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.
Optical bench is a device for measuring the optical performance of lenses. Optical glass is used for manufacturing lenses and prisms. It is specially manufactured to be free of defects and distortion, and to withstand heat and humidity. Each type f optical glass is classified according to its refractive index and light dispersive quality. Two or more types of optical glass are typically used in the component elements of photographic lenses.