A couple of my circuits centre on using LEDs on a bike. These would replace a conventional incandescent globe.

In my investigations recently, I have found how difficult it is to make that replacement.

Incandescent globes have been around for a long time. Their design is stable, almost standard, and as a result the folk who design lighting make certain assumptions that don’t hold true when the light source is changed to an LED.

1. The light is a point source.
   While the filament is not quite a point source, it is much smaller than an array of LEDs. This affects how the light is gathered and distributed by the reflectors and lenses in the light fitting.
   Light from an LED is directional, within a cone of about 15 degrees, so it won’t be spread off a reflector the same way as light from a filament. In many cases, no light at all will reach the reflector.
   Similarly, light from an LED won’t be collected and redirected by the lenses in a light fitting the same way as light from a globe.
2. The light is bright.
   A 21 W brake light or indicator is almost painful to look at. Many LED globes are dull in comparison. I recently ran some side-by-side tests of brake lights and was disappointed to see how dull the LED light was.
   People often choose a white LED to replace a standard globe that is used as an indicator or a brake light or tail light. The amount of amber or red light produced by a white LED is much less than that produced by an incandescent globe, and the LED will appear quite dull behind the coloured lens.
3. The globe has a low resistance.
   This is the driver for both the LED brake/tail light isolator and the LED ballast. LEDs have much higher resistance which means that LED indicators won’t blink at the right rate, if at all, and LED brake lights will trip the globe failure warning on modern bikes.
4. The globe is a standard size.
   As a result, the fittings are designed to accept a globe of a certain size. LED ‘globes’ are not necessarily the same size as standard globes. A 27 mm diameter LED ‘globe’ won’t fit into a hole made for a 1″ diameter globe.

So when replacing a standard globe with an LED globe, take some time to look at the assumptions that the light designer made, and choose the LED that is the best match to these.