Fluorescent lamp using high performance phosphor blend which is protected from color shifts by a very thin overcoat of stable phosphor of similar chromaticity

A fluorescent lamp having a predetermined correlated color temperature as defined by x-y coordinates as inscribed on the x-y chromaticity diagram of the ICI system together with combined high efficacy and good color rendition, said lamp comprising a sealed elongated light-transmitting envelope having electrodes operatively positioned therein proximate the ends thereof and enclosing a discharge-sustaining filling comprising mercury and a small charge of inert ionizable starting gas which when energized generates a discharge comprising ultraviolet radiations and a limited proportion of visible radiations, phosphor means comprising a predetermined amount of a first phosphor layer carried on the inner surface of said envelope and a predetermined amount of a second phosphor layer coated on and carried on said first phosphor layer so that said second phosphor layer is positioned nearest to said discharge:

• a. said first phosphor layer principally comprising a mixture of predetermined amounts and relative proportions of narrow-band blue-emitting phosphor having an emission substantially confined to the wavelength range of from 430 nm to 485 nm, green-emitting phosphor having an emission substantially confined to the wavelength range of from 515 nm to 570 nm, and red-orange-emitting phosphor having an emission substantially confined to the wavelength range of from 588 nm to 630 nm, and the predetermined amounts and relative proportions of said mixed phosphors comprising said first phosphor layer being such that the composite emission response from said first phosphor layer when excited by said discharge falls approximately within a predetermined color ellipse on the x-y chromaticity diagram of the ICI system, portions of said green-emitting phosphor component of said first phosphor layer if directly exposed to said discharge displaying a maintenance of light emission during lamp operation which is inferior to the maintenance of light emission displayed by said blue-emitting phosphor component and said red-orange-emitting phosphor component of said first phosphor layer, and the coating weight of said first phosphor layer falling within the range of from 2.3 mg/cm.sup.2 to 5.5 mg/cm.sup.2 ;
• b. said second phosphor layer principally comprising a relatively thin layer of stable finely divided phosphor means which when excited by said discharge has an emission of predetermined x-y coordinates which substantially correspond to said predetermined x-y coordinates of the composite emission of said first phosphor layer, and the coating weight of said second phosphor layer falling within the range of from 0.23 mg/cm.sup.2 to 0.7 mg/cm.sup.2 ;
• and
• c. said composite emissions of said first phosphor layer and said second phosphor layer coupled with the limited proportion of visible radiations generated by said discharge causing the total visible emissions from said energized lamp to have predetermined x-y coordinates which fall within said predetermined color ellipse as inscribed on the x-y chromaticity diagram of the ICI system;
• whereby said second thin phosphor layer screens said said green-emitting phosphor component from direct exposure to said discharge.