ANSI C79.1-2002 pdf download For Electric Lamps– Nomenclature for Glass Bulbs Intended for Use with Electric Lamps
4.2 SI (Metric) units 4.2.1 General In metric designations the first number symbol shall consist of two numbers expressed in millimeters (to the nearest whole number) and separated by a diagonal line. The number to the left of the diagonal shall indicate the average major bulb diameter above the CD line; the number to the right of the diagonal shall indicate the average diameter of the bulb neck at the CD line.
4.2.2 Fluorescent bulbs For fluorescent bulbs the two-number symbol described in 5.2.1 shall be used. The number to the left of the diagonal shall designate the bulb diameter in millimeters (to the nearest whole number); the number to the right of the diagonal shall designate the base line length to the closest tenth of a millimeter.
4.2.3 Round PAR Only one number shall be used to designate round PAR-type glass bulbs. This shall be the dimension of the maximum diameter, expressed in millimeters (to the nearest whole number).
4.2.4 Rectangular bulbs Rectangular glass bulbs shall be described by the letter symbol REC followed by two sets of numbers. The first number shall designate the dimension of the shorter side; the second number shall designate the dimension of the longer side. These two numbers shall be expressed in millimeters (to the nearest whole number) and shall be separated by the multiplication symbol (x).
5 Examples of designations
5.1 Customary units
See Table 1.
5.2 SI (metric) units
See Table 2.
6 Bulb shape classification
6.1 Basic shapes
The basic shape symbols are listed in Table 3. The descriptive information used in conjunction with the associated illustration in Figure 1 forms the definition of each shape.
6.2 Modifiers The basic shape designations in Clause 7.1 may be modified by the addition, as a suffix, of one of the symbols listed in Table 4. The descriptive information used in conjunction with the associated illustrations in Figure 1 forms the definition of each shape.