I have been using halogen 2.5Volt/0.8Amp lamps in my viewer for over ten years now. These lamps are truly excellent for viewer illumination and many Club members also use them, or similar high-output lamps. The retail price of the lamps is high - usually around $10 each in Australia. The lamp life seems acceptable to me, although many years ago I did experience a batch that had an unacceptably short life.
In my instance I have been using 4 Ampere-hour D-cell NiCad rechargeable batteries to supply the power to the lamp. These batteries can supply quite a large current to the lamp when the lamp is first turned on. This large in-rush current during the first fraction of second can reduce the life of any lamp. The life of the lamp could be increased by starting the lamp with a slowly increasing voltage, which is a little too complex to do in a portable battery powered viewer. However an approximation to this can easily be made by including a starting resistor in series with the lamp, which appears in circuit when the button is partially depressed, and is removed from circuit when the button is fully depressed.
The modification requires these
The switch in the Realist Red-Button viewer is modified as shown in the figure. The washer is placed under the mounting point for one of the switch contacts. This raises the contact up higher than the other one, and ensures that it will "make" with the switch-contact-ring before making with the other contact at the normal height. A resistor is added from the ring-contact using a flexible piece of wire to allow free movement of the red-button as normal. The wire needs to be soldered rather carefully so that the blob of solder doesn't interfere with switch operation. (The added wire will of course restrict the button from rotating which is possible in an un-modified viewer, but not required.) The other end of the resistor is connected to the lower of the two switch contacts as shown.