What Is Projector Throw Ratio And Why Is My Projector Dim?


By Arindam Roy

Projector throw ratio refers to the ratio of light that projects out of the projector compared to the amount of light reflected back on to a surface that is near the projector”.

Why do this? The projectors used in modern classrooms are placed at a distance from students. The goal is for as much light as possible to project on to the walls and ceiling. This makes it impossible for most students in these classrooms, even those sitting right next to the projector, not be able to see projected material. This creates an embarrassing situation- students can’t see what’s happening on screen and are forced into awkward interactions with each other or with their teacher.

A way around this is to project the computer screen directly into the ceiling or wall. This requires a projector with an optical axis that is aligned parallel to the surface of the ceiling, wall, or other surface that it is projected on. This makes it easier to see if you are in an environment where light reflection from walls and ceilings will be a problem for students.

Also read: What is contrast ratio in projector?

The lens size of a typical projector can be adjusted to achieve this same effect (“throw ratio”) for any given distance depending on how far the image needs to project. The farther out from the center of the lens, the more light reflects back at students and/or faculty. The closer the lens sits to the surface, the less light reflects back.

Thinking back to high school physics, think of it like this: An object traveling in a straight line will reflect and refract off of surfaces. The angle at which it leaves is determined by how far away the surface is from its center. The angle at which it reflects off of the surface is determined by the angle at which the object hit the surface. If you imagine this scenario with a line of students sitting in a straight line and in their seating position (as most people do in high school physics class), then the farthest away students will see the screen first, followed in order by those more closely to them.

Typically, a lens will have to be adjusted toward and away from light sources for most of its travel through space. Most lenses will have some amount of flexiity when it is adjusted because any optical property has an upper and lower limit.

The key here is to find the point on the lens where its maximum distance away from a light source is the closest to the projector (without getting too close and causing distortion). This is what we call the “throw ratio”.

Read more: What is a good lumens for a projector?

It is also important to look at how near an image can be projected onto a surface. If you remember from your physics classes, when something travels in a straight line away from you, it appears smaller and smaller. There’s some difference between how far away you can project onto a surface without making it appear distorted and how far away it can be projected while still being able to see anything.

These two concepts are opposite of each other, and so it’s best to start moving the lens away from a light source closer to where you want the image projected. The throw ratio is the farthest point on the lens from a light source that produces good results.

When using this method in modern classrooms, teachers will often have problems with desks blocking the path of students who sit too close to a projector. Thus, it’s important to make sure that desks are either moved or that students sit in an appropriate distance from where you want your image projected onto your walls and ceiling.

In the end, this isn’t a perfect method, but it’s one of our best options for minimizing light reflectance onto students and teachers in modern classrooms.

Relevant reading: What is a good contrast ratio for a projector?

Why is my projector dim?

A projector is said to be too dim when it does not project a clear or bright enough image on the screen. This means that your audience will have trouble viewing your presentation, which can have a negative effect on any speaking engagements.

There can be several causes for a projector to be too dim, many of which are completely out of your control, and some of which you may be able to do something about:

  • Sparks from an electrical power line or light switch could reach the lamp inside the projector and cause it to flicker. This is most likely if you are using an older model with metal parts in the lamp housing. A newer projector should have a plastic housing, so these sparks won’t reach the lamp. If you’re still getting flickering, see if you can replace the lamp with a new one.
  • If your power comes in through an outlet near you, you might have an electrical shock or electrocution on your hands if you touch the lamp while it’s on. Best to move the lamp to another outlet or use a surge protector with outlets spaced far apart.
  • If you’re using a high-wattage bulb and voltage regulator in your projector, it may become too dim due to overheating and failing. This is more likely if the projector has trouble getting enough power from its source (a small amount of flickering may occur on startup). An available alternative is to replace the bulb with a lower-wattage one (or two “warmers” in series).
  • If you’re using the same bulb as a friend who has a different brand or model projector, or if you’re using one from your old projector, there may be something about its construction that causes it to burn out younger than other bulbs. Try replacing it with something from the manufacturer.
  • Every time your projector is turned on, moving pieces inside are breaking down small amounts of lubrication and moving parts are wearing down more. Eventually, these tiny quantities will add up to be enough to cause the projector to fail on startup. Try replacing the lamp with a new one as soon as possible.
  • A fan cooling the coils of your power supply may be getting too dirty and losing its ability to regulate the amount of electricity going to your bulb. This can cause it to burn out if it’s particularly dirty, or in extreme cases, to overheat, melt, and start a fire. This is only likely if you’ve left your projector on for several hours without replacement (see “Torchlight” below). Replace the bulb or power supply as needed.
  • Even though halogen bulbs are supposed to last longer than standard incandescent bulbs, they still need time to cool down between uses. They can start to melt and fail if they are frequently turned on and off. This is especially likely if you’re using a lamp with a high-wattage rating. Save the cool-off time for when you turn off the projector, don’t do it while your’re using it.
  • If your bulb isn’t large enough for the size of your screen, it will be dimmer than it should be. Your audience may not even be able to see what’s on the screen. If this is the case, get a larger bulb that will work better for you or replace your screen with a larger one.
  • If your bulb isn’t rated for being projected onto a screen, it will become significantly dimmer than if it were. Get a bulb that’s been approved by the manufacturer for projecting onto screens.
  • With older models of projectors, a simple connection between the lamp and power supply can become bad and let too much voltage through to the lamp. This could cause an increase in heat around the lens or image distortion, as the voltage will fry whatever happens to be in its way. Try using another bulb from your manufacturer or get it fixed by your local electronics store.
  • The solder holding the circuits of your power supply together may be breaking down due to dust buildup and age (this is more likely with older models).

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