Often used for general task lighting and in flashlights, projectors, and microscopes, these bulbs are small but very bright. To replace a bi-pin bulb without a trade number, match the base number, voltage, and wattage. The base number reflects the pin spacing from center to center in millimeters—for example, G4 base pins are 4 mm apart. Brightness is measured in lumens; the higher the lumens, the brighter the bulb.