Do the maths!
Calculating and demonstrating the superiority of ALR screens.
Why using the right screen is both better and will save you money.
Of critical importance is choosing the right screen material. The default of focusing on the projector and not what the viewers actually look at - the screen - is contradicted by both the science and the savings it affords in conforming to the ISCR/PISCR standard. Optical-grade specialist screens reject ambient light and, reflect light evenly and without distortion, resulting in high-contrast, detailed images with accurate colour rendition. You’ll see the maths laid out in the table below.
Using the product-agnostic calculator you’ll find here let’s compare dnp Supernova 08-85 material (Scenario 1) with standard matt white (Scenario 2) material.
Step one, define the room conditions, here set at an average of what we measure. We’re looking to achieve a 15:1 contrast ratio, as per PISCR/ISCR’s minimum conformance. But see how we can fail at the very first step. A matt white screen will need 600cd/m2 (‘NIT’) to punch its white levels to 15 x its black level when the asthenopic (‘eye strain’) threshold is 477.5cd/m2 (using Task Luminance Ratio at 3:1, CIBSE/SLL’s maximum).
Now let’s input image size and screen performance specifications to discover projector lumens needed to fulfil the required minimum 15:1 image contrast ratio. This is quite startling: in round terms you only need 2000 lumens using the correct ALR (ambient light rejecting) screen material, yet 10,000 lumens on matt white material.
The above is the theory. Let’s add in the practice, a reality check.
When your projector is adjusted to its source using a PLUGE or grey scale (as per ANSI standards) the effective lumens drop. And even laser (laser phosphor, more correctly!) degrades over time. Even adding in these reality checks, you’ll see we only need at least 3000 lumens using ALR (ambient light rejecting) screens and over 14000 lumens with matt white!
In the interest of accuracy, these numbers are for on-axis viewing. At wider viewing angles the differential decreases slightly. But within the DISCAS standard viewing angle parameters, there differential is still typically over 3 times in outlying viewing positions.
Tools and Resources
Visit our Tools & Resources section for many more great articles, calculators and material to help you make the best-informed choice about your screen requirements.
Posted: 24th February 2021