Optical bonding and AR versus a brighter backlight.

The problem is contrast under ambient light.

A display’s dark-room contrast ratio does not describe how it will look with a bright reflection over the image. Ambient light reflected toward the observer raises the apparent black level as well as the white level, making displayed information harder to distinguish.

Optical bonding replaces an internal air gap with a suitable transparent bonding material, reducing reflection at those interfaces. An anti-reflective (AR) coating on the outer surface addresses a different interface: the surface facing the viewer. Combining them can be particularly effective for a reflection-limited assembly. Planar’s technical primer explains these complementary mechanisms.

A transparent, illustrative comparison.

The following example is a calculation, not a measured FPD.DEV product result. Assume a 500 cd/m² white level and 0.5 cd/m² black level, measured at the front of the assembly without ambient reflection. Add 100 cd/m² of reflected luminance equally to white and black for one fixed viewing condition.

Ambient contrast ratio = (white luminance + reflected luminance) / (black luminance + reflected luminance).

Doubling the backlight is assumed to double both the LCD’s emitted white and black levels. The reduced-reflection case assumes the same emitted luminance and a tenfold reduction in reflected luminance from an improved optical stack. Under these assumptions, reflection reduction produces much more ambient contrast than doubling brightness.

Where a brighter backlight still belongs.

Reflection control and brightness are complementary design variables. If the remaining emitted light is insufficient for the task, additional backlight output may still be needed. Evaluate it with the optical stack in place and at operating temperature.

In a project comparison, include electrical power, heat rejection, low-light dimming, mechanical space and the ability to maintain the required output. Avoid selecting a nominal panel brightness that cannot be sustained in the finished enclosure.

For a dual-mode assembly, repeat the review in NVIS mode. A daylight improvement must fit the night vision requirements, controls and final test configuration. See the daylight/NVIS specification checklist.

Make the comparison useful for your application.

• Define the task: character size, content, viewing distance, viewing angles and operating environment.
• Describe the stack: LCD, touch layers, cover substrate, coatings, bonding and any filters.
• Measure consistently: compare white, black and reflected luminance under stated illumination and geometry.
• Check integration: power, temperature, sealing, durability, repair strategy and supply availability.
• Agree acceptance: establish what the finished assembly must demonstrate, not just a component’s brochure value.

That gives a useful basis for deciding whether to change the front surface, bond the stack, increase backlight output or combine these approaches.

For avionics, plan DO-160 qualification alongside readability.

A sunlight-viewable display must also satisfy the environmental requirements of its installation. Specify the applicable RTCA DO-160 issue, sections and categories, and the performance to maintain under those conditions. Environmental qualification and sunlight readability require distinct evidence.

FPD.DEV helps find solutions when standard approaches fall short, balancing optical improvements with thermal, electrical and mechanical constraints. See the DO-160 requirements and evidence checklist.