Backlighters for illuminated LCD displays normally have four or more surface mount diodes to ensure a bright enough display. This is a considerable drain on a battery, as up to 20mA current is consumed when the display is illuminated. The problem can only get worse as the communications industry moves towards permanently lit displays.
One method of addressing battery drain is to lower the operating current by reducing the number of diodes needed to light the display. CTP COIL optical design expertise coupled with Hewlett Packard’s LED technology has enabled a reduction in the number of LEDs on the backlighter to two. One benefit offered to equipment manufacturers is a time saving in selecting diodes of similar brightness for an application, as diode brightness varies from batch to batch.
Both companies wanted to reduce the overall thickness of the viewing area from the current industry standard of 1mm to 0.5mm. At the outset, COIL did not know whether it would be possible to fill such a narrow injection mould cavity and still retain the required surface definition and flatness.
Initial concepts were tested using 2D raytracing software, and as the CAD model developed, with ASAP, a 3D radiometric raytracer that could accurately predict illumination levels was produced. Diode selection was critical, and on completion of the initial concepts, it became clear that a side firing diode with a beam half angle of about 120 degrees would be most effective.
The input device was to consist of a cylindrical lens leading to a tapered section (light guide) that channelled the light into the 0.5mm thick viewing area. The thicker input section was to sit in a rebate in the LCD’s underside so that the whole assembly, consisting of PCB, light guide and LCD, would be no thicker than 3.2mm.
The diode was modelled in ASAP as an incoherent light source and apodised to match the supplier’s catalogue beam pattern. Using IGES, the 3D CAD model was imported into ASAP and raytraced many times to ascertain the optimum position for the diode. The best results were obtained when the distance between the diodes was approximately four-fifths of the width of the light guide, with the diodes tilted at 23 degrees towards the centre of the viewing area. Without the tilt, the edges of the display showed all the brightness while the centre was dim.
At each source 1,000,000 rays were created and, as they split and scattered, 4,000,000 rays were traced, 2,000,000 of which reached the collector surface. Eventually, a contour map of ray patterns was produced to show the expected illumination and likely distribution of light within the viewing area.
Next, a spatially distributed scatter pattern was formulated to ensure that the light level was proportional to distance from the LED light sources. To provide additional reflection and scatter, a Melonex wafer was sandwiched between the PCB and the light guide.
COIL’s next task was to assess the viability of moulding a component as thin as 0.5mm. Extensive trials using Moldflow analysis software indicated that the part would mould at 0.5mm, but that the light input would not fill right into the thinnest edge.
Eventually, a flowleader was added between the gate and the input feature to ensure the flow to the input feature remained molten until 2.7s into the packing cycle. Shrinkage and Moldflow analyses highlighted other areas of concern and showed that the gate position could not be on the furthest edge from the input features.
Once the mould was complete, performance measurements were made with a lightmeter with the LCD fitted and removed. The figures obtained compare favourably with measurements taken from an eight-diode display.
Combined Optical Industries Tel: 01753 575011