Before modern cameras with built in flashes, large, delicate, flash units such as this were bought separately.
Flash bulbs are individual, disposable items–used once, then discarded. These are fitted into a flash gun, which may be built-in to the camera, or an add-on accessory. Bulbs improved on earlier flash systems by safely enclosing the material which could otherwise throw burning embers over the subject and emit large quantities of smoke. Bulbs contained various materials, often magnesium or aluminum wire or foil. Small amounts of highly explosive zirconium paste are typically used as the means of igniting the magnesium or aluminum primary combustible materials.
Early bulbs resemble domestic light-bulbs, including a metal base with a screw (ES/Edison Screw, or “Medium” base)–these include the common “Press 40” bulbs of the 1930s and 40s. By the end of the 1930s a new type of “midget” flashbulb was introduced that uses a bayonet (BC/Bayonet Cap) fitting. The No.5/No.25 became the standard bulb in this size and remained extremely popular through the 1940s and 50s, and continued to be sold up until the 1970s. In the early 1950s, in an effort to further reduce the size of bulbs and the equipment needed to fire them, the “miniature” M2 and M3 bulbs were released, which use a smaller bayonet fitting than the No.5 bulbs. In the late 1950s and through the 1960s the even smaller “capless” bulbs such as the AG-1 became popular as flash equipment continued to become smaller and more compact. Capless bulbs do not have a separate base, but instead provide wire contacts emerging directly from the glass, omitting the base.
By the end of the 1940s most bulb designs incorporated lacquer or plastic coatings to prevent bulbs from shattering. Although relatively effective, many flash units continued to be sold with special shields designed for use when using flashbulbs for close-up portraits, as an added safety measure. One particular cause of bursting is cracking and subsequent leakage of air into the low-pressure oxygen of the bulb; to reduce the chance of this, most bulbs had a blue indicator spot on the inside, which would change to pink if exposed to normal air. Use of this blue spot was continued in flashcubes & magicubes.
Prior to around the mid-1950s all flashbulbs were clear, and emitted light with a color temperature around 3800K. Clear bulbs work well with black-and-white or tungsten-balanced color film, but are not well suited for use with daylight color film. As color film became more readily available after World War II, to use clear flashbulbs with color film required that the photographer use a blue filter either on the camera lens or on the flash unit itself. Many flash units were sold with blue accessory covers for use with clear bulbs and color film. As color film became more popular through the 1950s, new flashbulbs with blue coatings on the bulbs themselves became available to reduce the hassle of using additional filters for color photography. The downside of the blue-colored bulbs is that their output is generally cut by about one-half, or one full stop.