I've been reading up about and experimenting with aspheric lenses for some time so thought I'd share some information in case it's useful to anyone else. I hasten to point out that others such as Marky have obviously already sussed this issue out, so I'm definitely not the first to discover this stuff.
Put simply, an aspheric lens carries out the same function as a traditional torch's reflector, to focus or concentrate the beam of light. However, it focuses the light coming out of a bulb or LED using the refractive abilities of glass (or acrylic, etc) rather the reflective abilities of something like polished aluminium or silver, i.e. a mirror. Because it’s an aspheric lens, and therefore not evenly spherical, the image it projects is distortion free. Thisis a good website if you want to know more about aspheric lenses, but there are also many others.
Aspheric lens back focal length
Aspheric lenses capture the light coming out of the LED and can focus it into a pencil beam or a wide flood pattern depending upon how far away the back of the lens is from the LED. If you adjust this distance to the point where the pencil beam is as narrow as you can get it, the lens will be at its back focal length (distance) from the LED (that’s the important bit. Oh, and every time I mention focal length from now on it means the back focal length, not the effective focal length). Very often you can see quite a clear an image of the LED's die when you shine the beam at a flat surface. The focal length is the distance measured from the BACK of the lens to the die of the LED, NOT to the front of the clear bead which covers the LED's die. If you move the lens nearer to or further from the LED, the image of the LED goes out of focus because the lens is no longer at its focal length (distance) from the LED.
So to get the longest throw from a torch you need to have the aspheric lens at its focal length in front of the LED. At its focal length the beam coming out of the lens will be at its most concentrated, so will travel the furthest. This is obviously important when you're hunting at longer ranges.
Unfortunately many suppliers don't specify the focal length of the lenses they sell, although some do. But as a general rule of thumb the THICKER the aspheric lens the SHORTER the focal length, and vice versa. If you look online you'll see lenses with the same diameter but different thicknesses, i.e. they have different focal lengths (assuming they’re made from the same material).
Lens diameter (and relation to beam pattern from LED)
Having worked out the optimum distance to position your lens in front of the LED to get the most throw, i.e. the greatest range, the diameter of the lens is the next issue. To work out the optimum diameter you ideally need to know the pattern or spread of the beam that comes out of your LED.
If you think about the shape of the beam that comes out of an LED, it's like a cone which gets wider as it gets further from the source. As you move the lens further from the LED, which you want to with LONGER focal length lenses, you need a wider aspheric lens to collect all the output from the LED without some of the LED's output "spilling" around the edges of the lens. But with a lens with a SHORTER focal length, which can be placed closer to the LED to get the concentrated pencil beam needed for longer ranges, the lens can be smaller in diameter as the cone is narrower closer to the LED.
The SIMPLEST way to find out about the beam pattern is to look up the LED's "Angle of half intensity" or "Half angle" on the LED's datasheet, such as here for the Oslon Black (2/3rds of the way down the second page). LEDEngin datasheets refer to it as the "Viewing angle" instead. For the Oslon Black LED, which is probably the most popular IR LED at the moment, the angle of half intensity is 45 degrees, but what exactly does this mean?
Imagine putting your eye directly in front of the centre of the LED (don't actually do this at home, or anywhere else, as you might damage your sight! Mind you I did say "imagine", so you could probably safely do this "imagining" almost anywhere). The beam from most LEDs is at its strongest intensity at the point directly in front of the die (known as zero degrees). Now imagine (here we go again!) moving your eye to one side. The intensity of the LED appears to decrease. Move your eye until the intensity is HALF what is was when your eye was directly in front of the LED. The difference between these two points is the angle of half intensity when measured as an angle at the LED. Hence the name "angle of half intensity" (obvious ain't it now you've thought about it).
(Note: For some LEDs the maximum intensity is NOT directly in front of the LED, but that would complicate my explanation even more than it is already!)
So for the Oslon Black with a half angle of 45 degrees, MOST of its light comes out in a 90 degree cone shape (45 degrees on each side). But the problem with simply using the half angle is that all LEDs will still be putting out light BEYOND the angle of half intensity. In some cases as much as one third of the output from the LED is outside the angle of half intensity.
Note: Some LEDs on suppliers' websites are labelled something like "OSLON Black LED, 850nm, 90deg", so it could give the impression that the half angle is 90 degrees. Always check the specification/data sheet as the degrees quoted in the title may be the half angle OR the full angle.
To understand the full picture it's back to the LED's datasheet to look at what is usually called the radiation characteristics polar diagram. Here's the diagram for the Oslon Black, taken from the 4th page of the datasheet.
Both sides of the diagram show the relative light intensity of the LED at different viewing angles, so they're both showing the same info but in different ways. Straight up on the diagram is directly in front of the LED (zero angle), and in the Oslon Black this is where it has greatest intensity, so it is given the value 1.0 on the vertical scale. As the viewing angle from the LED is increased (like moving your head to one side) the intensity decreases, so the graph goes down. From this chart you can work out the angle of half intensity by seeing where the graph crosses the 0.5 intensity line and then look on the scale of angles to see what angle this is at. In this case it's a 45 degrees.
If you're sad like me you can use the information on the graphs to work out how much of the LED's output is captured by a lens of a particular diameter positioned at a particular distance from a particular LED (it's all to do with the area under the curve if you're interested. No? Ok then, I don't blame you). As hunters we want the maximum illumination at longer distances, and you all now know that the light is most concentrated (in a pencil beam) when the aspheric lens is at its focal length. So this gives us a means of comparing the performance of lenses with different diameters and focal lengths in combination with different LEDs. Simples...
Note that the aspheric lenses which we use have a small shoulder around the lens for mounting in the head of the host. So the physical diameter is greater than the optically active diameter of the lens. It’s the latter which is used in the calculations.
I've written an Excel spreadsheet to work this all out for me, and am using it to calculate values for some of the common lens/LED combinations so that I can post them up here later. I’ll also post up a list of the aspheric lens suppliers that I’ve come across.
Effect on beam concentration/intensity of LED die size and distance from lens to LED
I’ve included this section because although at first glance the best lens would seem to be one which has a short focal length (so it can be placed closer to the LED where the LED’s beam is less spread out, and the lens doesn’t need to be as large in diameter to capture all/most of the LED’s output for long ranges). However, this is not necessarily the case…
Aspheric lenses produce the narrowest and hence most concentrated (brightest) beam of light when the light source is a single point source at the focal point of the lens. A point source just means an infinitesimally small pinprick of light. This should be at the precise point where the lens focuses; again this is a very small area (the focal point). In these circumstances the lens produces a parallel beam (pencil beam) of light of the maximum intensity for that lens/light source combination. If the light source is large some of the light is produced away from the focal point, and this will not be properly focused by the lens, so will be sent off at an angle by the lens rather than being concentrated into the parallel beam. So it is effectively wasted.
Therefore a very small LED die is much better than a large one as it’s more like a single point source, and therefore a higher proportion of its output will be concentrated into the parallel beam. However the size of the die as seen by the lens also varies depending upon how far away the LED is from the lens. As far as the lens is concerned the LED APPEARS to be smaller when it’s further away from the lens than when it’s closer. It’s all to do with what’s known as the solid angle, and is measured in steradians (I've included links if anyone wants to find out more).
So a lens with a LONGER focal length allows you to place the LED further away (more like a point source), and will produce a more tightly focused and hence a brighter beam at distance. There's a limit to what lenses can do, but generally the smaller the light source the tighter and therefore brighter the beam coming out of the lens. From my experiments in the field a small LED die is critical to getting decent illumination at range. Fortunately the Oslon Black IR LED is very good in this respect, with a 1mm x 1mm die.
Remember that to get the best results it is critical to mount the LED exactly centrally behind the lens, and not slightly to one side.
Basically a high quality lens will let through more light than a low quality one. Also the shape of a quality lens will be more precisely engineered to concentrate more of the light into the pencil beam. Unfortunately cost and reputation seem to be the only indicators of quality in aspheric lenses. Like so many things you probably get what you pay for.
I’m not aware of any coatings applied to aspheric lenses, similar to coatings applied to scope lenses to reduce reflections and hence to let more light through, but be aware that some coatings block IR light.
In my view the IDEAL setup would be:
- A high output LED with narrow beam (small angle of half intensity) and a small die size.
- An aspheric lens with long focal length so that the LED is a small point source, but with the lens being sufficiently wide to capture all of the LED’s output.
- A zoomable illuminator to adjust distance from lens to LED, so that you can vary the intensity and width of the beam to suit the range you’re shooting at. Remember that if you’re using a zoomie to adjust from spot to flood, if possible always move the lens CLOSER to the LED rather than further away. This is to ensure that you’re capturing more of the LED’s beam rather than loosing some around the edge of the lens, due to the way the way the beam spreads out as it gets further from the LED.
I think this explains why Marky's Markylights are so good, although according to the initial calculations I’ve done even these could be improved by using either a wider lens or an LED with a tighter beam (smaller angle of half intensity). I’ll keep you in suspense at the moment but will post these calculations up later.
Hope that helps. Sorry for the long post but I didn't have time to write a shorter one.