Comment choisir la résolution et la taille d'écran LCD adaptées : guide de sélection à l'intention des ingénieurs

Introduction

Choosing the wrong LCD size can force a complete mechanical redesign. Picking the wrong resolution makes text blurry, icons jagged, and your product look unpolished. Every hardware engineer and product manager has faced this dilemma.

This 4,000‑word guide gives you a systematic decision framework. You will learn:

• How screen size is measured (and why the diagonal alone is misleading).
• The real meaning of resolution and PPI (pixels per inch).
• How to balance size, resolution, cost, and performance.
• Recommended size‑resolution combinations for common applications.
• How to calculate the minimum PPI needed for a given viewing distance.
• Les erreurs courantes et comment les éviter.

Un LCD Size/Resolution Selection Spreadsheet (Excel) is included – enter your viewing distance and desired PPI, and it recommends the best match from our standard product library.

For a deeper understanding of how interfaces affect resolution choices, see our “Ultimate Guide to TFT‑LCD Interfaces”.

Part 1 – Basic Concepts: What “Size” and “Resolution” Really Mean

1.1 Size: The Truth About Diagonal Inches

The “size” of an LCD is the length of its diagonal, measured in inches (1 inch = 25.4 mm). This is an industry standard, but it can be misleading.

Why the diagonal alone is not enough
Two displays with the same diagonal can have very different aspect ratios (4:3, 16:9, 16:10, 5:4). A 7″ 16:9 screen is much wider and less tall than a 7″ 4:3 screen. Always check the mechanical drawing.

How to calculate the diagonal
If you know the active area width (W) and height (H) in mm:

\[
\text{Size (inches)} = \frac{\sqrt{W^2 + H^2}}{25.4}
\]

[Internal Link: Browse our LCD mechanical drawings library – each product page includes a dimensional drawing.]

1.2 Resolution: The Pixel Matrix

Resolution is the number of horizontal pixels × vertical pixels, for example 800×480.

Common resolution tiers

1.3 PPI (Pixels Per Inch) – The True Sharpness Metric

PPI tells you how densely pixels are packed. It is the only real measure of sharpness.

Formula

\[
\text{PPI} = \frac{\sqrt{\text{horizontal pixels}^2 + \text{vertical pixels}^2}}{\text{diagonal inches}}
\]

Exemple
A 7″ display with 800×480 resolution:
√(800² + 480²) = √(640,000 + 230,400) = √870,400 ≈ 933
933 / 7 ≈ 133 PPI

Recommended PPI by application

Warning
Higher PPI is not always better. It increases:

• Bandwidth requirements (need faster interfaces like LVDS or MIPI).
• Host processing power (larger frame buffer, more GPU load).
• Backlight power (denser pixels reduce aperture ratio).
• Cost (more expensive ICs, lower manufacturing yield).

Part 2 – Trade‑offs Between Size and Resolution

2.1 What happens when you increase resolution at a fixed size?

Benefits

• Sharper image.
• More content on screen (e.g., more rows in a data table).

Costs

• Higher interface bandwidth → may require moving from RGB to LVDS or MIPI.
• Larger frame buffer → may need an external RAM or a more powerful MCU.
• Reduced aperture ratio → lower light transmission → requires brighter (and hotter) backlight.
• Higher IC cost and lower yield → module price increases 20‑50% when moving from WVGA to WXGA on the same size.

2.2 What happens when you increase size at a fixed resolution?

Benefits

• Larger active area → easier touch targets, better readability for older users.
• Same software UI works without changes.

Costs

• PPI drops → pixels become visible (“screen door effect”).
• Same resolution on a larger screen looks blocky and jagged.
• Backlight uniformity becomes harder (more LEDs needed, higher risk of hotspots).

2.3 Golden combinations: size vs resolution by application

Part 3 – How to Calculate the Minimum PPI for Your Viewing Distance

3.1 The Retina formula (simplified)

Apple’s “Retina” concept says a display is sharp enough when the human eye cannot distinguish individual pixels at a normal viewing distance. The angular resolution limit of the human eye is about 1 arcminute (1/60 degree).

From this, the minimum PPI required for a given viewing distance (in inches) is:

\[
\text{Min PPI} = \frac{3438}{\text{viewing distance (inches)}}
\]

Why 3438?
It comes from: 1 / tan(1 arcminute) ≈ 3438. This formula assumes a 20/20 vision.

3.2 Examples

3.3 Acceptable PPI vs ideal PPI

• Industrial / medical: Meeting the minimum PPI is enough. Exceeding it adds cost without tangible benefit.
• Electronique grand public: Aim for 20‑30% above the minimum for a “premium” feel.

Interactive tool
We provide an online calculator: enter your viewing distance (cm) → get recommended PPI range and matching LCD models.

Part 4 – In‑Depth Selection Advice by Application

4.1 Handheld & wearable devices (small size, high PPI)

Key constraints

• Power consumption (battery life).
• Mechanical thickness.
• Host processor capability (often a low‑power MCU).

Recommendations

• Interface: MIPI DSI (low power) or SPI (very low resolution).
• PPI: 200 – 300.
• Avoid RGB interface – it drains battery quickly.

Erreur courante
Using a high‑resolution panel (e.g., 1080p on a 4″ screen) with a low‑end MCU. The MCU cannot drive the frame buffer, leading to sluggish UI.

4.2 Industrial HMI (medium size, medium PPI, wide temperature)

Key constraints

• Reliability (24/7 operation, vibration, dust).
• Sunlight readability (brightness > 500 nits).
• Long‑term availability (5‑10 years).

Recommendations

• Classic combos: 7″ 800×480, 10.1″ 1024×600.
• Interface: RGB or LVDS (good noise immunity).
• PPI: 100 – 120 (enough for 60‑100 cm viewing distance).

The 1080p trap
Some engineers specify 1920×1080 on a 10.1″ screen because “higher is better”. At 60 cm viewing distance, the human eye cannot resolve beyond ~150 PPI. The extra pixels only increase host cost, power, and EMI – with zero visible benefit.

4.3 Automotive displays (large size, high brightness, extreme reliability)

Key constraints

• Wide temperature: -40°C to +85°C.
• Vibration resistance (MIL‑STD or ISO 16750).
• LVDS interface (standard in automotive).
• Long backlight life (>50,000 hours).

Trends

• Moving from 12.3″ 1920×720 to 15″+ 4K panels, but requires a powerful GPU.
• Curved displays and local dimming are becoming common.

Recommendations

• For central infotainment: 8″-10.25″ 1280×800.
• For digital cluster: 12.3″ 1920×720 (ultrawide).

4.4 Medical monitors (medium size, high contrast, DICOM compliance)

Key constraints

• DICOM Part 14 grayscale standard.
• Luminance stability (backlight feedback sensor).
• Anti‑glare / anti‑reflective surface.

Recommendations

• Size: 5″ to 8″ for portable monitors; 15″+ for diagnostic workstations.
• Resolution: 800×600 (SVGA) or 1024×768 (XGA).
• Technology: IPS for wide viewing angles and colour accuracy.

4.5 Smart home appliances (small size, low cost, simple UI)

Key constraints

• Cost – the display must not double the BOM.
• MCU resources – often a small 8‑bit or 32‑bit ARM Cortex‑M.
• Simple menu or status display.

Recommendations

• Size: 2.4″ – 3.5″.
• Resolution: 320×240 (QVGA) or 480×272.
• Interface: MCU (8080) or SPI.

Erreur courante
Using a 480×800 MIPI display because it looks good on a smartphone, then being forced to upgrade to an expensive MPU with MIPI DSI host.

Part 5 – Common Mistakes and How to Avoid Them

❌ Mistake 1: Copying laptop resolution to a small panel

A 15.6″ laptop with 1920×1080 (141 PPI) looks sharp. But a 7″ panel with 1920×1080 has 315 PPI. At normal viewing distance, text becomes extremely small, Windows scaling is poor, and the host processor struggles.

Corriger
For 7″, do not exceed 1280×800. For 5″, stay at or below 800×480.

❌ Mistake 2: Only looking at diagonal size, ignoring aspect ratio

A 5″ 4:3 screen has a very different active area (width ~102 mm) compared to a 5″ 16:9 screen (width ~110 mm). UI layouts designed for one may overflow on the other.

Corriger
Always request the mechanical drawing and verify the active area width and height.

❌ Mistake 3: Using multiple different resolutions across a product family

One product uses 800×480, another uses 1024×600, a third uses 1280×720. The software team must maintain three separate UI layouts, increasing development and testing time.

Corriger
Standardise on one resolution per product family. If you need to support different screen sizes, use a display controller that can scale the UI.

❌ Mistake 4: Chasing very high PPI without considering reliability

In industrial environments with vibration, extremely fine ITO traces (required for >250 PPI) are more prone to cracking or bonding failure.

Explanation
High PPI forces narrower line/space design and tighter bonding pitch. Not all module factories can reliably produce such panels for harsh environments.

Corriger
For industrial applications, stay below 200 PPI unless you have a validated module and factory.

❌ Mistake 5: Designing tiny touch targets on a high‑PPI small screen

A 5″ 1080p screen (440 PPI) has very small physical pixels, but a touch button of 50×50 pixels might be only 2.9 mm wide – far below the recommended 7 mm minimum for finger touch.

Corriger
Design touch targets based on physical size, not pixel count. Minimum 7×7 mm for gloved fingers, 5×5 mm for bare finger.

Conclusion et prochaines étapes

Choosing the right LCD size and resolution is a balancing act between viewing distance, UI complexity, host capability, cost, and reliability. Use the decision tables and PPI formula in this guide to narrow down your options.

Que faire maintenant ?

✅ Download the free LCD Size/Resolution Selection Spreadsheet – an Excel tool that takes your viewing distance and desired PPI, then recommends the optimal size and resolution combination from our standard library.

✅ Vous hésitez encore ? Submit your application (industrial, medical, automotive, etc.) and typical viewing distance. JICLCD engineers will recommend three optimal configurations within 48 hours – free of charge.

✅ Browse our standard products – filter by size, resolution, and interface to find a display that matches your choice.

Download TFT LCD display module specifications

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