Obviousness

The obviousness of US patent 8408778 under 35 U.S.C. § 103 can be assessed by combining teachings from multiple prior art references, particularly US2007/0147081 A1 (Shin), US2006/0268537 A1 (Kurihara), and US7350937B2 (Hitachi Displays). A person having ordinary skill in the art (PHOSITA) would have been motivated to combine these references to improve the color accuracy, uniformity, and manufacturing efficiency of LED-backlit display devices.

Motivation to Combine Prior Art References

By the priority date of US8408778 (February 17, 2009), LED-based backlights for liquid crystal displays (LCDs) were well-known, and significant research focused on achieving stable and accurate white light while optimizing manufacturing costs.

1. Shared Problem and Predictable Results: All three references (Shin, Kurihara, Hitachi Displays) address the common problem of creating a uniform and desired color output (typically white light) from LED light sources in display backlights. Combining their teachings would lead to predictable results in improving color quality and light distribution, which was a well-understood goal in the art.
2. Complementary Technologies: Shin and Kurihara provide comprehensive structures for LED backlight units, including light guide plates, optical sheets, and color conversion layers. Hitachi Displays specifically focuses on managing variations in LED color output. A PHOSITA would be motivated to combine these complementary solutions to create a more robust and cost-effective backlight system.
3. Cost Reduction and Yield Improvement: The '778 patent explicitly aims to increase the "use yield of LED chips and light emitting diodes" by accommodating "multi ranks" or LEDs with color coordinate distributions outside a target range [cite: US8408778B2, Paragraph 0122]. Hitachi Displays teaches using LEDs with "different light emitting colors" or "slightly different wavelengths within the same color" to improve color rendering and efficiency [cite: US7350937B2, Paragraphs 0013, 0020]. A PHOSITA would recognize the economic advantage of using a wider range of LED ranks and combining this with color conversion layers (as taught by Shin and Kurihara) to correct the output to the desired target color.

Obviousness Analysis of Independent Claims

Claim 1 (Light Source with Conversion Sheet on Optical Sheet)

Claim 1 describes a light source including a light guide plate, an LED light-emitting module emitting light incident on the light guide plate, an optical sheet on the light guide plate's top surface, and a conversion sheet on the optical sheet's top surface, which converts the light's color to a target color. The optical sheet includes a diffusion sheet and a prism sheet, and the conversion sheet converts light passing through them.

• Light Guide Plate & LED Module (Edge-Lit): Both Shin and Kurihara teach a light guide plate (Shin 120, Kurihara 11) with LEDs (Shin 110, Kurihara 12) positioned to emit light into a side surface of the plate (edge-lit configuration). [cite: US2007/0147081 A1, Paragraph 0021; US2006/0268537 A1, Paragraph 0026]
• Optical Sheet on Top Surface of Light Guide Plate: Both references disclose optical sheets (Shin 140, Kurihara 13) positioned above the light guide plate. [cite: US2007/0147081 A1, Paragraph 0025; US2006/0268537 A1, Paragraph 0027]
• Optical Sheet including Diffusion Sheet and Prism Sheet: Kurihara explicitly details its optical sheet (13) as comprising a diffusion sheet (13A) and a prism sheet (13B). [cite: US2006/0268537 A1, Paragraph 0027] Shin also generally mentions optical sheets including diffusion and prism sheets. [cite: US2007/0147081 A1, Paragraph 0025]
• Conversion Sheet on Top Surface of Optical Sheet, Converting Light to Target Color: Shin discloses a wavelength conversion layer (150) placed "between the optical sheet and a liquid crystal panel," and Figure 1 clearly shows it on top of the optical sheet. This layer converts light (e.g., blue) to a target color (e.g., white). [cite: US2007/0147081 A1, Abstract, Paragraph 0026, FIG. 1] Similarly, Kurihara describes a phosphor film (14) "disposed on the optical sheet (13)" to convert light to achieve white light. [cite: US2006/0268537 A1, Paragraph 0028, Paragraph 0030, FIG. 1]
• Conversion of Light Passing Through Diffusion and Prism Sheets: Since both Shin and Kurihara position the conversion layer/film on top of the optical sheet (which includes diffusion and prism sheets), the light necessarily passes through the diffusion and prism sheets before being converted by the conversion layer.

Therefore, the combination of Shin and Kurihara renders Claim 1 obvious.

Claim 20 (Display Device with Conversion Layer Between Optical Sheet and Display Panel)

Claim 20 describes a display device including a cover, an LED light-emitting module on the cover, a light guide plate at a side of the module, an optical sheet (diffusion + prism) on the light guide plate, a display panel on the optical sheet, and a conversion layer between the optical sheet and the display panel, converting the light's color after passing through the diffusion and prism sheets.

• Cover & Display Panel: Shin describes a backlight unit for a liquid crystal display (LCD) and explicitly shows a liquid crystal panel (160) on top of the entire backlight unit. [cite: US2007/0147081 A1, Paragraph 0021, FIG. 1] The components would inherently be housed within a cover, a standard feature of display devices. Kurihara also discusses a display device using the phosphor film and lighting device. [cite: US2006/0268537 A1, Abstract, Paragraph 0025]
• Conversion Layer Between Optical Sheet and Display Panel: Shin precisely places the wavelength conversion layer (150) "between the optical sheet (140) and a liquid crystal panel (160)." [cite: US2007/0147081 A1, Abstract, FIG. 1] This layer converts the light's color to a target color, and given its placement, the light from the light guide plate would have passed through any intervening optical sheets.
• Other Elements: The other elements (light-emitting module, light guide plate, optical sheet with diffusion and prism sheets, and color conversion function) are covered by the combination of Shin and Kurihara as discussed for Claim 1.

Thus, Claim 20 is rendered obvious by the combination of Shin and Kurihara.

Claim 23 (Light Source with Reflective Plate and Conversion Sheet Coated on Diffusion Sheet)

Claim 23 describes a light source similar to Claim 1 but specifically adds a reflective plate under the light guide plate and states the conversion sheet is "coated with a color coordinate material on an upper surface of the diffusion sheet" such that it converts the light diffusing by the diffusion sheet.

• Reflective Plate: Shin explicitly includes a reflective sheet (130) positioned "below the light guide plate" (120). [cite: US2007/0147081 A1, Abstract, FIG. 1] The use of a reflective plate to enhance light efficiency is a conventional and obvious practice in backlight units.
• Conversion Sheet Coated on Upper Surface of Diffusion Sheet:
  • The '778 patent itself describes variations in placement, noting that a "color conversion material may be added into the diffusion sheet 141" or "coated with a color conversion material on an upper surface thereof" in its FIG. 3 embodiment. [cite: US8408778B2, Paragraph 0070]
  • While Shin and Kurihara show the conversion layer on top of the entire optical stack (diffusion + prism), the concept of coating a transparent sheet with fluorescent material (Shin [cite: US2007/0147081 A1, Paragraph 0026], Kurihara [cite: US2006/0268537 A1, Paragraph 0036]) was known. A PHOSITA, seeking to optimize the interaction between diffused light and the color conversion material, would find it an obvious design choice to coat the fluorescent material directly onto the diffusion sheet, either as a layer or by integrating it, particularly since the '778 patent explicitly contemplates this arrangement as a variant. Such a placement could optimize light mixing and subsequent direction by prism sheets.

Dependent Claims (e.g., Multi-Rank LEDs)

Dependent claims, such as Claims 8, 9, 24, and 26, relate to the use of LEDs with "different color coordinate distributions within a same color spectrum" or "multi ranks with a wavelength difference within a blue color spectrum."

• Multi-Rank LEDs: Hitachi Displays (US7350937B2) directly addresses this by teaching a lighting unit that uses "a plurality of light emitting diodes (LEDs) of different light emitting colors" or "slightly different wavelengths within the same color (e.g., blue LEDs with peak wavelengths of 450nm and 460nm)" to improve color uniformity and white light. [cite: US7350937B2, Abstract, Paragraphs 0013, 0020]
• Motivation: The motivation to combine the multi-rank LED teaching of Hitachi Displays with the color-converting backlight structures of Shin and Kurihara is to reduce manufacturing costs by allowing for a broader tolerance in LED production (using "multi ranks" or "failure ranks") while still achieving a precise target color output through the conversion layer. This directly aligns with the stated advantages of the '778 patent.

In conclusion, the independent claims (1, 20, 23) and related dependent claims of US8408778 would be considered obvious under 35 U.S.C. § 103 given the combined teachings of Shin, Kurihara, and Hitachi Displays, alongside common knowledge in the field of backlight unit design. The various placements and compositions of the color conversion layer, and the use of multi-rank LEDs, represent predictable design choices and combinations driven by well-known engineering principles and economic motivations in the context of improving LED backlight performance and manufacturing yield.