The science behind ambient contrast ratio (ACR)

Jose Romero, Azumo | May 25, 2023

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Contrast ratio is one of the go-to measurements to define the picture quality of a consumer electronic display. In its simplest definition, contrast ratio is the relationship between the maximum and minimum light intensity that can be generated by a display (the whitest white versus the richest black). Typically, a high contrast ratio is desirable — displays with a big difference between black and white will look crisp and clear. One does need to be careful when looking at the numbers, especially when comparing across different display technologies.

Figure 1: An illustration of ACR. Source: Azumo

The environments used for most current standard contrast measurements do not always represent the lighting conditions where screens are actually viewed nowadays. The metrics were created for the home theater experience (complete darkness). In other words, they were created when we thought of screens only as large TVs, before we put them in our pocket or backpack and took them everywhere.

Yet, the question rarely asked by users, “What is the contrast ratio we should be considering when the screen is outside?”

A look at contrast ratios

There are three usual ways contrast ratios are measured and reported as:

Static — This is the ratio between the light intensity of white and black areas that the display can show at the same time for a given brightness setting.
Dynamic — This is the ratio measured between the light intensity of a full screen of white versus a full screen of black. The difference seems subtle but it allows many display technologies to “game” this number as they can usually do things like dim the backlight on dark scenes or bump it up in bright ones to get impossible large numbers; this figure is usually found in advertising materials.
ANSI — This measurement is done with a checkerboard patterned test image where the black and white luminosity values are measured simultaneously and averaged. This is a standardized version of the static contrast ratio discussed previously, usually used for comparing projectors.

The one thing they all have in common? These measurements are all taken in pitch black rooms. There’s no ambient lighting taken into account. So the real-life setting of most non-TV displays isn’t considered in these tests at all. What good is a metric when it’s so far from reality?

Taking ambient light into account

Azumo is working to solve real problems for how to actually use devices — how to make devices hold a charge longer and be viewable in the full range of real-world conditions. Part of that is taking ambient light into effect. When measuring contrast ratios, it’s important to ensure that it is done in all sorts of conditions, and not boiled down to engineer impressive numbers.

Figure 2: No ambient light represents the ideal test conditions. Source: Azumo

Figure 3: Realistic test conditions under ambient light. Source: Azumo

Contrast ratios: Transmissive LCD versus reflective LCDs

Assumptions:

10 inch 4:3 tablet
2% top AR glass reflection
Minimum 50 nits

Transmissive LCD:

10% transmission
170% gain BLU (backlight unit) with DBEF (dual brightness enhancing film)
80% optically efficient light guide

Reflective LCD:

7% ambient reflective (CR 5:1)
18% direct reflective (CR 40:1)
FLP (front light panel) at ideal illumination
80% optically efficient light guide

Figure 4: Metrics associated with transmissive LCD with backlight (LCD + BLU) and reflective LCD with front light (RLCD + FLU) under various lighting environments. Source: Azumo

This chart may not be consumer-friendly, but it reveals a few things. Ambient light makes a huge difference in contrast ratio in terms of power consumption and readability. If a product is used in an environment other than complete darkness, the user will want to also consider ambient contrast ratio metrics when selecting a display.

If a product goes outside or requires a long battery life, an RLCD might be the right choice. Not every device will be used in the perfect lighting environment — most devices should be designed with flexibility in mind.

Azumo at Work

The important thing for device makers to keep in mind is that most of the metrics for display quality come from a time when screens were experienced very differently. The data is meaningless, and it’s actually keeping device makers away from being able to innovate and create devices that match actual use cases.

Azumo knows we’ve only just begun thinking of all the ways to use screens to dramatically improve the quality of medical or industrial devices that need to work outdoors and hold a charge for a longer time. Azumo knows there are more consumer applications for adventure seekers ready to come out of their caves and explore, with help from weather and GPS technologies, and tablets that are ready to travel. Azumo is excited by all the possibilities and has prided themselves on being in front of all these new discoveries. Watch a clip on ACR from their webinar, Choosing the Right Display for Your Product, on YouTube here.