Illuminating Display Measurement Metrics

Display technologies have evolved rapidly over the past decades, from bulky CRT monitors to sleek modern LCD and OLED screens. As display quality improves, accurate measurement of performance characteristics becomes increasingly important. 

Two key metrics for quantifying display brightness are candelas per square meter (cd/m2) and nits. Candelas per square meter measures luminous intensity per unit area, indicating how much light is emitted from the display surface. 

Nits are closely related, providing a measure of total luminous flux over the screen area. Both cd/m2 and nits quantify display brightness, an essential attribute influencing perceived image quality, viewing comfort, and device power consumption. 

This overview introduces these important display metrology concepts and their relevance, setting the stage for a more in-depth examination of display measurement methodology and interpretation of brightness specifications. 

Precise brightness quantification enables meaningful comparisons between displays and informs optimized calibration for intended usage scenarios.

Display Brightness Metrics: Exploring cdm²

Cdm2, or candelas per square meter, is a metric used to measure display brightness. It quantifies how much light is emitted from a display surface per unit area. The candela itself is a unit measuring luminous intensity - how bright a light source appears to the human eye.

So, cdm2 tells us the luminous intensity per square meter coming from a display. The higher the cdm2, the brighter that display will look.The candela and lumen (brightness) units originated in the late 1800s during the early days of photometry and light measurement. The term "candela" was formally adopted in 1937.

Cdm2 became a standardized metric for display brightness during the rise of television and computer monitors in the 1960s-1980s. Industry organizations like the Society of Motion Picture and Television Engineers (SMPTE) recommended cdm2 guidelines for studio monitors and projectors.

Today, cdm2 remains the predominant way to quantify display brightness across televisions, phones, tablets, and laptop screens. Manufacturers advertise cdm2 levels to hype up peak or sustained brightness. Many devices now reach over 500 or even 1000 cdm2.

Despite its widespread use, cdm2 does not perfectly align with human perception. Two screens with an identical cdm2 rating may look slightly different in terms of brightness.

That's because the candela is based on photopic vision - the response of our cones in well-lit conditions. It does not account for our rods which shape perceived brightness in dark environments. Still, cdm2 serves as a good approximation for comparing brightness across devices.

Pros & Cons of cdm2

The main advantages of cdm2 are universality and traceability to a SI standard. Any lab can accurately measure a display in cdm2. Values can also be converted to other photometric units like lumens or lux.

However, cdm2 does not consider the viewing environment, screen size, or visual response. More advanced metrics like nits (cd/m2) attempt to better model perceived brightness. But cdm2 remains a simple, standardized way to quantify display output.

Display Luminance Metrics: Unraveling Nits 

Nits, abbreviated as cd/m2, are a luminance units commonly used for displays. One nit is equal to one candela per square meter, the same as the cdm2

So, what's the difference between nits and cdm2? Simply put, nits factor in the light's angle and take a "normal" perpendicular view. Cdm2 just measures total light output.

This makes nits better suited for quantifying display brightness, as screens are meant to be viewed straight on. 500 nits closely match how bright 500 cdm2 looks to the eye.

The nit evolved from the concept of luminance - the intensity of light emitted in a given direction. Pioneers in photometry realized oblique angles changed perceived brightness.

In the 1920s, nits were established as cd/m2 for a light source emitting into a solid angle of one steradian. The steradian quantified emission angle, allows nits to model viewing position
For years nits were used in cinema projection and lighting. But in the 21st century, they became the norm for displays. Phone and laptop manufacturers now market screens based on nit values.As mentioned, one nit equals one candela per square meter. The candela measures total luminous intensity. The "per steradian" clarifies the candela is over a steradian - the 3D cone emanating light from a source. For a flat display, that's simplified to cd/m2.

So, nits incorporate both output intensity (candela) and directionality (steradian). This makes them well-suited for displays viewed head-on. Besides cdm2, nits can be converted to lamberts and foot-lamberts. One nit is equal to:

1 candela/m2

π candela/ft2 (lamberts)

0.2919 foot-lamberts

While sometimes still used in display specs, lamberts and foot-lamberts are considered obsolete. Nits in the form of cd/m2 remain the standard today.

Technological Landscape: Applications of cdm² and Nits

In the world of displays and screens, two key metrics are often used to measure brightness: candelas per square meter (cdm2) and nits. These units have become integral to various devices and industries that rely on quality visual experiences. 

Understanding where these metrics are applied provides insight into their technological importance.

Smartphones and tablets

Cdm2 is frequently used to measure display brightness in smartphones and tablets. With mobile devices, sufficient luminance is critical for outdoor visibility and usability in bright environments. Most modern phones and tablets aim for screen brightnesses between 400-800 cdm2. 

Top-end models even exceed 1000 cdm2 for enhanced visibility. Tablet manufacturers also utilize cdm2 when designing for use cases like professional drawing applications, where accurate colour reproduction is vital.

Computer monitors and laptops

Desktop monitors and laptop panels are optimized using cdm2 as well. Monitor manufacturers recommend brightness settings between 120-350 cdm2 for regular use, dependent on ambient lighting. 

High-end desktop displays for gaming and graphics work can achieve 500+ cdm2 for greater dynamic range. Laptop brightness is measured in cdm2 as well, with most models ranging from 200-300 cdm2 for comfortable use in various environments.

Television screens

Television brightness standards are also defined using the cdm2 unit. LED and LCD televisions usually aim for 500 cdm2 or greater for the best representation of highlight details. 

Higher brightness in nits improves picture quality in brighter viewing environments. Outdoor televisions can achieve up to 2000 cdm2 for visibility in sunlight.

Nits as a critical metric in high-dynamic-range (HDR) displays

Enhanced visual experiences through increased luminance

In HDR television and monitor standards, nits play a major role in improving the visual experience. HDR expands a display's contrast and colour for more realistic, life-like images. 

This is achieved by increasing the brightness, or luminance, to highlight levels measured in nits. More nits produce brighter highlights that pop and appear more natural to the human eye.

Nits' role in achieving extended contrast ratios

Nits help HDR displays accomplish substantially higher contrast ratios. The contrast ratio is the range between the darkest blacks and brightest whites a display can produce. HDR standards require these peak brightness levels in nits to reach certain thresholds. 

For example, entry-level HDR needs at least 500-600 nits, while premium standards like HDR10+ require upwards of 1000 nits for peak brightness. This expands the contrast ratio significantly.

Importance in outdoor visibility of screens

Additionally, higher nit counts improve outdoor visibility for HDR televisions and mobile devices. Having peak brightness exceed 1000 or even 2000 nits allows HDR displays to remain vivid and easy to view in bright outdoor environments under sunlight. 

This expanded luminance range enriched by measuring in nits delivers clear, discernible images regardless of ambient lighting conditions.

In summary, cdm2 and nits serve important roles in current display technologies across various devices. Understanding these metrics provides deeper insight into how manufacturers optimize screens for ideal visibility, colour, and dynamic range according to specific use cases. 

As display capabilities continue advancing into the future through technologies like micro-LED and QD-OLED, the foundational importance of brightness quantification through units like cdm2 and nits is likely to remain a core design consideration.

Precision and Accuracy: Measuring cdm² and Nits

Precise and accurate measurement of display brightness is critical for calibrating monitors, TVs, and mobile devices. Two key metrics for quantifying display luminance are candelas per square meter (cdm2) and nits. While related, these units have important differences in how they are defined and measured.

Measuring cdm2 Accurately

The candela (cd) is the base SI unit for luminous intensity, defined as the brightness of a light source emitting monochromatic radiation at 555nm. 

One candela equals one lumen per steradian. Cdm2 indicates the luminous intensity emitted from or reflected by a surface per unit area, measured perpendicular to the surface.Colourimeters and spectrophotometers are commonly used to measure cdm2. Both instruments rely on photodetectors to quantify light intensity across the visible spectrum. The accuracy of cdm2 measurements depends on several factors:

Ambient lighting conditions - Stray light can introduce errors, so measurements should be taken in darkened rooms.

Viewing angle - Luminance decreases at wide angles due to the cosine law. Measurements should be made perpendicular to the display.

Detector sensitivity - Photodetectors have differing spectral responses, so calibration with a known standard is essential.

Display uniformity - Luminance can vary across the display, requiring multiple measurement points.

Proper instrument selection, calibration, and measurement technique are necessary to achieve cdm2 accuracy within 5-10%. Tightly controlled testing conditions also help minimize errors.

Measuring Nits Radiometrically

The nit is a non-SI unit commonly used to quantify display brightness. One nit equals one candela per square meter. However, nits are measured differently than cdm2. Whereas cdm2 quantifies visible light intensity, nits measure total radiant power across all wavelengths. This radiometric measurement approach disregards the eye's wavelength-dependent response. Two techniques are primarily used to measure nits radiometrically:

Spectroradiometers - These instruments quantify spectral radiance, allowing the total luminous flux to be calculated. However, they may not account for display light scattering.

Integrating spheres - A display is placed inside a hollow spherical cavity coated with a diffuse white reflectance material. A photodetector measures the total reflected light, providing an accurate nit reading.

Radiometric nit measurements do not differentiate between photopic vision (daytime, cone-dependent) and scotopic vision (nighttime, rod-dependent). The human eye perceives short-wavelength light as brighter. Weighting radiometric data by the photopic luminosity function yields measurements closer to cdm2. But this correction is not applied for nit measurements.

In summary, cdm2 and nits both quantify display brightness but use different measurement approaches. Cdm2 measures visible light intensity to characterize visual perception. Nits measure total radiant flux, providing quantification convenient for engineers and manufacturers. 

Both metrics play important roles in display testing and calibration. Careful attention to instrument selection and testing conditions is necessary for accurate and comparable results.

Signworld: Using the Advanced Technology

Signworld utilizes the most advanced technology available in the digital signage industry. By leveraging high-nit, high cdm2 displays, Signworld provides superior brightness and image quality optimized for any lighting environment. 

Their expertise in display metrics like cdm2 and nits allows Signworld to engineer solutions that deliver exceptional visibility and readability to viewers. Furthermore, Signworld stays ahead of the curve by continuously integrating new innovations that take digital signage to the next level. 

With a keen understanding of display brightness standards and an eye towards the future, Signworld creates immersive digital experiences that engage audiences and provide maximum value to clients. 

The combination of technical leadership and customer-focused design is what makes Signworld a premier provider of cutting-edge digital signage.

Conclusion

In conclusion, this paper has explored the important display brightness metrics of candelas per square meter (cdm2) and nits. We have seen that cdm2 is an absolute measure of display brightness based on scientific standards, while nits are a relative measure based on the cdm2 value divided by the display area.

Accurately measuring and stating display brightness is critical for several reasons. First, it allows meaningful comparisons across different displays and devices. Using the absolute cdm2 scale prevents discrepancies that can occur when only using the relative nits unit. 

Second, proper brightness measurement is essential for ensuring displays are legible and comfortable for users. Screens that are too bright or too dim can cause eye strain and impede readability.

Looking ahead, cdm2 and nits will continue shaping technological innovation and user experiences. As displays become brighter, higher cdm2 values and nit counts will be needed. This drives advancement in display technologies to safely produce more luminance. 

Additionally, optimizing brightness to match ambient conditions and user preferences requires accurate brightness awareness. Overall, cdm2 and nits inform both engineering and design decisions to create better displays and experiences.