One shot developer is a developer that is used on a single occasion and then discarded. Opacity is the light stopping power of a a material. The greater the opacity of a substance, the more light its stops. In photography, opacity is expressed as a ration of the amount of light falling on the surface of the material to the amount of light transmitted by it. Opalescent is like opal, a material with a cloudy-white translucent appearance.
Opal lamp is a filament lamp with an opal glass bulb for optimum diffusion.
Opalotype is an obsolete printing process in which a carbon-process image is transferred on to translucent opal glass. Opaque liquid is a dense red or black pigment, dissolved in water to form a liquid paint used to fill in film areas that are required to pint as pure white. Usually shutter duration is unimportant since the available light is much dimmer than the flash. Opening up is increasing the size of the lens aperture or decreasing the shutter speed to admit more light to the film. Optical axis is an imaginary line passing horizontally through the center of a compound lens system.