How do headlamp beam deflectors/reflectors work?

[Vroomer]

I am taking a 2004 VW Polo to Europe and have bought a pack of headlamp deflectors.

The fitting diagram shows that, looking from the front, the reflector circle should be positioned left of the bulb and slightly below it (about 8 o'clock).

How does this work? Instinctively one thinks one needs to block light coming from the RHS that shines light to the kerb in the UK but would point at oncoming drivers in Europe.

You explanation would be gratefully received!

[Mr2Mike]

Because the light that is directed upwards comes mainly from the lower part of the reflector.

[Superhoop]

They don't do what they used to do years ago when the beam was determined by the glass - in older stuff, it would actually cover the part of the glass that shaped the beam, and made it flat.

On newer stuff, it just defuses the light, rather than actually change the shape of the beam. This still stops the lamps causing a problem for oncoming drivers though

[Vroomer]

Thanks, but why is the reflector to the left of the bulb, not the right?

[LuS1fer]

I'm guessing because the light goes via the reflector so bounces off at 90 degrees.

[Superhoop]

As above, it's now the reflector that determines the. beam, and it tends to be determined by the bottom of the reflector, and works like a mirror image, so the. beam deflector always looks like its on the wrong side.