HIGH CONTRAST–A term expressing a relationship of image tones in which highlights and shadows are represented by extreme differences of density. HIGH  KEY–A term applied to a photographic print or subject consisting entirely of light tones with little contrast; also applied to a method of lighting a subject. HIGHLIGHT–The bright parts of a picture or subject that are rendered as dense areas in the negative and by very low density in the print. HUE–That attribute of certain color perceptions in respect to which they differ characteristically from the gray of the same lightness and which permits them to be classed as reddish, yellowish, greenish, or bluish. HYDROMETER–Generic     term     for     various instruments designed to determine the specific gravity of liquids. (PIA) HYDROQUINONE C₆H₄(OH)₂–Common  photo- graphic  developing  agent  para-dihydoxybenzene. HYPERFOCAL DISTANCE–The distance from the optical center of lens forward to the nearest plane in acceptable focus when the lens is focused at infinity distance. AI-9 ILLUMINANCE –Luminous flux incident per unit area of a surface. Widely known as illumination. IMAGE, LATENT–The  invisible  image  produced  by the  action  of  radiant  energy  on  a  photosensitive surface. It may be made visible by the process of photographic   development. IMAGE, NEGATIVE–A  photographic  image  in  which the values of light and shade of the original subject are represented in inverse order. In a negative, light objects are represented by high densities and dark objects are represented by low densities. IMAGE, POSITIVE–A photographic image in which the values of light and shade of the original subject are represented in their natural order. In a positive, light objects are represented by low densities and dark objects are represented by high densities. IMAGE PLANE–The plane in which the image lies or is formed It is perpendicular to the axis of the lens. A real image formed by a converging lens would be visible upon a screen placed in this plane. INCANDESCENT –Glowing with heat, such as the tungsten  filament  in  an  incandescent  lamp. LAMP HOUSE–That portion of an enlarger, reader, or projector  that  contains  the  light  source  and condensers  or  mirror. INCIDENCE–The act of falling upon or affecting, as light upon a surface. INFINITY–A distance so far removed from an observer that the rays of light reflected to a lens from a point at that distance may be regarded as parallel. A distance  setting  on  a  camera-focusing  scale. INFRARED–Pertaining  to  or  designating  those  rays which lie just beyond the red end of the visible spectrum. They are invisible and are detected by their thermal, photoelectric, and photographic effects. Their wavelengths are longer than those of light and shorter than those of radio waves. INTERNEGATIVE –An internegative film is a nega- tive derived directly from a color reversal original film. All other color-duplicating negatives derived from any other than reversal film are known as color-duplicating  negatives  regardless  of  the generation. INVERSE SQUARE LAW–The  intensity  of  light received at a point (irradiance) varies inversely as the square of the distance from the source. The law holds for relatively small sources only and is useful in  calculating  photographic  exposures.  (PIA) IRIS DIAPHRAGM–Term  applied  to  the  adjustable aperture fitted into the barrel of the photographic lenses and so-called because of the contraction of the aperture resembles that of the iris (pupil) in the human eye. It consists of a series of thin metal tongues overlapping each other and fastened to a ring on the lens barrel, the aperture made smaller or larger by turning the ring. (PIA) JOGGING  –Frame-by-frame    advancement    of videotape. KELVIN (°K)–Measurement of the color of light in degrees. Numerically, the Kelvin temperature is equal to the Centigrade temperature plus 273 degrees. KEY  LIGHT–The  main  source  of  illumination  on  a subject. (PIA) LAMP,  PHOTOFLOOD–A lamp designed to yield brilliant diffuse illumination. These lamps are generally short-lived. (NMA) LAMP, REFLECTOR FLOOD–Light  bulb  with self-contained  silvered  surface  to  act  as  a  reflector.