What is SML?(Sub-Miniature Lamp)

Sub-Miniature Lamps (SML) are small tungsten filament vacuum lamps. Key characteristics and terminology for these is as follows.

Standard Size

Small size vacuumed lamps are called "SML" and are produced in a size (glass bulb diameter) range of 2.3~7.0㎜. These lamps are normally referred to as T-1, T-1¼ etc.
This refers to the bulb diameter. T-1 equates to 1/8-inch and therefore size is calculated by the following formula.

T-1 = 1/8inch = 25.4㎜ x 1/8 = 3.175㎜ → Bulb diameter 3.05mm
T-1 ¼ = 1/8inch x 5/4 = 25.4㎜ x 1/8 x 5/4 ≒ 3.969㎜ → Bulb diameter 4.1mm

The following table shows the relationship between Standard-size vs Bulb-diameter.

Main Parts

The main components of SML products are as follows:
1.Wire Terminal
3.Silicone Rubber Boots


Our SML filament shapes are based on the global specification JIS C 7711. Please see the following table and image for reference.

Glass Bulb

There are various shapes of glass bulbs for supporting customer's requirement.

Wire-Terminal Manufacturing Process

1. Welded stem
Dumet wire is welded to glass bead.

2. Connecting filament
Filament is mounted and formed into the correct shape.

3. Inserting glass bulb and Vacuum
Stem(2.) is inserted into glass bulb and vacuum is created by exhaust machine.

4. Sealing
Finally, stem and glass bulb are welded with burner. Wire-Teminal is completed.

Base / Terminal

Bases/Terminals are the items covering the SML electrodes. These materials can be either plastic or metal.


The characteristics of tungsten filament vacuum lamps are related to each other. Gaining an understanding of this relationship will help explain an SML's capabilities and limitations.

■The Voltage, Current, Luminance and Life-time relationship

SML's 5 main specifications are 1.Size, 2.Voltage, 3.Current, 4.Luminance, 5.Life-time.
Voltage, current, luminance and life-time are strongly inter-related. When 3 of these values are set during lamp design, the 4th is automatically decided.
For example, if you want to increase luminance, it is impossible to maintain the same voltage, current and life-time values.

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