Ask the Expert: How bright a projector do we need for our size of screen? Does distance from the back of the room matter?
Russell King asks:
“Our projector screen will be 50ft away from the back row. Given that we are a small congregation, but we want maximum picture clarity and visibility, what size screen should we purchase?”
Tim Horton from SFL Group answers:
As well as the size of the screen you need to be thinking about the brightness of the projector and the kind of image you want to project. The bigger the screen, the brighter the projector! It’s a simple calculation: there should be at least 80 lumens of projector intensity to every square-foot of screen surface. Therefore an 8×6 screen = 48 sq-ft needs a 4,000 lumen projector.
How is projector brightness measured?
Projector brightness is measured in ANSI lumens (lm). Projectors range from just a few hundred lumens (suitable for small offices or home use) to tens of thousands of lumens (for very large screens).
The 80lm/ft² rule
The intensity of a projected image is determined by three factors:
The amount of ambient light
If these three factors are not balanced correctly the image will appear “washed out” making it hard to see pictures or read projected text.
Assuming a typical level of ambient light in a lit indoor space, SFL’s design rule is to allow for 80 lumens per square foot of screen surface. We know from experience that this produces a satisfactory image quality in the majority of venues, although in vanues with very high levels of ambient light this figure may need to be revised upwards to accommodate.
The 80lm/ft² rule implies a constant image intensity by establishing a direct relationship between projector brightness and screen size. The larger the screen, the brighter the projector you will need to achieve satisfactory image intensity. Think of this as being like spreading butter on toast – the larger the piece of toast, the more butter you need to ensure it’s not spread too thinly.
Do I need a brighter projector for long throw?
No. it’s a common misconception that projectors need to be more powerful if they are further away. Light does not dissipate over distance, so the throw distance makes no difference to required projector power. For a long throw we simply need a suitable lens to focus the light into a smaller angle so that when it arrives a the screen we achieve the required intensity per unit area.
So to answer your question, it’s not a case of the distance from the back row, merely choosing a screen to the optimum size for your congregation to see and ensuring that your projector is powerful enough to deliver a bright enough image. For example, if you have a 10ft wide screen, and you are 20ft away then you need a 2:1 ratio lens. If you are 40ft away you need a 4:1 lens, and so on…