Patent GB-191222972-A (GB)
Title: Improvements in or relating to the Production of Light and other Radiations.
Inventor: HOFFMAN SAMUEL OSCAR
IPC: H01K1/26
Filed: 1912-10-08
Granted: 1913-05-22
Abstract: 22,972. Hoffman, S. O. Oct. 8. Colour filters. - An electric - lamp bulb or other globe is constructed to transmit a selected part of the radiation, such as the luminous part, and to return the remainder to the source, such as a filament or flame, the energy consumed corresponding to the transmitted radiation only, and the remaining radiation acting regeneratively. The following constructions of electric - lamp bulbs, which transmit the luminous radiation and reflect the infra-red are described: (1) A glass bulb is coated with a thin film of a material which reflects selectively, such as copper, gold, or silver. (2) A thin bulb within the ordinary bulb is constructed as shown in enlarged section in Fig. 2, with prismatic cavities and projections such that a beam of light K incident normally is divided at the surface M of a cavity, the luminous portion H being totally reflected and ultimately being transmitted after total reflection at P, and the non-luminous portion T being returned to the source after total reflection at O and R. Parts E where this action cannot occur have total-reflection prisms X. The angle of incidence at M in the form shown is 41 degrees, which is just above the critical angle for red light for a particular glass. If this angle is diminished, some red light as well as infra-red will not be transmitted. This enables a white light to be obtained from a source rich in red. (3) An inner bulb is divided lengthways into small lenticular prisms L, Fig. 10, which form corresponding spectra focused on the outer bulb A, which is silvered in strips to reflect the non-luminous portions. (4) A wide source, such as a flame or a filament subjected to vibration, may be surrounded by slits T, Fig. 10, which may comprise silvered strips on the inside of another bulb Q, the outside of which is shaped to form lenses corresponding to the slits and prisms. (5) An inner bulb, Fig. 9, has small prisms G formed on three parabolic surfaces m, silvered externally; these produce spectra at the intermediate parts k, which are silvered to reflect the non-luminous portions. Prismatic bulbs as described above may be made in sections. They may be cast from easily-fusible glass in cast-iron moulds, on which the pattern has been engraved by means of templets after being reduced from a large drawing and transferred photographically to the polished iron surface. The moulds are heated nearly to the fusing-point of the glass and are exhausted during the casting, so that the atmospheric pressure forces the glass into the fine lines of the pattern. Silvered strips arranged to reflect the non-luminous parts of the small spectra are formed on a bulb by silvering it completely, then coating it with bichromated gelatine, to which dyes may be added, then exposing this to the non-luminous parts only of the spectra by interposing between the small prisms and the source of light a colour filter, for instance a solution of iodine in carbon disulphide, then placing the bulb in hot water which dissolves the unaffected parts of the gelatine, then dissolving the silver strips so exposed, and finally removing the remaining gelatine by rubbing, or by means of a heated oxidizing-agent such as potassium nitrate.