Reflectors

From Lumenlab

So, you want more light from your DIY projector? This is a common concern, and there's more than one way to skin this particular cat.

So what is a reflector, anyway? Simply put it's a method of putting some of that light that isn't going into the collimator fresnel and making it useful. There are several different types, with varying degrees of usefulness.

A parabolic reflector is the kind used for satellite dishes; they take parallel rays and allow them to converge on a point source. In the case of a light they take light from a point source and turn it into parallel rays. This would be great for a projector, if we could get one large enough. It would eliminate the need for a collimator fresnel, since that's the job that the fresnel is supposed to be doing. Unfortunately, parabolic reflectors of adequate quality and size are rare and expensive. Though if you happen to have a spare 18" sattelite dish, you can try to get it mirror coated.

An elliptical reflector takes light from one point source, and focuses it onto another point. This could be used in a couple of ways for a projector, either treating the second point source as the point source for your fresnel, or eliminating both fresnels to have the reflector act as both collimator and field lens. Again, though finding an appropriate part is unlikely and may not yield significant results.

A spherical reflector is a section of a sphere. This takes light originating at the lamp and reflects it back to the source. These are common enough to find, as spherical sections are used in many other applications. Many people have used stainless steel mixing bowls, or the Ikea Soare napkin holder, which works very well with the S400DD lamp. This is by far the most common type of reflector used for the purposes of a DIY projector. Light reflected from directly opposite the direction that it needs to be to enter the collimator fresnel and become projected light can be reflected back. This light which would have simply been wasted can become useful.

So, how do you ensure that your reflector is positioned properly? The place to start is by figuring out the radius of your spherical section. In a perfect world, you could simply place the reflector so that the light source is at the geometric center of the sphere and call it done. Unfortunately, this isn't a perfect world. We need to account for a number of complicated factors, the math for which makes my head ache. So we'll "cheat." We could assume that it's a perfect world, and get the reflector close enough to help, but it's not the most help that it can be, and after all, that's the point, right? Now because we're using a spherical reflector, we can use it's properties to our advantage. To check the position of the reflector in relation to the lamp, all we need is something to cast a shadow. I simply used the edge of the projector box, after the projector was set up, but other people have used slits cut into cardboard with excellent results.

My theory is that if the reflector is in the best position possible, the lamp will act as a single point source, which casts sharp and clearly defined shadows. If the reflector is out of position, the shadow edges will become less defined. This can mean just blurry, or, at worst case, a double image, kind of like 2 separate lights. Your eyes here will be more accurate than any ruler can be, because they will tell you the real world status of the lamp and reflector combination, taking into account any discrepancies in the position and size of the light arc which differ from the theoretical center of the lamp. This will allow the most accurate positioning of the reflector possible, which in turn will help give you the brightest possible image on your screen.

Since your reflector is going to be in close proximity to your lamp, it needs to be made of something heat resistant. Stainless steel is a common material, although glass reflectors could also be used, if available. Cold mirror material would be ideal. A cold mirror allows infra-red (IR) light to pass through, but reflects visible light. This keeps the heat from concentrating at the lamp, and will help keep your projector cooler overall. If you're going to use stainless steel, be prepared to polish it. Though many good reflector items are already well polished, and some are even chromed, this is one thing that can never be too shiny. If you have a mirror surface, that is, of course, ideal.

To get the best results the pro reflector is highly recommended. It has been designed to match the Pulse-strike T15 or the S400DD. It has a dichroic coating (cold mirror) and made from Pyrex so heat isn’t a problem.