Contrast has long been considered a key display specification, but in todays environment with projectors having dynamic irises (DI), fixed irises, high-ANSI but average on/off CR and high on/off CR, but average ANSI contrast it's clear that a better description of contrast is needed. The traditional metrics used in the industry are sequential (e.g. Full White / Full Black) contrast and the ANSI checkerboard contrast test pattern. Sequential contrast is a useful metric for gauging the maximum contrast achievable in the darkest of scenes as luminance approaches 0 while the ANSI checkerboard contains 50% peak white luminance which is much brighter than a typical movie or video scene. One drawback to using only these two tools for gauging contrast is that both of these test patterns describe only the fairly extreme bright and dark ranges that would occur in a typical movie scene. Film based content is a dark media with ~20% average picture level (APL) so it's clear tht most scenes fall in between the traditional measures of sequential and ANSI CR. We will also show that trying to apply traditional sequential contrast is no longer an applicable figure of merit for gauging the static contrast for dynamic iris projectors at 0 luminance and we will provide an alternative definition that describes both non-DI and DI equipped projectors alike.
What this project attempts to do is gain a better understanding of projector performance in several interesting ways. One is through the definition of a new suite of contrast test patterns (The "Phelps/Petersen Contrast Test Patterns"), Another is through mathematical modeling methods implemented by Erik Garci and another is by relying on individual forum members to supply data on their own projectors which can be maintained and displayed in a database as an ongoing effort.
A word on Contrast
In the following discussion it's best to think of contrast as a function of luminance, the higher the luminance the lower the contrast. The measured (and in some cases modeled and predicted) contrast functions for several projectors can be found in the contrast chart and each point along the luminance curve gives a person a rough feel for the contrast with a specific amount of luminance. Rather than complicate this project by using pixels with grey luminance which in turn can deviate based on projector gamma, this project defines test patterns with pixels using only full white and full black. Varying degrees of luminance are achieved by using more white pixels organized into boxes of different sizes and numbers. Since contrast is always a relationship of white to black, it's useful to think of these test patterns as representing "static" or "simultaneous" contrast because like traditional ANSI test patterns, the contrast in these test patterns can be can be seen and measured at one time and even with a a dynamic iris projector the measured contrast is fixed (ie static)..
The Phelps/Petersen Contrast Test Patterns
These test patterns will soon be available for public download and they contain multiple test patterns that allow consistent measurements of contrast with varying degrees of full white luminance. These test patterns range in luminance from 0.1% to 100% (full white) and also include the equivalents of ANSI CR and Full On/Full Off patterns. These test patterns were designed for constant probe position so that all a person needs to do is set up instrumentation and then go through the test patterns one by one and record the white and black readings. Once the test patterns have been finalized a link or e-mail address ffor download will be provided here.
Mathematically modeled contrast
In addition to directly measuring contrast as a function of luminance, Erik Garci has created the contrast calculator which allows us to extrapolate contrast values from ANSI and On/OFF CR. In some of the curves below are several diagrams comparing theoretical performance of projector types as well as a comparison between actual and predicted values. Here is a link to the contrast calculator for those who would like to use it.http://home1.gte.net/res18h39/contrast.htmAVS Member Supported Contrast Dabase
In this link (AVS Member Supported Contrast Database
) an Excel spreadhseet is available for download which contains the data for current measurements for several interesting projectors. As the project goes forward, we hope that others will use the test patterns and measure their projector performance and add their results to this database. White level, black level and derived contrast for luminance ranging from 0.1-50% can be found for various projectors in this file.A word on Intra-Scene Contrast
Ultimately intra-scene contrast (the contrast between different areas in a specific scene) is what is most important to people. Unfortunately the projector video gamma, spatial relationships and pixel luminance histogram complicate an attempt at equating test pattern derived contrast with real world video content. The test pattern contrast data is still very useful in providing a consistent benchmark for gauging technology differences between projectors but we are also working on relating this data to a few specific scenes to help people gauge how scenes with varying degrees of luminance affect contrast. To this end, Erik Garci has provided an intra-scene contrast calculator (http://home1.gte.net/res18h39/intrascene.htm
) and we hope to integrate these few select scenes into this contrast project at a later date.Contrast at 0 luminance
As mentioned earlier, these test patterns with varying degrees of luminance (% APL) allow contrast to be measured directly in a similar fashion as the traditional ANSI benchmark and we call this static contrast because both white and black points can be measured at the same time and the values don't change over time (ignoring bulb drift, etc). Attempting to directly measure static contrast at 0 luminance is of course impossible because there is no white content to relate to black. In the past, sequential full on/full off contrast was a useful equivalent for contrast at 0 luminance, however this breaks down for dynamic iris projectors because the iris can never be open (full white) and closed (full black) at the same time.Contrast at 0% APL - a new definition
Because the full on (iris open)/ full off (iris closed) contrast is never achievable at the same time with a DI projector, it's useful to extrapolate what the contrast would be at 0 luminance by creating a new definition that we're calling contrast at 0%APL. This is defined as the measured white level at the smallest available luminance / measured full black. As can be seen from the charts below, this definition accurately portrays the contrast as one approaches 0 luminance for both dynamic iris and non DI equipped projectors.The chart below graphically shows the problem trying to plot both DI and non-DI data when attempting to extrapolate contrast at 0 luminance using traditional sequential contrastThe chart below graphically shows how using the new contrast at 0% APL definition leads to accurate values for contrast at 0 luminance for both DI and non-DI projectors alike.Plotted Contrast vs luminanceThe chart below shows a sample of contrast vs luminance (using 0% APL definition) for various projectors as well as a sample of DI settings. Relative White level and Relative Black Level vs Luminance
Because contrast is a relationship between white and black, it's useful to examine white level vs luminance and black level vs luminance independently to see how simultaneous deviations in both can affect overall contrast. Unfortunately, due to setup and measurement differences, absolute comparisons of white and black levels between projectors cannot be made. These graphs do the next best thing which is to normalize the black and white levels so that the change in black level or the change in white levels from 50% APL (ANSI) can be compared. Please don't assume that projector A or B has better black levels or whiter whites from these charts as this is not what this data is displaying. Instead it shows how the deviation in white or black level from 50% APL as a function of luminance is changing. As an example a person can see from this chart that the black level at full off has decreased from 50% APL by roughly similar amounts between the HD1 and the VW50 Iris 1. This graphically shows the benefit of enhanced black levels with this iris mode. The Relative White Level chart also graphically shows the drawback that this mode creates with white levels. Interestingly enough, the overll contrast curve shows how these two relationships work together to boost overall contrast over the non-DI mode.The chart below shows relative white level deviation vs luminance for a few selected projectors (DI and non-DI)The chart below shows the relative black level deviation vs luminance for a few selected projectors (DI and non-DI)
As a side note, it's also possible to very roughly extrapolate absolute black levels between the HD1 and VW50 Iris 1 mode from this relative black level chart because both have similar 50% APL (ANSI) performance. In other words assuming both projectors had equal brightnesses at full field white (full on), the black level at 50% APL will be roughly similar because their 50% APL contrast performance are similar. So, if they both have similar black levels at 50% APL then this black level performance chart shows us that the deviation from 50% APL (where they are both equivalent) will be fairly close at 0% APL and nearly 4x better than the VW50 with Iris off. I should point out that this exercise is not the purpose of this project or benchmark, but it is an interesting exercise nonetheless.Final thoughts
This project is unique in one respect in that it's a public work in progess and is therefore subject to change and revision at any time. For example, no attempt has been made to relate this contrast data with intra-scene contrast or to gauge other important aspects of projector contrast such as DI behavior (outside of full open and fully closed apertures). But this doesn't mean that work along these lines won't be done at a later date.
To see the full discussion on this project and to provide comments, criticism and to add data to this project, please use this thread:http://www.avsforum.com/avs-vb/showthread.php?t=761806