The Choice of Observer in Video Signal and Display Analysis: Championing the 2° Observer

by Florian Friedrich | June 2023

Very recently after displayweek 2023 in Los Angeles, I was approached with an intriguing question, “In the world of video signal analysis and display measurements, which observer is more relevant – the 2° or the 10° observer?” Knowing that we use the observer functions for many tasks in display measurements and signal analysis, the query stoked the fires of my enthusiasm for picture quality and display metrology, plunging me into the depths of this fascinating discussion. Here’s my take.

The world of color is as complex as it is captivating. Over the years, scientists have endeavored to understand and quantify color perception, leading to the development of numerous color spaces and observer models. The observer model is particularly important because it attempts to codify how we humans perceive color. In the standards set by the International Commission on Illumination (CIE), two observer models stand out – the 2° and 10° observers. Both have their merits, but let me tell you why I lean towards the 2° observer when dealing with video signals and display measurements.

What the 2° and 10° observer is all about

The 2° observer, based on the 1931 CIE standard, is derived from experiments using a small field of view in the center of the visual field. This area, called the fovea, is where our eyes are most sensitive and where most of our color perception occurs. On the other hand, the 10° observer, introduced in 1964, includes peripheral vision and provides a broader view.

Display measurements, observers and modern colorimeters

However, when it comes to color precision in video signal analysis and display measurements, the devil lies in the detail. The majority of the color-critical work in visual perception occurs in the foveal vision. Hence, the 2° observer, which encapsulates this area of the retina, presents an accurate representation of how we discern colors in our most sensitive visual field.

Interestingly, the evolution of colorimeter technology now offers physical filters that are impressively matched to the 2° observer functions. These functions are integral when readings from a spectroradiometer get converted to color coordinates, such as X,Y,Z or x,y,Y or u’,v’,Y. Take, for example, the Konica Minolta CA-410. This cutting-edge colorimeter, which I frequently employ for various display measurement tasks (including our innovative InnoPQ measurement system), features filters impeccably matched to these functions. Consequently, the use of slower spectroradiometers becomes impractical in many scenarios. I’m eager to test out the ADMESY Prometheus Colorimeter soon, curious to see if it meets or even surpasses my expectations set by the CA-410.

As we move forward with these innovative technological advancements, it’s important to remember that the cornerstone of color space definitions, the choice of observer, remains a key component in maintaining consistent, high-quality results.

Wider color gamuts

This brings us to the era of wider color gamuts such as DCI-P3 and BT.2020, which -as wel all know – offer richer, more vibrant colors than ever before. It’s an exciting time for anyone in the field of display technology. But this innovation begs the question – should we switch to the 10° observer, given that wider gamuts include more highly saturated colors, where the 2° and 10° observers differ most?

Interestingly, despite the push for wider color gamuts, many color spaces, including the celebrated sRGB, DCI-P3, and even the expansive BT.2020, are defined concerning the 2° observer. This is no coincidence but a pragmatic choice born out of the recognition that the 2° observer still provides a reliable and robust reference for colorimetry in most practical applications.

In our quest for broader color horizons, we should not lose sight of the critical factor – consistency. When analyzing video signals or measuring display performance, it is essential to ensure that the observer model aligns with the one used in defining the color space. Here, the 2° observer stands tall as the de facto choice in numerous color space definitions.

Moreover, considering the display’s characteristics, the viewing conditions, and the idiosyncrasies of human visual perception, the choice of observer model is just one piece of a much larger puzzle. While the 2° observer can introduce some variability, this impact is generally negligible in most applications.

The final take
To put it simply, the choice of the 2° observer for signal analysis and display measurements is akin to an experienced sailor navigating the turbulent seas with a trusted compass. It’s not that the 10° observer is flawed. Far from it. However, in the complex and ever-evolving domain of video signal analysis and display measurements, the 2° observer offers a beacon of consistency and reliability.

In conclusion, as we continue to push the boundaries of color spaces and stride into an era of wider color gamuts, it’s essential to hold on to the stalwart standards that have served us well. Despite the rapid advancements, the 2° observer remains a timeless choice in video signal analysis and display measurements. And as an ardent devotee of the magic of color, I heartily recommend the continued use of the 2° observer as a solid reference point in this captivating world of color and display technology. Here’s to the vibrancy, the depth, and the precision of color – all seen through the lens of the trusty 2° observer!


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