Subject: Re: [visionlist] LED monitors
From: Andrew Watson <[email-masked]>
To: [email-masked]
Date: Thu, 19 Feb 2009 08:05:00 -0800
List-Id: Vision Science Mailing List <visionlist.visionscience.com>

Deborah,

Below is a section of a displays standards document that Louis  
Silverstein and I wrote about LED displays.
I hope it is helpful.

OLED displays may well be useful in psychophysics, though I have not  
tried them yet myself.
I believe they are used in some off-the-shelf adaptive optics systems  
from Imagine Eyes  (http://www.imagine-eyes.com/).

Andrew B. Watson
Senior Scientist for Vision Research
NASA Ames Research Center
Moffett Field, CA 94035-1000
(650) 604-5419

10.5.4.2.3   Light-Emitting Diodes (LEDs) and Organic Light-Emitting  
Diodes (OLEDs)

The LED is a semiconductor device consisting of a single p-n  
junction;  light is emitted when the junction is forward biased by the  
application of a suitable voltage. The first commercially available  
LEDs were introduced into the marketplace in 1968 and quickly became  
an important technology for indicator lamps and small segmented  
alphanumeric displays.

  LEDs are now capable of producing a full spectrum of colors with  
very high color saturation and good luminous efficiency.   
Nevertheless, relatively high cost, interconnect complexity and  
manufacturing limitations in building high-density arrays of full- 
color LEDs have restricted their usage primarily to very large  
displays for digital signage and electronic billboard applications.

Organic LEDs (OLEDs) and polymer LEDs (PLEDs) are LEDs whose emissive  
electroluminescent layer consists of a film of organic compounds  
(Bulovic, 2005; King, 1994).  These films emit light when subjected to  
an electric current. The SPD of the emitted light depends on the type  
of organic molecule in the emissive layer.  Full-color OLEDs are  
generally achieved by either spatial patterning of R, G and B emissive  
materials or by use of a broad-band emissive material in conjunction  
with a spatial pattern of R, G, and B color selection filters.  The  
intensity of the emitted light depends on the amount of electrical  
current applied.  The nonlinear relationship between voltage and  
current provides OLEDs with a nonlinear transfer function which is  
well characterized by a power function.

  Claimed advantages for OLEDs include:  simplified manufacturing  
structure; compatibility with flexible substrates; lower manufacturing  
costs than LCDs or PDPs; high contrast with true black level; high  
luminance with good luminous efficiency; low power consumption; fast  
temporal response; wide viewing angle; and excellent grayscale  
performance (Bulovic, 2005; Ghosh & Hack, 2004).  The principal  
disadvantages include limited lifetime of OLED and PLED emissive  
materials, lack of a stable short-wavelength emissive material with  
good lifetime, differential aging of emissive materials with different  
SPDs and susceptibility to contamination and damage from moisture.  In  
addition, reflections from metal cathodes and other metallic  
structures within these devices can result in high levels of internal  
specular reflection under ambient illumination with commensurate  
degradations of ambient contrast.  Circular polarizers may be used to  
mitigate these reflections, but their use dramatically reduces the  
luminous efficiency of the display.

OLED and PLED display technologies remain in a very active state of  
development with a great deal of ongoing research on OLED / PLED  
materials, display system architectures and manufacturing processes.   
To date only a few small, mobile display products using OLEDs and  
PLEDs have appeared and stayed on the market.

Display Technology Attributes

General Rating

Comments

Spatial Addressability

Spatial Resolution

High

High

-Spatial resolution can be enhanced by vertically stacked color pixel  
structure

Temporal Response

Medium to High

-Native response in sub-millisecond range

-Temporal aperture extended by sample-and-hold mode of operation in  
active-matrix configurations

Luminance

Medium to High

-Material dependent

-Use of circular polarizer to enhance ambient contrast reduces luminance

Contrast

Ambient Contrast

High

Low to Medium

-Excellent black levels

-Specular reflectance from metal cathode limits ambient contrast

-Circular polarizer can be used to enhance ambient contrast

Grayscale Performance

Very High

-Continuous analog grayscale capability

-transfer function a true power function

Viewing Angle

Very High

-Effectively Lambertian viewing

-Contrast enhancement filters can attenuate luminance off axis

Color Gamut

Ambient Color Gamut

High

Low to Medium

-Color gamut defined by OLED / PLED materials

-Specular reflections from metal cathode reduces color gamut under  
ambient

-Circular polarizer can be used to enhance ambient color gamut

Physical Package

Very High

-Very small footprint & light weight

-Very small depth required

Application Flexibility

Very high

-Very amenable to portable or battery-powered applications

-Great flexibility in screen sizes

Other

-Prone to internal specular reflections under ambient

-Circular polarizer reduces luminance

-Differential aging of color materials can cause color shifts over time

-Active-matrix configurations prone to motion blur with dynamic imagery

Table 6. OLED / PLED Technology Attribute Ratings and Comments

On Feb 18, 2009, at 5:36 PM, Deborah Apthorp wrote:

> Hi,
>
> Does anyone know anything about the use of LED (NOT LCD) monitors  
> for psychophysics? I am not sure if there are any available yet with  
> a high enough refresh rate, but it seems as if potentially these  
> could be very useful for vision research. I can't find much about  
> them via Google so I wondered if anyone else in the vision community  
> is exploring these possibilities.
>
> Thanks,
>
> Deborah Apthorp
>
> (University of Sydney, Australia)
> _______________________________________________
> visionlist mailing list
> [email-masked]
> http://visionscience.com/mailman/listinfo/visionlist

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