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Factors Affecting the Brightness of a Projected Image

Posted by George Spoleto on

Brightness sits at the top of the list when thinking about the quality of a projected image whether for conference rooms, classrooms, houses of worship or home theater applications.

What are the factors that affect the brightness of a projected image?

Projector Brightness
When buying a digital projector one of the key metrics found on manufacturer specification sheets is brightness. Measured in lumens, this quantifies the light output of the projector. More lumens, more brightness from the projector.

Image Brightness on the Screen
What most of us are thinking when the topic is brightness is image brightness – how bright does the projected image appear to the viewers. Projector brightness is only part of the answer to “How bright will the image appear?”

The characteristics of the surface where the image is projected – specifically its reflectivity - also has an impact on the apparent brightness of the projected image. Reflectivity is indicated by "gain factor". Gain is a number (eg. 0.5 to 2.0) that represents the ratio of light reflected from the projection surface compared to a neutral reference surface (chalk matt white).

The gain factor of a projector screen can make the image appear brighter or less bright than what the lumen output of the projector might indicate. For many applications a normal gain of 1.0 will be suitable. However, screen gain can be used to fine tune the apparent image brightness for specific applications. 

High Gain screens (>1.0)
High grain screens may be useful for rooms with medium to high ambient light. However, these screens focus more light toward the center viewing position, less toward the angled viewing positions. This effect may compromise the brightness for viewers sitting at the sides of a wide seating arrangement. If you know that most viewers will be centered on the screen, and you already have a high gain rated screen, this may allow for a less expensive, lower lumen output projector purchase. Example: A screen with a gain of 1.2 will have a 20% brighter appearance at the center than a standard 1.0 gain screen.

Low Gain screens (<1.0)
Low gain screens are better when many viewers may be facing the screen at an angle. All viewers will then get a similar image brightness. In addition, low gain screens may be useful for dark rooms (home theater) where hot spots and glare could be an issue especially from a high lumen output projector.

Bottom Line
Projector lumen output and screen gain together determine the perceived brightness of a projected image.

For additional information on this topic see:

Buying Guide for Business Projectors (Projector Central) 

Your Guide to Understanding Screen Gain (eBay) 


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