Tips for using the halogen bulb
Power Consumption: These bulbs are rated 2.5V, 800mA. To operate at full power they need more current than ordinary no. 14 (300mA) or no. 245 (500mA) light bulbs. Use them with D size alkaline cells, Ni-Cd rechargeable batteries, or transformers rated 800mA. Avoid inexpensive zinc-carbon cells, smaller cell sizes (especially AA) or transformers that output less than 500mA.
Installation: Halogen bulbs are taller than ordinary bulbs, mainly due to a protruding glass nipple at the top. My halogen bulbs are trimmed at the top. These reduced-height bulbs will fit in most viewers but might still need to be reduced a bit to fit in certain viewers (like the VM model D). In this case I recommend filing off part of the rounded soft contact at the bottom.
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To reduce halogen bulb height, I file off the tip of the bulb. To reduce it even further, I file off the soft bottoms. |
Light Uniformity: Since bright light is emitted from a small filament, there are potential light-uniformity problems. The filament supporting wires tend to cast shadows and the glass-bending at the top creates a bright ring. One way to avoid these shadows and hot spots is to orient the bulb (if possible) so that the shadows are formed outside the viewing area. Another solution is to use a frosted bulb.
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A clear halogen bulb tend to form shadows from the filament supporting wires and hot spots from the bending of the glass. |
To Frost or Not to Frost? In the frosted bulb the glass is ground to result in the frosted appearance. When this is done, the light is not emitted from the very small filament but from the considerably larger glass envelope. The result is uniform illumination at the slight expense of light intensity. Bulb-induced shadows disappear and viewer-induced shadows are softened, just like they are in real life when the sun is blocked by a cloud.
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When light uniformity is important, I recommend the frosted halogen bulb. |
Light Flickering: When using the high-current halogen bulb it is important to keep the switch contacts clean or light flickering will result.
When is Bright Too Bright? There is an alternative to the 800mA bulb, rated 500mA. The two bulbs look identical but the 500mA is less bright. This can be useful in cases where the 800mA bulb is too bright or when battery drain is a problem. One idea is to carry both the 500mA and 800mA bulbs and switch as needed to get two levels of illumination.
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Viewer illumination as a function of voltage for 4 different light bulbs: No. 14 (was the only choice in the 50s, still available today), No. 245 (inexpensive alternative to the halogen bulb), 0.5A halogen and 0.8A halogen. |
Updated: February 2003