It often happens that you find flashlights that have good characteristics, but the light cone does not quite fit. Especially with tactical flashlights the control or strobe mode is often interesting for light painting, but the light cone is too narrow and has a strong spot. But with simple means these light cones can be made softer and in some lamps even very homogeneous.
The trick here is to use privacy film. These films are specially designed to diffuse light while still letting as much of it through as possible. If you attach such a foil on the glass of the flashlight you get a more diffuse light.
The d-c-fix films, numbers 26 and 35, are particularly suitable for this purpose and are available in most hardware stores. Other films are also suitable, you should only make sure that they are films with the highest possible light transmission.
These foils are very economical, because you only need a small piece of them. One roll is therefore enough for a hundred or more flashlights. The easiest way to start is to cut a rectangle that is a little bit larger than the lamp head.
With some lamps the head can be opened quite simply, so that one can take out the glass and use this as a template for cutting out. With such lamps, the foil can also be easily clamped between the glass and the lamp housing, so there is no need to worry about fastening.
With some lamps, however, you cannot open the head easily, so you have to cut the film on the glass so that the film fits exactly into the head. It is not necessary to clamp the foils, as they are held statically if they are attached correctly.
Here we show an easy way to modify lamps where you can open the head, because this is a little more work. It is best to start measuring the exact diameter required. This is very easy with a caliper gauge.
Then draw a circle with the measured diameter on a sheet of paper.
Now cut out the circle as exactly as possible and use it as a template to cut this circle out of the foils. With slow short cuts with scissors you can do this quite well if you show patience.
When you cut out the circle with the template you check if it fits on the torch. It is very likely that the circle is a bit too big at the first attempt, since you cut along the outer edge of the circle, and the circle then becomes larger than the template.
But that's no problem, just cut small pieces around the circle with scissors until the cut out film fits perfectly into the head of the lamp.
Now clean the film carefully with alcohol so that traces of oil that you always have on your fingers are removed. This makes the film adhere much better to the glass. It is then sufficient to moisten the underside a little and then to press the film onto the glass. It is best to turn on the lamp for a few minutes at the highest level to allow the moisture to evaporate. You don't have to worry about the heat, these foils can withstand a lot, so that even very strong lamps that heat up quickly can be modified without problems.
To show how the films work we have here an example with the Thorfire VG10S, which has a very strong spot. Here the light cone without film:
The pearled film (number 35) is very well suited to disperse the spot and soften the overall image. The center is still brighter than the edge, but the transition is much smoother, which is especially useful for illumination. The light colour remains the same and the brightness is hardly affected (only about 7% darker).
The milky film (number 26) makes the light much more diffuse and distributes the spotlight over the larger part of the light cone. In addition, the light cone is also extended, so that the lamp illuminates a larger area. The light gets a little warmer through the material and decreases only slightly in brightness (only about 9%). In the example photo you can see a small corona around the light cone which is caused by the fact that the film was not cut quite perfectly and is a tiny gap to the edge of the lamp head. If one can clamp the film between glass and housing, or the film is cut out better, this corona disappears.
It should also be noted that both films are suitable for removing coloured artifacts in the light cone. Especially with lamps with strong spots it happens frequently that one sees a halo around the spot which is somewhat color shifted (one often recognizes a greenish or yellowish halo). This halo is completely scattered by the films so that no color shading can occur.
If you use these foils with lamps that already have a quite uniform light cone, you get an absolutely homogeneous light cone as you know it e.g. from LED Lenser (but then even without the colored edge the LED Lenser often have).