DARK SLIDE The opaque slide that covers the film in a film holder, plate holder, film pack, or film magazine.

DARKROOM A room in which all light of color quality not safe for undeveloped sensitized materials has been excluded. It is used for loading and unloading and the developing of exposed photographic film or paper.

DENSITOMETER Device to measure the optical density of an image or base by measuring the amount of incident light reflected or transmitted.

(NABDC)

DENSITY The light-absorbing quality of a photo-graphic image is usually expressed as the logarithm of the opacity. Several specific types of density values for a photograph may be expressed; however diffuse transmission density is one of the most important for photographic transparency materials, such as negatives. Diffuse reflection density is generally of interest for photographic prints.

DEPTH In a nonstereoscopic picture, an illusion of three-dimensional space that is sometimes created by a combination of favorable lighting and coloring of the set and favorable viewing conditions for the reproduction.

DEPTH OF FIELD The distance between the points nearest and farthest from the camera that are acceptably sharp at a given lens setting.

DEPTH OF FOCUS The allowable error in lens-to-film distance within which an acceptably sharp image of the subject focused upon will still be obtained.

DEVELOPER (CHEMICAL) A chemical reagent used to produce a visible image on an exposed photographic layer.

DEVELOPER, COLOR A photographic developing solution capable of reducing silver halides with the simultaneous production of an insoluble colored oxidation product in the regions where silver is deposited.

DEVELOPING AGENT Chemicals used in the photographic processing baths to convert the latent image into a visible and photographically useful image.

DIAPHRAGM A device, such as a perforated plate or an iris, that limits either the aperture of the lens, the field covered by the lens, or both, depending upon its location.

DICHROIC FOG A deposit of colloidal silver on a photographic film caused by improper processing. This deposit commonly appears red by transmitted light and greenish by reflected light.

DIFFRACTION (OPTICS) The bending of light waves around the edges of opaque objects.

DIFFUSER Device for obtaining diffuse direct illumination, such as a wire screen, piece of cloth, or translucent membrane, placed between a light source and the subject illuminated.

DIFFUSION (l) (Optical) The scattering of light rays so as to cause the light falling on a surface or passing through an aperture to be coming from all directions in contrast to the regular radiation of light from a point source. Diffusion may be introduced by reflection from a matte surface, by transmission through a frosted or opal glass, or by the use of an integrating bar. When diffusion is complete, a sharp image of the light source can no longer be seen, and its place is taken by a uniform, extended source that emits light equally in all directions. (2) (Chemical) The migration of molecules or ions in a solution tending to reduce a difference in concentration between two adjacent regions.