Determining correct bulb height in reflector

[KevinL]

Some help needed building something custom that we'll tentatively call Project Lemonade

ok, let's say you have a reflector, with unknown proper focus height. Noted that it is quite a pain to wait for a hotwire to cool down to a safe temperature after lighting it up to check beam pattern, then open up the head and readjust the bulb.

I will dry-fit the bulb, un-lit of course, into the reflector, to assess how far it needs to go into the reflector.

I am aware with LEDs that one can stare into the front of the reflector and slide the LED back and forth until the entire reflector fills with the yellow phosphor, then note the height. I will probably use this technique... except there is no yellow.

What should I be looking for when aligning an incandescent bulb this way?

 

[LumenHound]

What should I be looking for when aligning an incandescent bulb this way?

For small reflectors just look for a magnified image of a few of the filament coils spread all the way around the middle part of the reflector. If the bulb has a large filament the image may fill almost all of the reflector. For larger deep reflectors look for the coil image close to the bulb opening.

It helps to have a good strong light directly above the reflector when you are looking down into it.

 

[Justin Case]

Three points uniquely determine the equation of a parabola. If you can determine three such points (at least two should be reasonably easy to get -- at the top and at the bottom), you can get the reflector's equation and then determine the focus.

 

[KevinL]

For small reflectors just look for a magnified image of a few of the filament coils spread all the way around the middle part of the reflector. If the bulb has a large filament the image may fill almost all of the reflector. For larger deep reflectors look for the coil image close to the bulb opening.

It helps to have a good strong light directly above the reflector when you are looking down into it.

Great, thanks! Will try that

Three points uniquely determine the equation of a parabola. If you can determine three such points (at least two should be reasonably easy to get -- at the top and at the bottom), you can get the reflector's equation and then determine the focus.

Shush.... you are reminding me of how I was always insulted by my HS teachers for never being able to figure out this maths stuff

 

[Mettee]

math aside, move the bulb in and out under low voltage and watch the image on a wall. You will see it focus, move it in and out of focus until you are happy. Typically that works the best for me, obviously knowing the F.L. is going to help. If you can take it out at night and use the beam to show you its shape, you can read that and adjust it as well.