Ground glass is

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

whose surface has been ground to produce a flat but rough (

matte Matte may refer to: Art * paint with a non-glossy finish. See diffuse reflection. * a framing element surrounding a painting or watercolor within the outer frame Film * Matte (filmmaking), filmmaking and video production technology * Matte p ...

) 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 refers to a variety of equipment used in scientific work, and traditionally made of glass. Glass can be blown, bent, cut, molded, and formed into many sizes and shapes, and is therefore common in chemistry, biology, and anal ...

Uses

Photography

photography Photography is the art, application, and practice of creating durable 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 employed ...

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

still A still is an apparatus used to distill liquid mixtures by heating to selectively boil and then cooling to condense the vapor. A still uses the same concepts as a basic distillation apparatus, but on a much larger scale. Stills have been use ...

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 on an image sensor or onto film stock, in order to produce a moving image to project onto a movie s ...

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

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

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

(or

loupe A loupe ( ) is a simple, small magnification device used to see small details more closely. They generally have higher magnification than a magnifying glass, and are designed to be held or worn close to the eye. A loupe does not have an attached h ...

). 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 A reflex finder is a viewfinder system with a mirror placed behind a lens. The light passing through the lens is reflected by the mirror to a focusing screen, usually ground glass. The image formed on this ground glass can be observed directly, gi ...

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 A rotary disc shutter is a type of shutter. It is notably used in motion picture cameras. Rotary shutters are semicircular discs that spin in front of the film gate, alternately allowing light from the lens to strike the film, or blocking it. D ...

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, 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 screen of an oscilloscope, ...

, 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. Because most films shot with

spherical lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

es are shot full-frame and later masked during projection to a more

widescreen Widescreen images are displayed within a set of 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 ratio greater than t ...

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 Frosted glass is produced by the sandblasting or acid etching of clear sheet glass. This creates a pitted surface on one side of the glass pane and has the effect of rendering the glass translucent by scattering the light which passes through ...

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 daylig ...

, namely on glass covers or enclosures for lamp fixtures, and sometimes on

incandescent bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...

s. Its functions include reducing glare and preventing

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

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 where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, o ...

Chemistry

Ground glass surfaces are often found on the glass equipment of

chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...

laboratories A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physici ...

. 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 A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

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 microsco ...

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 b ...

s are often ground on the sides and beveled 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 Henri René Albert Guy de Maupassant (, ; ; 5 August 1850 – 6 July 1893) was a 19th-century French author, remembered as a master of the short story form, as well as a representative of the Naturalist school, who depicted human lives, destin ...

short story "''La Confession''" 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 ''grain d'eglise,'' the term given by the French in India to the seeds of the Jaquirity or Rosary Pea plant (''

Abrus precatorius ''Abrus precatorius'', commonly known as jequirity bean or rosary pea, is a herbaceous flowering plant in the bean family Fabaceae. It is a slender, perennial climber with long, pinnate-leafleted leaves that twines around trees, shrubs, and hedge ...

''). The seeds contain a highly toxic lectin,

abrin Abrin is an extremely toxic toxalbumin found in the seeds of the rosary pea (or jequirity pea), ''Abrus precatorius''. It has a median lethal dose of 0.7 micrograms per kilogram of body mass when given to mice intravenously (approximately 3 ...

. The 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 types Laboratory glassware