Color light signals are superior to semaphore signals in terms of multi-aspect, maintenance and control. The only minus is the relatively short 1,000 to 2,000 hours life of the incandescent light bulb as the burnout of a signal bulb (single-filament type) would affect train operation directly. Therefore, bulbs have to be exchanged periodically given the average lighting hours differ in positions G, Y and R. Signal bulbs are used with 80 - 90% of rated voltage to extend their life without reducing the required visibility; 600 m in Japan.
(Note: The theoretical life of a filament is in reverse proportion to the 13.5 - 14th power of the voltage.)
Signal bulbs were imported from America and were first produced domestically in around 1925. These were superior in performance but inadequate in terms of productivity because of the peculiar shape of their filaments. Therefore, a new signal bulb (G-type, 30 V, 45 W, double-filament, life: main 9,000, sub 5,000 hours) which introduced a car-bulb standard and improved it for railway signal use was adopted in 1970. In the case of multi-aspect signals, significant differences between the resistance values of two lighting bulb circuits (at its maximum when a filament burns out), may cause a driver's misunderstanding because of the low brightness of a bulb, therefore, voltage balancers are inserted as shown in Fig. 11. A bulb-burnout detector is also attached and connected to a monitor-line to send bulb-failure information to maintenance depots.
Fig. 11 G-type signal bulb and voltage balancer
LED (Light Emitting Diode) has progressed greatly in terms of availability of colors and brightness in the past 10 years, and has been applied for various kinds of indicators. Excluding red LED, especially blue LED with enough brightness for outdoor use, difficult to manufacture in the past, has finally been adopted for practical use recently.
