会社のニュース Monochrome PMOLED in Direct Sunlight: Which Emission Color—White, Blue, Yellow, or Green—Performs Best for Industrial HMI?

Introduction: The Sunlight Visibility Dilemma in Industrial HMI Design

For engineers in Germany, the U.S., and other industrialized markets, selecting a display for harsh outdoor environments isn’t just about resolution or interface—it’s about readability under extreme ambient light. In sectors like energy, agriculture, logistics, and defense, human-machine interfaces (HMIs) must remain legible in full daylight, often without backlight assistance.

Enter the 3.12-inch monochrome passive-matrix OLED (PMOLED)—a rugged, high-contrast solution offered by Saef Technology Limited with four optional emission colors: white, blue, yellow, and green. But which of these performs best in direct sunlight? And how do you choose when your product must function reliably from Arizona deserts to Bavarian alpine fields?

This article dives deep into the optical physics, human vision science, and real-world performance data—anchored in the technical specifications of the SFOM312YZ7-25664WBYG-01 module—to deliver a definitive answer.

Why Monochrome PMOLED? The Foundation for Sunlight Readability

Unlike color LCDs that rely on polarizers and backlights (which wash out in bright sun), monochrome PMOLEDs emit their own light and boast intrinsic contrast ratios exceeding 10,000:1 (per Saef’s spec sheet). This eliminates glare-related legibility loss—a key advantage for outdoor HMIs.

The SFOM312YZ7-25664WBYG-01 features:

1. 256×64 resolution on a 3.12-inch diagonal
2. 16-level grayscale via SSD1322 driver IC
3. Operating temperature range: –40°C to +80°C
4. Optional CTP/RTP touch integration for modern HMI responsiveness

But among its four emission options, which chromaticity offers the highest perceptual contrast against a bright sky background?

Human Vision & Solar Spectral Overlap: The Science Behind Legibility

Sunlight peaks in the green-yellow region (~550 nm) of the visible spectrum. Counterintuitively, this doesn’t mean green is best—it means high ambient irradiance in that band can reduce perceived contrast if the display emits in the same region.

Let’s examine each option using CIE 1931 chromaticity data (from Saef’s specification):

Now consider human photopic luminosity function (V(λ)), which peaks at 555 nm (green). While green appears brightest in darkness, in high-luminance conditions, the eye’s dynamic range compresses, and color contrast against the sky background becomes dominant.

Research (e.g., Journal of the SID, Vol. 25, 2017) shows that blue and white emissions often outperform green/yellow in direct sunlight because:

1. The sky background is already rich in green/yellow photons, reducing relative contrast.
2. Blue light scatters less in the eye’s optics, yielding sharper edges.
3. White OLEDs (typically blue emitter + yellow phosphor) provide broad-spectrum output that maintains high luminance-to-background ratio when calibrated properly.

Saef’s spec lists a typical luminance of 140 cd/m² (all pixels ON). While modest compared to sunlight (~100,000 lux ≈ 30,000 cd/m² reflected), the infinite black state of OLED ensures text/icons remain crisp—if the emission color avoids spectral overlap with ambient light.

Field Evidence: What Do Engineers in Germany & the U.S. Prefer?

In practice, European industrial designers (especially in German machinery and automotive test equipment) favor white or blue PMOLEDs for outdoor handheld diagnostics. Why?

• White: Offers neutral appearance, aligns with ISO 13187 ergonomic standards for control panels, and integrates seamlessly with dark UI themes.
• Blue: Provides high edge acuity and stands out against green foliage or concrete—common in agricultural and construction settings.

Conversely, yellow is preferred in low-light or foggy conditions (e.g., maritime or aviation backup displays) due to its long-wavelength penetration, but it washes out faster in midday sun.

Green, despite historical use in legacy military displays, is now less favored for new outdoor designs due to poor solar contrast—unless used with aggressive anti-reflective coatings (not standard on PMOLED).

Practical Recommendation: Optimize for Your Use Case

Based on Saef Technology Limited’s SFOM312YZ7-25664WBYG-01 platform:

1.  Choose WHITE if your HMI requires neutral aesthetics, compatibility with dark-mode UIs, and balanced daylight performance across diverse geographic conditions.
2.  Choose BLUE if your application operates in high-glare, high-detail scenarios (e.g., precision instrumentation, drone controllers).
3.  Avoid GREEN for primary outdoor use unless paired with optical bonding and AR filters.
4.  Use YELLOW only for dawn/dusk or indoor-outdoor transitional environments.

Pro Tip: All variants support optional capacitive (CTP) or resistive (RTP) touch overlays, enabling glove-compatible or wet-environment interaction—critical for industrial HMIs in Europe and North America.

Conclusion: Sunlight Legibility Is a System-Level Decision

Selecting the right emission color isn’t just about preference—it’s about matching photometric performance to environmental physics. For engineers building next-gen industrial devices, the 3.12-inch PMOLED from Saef Technology Limited offers a rare combination of ruggedness, contrast, and chromatic flexibility.

By choosing white or blue emission, you maximize daylight readability while leveraging OLED’s inherent advantages: zero power for black pixels, wide viewing angles (160°), and instant response (<10 µs).

Ready to prototype? Saef Technology Limited provides engineering samples, touch integration kits, and custom mechanical designs to accelerate your HMI development—backed by ISO-compliant quality control and global logistics.