actually, a 'normal' lens is a spherical lens. This means that the curved surface of the lens is a part of a sphere. If you would draw a line over the surface, this would be a part of a circle.
This is very interesting from the point of view of the manufacturars, since this is easier to make.
an aspheric lens has a surface that is not a part of a sphere. there is some more complicated mathematics involved in describing the surface mathematically. Also, this is more difficult to manufacture.
Since we see a lot of plastic lenses lately, these are easier to make because you just make a mold and extrude the plastic in the mold. You just need to make the shape of the lens.
This is correct. Most of the aspheric lenses we are interested in for our flashlgihts are parabolic, rather than spherical. If you mathematically look at a parabola (the desired surface for focusing light) and a sphere, they are approximately same for lenses where the focal distance / lens diameter, or f-number is large. For lenses with a high focal length / diameter, a bigger section of the parabola is required, which will start to differ from a spherical surface.
aspherical lenses give you the advantage that, when designing the lens, you have extra parameters to compensate for unwanted effects like abberations, but this only works good if you don't deviate too much from the optical axis of the lens (here, we would say when the beam angle is too big, an aspherical lens wouldn't be very useful or better compared to a normal spherical lens).
Beam angle and being on-axis aren't really related. Staying on axis means that 1) your LED die should be well centered underneath the lens, and 2) the die size should be small compared to the size of the lens. If the die has a large extent, then the frignes off the lens will be off-axis and tend to be distorted.
This is also a reason why these lenses are, as far as i know, only available for small angle beams.
hope this makes a little sense to you guys :thinking:
The beam angle exiting the flashlight isn't really the issue. The bigger issue is the acceptance angle, which relates to how large of an angle of light the lens can capture from the LED. This is described in photography by the f number (f/d, or the focal length divided by the diameter). Low f-number indicates more light gathering power.
One lens I have in one of my maglites is an apsheric lens that is 52mm diameter, 33mm focal length (about 0.6 f-number). Because this lens can be placed close to the LED, it can capture light from a wide range of angles.
Now if I were to use a 50 x 200mm spherical lens (f number of 4), I could still project an image of the LED de on the wall just like I could with the aspheric lens. Beam angle (output) would be identical. The difference is with the lens 200mm away from the LED, the vast majority of the light from the LED is being lost as only the light from the LED going "forward" initially will be collimated.
Beam angle is related to the size of the LED die compared to the diameter of the lens. Beam angle can be larger if there are defects in the lens or reflector -- including intentional defects such as diffusers / stippling etc. but that isn't what determines the actual angle itself.
