A quick refresher: contrast ratio is a method for expressing the range of difference between the darkest area of an image and the brightest area of an image. The ratio is generally expressed in the form of:
Where 1 equals the lowest light value x equals the maximum brightness value divided by the lowest brightness. For instance, if the lowest light value is 10 units and the maximum light value is 1000 units, then we have:
Projector manufacturers publish a Contrast Ratio specification based on a full on/off measurement method. This means that the dark and the bright values that define their contrast ratio are displayed and measured separately. The dark value is established by measuring the output of the projector with no input signal or content. The bright value is established with a peak white input signal and with the projector's brightness and contrast controls set to maximum.
Now, given that what we're really interested in is how well the projection system will perform when displaying dark and bright values simultaneously the full on/off specification is of limited value. In the real world we would never use the projector with the controls set to maximum brightness and contrast because the image quality will be terrible, we will eventually content to watch too!
So, what is a more useful way to define Contrast Ratio? The best way, and incidentally the method chosen by the ANSI standards committee, is to project a grid pattern of 16 rectangles, alternating between black and white; then measure the light coming off of the black rectangles and measure the light coming off the adjacent white rectangles. It is important to emphasize that these black and white rectangles are being projected simultaneously. For example, if we measure 2 light units from a black area and then we measure 40 light units from the adjacent white rectangle, then we have a real world Contrast Ratio of 20 :1
Let's return to the idea of a projection system. A projection system consists of three parts: the projector, the screen and the room. If we consider these three parts and establish values for them, we can make a reasonable prediction of what our real world contrast ratio will be. Further, we establish what our real world Contrast Ratio will need to be for different projection applications, based on a recent standard developed by Infocomm called PRISC or Projected Image System Contrast Ratio.
Ambient light can easily be measured by using free light meter apps for smart phones. Here are links to free light meter apps for Android, Apple and Blackberry devices respectively, just click to open in a new window:
The lighting conditions in the room when you make your measurement should be at the same light level you intend to use your projector in. Place your meter in the centre of the planned image location to make your measurement. This is your Ambient Light measurement, record it for later use.
If your projector is already in place and set to the image size you plan to use it at, you can turn the projector on, let it warm up for at least 15 minutes and then display a full field white test pattern. You can find a good 100% White test pattern here:
Place your meter at the same position you made your ambient light measurement from and measure again to ascertain your Projector Light value.
Once you have taken both measurements simply multiply your measured projector light by the reflectance factor of your screen and then use the resulting value to determine your real world Contrast Ratio. Expressed a bit differently we get this:
We now know how to calculate Real World Contrast Ratio, so let's revisit PRISC and see which ratios have been specified as adequate for a particular task:
Category: Passive Viewing
Description: The viewer is able to recognize what the images are on a screen and can separate the text or the main image from the background under typical lighting for the viewing environment. The content does not require assimilation and retention of detail but the general intent is understood. Passive engagement.
Minimum Contrast Ratio: 7:1
Examples: Non-critical or informal viewing of video and data; best for use in retail stores, family (TV) rooms.
Category: Basic Decision Making
Description: The viewer can make basic decisions from the displayed image. The decisions are not dependent on critical details within the image, but there is assimilation and retention of information. The viewer is actively engaged with the content.
Minimum Contrast ratio: 15:1
Examples: Information displays, presentations containing detailed images. Best for use in classrooms, boardrooms, multi-purpose rooms and product illustrations.
Category: Analytical Decision Making
Description: The viewer can make critical decisions by the ability to analyze details within the displayed image. The viewer is analytical and is fully engaged with the details of the content.
Minimum Contrast Ratio: 50:1
Examples: Examination of engineering and architectural drawings, electrical schematics, forensic evidence, failure analysis, photographic evaluation. Best for use in courtrooms, medical galleries, control rooms etc.,
Category: Full Motion Video
Description: The viewer is able to discern key elements present in the full motion video, including detail provided by the cinematographer or videographer necessary to support the story line and intent.
Minimum Contrast Ratio: 80:1
Examples: Controlled viewing environments; best for use in home theatre settings, business screening rooms, broadcast post-production environments.
Source: ANSI /INFOCOMM 3M-2011
Stay tuned for our next post where we will be discussing how our knowledge of Real World Contrast Ratio will be incorporated into the next generation “Which Goo” calculator, a tool that we have developed to help you achieve the best results possible for your projection system.