Why are pre focused headlights are focused?

[Dr.Mabo]

img_2166.jpg

These headlights are generally referred to as pre-focused. As I understand it, this means that they have no left/right orientation and can therefore be used for both left-hand and right-hand traffic.

img_2165.jpg

However, there are actually two variants of these headlights. One with a round logo (first pic) and the words Right Hand Drive and one with a square logo and the words Left Hand Drive embedded in the top left or right of the glass.

img_2167.jpg

There are also stickers on the back for the bulbs to be used. These also differ between the two headlights. 414 for RHD and 355 for LHD.

Why are these headlights now referred to as Pre Focused?

It would be nice if someone could shed some light on this.

[GraemeC]

I always understood it that the lens is designed to focus the light output from the correct bulb to suit the right or left hand drive application. ie you need both parts to be correct.
Later H4 lenses were then designed for a common design of bulb, so only one part of the equation was specific to right or left hand drive.

[hanlminiman]

Not sure what the answer is but a very interesting question.
Looking at the images of the lenses my first thought was about the "circle" and the "square" in the middle which I thought might affect LHD/RHD as there are up/down dip lenses available (round vs square?) but then I saw what I think are Euro markings on the round lens. Is the first lens a modern one?
Makes me think of Spot, fog & long range lens patterns which I remember from historic Cibie adverts.
I'll have a word with a friend, who as a Physicist developed and then promoted the Triplex heated windscreens at Motor Shows way back! Post 1964 Monte screens!

[DougR]

Hi, pre-focused headlights were the originals that had a bulb clipped in. They were replaced by sealed beam units that had the filaments built in. The question of LHD versus RHD was entirely down to the lens. There never was a lens that could cater for both. If you had a UK/IRL car you had to fit stickers and other accoutrements to drive in Europe.
Cheers
Doug.

[Spider]

I always understood it that the lens is designed to focus the light output from the correct bulb to suit the right or left hand drive application. ie you need both parts to be correct.
Later H4 lenses were then designed for a common design of bulb, so only one part of the equation was specific to right or left hand drive.

That's been my understanding too.

The light patterns are thus;-

[Tim]

My understanding was that pre-focus refers to the location of the high and low beam filaments in relation to the focal point of the reflector. Close to the focal point gives a narrow parallel beam, further away will give a more diffused spread pattern. The ribs in the lens deal with directing the beam toward the side of the road when it dips, and possibly also the rotation of the globe in the holder. When comparing the LHD and RHD lamps, are the globe holders oriented on the same angles?

You'll also find motorcyle pattern lenses. I'm not certain what the difference is, but I think it relates to the lean of the bike as you ride through bends. Having ridden a bike with a car headlamp, it gets pretty dark in the twisty corners.

Tim

[Dr.Mabo]

Thank you for the answers so far. I think I was a little unclear with my question . I am fully aware of what the focusing of the headlights entails and that there should be different illumination for LHD and RHD.

I just got stuck on the pre-focused term. I interpret pre-focused as before focusing, i.e. the last unfocused headlights before LH and RH focusing was introduced. Just like pre-war cars are cars that were built before WW2.

That's why I've always wondered why pre-focused headlights have a focus for LH or RH after all.

But I'm beginning to think that pre-focused in this case means that the headlight unit is only prepared from the glass and becomes a focused headlight in conjunction with the appropriate bulb.

[timmy201]

I think in this context “pre focused” means the focus point was preset and is constant between all BPF bulbs and no further focusing is required

Here’s an old instruction on focusing bulbs, with a note at the end saying it’s not needed for pre-focus bulbs

IMG_0173.png

[GraemeC]

I think Tim has hit the nail on the head there.

[Peter Laidler]

From an optical point of view, those internal lines, shapes and patterns are in fact prisms that you'll all remember from your 4th form physics lessons at school. All light travels in straight lines and all that stuff and those prisms just deflect the straight lines of light into another direction. Dip left or right or flat (for fog). Nowt for spots except the...... forgotten the word (parabolic ?) shape of the highly polished bowl