottom. Test tubes are usually placed in s ...

s often have a small ground-glass label area on the side. (Pencil writing on ground glass is largely inert, rub-proof and waterproof, but can be easily erased.)

Technical optics

optical microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of micros ...

may include a ground- or frosted-glass

diffuser Diffuser may refer to: Aerodynamics * Diffuser (automotive), a shaped section of a car's underbody which improves the car's aerodynamic properties * Part of a jet engine air intake, especially when operated at supersonic speeds * The channel bet ...

to evenly illuminate the field behind the specimen.

Microscope slide A microscope slide is a thin flat piece of glass, typically 75 by 26 mm (3 by 1 inches) and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is mounted (secured) on the slide, and then ...

s are often ground on the sides and

bevel A bevelled edge (UK) or beveled edge (US) is an edge of a structure that is not perpendicular to the faces of the piece. The words bevel and chamfer overlap in usage; in general usage, they are often interchanged, while in technical usage, they ...

ed on the corners to soften the edges for safer handling.

Abrin

Popular belief for many centuries is that ground-up glass (i.e., glass broken into tiny fragments) can kill if swallowed. In fact, this is a myth, as it is largely ineffective. The Guy de Maupassant short story concerns a jealous girl who poisons her older sister's suitor by inserting ground-up glass into cake. The term "ground glass", as it relates to poisoning, is a corruption of , the term given by the French in India to the seeds of the jaquirity or rosary pea plant ('' Abrus precatorius''). The seeds contain the extremely toxic lectin abrin, whose toxicity is over 30times that of

ricin Ricin ( ) is a lectin (a carbohydrate-binding protein) and a highly potent toxin produced in the seeds of the castor oil plant, ''Ricinus communis''. The median lethal dose (LD50) of ricin for mice is around 22 micrograms per kilogram of body ...

. These seeds have been used in India to kill cattle, and in homicides. Captain F. C. Briggs, adjutant to General Reginald Dyer, died of 'powdered glass' poisoning before he could give evidence to the Hunter Commission examining the Jallianwalla Bagh massacre.

References

External links

Ground glasses available from PanavisionGround Glass focusing screens available from hopfglass
{{From Camerapedia, Ground glass back Optical components Photography equipment Glass shaping Glass types Laboratory glassware