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Lens Spacing vs Cutoff “step” size

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  • Lens Spacing vs Cutoff “step” size

    I recently swapped a set of ZKW-R clear lenses into the E46 style projectors in my European spec headlights on my B8.5 Audi S4. The clear lenses are slightly larger diameter than the stock ones and therefore do not fully seat in the lens holders, effectively positioning the back of the lens closer to the projector bowl than where the stock lens is positioned. To compensate for it, I installed two small washers acting as spacers at each of the four screws where the lens holder mounts to the projector bowl.

    The result is that the “steps” in the beam cutoff are being projected much larger now than they were with the headlights in stock form. Essentially the cutoff pattern is being magnified too large now. In order to decrease that magnification and get the cutoff pattern size smaller, do i need to mive the lens farther from the projector bowl (add more washers), or move the lens closer? Unfortunately i do not have a wokbench power supply or else i could answer my own question. I would like to understand how the spacing and projected image are related before diassembling and modifying the headlights yet again. Thanks in advance for any guidance!

  • #2
    When swapping lenses and attempting to retain the same beam pattern, it is important to maintain the same relative position of the replacement lens' focal point as close as possible to that of the original's. The fact that the replacement lens' back surface (i.e. one that is closest to the bulb instead of the one facing forward outside) is closer to the projector doesn't necessarily mean that the replacement lens' focal point is closer to the bulb now. The reason is that the lens surface itself, including the edges where it mounts, do not uniquely dictate this focal point position.

    For example, if the original and replacement lens' forward surface has the same curvature (which is one of the factors for the lens focal length), has the same glass material (because index of refraction, which also affects focal length, depends on the glass material type), but one has completely flat back surface (i.e. plano) and the other has a convex surface, then even if the position of the edges that sit on the lens holder is identical, the two lenses' focal points may be placed differently relative to the reflector-bulb optics.

    The best bet is to set up a fixture to keep the bowl and bulb in one place in front of a wall that is not too close (as projecting it closer to 25ft will yield a not so useful assessment), and with the replacement lens mounted, see if moving the lens holder away from the bowl gives you a better beam image. I'd do this before adding spacers etc.
    Jul 2012 ROTM (3-way quad headlight) ; Sep 2015 ROTM (custom muli-lens 7" fogs)

    3-way quad wiring; foreground limiter; squirrel finder;

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    • #3
      Thanks for that input. It all makes sense to me - however, unfortunately I do not have a way to bench-test this because I do not have a way to power a D3S bulb besides plugging the headlight assembly into the car. So I guess I am trying to understand the theory a bit better so I can understand how the lens location affects the size of the projected image, which may help me to achieve the desired outcome with minimal iterations. To that end, let's ignore the aftermarket clear lens for the moment and consider only the stock lens and projector bowl, which projects the cutoff shield image at a size of "X". As I move the lens farther away from the projector bowl, does "X" increase or does "X" decrease?

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      • #4
        Is the ballast attached to the headlight? If so, take the whole headlight in and just apply 12V to the ballast. You can just a portable jump starter, computer PSU, or dedicated power supply.

        2000 Toyota MR2 Spyder
        2017 Lexus CT200h F Sport

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        • #5
          Originally posted by Haloruler64 View Post
          Is the ballast attached to the headlight? If so, take the whole headlight in and just apply 12V to the ballast. You can just a portable jump starter, computer PSU, or dedicated power supply.
          Yes, the ballast is attached to the headlight. I tried a few months ago to use an old computer PSU as my workbench ballast power supply but was unsuccessful. I have to admit that electronics are not my forte, and quite possibly I did something wrong in my setup, but I confirmed with a multimeter that the PSU was putting out 12V yet I could not get the bulb to light up. Now, it's also possible I shorted something out. The ballast appears to have 4 'prongs' into which a mating 4-socket plug fits, but there are only two wires going into that plug. I was using alligator clips to connect the ends of the 12V wires from the PSU to the two 'active' male prongs on the ballast but maybe that was not the right way to go about it? The prongs are pretty close together so it's quite possible that the two clips touched each other and caused a short circuit. The photo below is not the exact ballast used on my headlights, but mine have the same recessed 4-prong connection to the power supply as the one shown here (the larger plug toward the top of the photo).

          So anyway, I clearly have challenges setting up a good power supply to test these things on the bench. If I could get a bench test setup going, this would be a ton easier. If you have suggestions for how to set this up, I am open to them!


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          • #6
            I use one of these, great little thing. No switch and semi exposed contacts so it's not the greatest thing out there, but it's powerful enough to light two OEM Hella ballasts with ease. https://amzn.to/2RCwLWS

            All you'd need to do is run a regular 3 prong power cord to it, an old one from a computer will do. Cut the end off and stick the 3 wires in the right places. Then run a power and ground and that's it. Just be sure to wire everything up then plug the power supply in, not having a switch kinda sucks. I'm gonna put mine in an ammo can with an external switch sometime soon, make it all nice and pretty.

            2000 Toyota MR2 Spyder
            2017 Lexus CT200h F Sport

            Comment


            • #7
              Originally posted by Haloruler64 View Post
              I use one of these, great little thing. No switch and semi exposed contacts so it's not the greatest thing out there, but it's powerful enough to light two OEM Hella ballasts with ease. https://amzn.to/2RCwLWS

              All you'd need to do is run a regular 3 prong power cord to it, an old one from a computer will do. Cut the end off and stick the 3 wires in the right places. Then run a power and ground and that's it. Just be sure to wire everything up then plug the power supply in, not having a switch kinda sucks. I'm gonna put mine in an ammo can with an external switch sometime soon, make it all nice and pretty.
              I am going to send you a PM a bit later tonight. I have some specific questions on how to wire this up. For $18 it's definitely worth a try, I just need some confidence that I'm not going to fry my headlight (or myself!)

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              • #8
                I'll be happy to answer them.

                2000 Toyota MR2 Spyder
                2017 Lexus CT200h F Sport

                Comment


                • #9
                  Originally posted by Haloruler64 View Post
                  I'll be happy to answer them.
                  Thanks! Just sent you a PM.

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                  • #10
                    Originally posted by SteveYem View Post
                    Thanks for that input. It all makes sense to me - however, unfortunately I do not have a way to bench-test this because I do not have a way to power a D3S bulb besides plugging the headlight assembly into the car. So I guess I am trying to understand the theory a bit better so I can understand how the lens location affects the size of the projected image, which may help me to achieve the desired outcome with minimal iterations. To that end, let's ignore the aftermarket clear lens for the moment and consider only the stock lens and projector bowl, which projects the cutoff shield image at a size of "X". As I move the lens farther away from the projector bowl, does "X" increase or does "X" decrease?
                    If you move the lens far enough (i.e. in the order of inches, not fraction of millimeter from its focal point), the image size will decrease. However I'm not sure what the behavior is when it is slightly closer or farther from the focal point. I believe the latter range is the tuning range you're looking at.
                    Jul 2012 ROTM (3-way quad headlight) ; Sep 2015 ROTM (custom muli-lens 7" fogs)

                    3-way quad wiring; foreground limiter; squirrel finder;

                    Comment


                    • #11
                      Originally posted by satrya View Post

                      If you move the lens far enough (i.e. in the order of inches, not fraction of millimeter from its focal point), the image size will decrease. However I'm not sure what the behavior is when it is slightly closer or farther from the focal point. I believe the latter range is the tuning range you're looking at.
                      Correct, we are talking a couple millimeters at most. Is it plausible that the large change in projected image size is due to using a clear lens in place of the OEM dimpled lens? I thought the dimples served only to diffuse the light for softening the cutoff but of course I could be mistaken.

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                      • #12
                        Yeah dimpled vs clear won't change the step size usually. It might be the wrong lens, or not installed right.

                        2000 Toyota MR2 Spyder
                        2017 Lexus CT200h F Sport

                        Comment


                        • #13
                          Originally posted by Haloruler64 View Post
                          Yeah dimpled vs clear won't change the step size usually. It might be the wrong lens, or not installed right.
                          It is labeled as, and physically appears to be, the correct ZKW-R lens of which I had used another pair previously to retrofit into a set of US-spec B8.5 S4 headlights (the headlights that originally came with the car) and I did not experience this "magnification" phenomenon. On that retrofit I did add two small washers at each mounting point for the lens holder, same as I did on these Euro-spec headlights, but the result was not the same. Granted, the Euro spec projectors have a more prominent step in the cutoff shield than the US-spec version, but it does seem that the change in projected image is much more drastic on the Euro version.

                          Hopefully this weekend I will have the chance to set up my bench power supply and do some experimenting with lens spacing vs. image magnification. I'll update this thread with my findings.

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                          • #14
                            It is possible that what appears to be the same projector and/or lens may be slightly different to match the specific needs of a particular OE vehicle of a certain model year. And since there is no SKU, barcode, QR code, or other identifiers, it is almost impossible for us to know. The projector manufacturer may subtly modify several parameters at the same time (e.g. lens material to alter index of refraction; slight bowl curvature; slight reflective coating thickness change) to make a specific version. And when we procure them for retrofit, a mix and match of the same projector family may appear like they are the same.
                            Jul 2012 ROTM (3-way quad headlight) ; Sep 2015 ROTM (custom muli-lens 7" fogs)

                            3-way quad wiring; foreground limiter; squirrel finder;

                            Comment


                            • #15
                              And aftermarket lenses vary. The old STI-R lenses had a few really bad batches that didn't work in many projectors.

                              2000 Toyota MR2 Spyder
                              2017 Lexus CT200h F Sport

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