What is the f-number of your aspheric lens?

[Walterk]

What is the f-number of your aspheric HID-Headlight lens?

(And what thickness and diameter while you have a measurer in your hand..)

Thx !

First input: f 0.71 - Typical Maglite-size, fitting in the existing housing.(Is it the best, or just the only one with focus on right plane? )
Found: f 0.72 - first Deft Subaru SVX '92 Hid Headlight lens - 79mm dia, bfl 54mm.

 

[Walterk]

Re: What is the f-number of your Aspheric lens?

Wow! Zero posts so far.
Changed the title from 'whats the f-number of your Aspheric lens' instead of 'your HID-lens' .

Shorter Focal length is no guarantee for more throw. It does to some extent, but I wonder up to what point?

With very low f-numbers the light bounces of the plano plane and reduces light getting into the lens. There is a thin optimium line between low-f/large spot and high-f/small spot......!

As explained here :

Rules reflections

Angle of incidence

I am very curious, what your experiences are. May it be a HID-lens, Deree, WolfsEye, Deft FTP/HO, Mag-mod, FlyDragon, found in an old projector or scrapyard....

The f-number is the focal length divided by the effective diameter (the bulb without rim). This ratio is fixed by proportions of the lens, independent of size.

 

[Al Combs]

Re: What is the f-number of your Aspheric lens?

Wow! Zero posts so far.
Changed the title from 'whats the f-number of your Aspheric lens' instead of 'your HID-lens' .
I care, why dont you..

May it be a HID-lens, Deree, WolfsEye, Deft FTP/HO, Mag-mod, FlyDragon, found in an old projector or scrapyard....

The f-number is the focal length divided by the effective diameter (the bulb without rim). This ratio is fixed by proportions of the lens, independent of size.

(Still curious, as there is a thin optimium line between low-f/large spot and high-f/small spot......!
As with very low f-numbers the light bounces of the plano plane and reduces light getting into the lens. )

I can't tell if you are taking a poll or just need help picking one. FWIW, the Melles Griot 01 LAG 123, now known as CVI LAG-52.0-37.0-C was one of the most popular around here. It has a focal length of 37mm and a diameter of 52mm, for a focal ratio of 0.71. I don't know about the Tiablo or Dereelight aspherics, but I suspect they are similar. Hope this helps.

 

[Walterk]

Re: What is the focal length of your HID-Aspheric lens?

As said; I am curious, because I want to understand.
I have plenty lenses but experience shorter FL isn't always better.

For example two lenses I have :
f 0.43 - 71mm Dia (focal length 30mm)
f 0.54 - 72mm Dia (focal length 40mm)

To my visual experience the 40mm focal length performs better then the 30mm lens. More people have similar experiences on this forum I've read.

I added a link and the graph that started my curiosity for reference above.
From that information, I copied and stretched the graph, made some simple but laborous maths.
( Remember I am no expert, just try to understand what meanings are of what I read here and there.)

I think for each Led there is an optimum, shown by the orange line in the graph, expressing 'throwrate'.
The blue line is the correction factor, due to losses with very short focal lengths.
The red line is the fixed viewingangle resulting from f-number.
The purple line is the percentage of flux versus viewingangle, from Led datasheet or for example 'White led testing'-thread.

Someone can verify or clarify on this?

If so, my assumptions, as far as my understanding is at the moment:
(Example 75mm Aspheric with f# 0.66 and Cree XRE Led.):

Intensity beam (800.000Lux ) = (Geometric correction factor (0,8) x Surface area of aspheric lens (4000mm2 ) x Lux surface brightness (250 Lux/mm2 )

Amount of light (180 Lumen) = Lumen of Led (1mm2 x (250Lux/mm2)) x Geometric correction factor (0.8) x Leds Flux according datasheet at given angle (90%)

 

[gcbryan]

Re: What is the focal length of your HID-Aspheric lens?

I don't have any numbers for you. More questions that anything else...

So, in a nutshell, is the issue too large a focal length and you don't have much light hitting the optic even though any light that does hit it gets focused very well (narrowly).

With a shorter focal length lens is the issue that you can get so close that light doesn't hit the lens properly as well and the light that does hit it isn't focused very tightly and therefore doesn't throw as well?

So there is a optimal line where you are getting a good amount of light hitting the optic and it's being focused narrowly as well.

Is that what we are talking about (in laymen's terms)?

The problem with my lenses are they are cheap DX lenses and they aren't sold by focal length (just diameter) so there is a lot of guessing involved.

 

[Walterk]

Re: What is the focal length of your HID-Aspheric lens?

(Assuming Led at focal length: )

too large a focal length and you don't have much light hitting the optic even though any light that does hit it gets focused very well (narrowly).

, and too short the focal length less-then-expected light is entering the lens.
(You would expect more, as it is closer to the Led and thus has a larger viewing angle.)

 

[gcbryan]

Re: What is the focal length of your HID-Aspheric lens?

I don't know what optics you have to experiment with but I'd think you could experiment with two 28mm diameter optics with different focal lengths and come up with some kind of rough answer to your question as to where that "thin line" is.

You have the knowledge and I would guess a lux meter as well. I have one of the cheap flood-to-throw flashlights from DX and it has a plastic optic that is 28mm in diameter.

I also have the glass 28mm optic sold on DX. I don't know the exact focal lengths of these two but they are different. The glass optic has a slightly longer focal length.

If you happen to have access to those two lenses you could measure the lux reading at 1 meter or whatever with those two lenses in a flashlight keeping everything else the same (emitter, battery charge, etc).

It would take a little work to mount one of them at it's focal length. One of mine works just as a replacement for the front glass lens. The other would require mounting deeper into the head.

Have you done any experiments like that?

 

[Walterk]

Re: What is the focal length of your Aspheric lens?

Edit:
Reading again this old post from me, from before the big crash. I think I had not totally clear the difference between lumen and candela.
For throw that's more then cleared up in the thread Calculating throw.
For Lumen I still want to find out how to predict lumen for given distance led-to-lens, by a given led, and a given lens.

Please help where you can, with explanation.

Losses in the system:
1-Lenses with shorter focal lengths have smaller aperture. Not the whole lens diameter is effective.
You can test to see what is the back focal length and the effective aperture.

2-There are losses from reflections, light emitted from the led is partly reflected instead of transmitted.
I THINK the graph Geometric correction factor takes this in account to show losses.

3-The viewing angle of the led doesn't match the aperture in our application.
Easy to calculate if you make a construction of angles and diameters in cad-software or on paper.

4-Not all lumen are distributed equal over all viewingangle of the led.
Needs mathematical integrating, or my preference; measuring area from graph led viewingangle.
Crosscheck with JTR's White led lumen test.

5-There are losses in the glass, each plane, just pick 85% for plastic, 90% for glass and 95% for AR-glass.
Can be refined later from manufacturers specs.

6-Lumen emitted from the led can be estimated by led datasheet at given current, Vf and temperature.
Crosscheck with several reviews and JTR's White led lumen test.

7-Lumen OTF = 1 x 2 x 3 x 4 x 6 ?
Can be checked with integrated sphere measurements from aspheric flashlights.


Edit2:
Did some calculations and I think there are a few flaws.
- Unsure about the graph, have to check that first. And the graph shows shift between lens and mirror. Could this be for reflectivity of the material or for the focal versus back focal length?
- Is it valid to measure a surface from the led-graph to have the correct lumen figure for that viewingangle?
Not sure yet. Some contradiction to figures from JTR White led lumen tests. The led might show different behaviour when overdriven also.

So, I will have to make measurements with a lightbox to make my own table that shows how many lumen from the led are transmitted at different focal distances.