Dave Rasmussen, age 11, of Burlington, Vt., for his question:

WHAT MAKES A FLASHBULB SO BRIGHT?

A flashbulb is a convenient source of artificial light used to take pictures indoors, and outdoors at night. The bulb contains a fine tungsten wire filament, a zirconium primer and finely shredded metallic foil. Electric current heats the filament which ignites the primer.

The burning primer in a flashbulb then ignites the foil. Because the lamp is filled with oxygen, the burning foil produces an intense white flame that varies in time and intensity with the type of bulb used.

Electronic flashlamps are gas filled tubes that require a high voltage power source. They can be used several thousand times.

Regular flashbulbs are rated and numbered according to their size and burning characteristics. The bulb is filled with aluminum or zirconium foil. Zirconium produces a brighter flash than aluminum, so bulbs using zirconium are smaller than aluminum foil bulbs of equal light output.

The burning of a flashbulb takes place so quickly that it creates heat and a small explosive force.

A lacquer coating on the inside and outside of the lamp helps absorb the heat and also prevent the bulb from shattering.

A cracked bulb that allows some air to enter will not fire properly and may explode. Some lamps have a spot that changes color to indicate that air has leaked in.

Most flashbulbs give the greatest light in one fiftieth of a second. SM or speed midget bulbs contain no foil but have a larger primer. This type produces an extremely bright light for only about one two hundreth of a second. It burns only the primer in an oxygen filled bulb.

A blue coated lamp used with ordinary daylight type color film makes it possible to take color pictures indoors.

Another type of bulb is coated with a special lacquer that holds back all visible light rays and permits only infrared rays to light the subject. When used with a film that is sensitive to infrared rays, pictures can be taken in complete darkness.

Often a flashbulb will improve the quality of pictures taken in bright sunlight by removing dark shadows.

The bulb is placed in a holder attached to the camera or held nearby on an extension cord. A reflector behind the bulb helps to direct the light.

The holder contains batteries or a small generator, which provides an electric current to ignite the bulb. The camera shutter has built in electrical contacts to fire the bulb when the shutter is wide open during the exposure.

It is easy to determine exposure when using flash. First, set the shutter speed for the scene. Then check the instruction sheet that comes with the film or the flashbulb to find the guide number for the bulb you are using.

To determine the proper f stop, divide the guide number by the number of feet between the flashbulb and the subject.