A wholesaler ordered a pallet of 500W linear halogen bulbs at the best price he had seen, and lost the pallet twice over in returns. The lamps were not defective. Half of the returns came from customers whose work lights take a 78mm tube and had been sent 118mm; the rest came from a batch marked 230V that went to a market wired for 120V. Three numbers decide whether an R7S halogen lamp works in a fixture — length, voltage and wattage — and none of them is obvious on the shelf. This is what those numbers mean and how to order them correctly.
R7S is a cap designation rather than a size: it means a recessed single contact with 7mm between the contact points at each end of the tube. What the buyer actually needs is the overall length, measured tip to tip rather than across the glass, and the two lengths that cover almost every fixture are 78mm and 118mm. A 78mm lamp in a 118mm holder stops short of the socket, so the ends arc and burn the ceramic; a 118mm tube forced into a 78mm fixture will not seat at all, and the pressure cracks the holder. The working rule for a distributor is to measure a lamp taken out of the fixture before ordering a replacement, and to keep the two lengths as separate SKUs. An R7S 118mm bulb sold into a 78mm application is a return no matter how well the lamp is made.
The 120V vs 230V halogen split is the most expensive mistake in this product family, because the two lamps look identical and the difference stays invisible until the fixture is switched on. A 120V lamp on a 230V supply fails immediately — the filament cannot survive roughly twice its design voltage. The reverse is quieter and worse for a distributor: a 230V lamp on a 120V supply lights, but at around half power it never reaches the temperature at which the halogen cycle works. That cycle is the whole point of a halogen lamp: tungsten that evaporates from the filament is carried back and redeposited only while the quartz wall stays above roughly 250°C. Under-driven, the tungsten settles on the glass instead, the tube blackens, output drops, and the customer returns a lamp that died in a few weeks with no visible defect.
Wattage in linear lamps runs from about 100W to 500W, and the fixture rating is a hard limit rather than a suggestion. The 500W tube is a heater that happens to glow: it belongs in work lights, security floodlights and paint-drying rigs, and its radiant heat is the reason it exists. Fit one into a holder rated at 300W and the ceramic and contacts will discolour and crack, thermal cutouts will trip, and reflectors will warp — often after weeks of apparently normal service. Ask the end user what the old lamp was rated at, and if the answer is a guess, quote to the fixture rating rather than to the brightest lamp in the range.
The rule that separates a lamp lasting a year from one lasting a month is handling. Quartz, unlike ordinary glass, cannot tolerate contamination at high temperature: oils from a bare finger carbonise on the hot surface, create a local hot spot, and the devitrification that follows cracks the tube. A lamp that leaves a factory with fingerprints on the glass fails in the field, and the failure looks like a manufacturing defect. Handle R7S halogen lamps by the cap or by the sleeve they are packed in, never by the glass; if the tube has been touched, wipe it with isopropyl alcohol and let it dry before fitting. Two smaller habits matter as much: avoid mounting where the tube takes continuous vibration, because the filament supports are designed for a static fixture, and check dimmer compatibility, since many ordinary dimmers cannot hold a halogen filament in its cycle and shorten life.
Not every R7S tube is meant for light. Ruby and gold coatings shift the output toward infrared and cut visible glare, which is why coated 118mm tubes are specified for brooding, wax melting, patio heaters and drying lines — applications where heat is the product and the glow is a nuisance. A coated lamp is not a brighter lamp; measured in lux it is dimmer than the clear version, and buying it for illumination wastes money. The mirror of that also holds: a clear tube in a brooder gives the birds glare and a focal hot spot instead of an even band of warmth. Match the finish to the application, not to the catalogue photograph.
The five-point check before ordering linear halogen bulbs: measure the length tip to tip and keep 78mm and 118mm as separate SKUs; confirm the supply voltage, 120V or 230V, against the destination market rather than the sample in hand; match wattage to the fixture rating rather than to the brightest option; specify clear or coated according to whether the job needs light or heat; and require the sleeves plus a handling note in the carton so the end user does not destroy the lamp at installation. Shunqi Lighting produces R7S halogen lamps in 78mm and 118mm, clear and ruby-coated, from 100W to 500W, with voltage and wattage marked on every lamp — see the R7S J78 quartz halogen lamp and the R7S J118 ruby infrared lamp, compare base types if your fixture is not a linear one, read the infrared heating lamps guide for heat applications, browse the full product catalog, or request a quote with your lengths, voltages and quantities and we will confirm the correct version before it ships.