There is a fixed limit in the design off all bulbs that use filaments, including vacuum bulbs. This section takes a closer look at small vacuum bulbs with tungsten filaments.
As shown in the standards graph in the details for each product, sub-miniature lamps have four limiting factors, in addition to their size. These factors are rated voltage, rated current, brightness, and lifespan, all of which are inter-related within fixed parameters. Thus when designing a lamp, defining three of these four will automatically decide the value of the fourth.
It is impossible to change just one of these factors when it changes in the design of a standard item are necessary. For example, it is impossible to increase the brightness of a lamp without changing the voltage, current, or lifespan. One of these factors must change. Let’s examine an example where the lifespan of the lamp is changed.
It is possible to make approximate estimates of the lifespan of a lamp from the differences in the efficiency (MSCP/WATT) of its filament. Generally speaking, high-efficiency lamps have a shorter lifespan than low-efficiency lamps. However, the lifespan of a lamp is not solely linked to high or low efficiency, but is primarily affected by filament thickness and the evaporation speed of tungsten at low temperatures. In the sample table below, comparing the OL-387 and OL-327LSV shows that although the OL-387 has a lower efficiency rating, it has a much higher filament temperature than the OL-327LSV, which suggests that it would have a shorter lifespan. However, the thicker filament of the OL-387 means that its lifespan is actually longer. These facts suggest two basic ways to change the design of a standard lamp to obtain a long lifespan at an identical voltage.
| Lamp no. | OL-327 | OL-387 | OL-385 | OL-327LSV |
|---|---|---|---|---|
| Design Voltage (V) | 28 | 28 | 28 | 28 |
| Design Current (A) | 0.04 | 0.04 | 0.04 | 0.06 |
| Watts (W) | 1.12 | 1.12 | 1.12 | 1.68 |
| Brightness (MSCP) | 0.34 | 0.30 | 0.20 | 0.34 |
| Efficiency (MSCP/W) | 0.306 | 0.270 | 0.180 | 0.201 |
| Average lifespan (hours) | 4,000 | 7,000 | 25,000 | 25,000 |
| Filament color temperature | 2,200°K | 2,075°K | 1,925°K | 2,125°K |
| Filament tungsten diameter (microns) |
11.24 | 11.78 | 12.62 | 15.54 |
| Filament tungsten length (millimeters) |
104.0 | 121.5 | 146.0 | 151.0 |
One method is to reduce the brightness without changing the current, and the other is to increase the current without changing the brightness. Although both methods will reduce the lamp’s efficiency and filament temperature, increasing the current uses more power, generating heat, which means that the filament temperature will tend to be higher than for the former method, increasing constraints on design. The first method requires a longer filament, which is limited by the size of the lamp. A common method used to loosen these restrictions is to use a double coil in the filament. The table above compares four types of T-1 3/4 size 28V items. Although the OL-327 is designed as a long life lamp, looking at the other three reveals the designs of the OL-387 and OL-385 have taken reduced brightness option of using longer filaments with very little thickening. In contrast, the OL-327LSC uses an increased current, and therefore more power, lowering efficiency and requiring considerable increases in filament length and diameter.
Conversely, resistance to shock tends to increase with filament width, and the OL-327LSV was designed with this in mind, as a shock-resistant version of the OL-327. Generally, low-voltage filaments are shorter in length that those designed for high-voltage, and filaments designed for large currents are thicker than those designed for use with small currents.
Thus a low voltage (5-6V), high current (100-200mA) lamp is ideal in terms of resistance to shock or vibration. Please note that of the above explanations for vacuum bulb limits, design changes and their effects, and other related matters are only valid for lamps produced using advanced manufacturing techniques.
Some people make the mistake of believing that the low temperature of the filament in a low-vacuum lamp indicates a long bulb life. In a low vacuum environment, filaments evaporate quickly and may break after on a short period of use. In comparison to high-vacuum lamps, low-vacuum lamps exhibit extremely rapid blackening of the inner surface of the bulb (in a matter of a few hours) and are very hot, making it easy to distinguish between the two. This also means that aging plays an important role in helping to locate lamps with imperfect vacuums.
Although the above is only an incomplete explanation of many aspects of small vacuum bulbs, we hope it will be useful to our customers whenever they use small lamps, or are evaluating a new use for such products.
