Not all lighting technologies are equally suited to task lighting applications

An OLED desk lamp capitalizes on characteristics unique to organic semiconductor lighting technology to provide targeted illumination for accomplishing desktop tasks. Desk lamps are intended to deliver effective and controllable illumination to support visual performance by placing lighting close to task surfaces, providing an ample amount of glare-free, spectrally optimized light, and permitting flexible positioning and/or directional control of the light.

Achieving quality task lighting involves more than simply providing sufficient light to make a given task visible. Localized task lighting is deemed of high quality it enables tasks to be performed comfortably, accurately and efficiently. Not all lighting technologies are equally suited to task lighting applications. With the ability to deliver uniform light distribution, soft illumination and high color quality, OLED lighting has a competitive edge in creating a supportive visual environment with localized task lighting.

An exciting application of OLED lighting technology

OLED desk lamps are an exciting application of organic electroluminescent devices which we call them organic light emitting diodes (OLEDs). Structurally, an OLED desk lamp is a straightforward combination of the light source and a desktop mount. The essentials of high quality task lighting almost exclusively originate from the light source which requires no board-level integration, no external optics and no heat sinks. In comparison, LED desk lamps are complex devices that integrate multiple components to produce light that is conducive to task performance.

An OLED is a surface-emitting device that utilizes thin layers of organic semiconductors to produce light. The functional principle of OLEDs is similar to that of LEDs. When a voltage is applied in the forward direction between the anode and the cathode, holes and electrons are respectively injected from the two electrodes and pass through the carrier injection and transport layers to recombine in the light emitting layer (EML) and form excited molecules (excitons). Light is emitted when the excited molecules drop to a ground state. This process can be known as electroluminescence. OLEDs are composed of carbon based (organic) molecules that have electroluminescent properties. LEDs are typically fabricated from gallium based crystals doped with various inorganic materials (aluminum, arsenide, phosphide, indium, etc.).

OLED light panel

Typically, an OLED desk lamp incorporates one or more bottom-emitting OLEDs in which light is emitted through a transparent bottom electrode. This transparent electrode is usually the anode made of a transparent conducting oxide (TCO), for example indium tin oxide (ITO). Over the anode are the organic layers which start with the hole injection layer (HIL). The hole transport layer (HTL), emitting layer (EML), hole blocking layer (HBL) and the electron transport layer (ETL) are then deposited in sequence. The top layer is the cathode, which is typically a reflective aluminum or silver layer designed to maximize light extraction. The bottom electrode (anode) is constructed on a transparent glass or plastic substrate. Internal light extraction layers can be interposed between the substrate and anode to extract light typically lost due to the difference in refractive indexes among the transparent substrate, anode and organic layers.

The organic light module is subjected to an encapsulating treatment so as to protect the active layers from external influences such as moisture and oxygen. The encapsulation layer is then interfaced to a flat heat spreader which can be for example a thin aluminum sheet, copper plate or graphite foil. The more uniform heat dissipation over the entire panel the more uniform the light intensity.

Soft light from a surface emission device

Lighting exists to enable vision. OLED lighting stands in an advantageous position to address the qualitative needs of desktop illumination. OLEDs provide the source-level ability of allowing luminances of the light source to contribute favorably to visual performance and visual comfort for the specific visual tasks involved. In contrast, their inorganic cousins are high intensity point light sources that require the uses of sophisticated secondary optics to remove the harshness and achieve uniform light distribution. This inevitably adds to the cost and leads to high optical losses. High luminance can cause uncomfortable sensation of brightness. Glare develops when luminances are excessively high in relation to the state of adaption.

Light emitted by OLEDs boasts a natural softness. The luminance of OLED light panels is uniformly kept below 3000 cd/m2 across the entire span of the light emitting surface so there is no disability glare, discomfort glare and veiling reflections. Uniformity, the even distribution of illuminance across the task plane, is as equally crucial as glare control to close-range task lighting applications. In general, the ratio of average to minimum illuminance values across the practical task planes should not be lower than 1.4:1. It is a breeze for OLEDs to achieve an excellent uniformity because they are inherently surface-emitting devices.

The luminous efficacy of an OLED light panel is around 40-90 lm/W, which is incomparable to that of an LED. Nevertheless, its naturally diffused emission and homogeneous light distribution make virtually every lumen usable for task lighting. Since there’re are no losses in efficiency in secondary optical systems, the luminaire efficacy of OLED desk lamps is very close to that of the LED counterparts.

Faithful color reproduction

The quality of task lighting is also defined by color quality of a light source, which depends on the spectral power distribution (SPD), or the amount of energy emitted at various wavelengths of the visible spectrum. OLEDs blend individual spectral bands of red, green, and blue light to form white light. The color mixing approach enables the light source to deliver the right amount of radiant power fairly uniformly across the visible spectrum from 380nm to 770nm. Since objects inherently do not have color, but simply reflect wavelengths of the incident light. The spectral power distribution of the incident light therefore determines how object colors are rendered. A balanced spectrum allows OLEDs to provide accurate color rendition.

OLED desk lamps typically have a 90+ Ra (commonly known as CRI) and a R9 (for red color rendering) value exceeding 50. They outperform average LED desk lamps by a wide margin in the ability to faithfully reproduce the colors of various objects. The most common white LEDs today use blue (450-460 nm) wavelength sources to pump a phosphor, or mix of phosphors. The resulting broad-band yellow light combines with the blue to create white. However, achieving a balanced spectrum with this method comes at a high cost and loss of luminous efficacy. This fundamental tradeoff often results in a compromise toward efficacy at the expense of color quality in LED products.

Visually pleasing, circadian friendly lighting

Aside from color rendition, a light source can be characterized by its color temperature. The measurable color attribute describes the color of white light, in terms of its apparent “warmth” or “coolness.” Selection of the color temperature is an important aspect of task lighting as apparent colors of white light affect the way people feel and work. White light with warmer tones which exhibit a correlated color temperature (CCT) in the range of 2700K to 3200K tends to create a sense of comfort and relaxation. Cool enough tones, which correspond to higher CCT white light, have a stimulating effect and enhance concentration.

The “warmth” or “coolness” is essentially a reflection of the spectral composition of a light source. Different combinations of wavelengths can have implications beyond simple vision and color perception. Desk lamps typically use light sources of 3500K to 4100K that help improve visual acuity and allow people to stay alert, active and productive. Lighting at an excessively high CCT such as 6000K, however, can cause people exposed to the greatest risk of circadian disruption and subsequent health effects. The high percentage blue content in the spectrum of the excessively high CCT white light can acutely suppress nocturnal melatonin production.

Absence of blue light hazard (BLH)

At very high intensities, blue light (short-wavelength 400-500 nm) can cause photochemical damage of the retina. The potential for retinal injury is referred to as blue light hazard (BLH). Task lighting at an excessively high CCT and high intensity should be prohibited as the amount of high energy blue light can induce a high oxidative stress. OLEDs are claimed by their manufacturers to have the highest photobiological safety among all type of light sources. This presumption is based on the absence of blue spikes in the SPDs of OLEDs. This is a misinformed theory. Different types of light sources at the same CCT do not vary widely in the total amount of blue in their spectrums. Even though phosphor-converted LEDs pumped with blue LED chips do generally exhibit a local peak in the SPD, the total amount of blue light in these LEDs is not necessarily higher than that of the OLED equivalents.

In practice, OLED lighting products are very safe because they are not available in high CCT options. The CCTs of currently available OLEDs are limited to 4000K. Due to the low efficiency and stability of blue emitters, increasing the percentage of blue in the spectrum of OLEDs for a cooler tone is inversely correlated with luminous efficacy and the lifetime of an OLED. In contrast, there is penalty in efficacy for LEDs to deliver white light with at low CCTs due to the energy loss associated with phosphor down-conversion (Stokes loss). As a result, low cost, high CCT LEDs that deliver blue-rich light have been heavily marketed.

OLED driver

OLEDs need to be properly driven to ensure that they perform to their full potential. A substantially constant current must be provided to the load as the transfer from DC power to light output in an OLED is nearly linear. The OLED driver is typically implemented as a switch mode power supply (SMPS) that converts alternating current (AC) line power to a DC output of the desired amplitude.

One of the critical performance markers of an OLED driver is the ripple value. Ripple is the residual periodic variation superimposed on the DC output current by alternating voltage. Sufficiently large ripples in the DC current provided to the OLED load can result in flicker and other visual anomalies. This ripple current must be suppressed to such a low amplitude that low frequency sinusoidal fluctuations in a waveform is completely eliminated to remove flicker visible to the human eye at a frequency of up to 80Hz, and also little high frequency ripple component is generated at the DC input. High frequency flicker is not apparent to the naked eye but still has negative effects on health, such as headaches or even seizures in extreme cases. High-frequency flicker also affect one’s ability to concentrate, which is extremely undesirable for task lighting.

In order to deliver flicker-free lighting high concentration tasks, the peak-to-peak current ripple value in the DC output of an OLED driver should be controlled within ±5%. Typically, 10 percent flicker at 120 Hz is the maximally acceptable value for detail-intensive, task-specific lighting.

OLED dimming

OLED desk lamps generally have a dimming function that allows multi-level intensity control (step dimming) or wide/full-range intensity control (continuous dimming). The preferred method for continuous dimming OLEDs is to adjust the current flowing through the OLED using constant current reduction (CCR) drivers. OLEDs do not respond to rapid on-off cycles of pulse-width modulation (PWM) in the same manner as LEDs, hence photometric flicker can be present under dimmed condition.