Emissive displays have proved useful for a variety of applications. For example, plasma display panels (PDPs) were at one time the leading flat panel display technology. More recently, displays using organic light emitting diode (OLED) technology have gained favor, most recently as a display component for useful devices such as mobile telephones, automobile radios, and many other consumer products. Some applications that are not display oriented have been postulated, including use as a pixilated emissive device in an additive manufacturing device. Additionally, an emissive array of pixels has been proposed for use as components for an automotive head lamp system.
More recently, emissive display system developers have demonstrated emissive displays based on backplanes driving small LEDs with a pitch between adjacent pixels of 17 micrometers (hereafter microns or μm) or less. These small LEDs are commonly termed microLEDs or μLEDs. LEDs are designed to exploit the band gap characteristic of semiconductors in which use of a suitable voltage to drive the LED will cause electrons within the LED to combine with electron holes, resulting in the release of energy in the form of photons, a feature referred to as electroluminescence. Those of skill in the art will recognize that semiconductors suitable for LED applications may include trace amounts of dopant material to facilitate the formation of electron holes.
The choice of semiconductor materials to form an LED will vary by application. In some applications for visual displays one monochrome color may be desirable, resulting in the use of a single semiconductor material for the LEDs of all pixels. In other applications, a full range of colors may be required, which will result in a requirement for three or more semiconductor materials configured to radiate, for example, red, green and blue or combinations thereof. In the case of additive manufacturing, a semiconductor material may be selected such that it emits radiation at a wavelength suitable for it to act as actinic radiation on a material used in the additive manufacturing process.
All potential variations are included within the scope of the present invention.
It is well known that LEDs made of different semiconductor materials may differ from each other in a number of important manners. First, the ideal drive voltage may differ from one material to another for various reasons. Second, the required current to reach a desired emission level may differ. For a multicolor display, it is important that the LEDs for each color be driven so that the relative intensities of the color meet the color balance desired for a display. This requirement is well known in the art although means for achieving it are variously understood.
It is well known that a preferred means for controlling the apparent intensity of an LED is pulse width modulation, also referred to as duty cycle modulation. Pulse width modulation is preferred because, as is well known in that art, voltage modulation of an LED often results in a shift in the color emitted by the LED, thereby complicating the task of maintaining color balance within the display. Such pulse width modulation necessarily requires that the rate at which pulses occur must be very rapid compared to the visual characteristics of human vision. This characteristic is typically referred to as critical flicker frequency or flicker fusion frequency. It is the frequency at which a human observer perceives a flashing light as a steady light.
As a practical matter, when gray scale is created using pulse width modulation, control of the intensity of an LED through pulse width modulation should be implemented separately from the creation of gray scale through pulse width modulation in order to ensure that full gray scale capability is retained. In that case, the pulse rate of the waveform used to create the duty cycle modulation of the LED to control the overall intensity of the LED should operate at a rate that insures that even the shortest duration gray scale modulation step receives a duty cycle “on state” during its operation. Otherwise, gray scale generation may be compromised through unintended aliasing.