Aspherical Lens and Incans?

[bullettproof]

Im sure this has been done before. How is an Aspherical Lens on a Philips 5761 bulb? How does the beam project and heat build up ? Does anyone have any beam shot pics or know what Lens is best to use?????

Thanks,

 

[thezman]

It will, if focused properly, project an image that looks like the filament.

Not a pretty sight in my opinion.

 

[Benson]

Im sure this has been done before. How is an Aspherical Lens on a Philips 5761 bulb? How does the beam project and heat build up ? Does anyone have any beam shot pics or know what Lens is best to use?????

Thanks,

The beam will look like a coiled filament with blue and/or red haze from chromatic aberration, unless you substantially defocus it (which kinda loses the point, imho). I've only tried one aspheric for this, a cheapo from DX. See LuxLuthor's aspheric Mag shootout to see how a higher quality lens would improve it. I'd probably get a good lens if I was making a light, but for sliding it over the end of various light with a cut-off pop can, I can tolerate the cheap one...

With the DX lens, a WA1185 projects a reasonably clean 5.4" x 14.8" spot at 19 feet. The 1185 is better for this; although it has a big filament, it has a smooth dome at the front, because they fill the bulb from the base. This gives good output out the front, and nice focus.

The Philips 5761, however, is filled from the top of the bulb. This doesn't bother reflectors, since they collect light from the sides into the spot. But with an aspheric, the fill-tube distorts the light coming forward, preventing a sharp focus, adding its own chromatic aberration, and probably decreasing OTF lumens. The filament is also wound deeper vertically, again hurting focus. The filament is smaller from the top, though, so it focuses to a smaller spot, 5" by 10.2" at the same 19 feet, but with worse blurring and color fringing.

Sorry, no beamshots, my digicam's dead. But, in case you don't use inches, or want to use it ranges other than 19 feet, here are the beam angles for those two bulbs with the DX lens:

• 1185: 24 x 65 mrad
• 5761: 22 x 45 mrad

If you've got some flexibility in your design, a side-projecting 5761 could work out real nice, with a squarer beam than either of these (I'd guess about 33x45 mrad) and rather sharp.

 

[LuxLuthor]

The problem you run into is that most aspherics have a specific focal length ("FL") point. They work with LED's because they are a flat die that you can match up with the lens FL focus point. When you get a 37mm FL aspheric, that is the actual distance from the lens to the LED surface if you have it precisely outline the LED lines on a whitewall. The incan or HID bulbs are not flat enough, so it doesn't work the same.

Some of us tried to find longer focal lengths and thinner magnification thicknesses that don't need such a precise (flat) focal length point. These have a higher (longer) focal length which moves the focus point way behind the bulb.

The idea being to have the larger bulb size mimic a middle point of a cone of light, that would be theoretically have been formed by a smaller light source back at the focal length point. The results have just not been that great with most bulbs. There are a couple threads with sketches demonstrating this idea, and lenses we tried.

 

[Patriot]

The WA1111 works ok but like all incans only light from the front of the filament is in focus and all the other light from the filament is out of focus. It's fun to experiment with but I can't think of any practical application where there wouldn't be an alternative which works better.