GE-Mazda Ribbon Filament Microscope Illuminator

In developing lamps for microscope illumination, the total quantity of light does not need to be very great but it is critical that it has a high brightness - i.e. the quantity of light per unit surface area of the light source must be high.

In case of illumination by the Köhler method coiled tungsten filaments are ideal, in particular with low voltage and high current to achieve small dimensions. However the uniformity of luminance across a short coil leaves a lot to be desired, due to their thermal gradient with a central hot-spot and relatively cold ends. This problem was overcome with the development of the tungsten ribbon filament lamp illustrated on this page. The homogeneity of the ribbon also allows brighter illumination by the more efficient Nelson method, since there is no coil whose image needs to be diffused out.

The filament is made from a thin strip of tungsten sheet about 2mm wide. It is bent into a U-shape whose central portion has substantially uniform luminance. There are significant heat losses at the filament ends, however these are bent backwards out of view and sufficiently long that the central region has a uniform thermal profile. The filament tails are clamped into heavy nickel leadwires, and the whole assembly has been electropolished for ultimate cleanliness. Most unusually a hard glass bulb has been used, despite the low watttage. This is probably due to the high operating current of 18A which requires heavy-gauge leadwires. Soft glass requires seals to dumet wire, which is not reliable in large diameters due to the thermal expansion mismatch. In this lamp the seals are to tungsten wires, which form well matched seals to borosilicate. The top exhaust tip indicates that this lamp was probably made before about 1925.
Manufacturer: General Electric U.S.A.
Lamp Power: 108 Watts
Lamp Voltage: 6 Volts
Lamp Current: 18 Amperes
Cap Type: E26s/25 Brass & Vitrite
Bulb Type: T-32 T-10 in eighths/inch
Bulb Finish: Clear Borosilicate
Filament Type: R-6 Ribbon Transverse 2x8mm
Atmosphere: Nitrogen
Luminous Flux: 1,800 Lumens
Luminous Efficacy: 16.67 Lumens per Watt
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: 3000 Kelvin CRI: Ra 100
Chromaticity Co-ordinates: CCx: 0.452 CCy: 0.409
Rated Lifetime: 150 hours
Burning Position: Vertical base down
Overall Length: 140 mm 51/2 inches
Light Centre Length: 85.7 mm 33/8 inches
Factory: U.S.A.
Date of Manufacture: Approx. 1925
Original Value: Unknown
References: 1) GE #9200 Large Lamp Catalog, 1979 USA, EDW Lamp Specification
2) Bright Light Sources, J.N. Aldington (Siemens UK), Proceedings of a Lecture at the Illuminating Engineering Society of London, Nov. 1944, pp.1-10.
3) Characteristics of Ribbon Filament Lamps, Leighton L.G., Illuminating Engineering Vol.LVI p.121, March 1962.
4) IES Lighting Handbook, 5th Edition 1972, pp.8.13-8.14.