When designing an optical system around an LCD module, understanding the panel’s optical characteristics is critical. These parameters directly influence brightness, contrast, color performance, viewing experience, and system-level trade-offs such as backlight power, optics size, and thermal design.
In this article, we use the LCD module E70RG88048LB2M450-C from FocusLCDs as an example to explain the most important optical characteristics that must be considered during optical system design.
1. Transmittance (Tr)
Transmittance is the ratio of the luminance passing through the LCD cell panel to the luminance of the backlight.
Transmittance = Panel Output Brightness / Backlight Brightness
- Measured with the panel electrically driven
- Includes losses from polarizers, LC layer, and color filters
- Higher transmittance → higher output brightness for the same backlight power
Design Impact
- High-transmittance panels allow lower backlight power, reducing heat and power consumption
- Low transmittance requires brighter backlights or optical enhancement films
2. NTSC (Color Gamut)
NTSC describes the color gamut, or how wide and vivid the display’s colors are relative to the NTSC standard.
- A higher NTSC percentage indicates more saturated RGB primaries
- Color gamut is defined as: Color Gamut = Panel Primary Color Area / NTSC Reference Area
Design Impact
High-gamut displays require:
High-quality backlight spectrum
Optimized color filters
Critical for applications such as medical imaging, HMI, and visualization
3. Viewing Angle
The viewing angle is defined as the angle at which the contrast ratio remains greater than 10:1, measured from the optical axis (normal to the panel).
- Measured horizontally (3–9 o’clock) and vertically (6–12 o’clock)
- Larger viewing angles reduce grayscale inversion and color shift
Panel Technology Comparison
- IPS / VA / LTPS: Naturally wide viewing angles
- TN panels: Traditionally narrow viewing angles, now improved using wide-viewing-angle polarizers
Design Impact
- Wide viewing angle panels reduce the need for additional optical compensation films
- Critical for multi-viewer or off-axis viewing applications
4. Contrast Ratio (CR)
The contrast ratio (CR) is the ratio of maximum luminance (white) to minimum luminance (black):
CR = Lwhite / Lblack
Measured at the center of the panel, where:
- Lwhite: luminance at R=G=B=255
- Lblack: luminance at R=G=B=0
Example:
- White luminance = 200 cd/m²
- Black luminance = 0.5 cd/m²
CR=400:1
Design Impact
- Higher CR improves image depth and readability
- Particularly important in outdoor displays and high-ambient-light environments
5. Luminance
Luminance (L) describes the visible brightness of the display and is expressed in cd/m² (nits).
L = Backlight Brightness × Panel Transmittance
Design Considerations
Too low → dim, unreadable display
Too high → eye fatigue and increased power consumption
Must be optimized based on:
Viewing environment
Application duration
Ambient light conditions
6. Chromaticity
Chromaticity defines the color tone and saturation, independent of brightness.
- Determined by the relative contribution of R, G, and B primaries
- Represented using chromaticity coordinates (e.g., CIE 1931)
Design Impact
- Affects color accuracy and consistency
- Important for color-critical applications such as medical, industrial inspection, and HMI systems
7. Response Time (RT)
Response Time (RT) describes how quickly the LCD transitions between brightness states.
RT = Tr + Tf
Where:
- Tr: Rise time (10% → 90% transmittance)
- Tf: Fall time (90% → 10% transmittance)
Unit: milliseconds (ms)
Frame Rate Relationship (Theoretical)
- 16 ms → ~62.5 FPS
- 8 ms → ~125 FPS
- 5 ms → ~200 FPS
- 4 ms → ~250 FPS
Design Impact
- Shorter RT reduces motion blur and trailing artifacts
- Essential for video, gaming, automotive, and dynamic HMI displays
Summary: Optical Design Considerations for LCD Systems
When designing an optical system around an LCD module, engineers must balance:
- Transmittance vs backlight power
- Color gamut vs optical efficiency
- Viewing angle vs panel technology
- Contrast ratio vs ambient light
- Luminance vs eye comfort
- Response time vs motion clarity
Understanding these optical characteristics allows designers to:
- Optimize backlight selection
- Reduce power and thermal load
- Improve image quality and user experience
- Ensure performance meets application requirements
References